Evolutionary Tree of Life (Full Series)

Hi, this is Matt Baker. One of the things that this channel is best known for is its family trees. Well, today I'm going to show you the ultimate family tree, the one that connects every single living thing on our planet, the evolutionary tree of life. If you want to get a copy, just head over to our website usefulcharts.com or click the link in the description or pinned comment. At the very bottom of this chart is LUCA, which stands for Last Universal Common Ancestor. This is the single-celled organism from which both bacteria and archaea evolved, as well as all other living things on the planet. Now as to where LUCA and its ancestors came from, well, that is a question that falls outside of the theory of evolution, and therefore I won't be addressing it in this video. However, this doesn't mean that scientists have absolutely no clue about how the first living cells arose from non-living matter. There are actually several theories out there, and if you're interested in learning about them, just do a search for the term abiogenesis. I should also point out that I've put viruses over here, unconnected to the main tree. That's because we're still not sure exactly how viruses are related to life. They may represent a stage before the emergence of life, or they might be the result of some DNA or RNA escaping from a living cell. Either way, the main difference between anything living and a virus is that life can reproduce on its own, whereas a virus needs a living host in order to replicate. Nowadays, viruses are categorized into realms, of which 3 are shown on this chart. Duplo DNA vira includes viruses that are based on a double strand of DNA, hence duplo. These include the viruses that cause herpes, mono, and chickenpox. Vari DNA vira includes various DNA-based viruses, such as the virus that caused smallpox, as well as the closely related monkeypox. However, most viruses that you're likely to be familiar with fall under ribovira because they are dependent on RNA. RNA rather than DNA. This includes the viruses that cause the common cold as well as those that cause the flu, which is short for influenza. Take note that there is in fact a difference between a cold and a flu, a flu being the more serious of the two. But of course more serious than both a cold or a flu is COVID, which is caused by a coronavirus. Coronaviruses fall under the Nidovirales category. Finally, take note that the Riboviridae also includes the viruses that cause measles, mumps, and rabies, as well as HIV. Okay, we're now ready to move up from LUCA to the earliest branches of life. 10 years ago when I first made an evolution chart, it was thought that life initially went in 3 directions, which is why the previous version had 3 domains: Bacteria, Eukaryote, and Archaea. However, nowadays the leading theory is that there should be only 2 domains, Bacteria and Archaea, with eukaryotes evolving out of Archaea. Let's look at Bacteria first. Bacteria is the type of single-celled life that we're most familiar with because, like viruses, they cause a lot of diseases. Take for example Streptococcus, which causes both strep throat and pneumonia, or Staphylococcus, which people often get in wounds. Both of these bacteria belong to the Bacillota phylum. An even larger bacteria phylum is Pseudomonadota. It includes the germs that cause E. coli, cholera, salmonella, meningitis, and worst of all, the bubonic plague, aka the Black Death, the most deadly pandemic in all of human history. But not all bacteria is bad. There's also good bacteria called probiotics, such as Lactobacillus, which is used to make yogurt and is good for our digestive systems. There's also pelagibacteria, pelagis being Greek for of the sea. These bacteria live in the ocean and are super important for the Earth's overall ecosystem. Archaea are far less familiar to the average person. They were initially lumped together with bacteria, but are now understood to be quite distinct. In fact, they include some of the most extreme forms of single-celled life, such as those that live in super hot environments called thermophiles and those that live in super salty environments called halophiles. However, Archaea also includes a proposed superphylum called Asgard, named after the home of the gods in Norse mythology. It's from Asgard cells that the very first eukaryote cell is thought to have developed around 2 billion years ago. Now eukaryote cells differ from bacteria and archaea cells in that they are much larger and more complex, being comprised of many different parts, most notably a nucleus, which is where the cell's DNA is located. This contrasts with bacteria and archaea cells, which have DNA floating around in the middle. Another part of a eukaryote cell that you might be familiar with are the mitochondria, Which are kind of like little energy factories. Interestingly, these came to exist within eukaryote cells via a process called endosymbiosis. Basically, a bacteria cell similar to those Pelagibacteria I mentioned earlier went inside an Asgard cell to create a sort of supercell. One example of these early type of supercells are amoebas. However, over time, these new kind of cells started to join together to form multi-celled organisms as well. Such as brown algae, which is now placed in the SAR supergroup. But then there was yet another endosymbiosis event around 1 billion years ago. This time, a type of cyanobacteria entered a eukaryote cell to create archiplastids, which have yet another special cell part called plastids. The most famous type of plastid is the chloroplast, which is used by plants to convert energy from the sun into a type of energy that can be transferred to other living things. As humans, all of the energy that we have comes from eating plants or from eating animals that eat plants or in rare cases from eating animals that eat animals that eat plants. Take note that some types of brown algae did not originally have plastids but perhaps inherited them from red algae. Now take note that all of these early eukaryote life forms exist outside of the 3 main kingdoms. Which are plants, fungi, and animals. At one time, these early eukaryotes were placed in a 4th kingdom called Protists, and then later they were divided into a bunch of smaller kingdoms. But nowadays, the exact classification has been left up in the air because scientists are still trying to figure out exactly how everything pieces together. Another big thing that has only been discovered quite recently is that fungi are actually more closely related to animals than they are to plants. This is because both fungi and animals evolved around a billion years ago from early opisthokonts, whereas plants did not. The term opisthokont comes from a combination of Greek words that means something like rear pole. That's because simple opisthokonts have these sort of tail-like things called flagellum. Now for the rest of this episode, as well as episodes 2 and 3, I'll be focusing entirely on the animal branch. However, take note that in episode 4, I will be coming back to talk about both plants and fungi, so stay tuned for that. Anyhow, the earliest types of animals probably looked something like sea sponges, which to be honest don't really seem like animals, even though they are. You see, despite the fact that sea sponges don't move, they are in fact animals because they have animal cells, just like you and me. Corals, which belong to the same phylum as jellyfish, also don't move, but they at least have a very simple nervous system. And it is the nervous system that eventually allowed animals to evolve the ability to move, react, and eventually to think. But I'm getting ahead of myself. The other big thing that developed early on in the animal branch is bilateral symmetry. You only have to look in the mirror to understand what this is. It's the reason why you have 2 eyes, 2 ears, 2 arms, 2 legs, etc. Basically, if you draw a line down the center of your body, or that of most animals, you get 2 sides that mirror each other. With the emergence of Bilateria, animals went in 2 very different directions. There's the Protostome branch and the Deuterostome branch. These 2 terms mean mouth first and mouth second. Mouth first and mouth second. Referring to the different ways that animal embryos can develop. Most animals have a digestive system with a mouth on one end and an exit hole on the other. Protostomes develop the mouth first and then the exit hole, whereas deuterostomes do the opposite. The other big difference is that the protostome branch went on to produce life forms with an exoskeleton, or outer shell, like insects, for example. Whereas deuterostomes went on to produce life forms with an endoskeleton, meaning that they have their hard parts called bones on the inside. Let's look at protostomes first. The earliest types of protostomes were the ones that don't have exoskeletons, such as flatworms and earthworms. But then came mollusks, which do, such as snails and clams. The most interesting type of mollusks are the cephalopods, which include squid and octopuses. Their ancestors would have had shells too, but at some point they lost them. However, they gained something potentially better: eyes, very similar to our own, which is a great example of something called convergent evolution. These are instances where similar structures evolved independently of each other on different branches of the tree. Octopuses in particular are also among the most intelligent non-human animals on the planet, which just goes to show that there's no one branch on the tree that is necessarily more advanced than any other. Now while I'm here, I'll point out the fact that most living things can be categorized according to the very old naming system developed by Swedish biologist Carl Linnaeus back in the 1700s. Linnaeus is the one who developed the ranking system that starts with kingdom and then proceeds through phylum, class, Order, family, genus, and species. So to make things clear, many species make up a genus, many genera make up a family, many families make up an order, many orders make up a class, many classes make up a phylum, and many phyla make up a kingdom. Now since the 1700s, it's become clear that things are actually more complicated, and that's why there are often other category names inserted between the main ones. such as Bilateria and Protostomia. In these cases, those divisions are simply called a clade. Okay, next I want to start back at the Protostome node and proceed up this time towards the insects. But first, I want to point out this event here, the Cambrian Explosion, which occurred around 540 million years ago. You see, this is the point in which animal life started to go in a bunch of different directions. as evidenced by the wide variety of fossils that can be found after this point. However, it's important to remember that creatures with bones or shells leave better fossils, so it might just be that 540 million years ago was when shells and bones became more common. Regardless, it was around this time that the first tactilepods evolved, tactilepod meaning organized feet. This category includes tardigrades, which look like tiny bears wearing hazmat suits. But more importantly, it includes the massive Arthropoda phylum, which has all the spiders, insects, and other creepy crawly things. These were the very first animals to leave the water and crawl onto land, although obviously some, like crabs and lobsters, still live in the sea. The very first arthropods would have looked something like this, kind of like a millipede. Note that it has a segmented body and lots of legs. All modern arthropods have the same basic body plan but with modifications. For example, insects evolved to have only 6 remaining legs. However, another 6 of their former legs evolved to serve as mouthparts. In addition to this, most of these segments fused over time so that insects are now left with only 3: the head, thorax, and abdomen. Contrast this with spiders who, as you probably know, have 8 legs. and only 2 segments. But what you probably didn't know is that most of their legs evolved from the same appendages that now serve as mouthparts in insects. Spiders' 2 mouthparts, or fangs, actually evolved from what used to be their antennae, and 2 of their former legs evolved to become pincers, which are much larger on the closely related scorpions. Another thing to note is that crabs are actually more closely related to insects, being that both of these groups evolved from early Insects, however, fall under the category of hexapods because, like I said, they all have 6 legs. Some insects also developed wings early on, such as dragonflies. But regardless of whether an insect crawls or flies, technically speaking, most insects are not bugs. Although the word bug is used casually to refer to any creepy crawly thing, from a scientific point of view, only those insects belonging to the Hemiptera order are true bugs. The largest category of insects is actually the Holometabola superorder. These are insects such as beetles and butterflies that go through a metamorphosis involving 4 stages: egg, larva, pupa, and adult. Included in this group is the Hymenoptera order, which includes both ants and bees, arguably some of the most intelligent insects due to their complex social structures. Okay, let's now go all the way back down to Bilateria and follow the deuterostome branch instead. However, the most important deuterostome phylum is Chordata. Chordata includes lancelets as well as all the vertebrates, which are animals with backbones protecting a column of nerves that run down the back. The earliest vertebrates would have looked similar to lampreys, which have a mouthful of teeth but no jaw. Now once the jaw evolved, things went in 2 directions. On the one hand we get fish with skeletons primarily composed of cartilage, like sharks and rays, and on the other we get fish with skeletons primarily made of bones, which are much harder. If we follow this bony fish line, we'll see that things once again split in 2. We get ray-finned fish and lobe-finned fish. The vast majority of fish that you're likely familiar with are ray-finned, meaning that they have simple fins like this that flap, helping the fish to swim in the water. All ray-finned fish belong to the Actinopterygii class, which contains about 50% of all known vertebrate species. Most of these in turn are teleosts, which can then be broken down into 2 main clades: the Otocephala and the Euteleoste. Otocephala includes catfish, sardines, tetras, and carp, with the carp order being the one that goldfish and koi belong to, as well as minnows. Whereas Euteleoste includes cod, salmon, and the perchomorphs. Perchomorphs are, evolutionarily speaking, newer fish in that they mostly developed after the extinction of the dinosaurs. They include edible fish like tuna, halibut, and perch, but also popular aquarium fish like guppies, cichlids, and bettas. And then there's one of the strangest fish of all: seahorses, named because they have upright heads that look like horses. Let me now tell you about lobe-finned fishes. Although far less common, they are super important in that it is from the lobe-finned fish, or Sarcopterygii, that all land vertebrates descend. So as you can see from this diagram, lobe fins are more complex than ray fins. They have bones that look more like little limbs, which is basically what they are. Although lobed finned fish just used them to flap and hence swim, this would later allow certain creatures to do a bit of crawling in shallow water as well. The main lobe-finned fish that are still around today are the coelacanths and the lungfish. Okay, so the first thing we need to discuss is how a creature like this eventually evolved into a creature like this. Both have skulls and a central backbone, so that's not the issue. The issue is how a fish's fins, which are used for swimming, evolved into a land animal's limbs, which are used for walking. Well, like I said last time, it has to do with a special category of fish called Sarcopterygii, which still exists to this day, but are quite a bit different from the dominant Actinopterygii class. You see, most fish have ray fins that look like this. They're basically just one strong bone with a bunch of really thin bones attached to it, forming a sort of fan that flaps back and forth. In contrast, Cercopterigi fish have fins that look like this. They too have a bunch of thin bones, albeit a bit thicker, but what's really interesting are these 3 bones here. If you were to look at an X-ray of your own arm, you'd see that the upper part between your shoulder and elbow has a single bone which is called the humerus, whereas the lower part between your elbow and your hand has 2 bones. called the radius and ulna. After that, you get all the small bones in your wrist, hand, and fingers. Well, this basic structure can be found in just about every 4-limbed animal on Earth, whether it's a lizard or a lion. Even birds have it, as well as bats and whales. That's because all 4-limbed animals are the descendants of sarcopterygii fish. But how did some fish go from having fin rays to having lobe rays? Well, it may be easier than you think. If you've ever had a home aquarium, you might be familiar with zebrafish. They're often used in scientific research, and nowadays you can buy a genetically modified version called GloFish, which glows in the dark. But just a few years ago in 2021, scientists were able to genetically modify a zebrafish in another interesting way. By creating a mutation in just one gene called HOXA11, They were able to breed zebrafish with 2 new fin bones similar to the radius and ulna. However, stronger limbs were not the only thing that were needed in order to make the transition from water to land possible. There was also the issue of breathing. As you probably know, most fish breathe using gills, which are organs behind the eyes used to extract oxygen from water. Well, it might surprise you to learn that lungs actually evolved around the same time as gills. In other words, the earliest fish had both. However, most fish ended up relying entirely on their gills, which then allowed their lung to evolve into the swim bladder, which they could then use to control their buoyancy. But notice that I said most fish. One of the surviving members of the Sarcopterygii branch is the lungfish, which as its name implies uses lungs to breathe. But even if some fish had lungs as well as stronger limbs, what led them to leave the water and crawl onto land? Well, it wasn't a sudden move like most cartoons suggest. Like most things in evolution, it took millions of years. First of all, it's important to remember that vertebrates were not the first animals to leave the water. Insects beat them onto the land by tens of millions of years, and plants and fungi were there even earlier. Which is good because once the vertebrates arrived, they needed something to eat. And eating is one explanation for why some fish eventually ventured into shallow water, and then into shallower water, and then even more shallower water, and so forth. You see, one of the great things about shallow water, from a fish's perspective, is that it has less predators. So once there were ample insects to eat there— I mean, Why wouldn't a fish move into that neighborhood? Now I've taken a little longer to explain this part of the tree because it's often the part that some people find hard to accept. In the past, creationists liked to talk a lot about missing links and how no one had ever found a fish with feet. Well, that's no longer an argument they use because in recent decades we've actually found fossils that look exactly like a half-fish, half-lizard-like creature. The most famous of these is Tiktaalik, discovered in 2004. Living around 370 million years ago, it had scales and gills like a fish, but also a flat head and strong downward-facing limbs like a lizard. Note in particular that it had 4 limbs. This is important because every single vertebrate after this point also only has 4 limbs. which is why they are called tetrapods. The earliest tetrapods, such as Ichthyostega, would have been amphibious, meaning that they could live both in water and on land. Some tetrapods still can, and these are placed in the Amphibian class, the 2 most well-known groups being salamanders and frogs. Take note that a toad is just a special type of frog with drier skin and shorter back legs. All non-amphibian tetrapods are amniotes. This is because they evolved to have an extra layer that surrounds their embryos, forming what's called the amniotic sac. Evolving around 320 million years ago, this served to better protect the embryo, and hence it allowed certain tetrapods to lay eggs on land instead of in the water, like frogs and salamanders still do. The end result was that amniotes were able to venture further and further away from the water. At this point, tetrapods split in 2 very different directions. We get the synapsid line, which eventually led to mammals, and the sauropsid line, which leads to reptiles and birds. For the rest of this video, we are going to focus exclusively on the sauropsids, Returning to the synapsids in episode 3. First of all, take note that the word sauropsid literally means lizard face, even though not all reptiles are actually lizards. Lizards are actually just one type of reptile, but because they are the most common type around today, people sometimes use the 2 terms interchangeably. All lizards, such as geckos, dragon lizards, chameleons, and iguanas, belong to the Squamata order, as do snakes. Snakes are included because they actually evolved from earlier lizards that went on to lose their legs. Another major reptile order that still exists today is the Testudines order, which includes turtles and tortoises. The difference between a turtle and a tortoise is kind of the same as the difference between a frog and a toad. Turtles live in more wet environments, whereas tortoises are just a special type of turtle that live in more dry environments. Finally, in terms of major extant reptiles, there's the crocodile order, which also includes alligators. The easiest way to tell the difference between a crocodile and an alligator is to look at their nose, although you obviously don't want to get too close. Crocodiles have a more pointy nose, like a V, whereas alligators have a more roundy nose, like a U. Note that crocodiles belong to a subgroup of reptiles called archosaurs, which means that they are more closely related to dinosaurs than either lizards or turtles. Now before I move on to talk about dinosaurs, let me point out 3 orders that are often associated with dinosaurs but were not actually true dinosaurs in the technical sense. These include pterosaurs such as pterodactyls, which most people tend to think of as flying dinosaurs, as well as ichthyosaurs and plesiosaurs, which most people think of as swimming dinosaurs. Note that ichthyosaurs looked more like dolphins, whereas plesiosaurs looked more like the supposed Loch Ness Monster. Actual dinosaurs are only those reptiles that belong to the Dinosauria clade. which evolved from early archosaurs. As you might know, the word dinosaur literally means terrible lizard. It was coined in 1842, around the same time that the very first partial dinosaur skeletons were found and preserved. Now humans around the world had been finding individual dinosaur bones for centuries before this, but it wasn't until the mid-1800s that geologists started to realize that they belonged to an extinct form of large reptiles. Prior to this, it was assumed that they must have belonged to giant humans or dragons. Nowadays we know that dinosaurs lived approximately 240 million years ago and can be divided into 2 main categories: Ornithischia, meaning bird-hipped, and Saurischia, meaning reptile-hipped. This is somewhat confusing because birds eventually evolved from Saurischia, Anyhow, ornithischian dinosaurs were mostly plant-eating, and they include well-known genera like Ankylosaurus, Parasaurolophus, Stegosaurus, and Triceratops. In contrast, saurischian dinosaurs include both plant-eaters, called sauropods, as well as carnivores, called theropods. Sauropods, like the familiar Brontosaurus, are those really big dinosaurs with long necks. In fact, they were the largest animals to ever walk the Earth. However, it's the theropods, such as Tyrannosaurus rex, that we tend to consider to be the mightiest of all the dinosaurs. Which is interesting because it's from the theropod dinosaurs that birds descend. The path to birds starts with Avialae, which simply means bird wings. This clade includes Archaeopteryx, which is the earliest known dinosaur to have feathers and wings. Remember, pterodactyls are not true dinosaurs, and their line died out. Archaeopteryx is therefore also considered to have been the first bird, even though technically it does not belong to the Aves, or bird, class. Now this is where I have to point out an anomaly on this chart. As I mentioned last time, living things are still classified using the old Linnaean naming system, which works like this: inside a kingdom are many phyla, inside a phylum are many classes, inside a class are many orders, and so on. There's never supposed to be a phylum within a phylum or a class inside a class. But in this case, we do have a class within a class. We have the Aves class nested inside the Reptile class. This is because when Linnaeus made the original system, he didn't know that birds were descended from reptiles. He thought they were a totally separate category, and therefore he classified them as such. But we now know that birds definitely descend from reptiles, from dinosaurs to be exact. So does this mean that birds are dinosaurs? Well, technically speaking, yes, although in common usage we tend to reserve the word dinosaur exclusively for the now-extinct non-avian dinosaurs. Birds are also technically reptiles, but again in common usage we tend to use the word reptile only for non-avian reptiles. Because of this change in understanding, some taxonomists want to stop calling Aves a class and simply consider it a general clade instead. I've opted to keep it as a class because I think generally people still tend to think of birds as being distinct from other reptiles, which is why they end up being a class inside a class. Okay, when it comes to birds, the oldest type cladistically speaking are ostriches and the related emus and kiwis. Ostriches cannot fly, but evolved from earlier birds that could. Next comes fowl, which can be divided into waterfowl, like ducks, and landfowl, like chickens. Some fowl, like geese, are very good at flying, while others, like chickens, can fly only a little bit. Of course, one of the most important things about fowl is that they are a major food source for humans, not only in terms of their meat, but also their eggs. All the remaining birds fall under the category of neoaves, or new birds, To explain their origin, I first need to introduce you to some of the terms used for measuring time in the deep past, which I've largely avoided up to this point. In episode 1, I mentioned the Cambrian Explosion, which is so named because it occurred at the beginning of the Cambrian Period, the first of 12 time periods that cover the last 540 million years, give or take a million. Now take note that the starting date of 540 million years ago was chosen not because that's when life began, but rather because that's when life started to go in a bunch of different directions, as evidenced by the fossil record. So after the Cambrian comes the Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary. Now throughout the history of life on Earth, there have been 5 major extinction events caused by either asteroid impacts or volcanic eruptions or some other huge catastrophe. In each case, these events caused the majority of species living on the planet to die out completely. However, this in turn helped drive evolution because the species that did survive could suddenly take advantage of a dramatically new situation. So what we find is that following each one of these major extinction events, we get what was once a small branch of the tree suddenly going in a bunch of new directions. So for example, after this one here, that's when we suddenly got lots of different types of insects and bony fish. And this one here, well, that's around the time that the first land vertebrates started to really thrive. But then came this one, the P-T extinction. It's named that because it marks the transition between the Permian and Triassic periods. Well, it was the biggest mass extinction event of all time, killing off about 80% of all ocean species and about 70% of all land species. In fact, the P-T extinction changed things so much that we use it to mark the transition to a whole new era. We call the first 6 periods the Paleozoic Era, Paleozoic meaning old life, whereas we call the next 3 periods the Mesozoic Era, meaning middle life. The Paleozoic is also sometimes referred to as the Age of Fish because the greatest diversity of life during this entire era could be found in the ocean, whereas the Mesozoic is referred to as the Age of Reptiles Because that's when the dinosaurs dominated. Although true dinosaurs didn't actually appear until the Jurassic, after yet another major extinction event. Which brings us to the final mass extinction, and the one that you're likely the most familiar with. Most people know it as the K-T extinction, K standing for Cretaceous and T standing for Tertiary, which was an older term used for the Paleogene and Neogene. The Paleogene and Neogene. We know that this one was definitely caused by an asteroid, and we even know exactly where it hit. It's right here in Mexico. But the reason most people know about it is that this was the event that killed off the dinosaurs, paving the way for mammals to dominate. That's why the current era, the Cenozoic, which means recent life, is also called the Age of Mammals. However, that's a bit misleading because it could also just as accurately be called the Age of Birds, which is the reason why I took the time to explain all of this. You see, it was right after the K-T event and the extinction of the dinosaurs that the neoaves really took off. Or to put it another way, it was after the K-T event that the non-avian dinosaurs were replaced by the avian dinosaurs. Okay, so let's now take a look at the Neoaves. First off, there's the seabirds, which include herons and pelicans, but also penguins. And then there's the shorebirds, which include seagulls and their relatives. The word Columbia comes from the Latin columba, which means a pigeon or dove. So that's why these birds are in the Columbiformes order. Take note that there's no real difference between a pigeon and a dove, besides the fact that we tend to use the word dove for the smaller species in this group. The word Apodiformes literally means footless, even though birds in this order, such as hummingbirds, do actually have feet. It's just that they have rather small feet. So small that they can't even use them to walk or jump. All other neoaves fall into the land birds category, which could more accurately be called the tree birds, category because the defining feature of this group is that they mostly live in trees. From here things split into the Afroaves, which originally evolved in what is now Africa, and the Australaves, which originally evolved in Australia and South America. Afroaves includes woodpeckers, most birds of prey like eagles and hawks, as well as owls. Whereas Australaves includes parrots, And the largest bird order of all, the passerines, or perching birds. Passerines are well known to most people because they include all of the songbirds, many of whom live alongside humans in our neighborhoods as well as in the forest. These include popular backyard birds like sparrows, cardinals, finches, chickadees, starlings, and robins. But it also includes a family of birds that ranks among the most intelligent animals in the world: corvids. Corvids include magpies and jays, as well as crows and ravens. The main difference between a crow and a raven is that a raven is much larger and goes caw, caw, caw, whereas a crow is smaller and goes caw. Ah! Ah! Ah! Ah! Okay, last time I pointed out this circle here, which represents the very first amniotes. Amniotes are so named because in their embryonic stage they grow inside an amniotic sac. In egg-laying creatures, this sac is located within the egg itself, alongside a separate sac called the yolk. which eventually gets absorbed. However, in live-bearing creatures and humans, the amniotic sac grows inside the womb and then ruptures just prior to birth, which is when we say that a mother's water has broken. The earliest amniotes would have all been egg layers, and the first ones likely evolved around 320 million years ago. But very soon after that, they split into 2 groups: Each going in a very different direction. This fork in the road, so to speak, is one of the most important on the entire tree because it basically splits the entire top into 2 parts. On the right side are the reptiles, dinosaurs, and birds, which we discussed last time. And on the left are the mammals, which we're about to get into. The technical term for these 2 groups are sauropsids and synapsids. As I mentioned last time, sauropsid literally means lizard face. But what about the word synapsid? Well, the word synapse means something like connecting arch or bridge. Therefore, we could say that synapsid means bridge face. To better understand this word choice, let's take a look at some skulls. This is the skull of a coyote, which is a synapsid. Notice this part here? It looks like a connecting arch or bridge. Most synapsids have this feature, whereas sauropsids, as represented by this T. rex skull, do not. Another key difference is that synapsids have only one opening behind each eye socket, whereas most sauropsids have 2. Now, we humans are also synapsids, so you can actually feel that connecting bridge on your own face. It's right below your temple, your temple being what's left of that single opening behind the eye socket. Basically this bridge connects your cheekbone to the place where your jaw begins. That's why when you chew you can feel the muscles moving in this area. The earliest type of synapsids, such as Dimetrodon, were originally called mammal-like reptiles. That's because on the outside they looked like a reptile, even though on the inside they were starting to develop mammal-like features. Later they were called polikosaurs, which is the term I use on this chart. But note that I put it in quotation marks. That's because officially this term is no longer in use either, even though in common speech it's still the name that most people use. Technically it's better to call creatures like Dimetrodon protomammals or stem mammals because even though they looked like reptiles, they were not. Now in episode 2, I introduced this chart called the Geologic Time Scale, which divides the last 540 million years into 12 periods. Well, pelycosaurs lived during these 2 periods here, the Carboniferous and the Permian. But then during the Permian, they were replaced by a new kind of proto-mammal, the therapsids, which included cynodonts. meaning dog-teethed. These creatures managed to survive the massive P-T extinction event and then really started to thrive during the early Triassic. So in other words, proto-mammals were around before the emergence of the dinosaurs. In fact, up until the mid-Triassic, the synapsids were the dominant land animals, not the sauropsids. It wasn't until the second half of the Triassic that the archosaurs And then later dinosaurs started to take over, reaching their peak during the Jurassic and Cretaceous. And then of course came the K-T extinction, in which a massive asteroid hit the Earth, wiping out all of the non-avian dinosaurs. The explosion would have been equal to about a billion Hiroshimas. But it wasn't just the actual impact that caused the extinctions. It was the aftermath too. Initially, there would have been radiation, firestorms, and tsunamis. But then perhaps worst of all, all that debris thrown up in the air would have blocked the sun for about 2 years, meaning that plants couldn't perform photosynthesis. But it was this event, as well as the previous 2 extinction events, that helped drive the evolution of mammals. One theory is that once the dinosaurs became dominant, the main proto-mammals to survive were the smaller nocturnal ones. because they were better at hiding from big predators. This helps explain some of the additional changes that occurred to synapsid skulls. You see, another feature of later synapsid skulls is that the jaw is made up of just one large bone, whereas the jaw in sauropsid skulls is made up of several bones. So what happened to those other jaw bones? Well, we synapsids actually still have them. But ours are really, really tiny and located in our ears. They help us to hear much better than reptiles and birds do. Synapsid skulls also developed a secondary palate which separates the nasal cavity from the oral cavity. This allows us to smell much better than reptiles and birds do. But whereas synapsids have superior hearing and smelling, Sauropsids have better vision. They have 4 cones in their eyes, whereas we have only 3, and most mammals have only 2. This all makes sense if we consider that our synapsid ancestors were mostly nocturnal. If you're moving around in the dark, being able to distinguish between colors isn't really all that important, but being able to hear well and smell well is. Another feature that proto-mammals developed was the ability to regulate their own temperature rather than simply relying on the sun for warmth. This requires a lot more energy, but it paid off because warm-blooded creatures are more adaptable, being able to grow faster and survive in different environments, including cold nights and deep holes underground. One way proto-mammals managed to regulate their temperature was by developing fur. Another was by developing the ability to secrete fluids, like sweat, for example. Which brings me to the feature for which mammals get their name: mammary glands. Mammary glands produce milk, which mothers initially used not to feed their babies but simply to keep their eggs moist. Over time, however, it ended up being the primary food source for mammal newborns, which allowed them to stay closer to their mothers and hence away from predators. Now we tend to think of all modern mammals as being livebearers, But there are still a few around that lay eggs, such as platypuses and echidnas, both of which live in Australia and belong to the monotreme order. Monotreme meaning one hole. This is a reference to the fact that they, like birds and reptiles, have just one hole that they use for all their bodily functions. All other mammals are therians, meaning beasts, and they can be divided into 2 categories: Marsupials, which again only live in Australia, and placentals. Marsupials give birth to live young but in a not-quite-ready state. Once they leave the womb, they crawl up into a special pouch called a marsupium where they continue to develop a little more. Now the marsupials that you're probably most familiar with are kangaroos, but there are others still around like koalas, wombats, and opossums. And up until around 60,000 years ago, there were many more as well, some of them quite large, like the rhino-sized Diprotodon and the cat-like Thylacoleo. The vast majority of mammals, however, are placentals, meaning that they develop placentas during pregnancy, which feed their young until they are fully ready to be born. So no egg and no need for a pouch. Now in order to explain the 2 main branches of placentals, Boreoeutheria and Atlantogenata, I need to first tell you a bit about prehistoric continents. As you probably know, there was once just one big supercontinent on Earth called Pangaea. It existed during the Triassic Period, which was right around the time that the very first mammals appeared. But then during the Jurassic Period, it started to break up, with Australia coming off first, which is why the mammals there are so different. Eventually though, the largest chunk of Pangaea split into 2 main parts: Laurasia in the north, being comprised of what would eventually become North America, Europe, and Asia, and Gondwana in the south, which was basically South America and Africa squished together. So the reason why we have 2 main types of placentals is that the boreoeutherian mammals initially evolved in Laurasia, whereas the Atlantogenatan mammals initially evolved in Gondwana. Let's look at Atlantogenata first. It can be split further into Xenarthra, which developed in West Gondwana, and Afrotheria, which developed in East Gondwana. Xenarthra means strange joints, and they are so named because they have some extra parts in their spine. Examples include armadillos, sloths, and anteaters. In contrast, Afrotheria consists of aardvarks, manatees, and most notably elephants, including the 2 main types still alive today: African elephants and Asian elephants. However, if we were to go back just 4,000 years, there would be a 3rd kind as well: mammoths. For the rest of this video, we'll be dealing with Boreoeutheria, which can also be split in 2. First, there's Laurasiatheria, which is named after that northern supercontinent I mentioned earlier. Here we find hedgehogs and bats, as well as all of the hooved animals called ungulates. These in turn can be divided into Perissodactyla, meaning odd-toed, and Artiodactyla, meaning even-toed. Those with an odd number of toes per hoof include horses, who have just one, and rhinos who have 3. In terms of even-toed ungulates, hippos, pigs, and deer all have 4 toes per hoof, whereas the rest have 2, such as giraffes, camels, and bovids. Bovids being the family that includes cows, sheep, and goats. But also included in Artiodactyla are a group of mammals with no hooves at all, at least not anymore. These are the cetaceans, like dolphins and whales, who now live exclusively in the water, but who evolved from hippo-like ancestors who once lived on land. Finally, within Laurasiatheria is the Carnivora order, where we find humanity's 2 best friends, cats and dogs. In fact, this entire order can be split in 2 between the cat-like carnivores called Feliformes and dog-like carnivores called Caniformes. In Feliformia we find hyenas as well as the Felidae family which includes cougars, cheetahs, bobcats, and the now extinct saber-toothed tigers, and panthers. Tigers, lions, and leopards are actually all panthers even though we tend to reserve the word panther only for black panthers which are actually black leopards. And then of course there's the Felis genus, which includes the African wildcat, from which the domestic cat Felis catus evolved. On the dog-like side are many different families, such as the ones with bears, skunks, raccoons, seals, and otters. But then of course there's Canidae, the dog family, which includes foxes, coyotes, and wolves. It was from the gray wolf, Canis lupus, that the domestic dog evolved. Okay, so that takes care of Laurasiatheria. Let's now look at Euarchokontoglires, which is so named because it is made up of Euarchokonta, meaning true rulers, and glires, which means mouse. Glires has more than just mice though. It includes rabbits, as well as all the rodents, which can be divided into beavers, porcupines, Squirrels, capybaras, rats, and hamsters. Euarchochonta includes tree shrews as well as the most important mammal order of all, at least to us, the primates. Even the word primate itself indicates its importance as it literally means of the first rank. But again, this is obviously from the human perspective. From an evolutionary perspective, no one branch of the tree is any more important or better than any other. As long as it's still there, it can be considered successful. And even from the perspective of our own definition of intelligence, there are many branches of this tree that have produced highly intelligent life forms. Besides primates, there's also cetaceans, elephants, parrots, and corvids. And way down here, there's octopuses, who are known to use tools. Even pigs are smarter than most people realize. But on the other hand, I don't think we have to apologize for having a particular interest in primates. After all, they are our closest relatives. So let's take a look. First, I'll mention the 2 prosimian orders: tarsiers and lemurs. All other primates, including monkeys and apes, are called simians, which literally means flat-nosed. Now a lot of people use the words monkey and ape interchangeably, but they shouldn't because These are actually 2 very different types of simians. Monkeys are generally much smaller and have tails, whereas apes are bigger and are the only mammals to lack tails. Monkeys themselves can be divided into 2 main types based on where they first evolved. So there are Old World monkeys from Africa and Asia and New World monkeys, mostly from South America. In terms of apes, most belong to the hominid family, or great apes. But there's also one family of lesser apes, the gibbons. The great ape family currently consists of 4 genera: Gorilla, Pongo, aka orangutans, Pan, which includes common chimpanzees and bonobos, and finally Homo, humans. As you probably know, humans are most closely related to chimpanzees, which is why I added an extra node here representing their last common ancestor. This ancestor was neither a chimp nor a human, but rather something in between that lived around 9 million years ago. Perhaps something like the now-extinct Sahelanthropus tchadensis discovered in 2002. What's more certain is that around 3.5 million years ago, our ancestors were Australopithecines, meaning southern apes. This now-extinct genus was first discovered in 1924, but was made famous in the 1970s with the discovery of this partial skeleton, nicknamed Lucy. Since then, more complete skeletons have been found, and the genus has been divided into many different species and perhaps even multiple genera. Now before I move on, I want to shout out Ettere Matza, who produced all of the images for this part of the tree. You may know his work from the Stefan Milo channel. But if you want to follow Etteray directly, you can do so on Instagram. I'll leave a link to his profile in the description. Eventually, one of the Australopithecus species evolved into a new genus called Homo, the earliest known species of which was Homo habilis, meaning handyman, because this species is known to have used stone tools. A little later came Homo erectus, meaning upright man, The first fossils of this species were found in Asia way back in 1891, but since then similar fossils have been found in Africa as well. Some researchers simply call these African specimens African Homo erectus, but others consider them to be a totally different species and instead label them as Homo ergaster, meaning working man. Either way, it is from the African hominid that all humans today descend. By way of an intermediate species called Homo heidelbergensis. But before we move forward towards Homo sapiens, I want to first point out this species here, Homo floresiensis, discovered in 2003 on the island of Flores in Indonesia. This early hominid stands out because it was much smaller than all the other Homo species, perhaps due to island dwarfism. Initially, it was thought that they might still have been around as recently as 10,000 BCE, but more recent estimates have pushed that back closer to 50,000 BCE. There are currently 2 main theories as to where these hobbit-sized people came from. One is that they descended from Asian Homo erectus. The other is that they were actually the remnants of an earlier species, perhaps as old as Homo habilis. But back to Homo heidelbergensis, a species that lived from around 700,000 years ago to around 200,000 years ago and lived in both Africa and Europe and perhaps even as far away as China. This species was the common ancestor of Neanderthals, Homo sapiens, and Denisovans. Neanderthal fossils were the first fossils to be recognized as belonging to some type of prehistoric human as far back as 1856, which was 3 years before Darwin's On the Origin of Species. Nowadays we have 400 Neanderthal specimens and have completely mapped out their genome, from which we now have firm evidence that Neanderthals and Homo sapiens interbred with each other. In contrast, we only learned about Denisovans in 2010, and we currently only have around 10 specimens. We have, however, mapped out their genome, and from this we have learned that they interbred with both Neanderthals and Homo sapiens. Neanderthals are named after the Neander Valley in Germany, where some of the very first remains were found, and they are generally now seen as being a unique species called Homo neanderthalensis. However, the Denisovans, which are named after Denisova Cave in Russia, have not yet been assigned an official name. Now, the main difference between the 3 kingdoms boils down to how each one obtains its food. Animals, including us humans, have to eat our food, meaning that we have to literally take other life forms, put them in our bodies, and then let our internal digestive systems do some processing. Similarly, fungi also need to obtain their food from other sources. But a fungus doesn't need to ingest its food because it has the ability to absorb it externally. But plants— plants are super special because they alone have the ability to make their own food through a process called photosynthesis. They can do this because they have little green cell parts called chloroplasts that take energy from the sun and turn it into sugar. Interestingly, the word chloroplast comes from the Greek and literally means green thing that makes. Now back in episode 1, I explained where these little green cell parts came from. They were originally a type of cyanobacteria. Again, the word cyan comes from the Greek and refers to a sort of greenish-bluish color. But then around 1 billion years ago, an endosymbiosis event occurred whereby that bacteria entered a larger eukaryote cell to create the first archaeplastid. From there, we got a form of multicellular life called algae, which comes in green, red, and brown. Algae are similar to true plants in that they can perform photosynthesis, but they differ in that they're far more simple and live exclusively in the water. Okay, so the first major split within the plant kingdom is between vascular plants and nonvascular plants. Vascular plants have a network of veins Kind of like we animals do, but whereas our veins carry blood, plant veins carry water and nutrients. Together, plant veins make up all the various features that we tend to associate with plants— things like roots, stems, branches, and leaves. But not all plants have these things. Those that don't, i.e., the nonvascular plants, belong to the Bryophyta division. Take note that when it comes to plants and fungi, the word division is used in place of phylum. The type of Bryophyta that you're probably the most familiar with is moss, that spongy green stuff that spreads out over surfaces but never grows very tall. This is because bryophytes need to stay close to moisture, since it was in water that plants originally evolved. However, vascular plants do much better on land and can grow much higher thanks to their stems and branches, which a) provide them with structure and b) help transport water up. If you've ever cut a carrot, you may have noticed that it has a little circle inside a bigger circle. Well, all plant stems and branches are similar. The circle in the middle is called the xylem, which in Ancient Greek means wood. Its job is to carry water and minerals up from the ground to the top of the plant. In contrast, the outer circle is called the phloem, and it serves as a 2-way pipe carrying sugar the byproduct of photosynthesis, to every part of the plant. Okay, the next split on the chart divides the vascular plants into those with seeds and those without seeds. Those without belong to the Pteridophyta division, pterid meaning ferns. So ferns, like most other plants, do have branches and leaves, but since they don't have seeds, they reproduce in a slightly different way. If you look on the underside of a fern leaf, you'll notice a bunch of tiny little bumps. These are called sori, and they are full of spores. Spores differ from seeds in that they are produced asexually. Which brings me to the whole topic of plant sex. Most plants produce seeds, and seeds are basically little embryos, meaning that in order to create one, you need both a female and a male, or at least a female part and a male part. But before I dive into that, I need to point out that seed-bearing plants can be divided into 2 main categories: Angiosperms and gymnosperms. Angiosperms are also known as flowering plants, but they could also be called fruit-bearing plants because only plants with flowers produce fruit. And what is fruit? Well, fruit is simply a special container for seeds. In fact, the word angiosperm literally means container seed. This is in contrast with gymnosperm, which means naked seed. Now when it comes to gymnosperms, most people tend to think only of conifers, or cone-bearing plants, almost all of which are evergreen. But there are actually a few other types of gymnosperms as well, such as the ginkgo tree, which loses its leaves. So what unites all gymnosperms is not cones or being evergreen, but the fact that their seeds aren't enclosed in fruit. Let's look at the well-known pine tree as an example. When I say pine cone, you probably think of this. But this is actually just one kind of pine cone, the female kind. On the same tree, you'll also find these ones, which are the male kind. So I guess we could say that pine trees, at least most species of pine, are hermaphrodites, meaning that they are both male and female at the same time. What happens is this: the male pine cone releases pollen, some of which blows in the wind and enters the female pine cones. where it combines with ova to form embryos, or seeds. These seeds then grow, and when they are ready, the female cone releases them in a mostly naked form, i.e., without fruit. Some angiosperms, such as pumpkin plants for example, reproduce in a similar way, except instead of cones they have flowers, and instead of producing mere seeds, they produce fruit with the seeds on the inside. So for example, on a pumpkin plant, you'll find 2 different types of flowers. Although they look quite similar, one type will be male and the other type will be female. Once the female flower is pollinated, the fruit, in this case the pumpkin, starts to develop at the bottom of the flower. Over time though, the flower fades away and the pumpkin gets bigger and bigger. Now there are actually 2 other ways that seed-bearing plants can reproduce. For example, most willow trees are either male or female. So in this case, some trees produce only male flowers and others produce only female flowers. But it's actually the final way that is used by most angiosperms. Most angiosperms produce just one type of flower, but that flower is both male and female at the same time. Sometimes called perfect flowers, they have both stamen which are the male parts, and carpels, which are the female parts. But angiosperms are not actually classified by how they reproduce. Instead, they are classified by how their seeds grow, the 2 main types being monocots and dicots. Cot refers to cotyledons, which are plants' very first leaves. If you've ever tried to grow plants from seeds, you've probably noticed that these sprouts usually look something like this. Those 2 initial leaves are the cotyledons, and since there are 2 of them, we know that this plant is a dicot. Grass, on the other hand, is a monocot, so when it sprouts, it looks like this. Another good example is to think of a corn kernel versus a peanut. A corn kernel is a monocot seed, which is why it's just a single piece, whereas a peanut is a dicot seed, which is why you can split it in 2. Most dicots are eudicots, meaning true dicots. But there's also magnolid dicots, which are some of the oldest angiosperms still around. As the name suggests, this group includes magnolias, but it also has avocado, cinnamon, laurels, black pepper, and radiator plants, which are one of the more popular types of houseplants, coming in lots of different varieties. If you have a Peperomia plant at home, you'll notice that its flowers look more like long thin cones than normal flowers. That's a reminder that it belongs to the oldest category of flowering plants, going all the way back to when flowers were still evolving from cones. Let's look at monocots next. This is where we find almost all of the plants that start out as bulbs, like onions and garlic. as well as lilies and tulips. But it's also where we find major tropical fruits like coconuts, bananas, and pineapples. Finally, and perhaps most importantly, it contains all of the grasses, which include the world's 3 largest food crops: wheat, rice, and corn. But now comes the big category: the eudicots, or true dicots. This group contains about 75% of all flowering plant species, most of which only evolved relatively recently, after the extinction of the dinosaurs. Eudicots are mostly split into either rosids or asterids, but before we look at them, let me point out a few exceptions. 2 of the oldest eudicot families are the lotus family and the buttercup family, which also includes poppies, from which we get opium. Then there's the Caryophyllales order, which includes all of the various types of cactuses, as well as the famous Venus flytrap, one of the few examples of a carnivorous plant. It also includes carnations and the Amaranthaceae family, which contains spinach and beets. Okay, let me now introduce you to Rosids and Asterids, the 2 main categories of eudicots. The terms come from the names of 2 famous flowers: the rose and the aster. Or daisy. Although take note that aster literally means star. That's because asters look like stars. Not this kind, but this kind, which we call an asterisk. Now the really neat thing about daisy-like flowers, such as the sunflower, is that each flower is actually a constellation of many tiny little flowers. Even each petal is technically its own flower, having evolved from multiple petals that fused Together. But the Asterid category contains more than just fancy flowers. It also includes many plants that we eat and drink, such as carrots, coffee, olives, and the Lamiaceae family, which includes most herbs such as mint, basil, oregano, and thyme. And then there's the Solanaceae family, also known as nightshades, which is ironic because sol means sun. So is this a sun-related family or a night-related family? Well, the origin of the term nightshade is debated, but one thought is that it's because some of the plants in this family are quite dangerous, ranging from poisonous berries that can kill you immediately to tobacco, which if you smoke enough of it will kill you over time. But it also includes many plants that are not dangerous, like potatoes and tomatoes. Which brings me to the age-old question: is a tomato a fruit or a vegetable? Well, the answer may surprise you. It's actually both! From a botanist's point of view, it's a fruit. That's because it's a seed container that grows out of a flower. However, from a culinary point of view, it's a vegetable. That's because the word vegetable is actually an arbitrary term used for any part of a plant that is edible but not sweet. So vegetables can be leaves like spinach, stems like celery, seeds like lentils, flowers like broccoli, roots like carrots, bulbs like onions, or even fruit like tomatoes. Another part of certain plants that can be edible are tubers, which are a part of the root system. The most famous tuber is of course the potato. So if you've ever wondered why potatoes and tomatoes can be so closely related, yet look so different, now you know why. They are simply different parts of a plant. Let's now move over to the rosids. There is actually no foolproof way to tell the difference between a rosid plant and an asteroid plant just by looking at them, because the technical difference has to do with seed formation. However, generally speaking, rosid plants usually have more obvious stipules, which are those little things that stick out between a branch and a leaf stem. Unlike Astrids, Rosids include both small plants as well as big ones, aka trees. A tree is defined as any large plant with a central trunk. We've already run across trees in the conifer section, but those were the evergreen ones. When it comes to deciduous trees, meaning those that lose their leaves in the autumn, they are mostly found here among the Rosids. such as elms, oaks, maples, and willows. However, Rosids also include many edible plants, such as squash, beans, grapes, citrus fruits, and cacao, which is used to make chocolate. And then very closely related to the rose flower itself are apples, as well as peaches, plums, and all the various red berries. Now I should point out that when it comes to roses, the ones you're likely familiar with don't actually occur in nature. A wild rose, like most eudicots, only has 5 petals. The many-petaled roses that people like to give to their lovers are the result of many decades of human cultivation. Okay, so that takes care of the plant kingdom. For the rest of this video, we're going to take a look at fungi. For many centuries, fungi were considered to be plants. But we now know that they are actually more closely related to animals, having split off from a common ancestor about 1 billion years ago. The study of fungi is called mycology, from the Greek word mykēs, meaning mushroom. But there are a lot of other types of fungi beyond just mushrooms. A good example is bread mold, which is considered a lower fungi. If you let it grow, you'll notice that it creates a huge network of tiny little threads. These are called hyphae, and even mushrooms have them. It's just that in that case, they are usually underground. It is the hyphae that does what fungi are best known for: decomposing organic material. So for example, if you let mold grow on an apple, the apple will eventually disappear and you'll be left with a puddle of brown goop. That's because the fungi will have basically eaten the apple. Higher fungi can be divided into 2 main divisions: Basidiomycota, meaning club-like fungus, and Ascomycota, meaning sac-like fungus. Ascomycota includes common molds as well as fungi that create a symbiotic relationship with algae to create lichen. However, it also includes yeast, both the type that causes infections as well as the type used to make bread. It does this by eating the sugar in the flour, creating gas that then causes the bread to rise. Another example of the fact that fungi can be either good or bad is these 2 closely related types. Again, one type creates infections for humans, such as ringworm and athlete's foot. But the other type is used to make the antibiotic penicillin. Remember, fungi are good at eating or destroying things. So in the case of Penicillium, what the fungi destroys is bad bacteria. So we've just learned how to use this to our advantage. Under Ascomycota is also truffles. Besides mushrooms, truffles are the other main type of fungi that humans eat. However, they are far more rare and thus usually quite expensive. The final thing I want to mention in terms of Ascomycota is Prototaxites. I haven't included them on this chart, but long, long ago, around the same time that the first vertebrates walked on land, this giant fungus was actually the largest organism on land, about as tall as a 3-story building. Okay, let's now look at Basidiomycota, and in particular the Agaricomycetes class. The word agaric is the technical term for any fungi that produces mushrooms. In particular the kind with a rounded cap with gills underneath. We tend to think of the mushroom part of an agaric as being its main part, but in actual fact it's simply the equivalent of its fruit. In other words, a mushroom is the part of the fungus responsible for reproduction, in this case by producing spores. The main part of an agaric, or any fungi for that matter, is actually the network of hyphae That I mentioned earlier, which in this case is located underneath the mushroom. The largest mushroom order is actually the Polyporales order, which includes all of the shelf fungi that grow on the side of trees. In contrast, the main type of edible mushroom, Agaricus bisporus, also known as button mushrooms, belongs to the Agaricales order, which also includes shiitake mushrooms, psychedelic mushrooms, and the super poisonous death cap mushroom. But perhaps the most recognizable mushroom of all, made famous by the Super Mario game, is Amanita muscaria, known commonly as the fly agaric because it can be used to kill flies. First off, I need to address the word theory. In everyday speech, a theory is simply an idea that may or may not be true. However, in science, that's not what a theory is at all. For ideas that may or may not be true, scientists use the word hypothesis instead, and they reserve the word theory for ideas that have been tested over and over again and shown to be extremely reliable. So take, for example, the theory of heliocentrism. Which is the idea that the Earth revolves around the Sun and not the other way around. You probably assumed that this idea is now known to be a fact. Well, it's not. It's actually still just a theory because no one has been able to travel far enough out into space to actually observe it happening. However, in science, we don't ever say that something is just a theory because in science, when something reaches the level of a theory, It's basically as close to fact as you're going to get. The other thing I want to clarify is what I mean by evolution. Most creationists accept what they call microevolution, which is an acknowledgement that species can and do change over time. For example, through human intervention, large dogs can be bred to become smaller dogs, and wild roses can be cultivated into complex hybrids. And even without human intervention, we know that white moths can eventually evolve to become black moths in order to better hide in their environment. But this is not what I'm talking about. What I'm talking about is what creationists call macroevolution, which is when one species ends up evolving into a totally new species, something which unfortunately we cannot directly observe because on average it It takes more than a million years. But it's even more than just macroevolution. What I also mean when I say evolution is the theory of common descent, which is the idea that every single species on Earth can eventually be traced back to a single common ancestor. Now the opposite of the theory of evolution is the theory of creationism, which is not just any belief in a creator god. But specifically the idea that God created each species on Earth at pretty much the exact same time, and that all of those species have, for the most part, remained the same ever since. So according to creationists, fish were created at the same time as birds, and mammals at the same time as insects. So why am I so sure that creationism is wrong and that evolution is right? Well, I'm going to give you 6 reasons, starting with this one. The older a geologic layer is, the less diverse and less complex the fossils found within it will be. You see, over time, changes occur to the Earth's surface, and new layers of rock end up forming on top of older layers of rock. This means that, generally speaking, the deeper down you dig, the further back in time you go. If creationism were true, you'd expect to be able to find, on average, the same sorts of fossils wherever you look. But this is not what we find. For example, lions, which according to evolution appeared only recently, are never found at deeper levels. They're only found in more recent layers, such as at the La Brea Tar Pits. On the other hand, fish, which have been around for much longer, are found both near the top as well as down in much deeper, older layers of the earth. It would take just one discovery of a fossil that goes against this pattern to disprove the entire theory of evolution. So for example, if any fossilized mammal were ever to be found at a very deep level where previously only Cambrian fossils had been found, then boom, the whole theory would crumble. But that has never happened because whenever fossils are found, they always adhere to a very clear pattern. Lots of different types near the top, including the most complex forms of life, fewer and more similar forms as you go down. So if you were to chart out all of the 250,000+ fossil species found to date according to how old each one is, you'd end up with a chart that looked like, well, something like this. Okay, reason number 2: all animals share the same basic anatomy. Even when it doesn't make sense. To put it another way, whether we're talking about reptile species, bird species, mammal species, or even humans, while each species may look very different, each one is actually just a variation on the same basic body plan. One that has 4 limbs and a head with 2 ears, 2 eyes, 2 nostrils, and just 1 mouth. This indicates that all animals evolved from just 1 original species and then went in different directions from there. You'd think that if a Creator had designed each species from scratch, there'd be a bit more variety in terms of body plans. Like, why no turtles with 6 legs? Or mammals with 4 eyes? Let's look at things in even more detail. Take for example our arms. As I mentioned in episode 2, humans have a single bone in our upper arm called the humerus, followed by 2 bones in our lower arm called the radius and ulna. These are then followed in our hands by carpals, 5 metacarpals, and 5 phalanges. Well, every single reptile, bird, and mammal— even bats and whales— have this exact same set of bones, just in different proportions. Again, this indicates that we all evolved from just one original species that had this arrangement. We can do the same sort of comparisons with insects and with plants. And we can even combine all the various ancestor species together to get even older ancestor species. In the end, we'd end up with, yet again, something like this. Now before you argue, well, God just used designs for basic body plans that work really well, and that's why we see them repeated. Well, let me remind you of the second half of point number 2. All animals share the same basic anatomy Even when it doesn't make sense. A good example of something not making sense is the laryngeal nerve, which goes from the brain to the throat and is an aspect of the general animal body plan that we all share. There's a bit of a design flaw in that the laryngeal nerve loops around the aortic arch instead of going straight to the larynx, which would have been easier. For most animals, this isn't much of a problem. However, for giraffes, It creates a rather ridiculous scenario in which that nerve has to go all the way down the long neck, loop around the arch, and then go all the way back up again. Doesn't seem like something a designer would do, but it does make sense if giraffe necks simply evolved to get longer over time. An example in humans is our faulty sinuses. If you want to learn more about them, I suggest you check out this video by Stefan Milo. Which I'll link to in the description. Now, all this talk of faulty design brings me to reason number 3. Many animals have body parts that they no longer need. Take for example dolphins and whales. Unlike most mammals, dolphins and whales have only 2 limbs, their fins, instead of the usual 4. However, if you look at their skeletons, you'll find that they actually have bones for back limbs that they don't use. In fact, those bones are located entirely inside their bodies. This is because once upon a time, the ancestor of all dolphins and whales was a 4-legged land creature. But over thousands of years, as that creature spent more and more time in the water, it no longer needed its back limbs, and therefore they just kind of whittled away. Even humans have features like this, which are known as vestigial structures. My favorite example is that of goosebumps. In most mammals, goosebumps serve the purpose of making an animal's hair stand up, either when it's scared so that it looks bigger, or when it's cold so that it can warm up. Humans still get goosebumps when they're scared or cold, but because we no longer have much hair, they no longer do anything except make our skin look like we've got a bunch of little pimples. Alright, moving on. One of the things that makes evolution so hard to accept is that it takes millions of years, and therefore we can't see it happening in real time. I mean, even I'll admit, the thought of a single cell evolving all the way to a human seems pretty crazy. But here's the thing: we can in fact see a single cell evolve into a human in just 9 Months. It happens every time a zygote grows to become a baby. So let me now give you reason number 4. As fetuses grow, they acquire features in the exact same order that they are predicted to have been acquired through evolution. The result is that at certain stages, all animal fetuses look pretty much the same. Then they go in different directions, just like they did in evolutionary history. In order to better understand the significance of this, imagine 5 different Lego sets, all of which start out with the exact same set of instructions, but then 4 of them get an additional set of instructions and 1 doesn't. Next, imagine that of the remaining 4, 2 get a 3rd set called Part 3A and the other 2 get Part 3B. Finally, imagine that those same 4 all get a 4th and final set of instructions, but that this time each set is unique. Now, if a creator was designing 5 different Lego sets, they could do it this way, but they wouldn't have to. If they wanted to, they could simply make 5 different Lego sets, all with different pieces and with different instructions. So even if you believe that God created every species at the exact same time, you can't deny that God did it in a way that made it seem like some species were designed off the plans of others. As if God was experimenting, which is weird because God is supposed to be perfect, and therefore God shouldn't have to use trial and error. So once again, if we put all these step-by-step building stages for each species together and then sorted them out based on what built upon what, well, you guessed it, it would look something like this. So either evolution is true or God went out of the way And I mean really out of the way, to make it look like evolution is true, going as far as to make sure that only the fossils of really simple life forms ended up in the deepest layers of the Earth. Okay, for my last 2 points, I'm going to get a bit more technical. As you probably know, the actual instructions for living things are stored in a structure called DNA. Well, a single strand of DNA is called a chromosome. And most animals have several chromosomes, a full set of which is stored in every cell in their bodies. The number of chromosomes a species has varies quite a bit, from this ant, which has just 1, to this butterfly, which has about 450. But keep in mind that the number of chromosomes doesn't indicate how old a species is in terms of evolutionary history. This is because chromosomes can both split and fuse, and they do so often, although by often I mean every few million years or so. And because they split and fuse so often and can do so in various different ways, some chromosomes end up being really long and others end up being really short. Now, how is this useful in terms of providing evidence for evolution? Well, even though chromosomes can vary quite a bit, you'd expect that 2 species that were very closely related would have very similar chromosomes, both in terms of number and size. So to test this, let's take humans. We have 46 chromosomes, or 2 sets of 23. According to evolution, our closest ancestor is the chimpanzee. So how many chromosomes do chimpanzees have? Well, they have 48, or 2 sets of 24, as do gorillas and orangutans. But let's place one of their sets of 24 next to one of our sets of 23. Take note that on this chart H stands for human and C stands for chimpanzee. Well, what do you know? Most of the chromosomes match almost perfectly. The only one that doesn't is our chromosome number 2. But in that case, there are 2 chimpanzee chromosomes that when put together match it almost perfectly. The most likely explanation for this is that our common ancestor, like most great apes today, had 2 sets of 24 chromosomes. But then at some point in the branch that led to humans, 2 chromosomes fused to become 1, reducing our total to 2 sets of 23. So my reason number 5 is, just like a DNA test can determine who your closest human relatives are, Comparing broad similarities in DNA can determine who our closest animal relatives are. Now before I move on, let me clear up a common misconception that people have about evolution. Most people seem to think that evolution says that humans evolved from chimpanzees, but it doesn't. What evolution says is that both humans and chimps evolved from a common ancestor, one that was neither a human nor a chimp. Although I'll admit, to the human eye, that common ancestor looks more like a chimp than it does a human. Another thing that creationists like to say is, if one species can evolve into another species— take for example a tiger and an elephant— how come we've never found a fossil of a half-tiger, half-elephant species? Again, this boils down to a misunderstanding of how evolution works. According to evolution, a tiger will never evolve into an elephant, and an elephant will never evolve into a tiger. However, if we go back far enough in time, we can find a common ancestor that was neither a tiger nor an elephant. You see, we can move forward along the branches of the evolutionary tree, and we can even make new branches, but one thing we can never do is jump from one branch to another or combine 2 branches to make a chimera. Okay, my last point builds on point number 5 but gets even more technical. It has to do with viruses. Viruses differ from living things in that they cannot survive on their own. In order to replicate, they need to infect a living host. Well, sometimes a piece of a virus's DNA or RNA actually gets inserted into the host's DNA and then from there continues to get copied to future generations. Whenever this happens, the new sequence is called an endogenous retrovirus, or ERV. Usually the ERV doesn't cause any harm. It's basically just the equivalent of the virus leaving a bit of graffiti saying, I was here. Now don't start freaking out over the thought of a virus being able to change your DNA. From the standpoint of a human lifespan, ERV events are extremely rare. But over hundreds of millions of years, they do add up. So much so that it is estimated that around 8% of the human genome is basically viral graffiti. Another way to look at it is to see these ERVs as being sort of like fossils, but in our DNA. Each one is a record of something that happened a very long time ago. Species that are closely related to us, such as chimpanzees, have virtually the exact same ERVs as we do in the exact same places, whereas species that are less related have less ERVs in common. Again, we can use ERVs to map out the past and to find out which species are more related to which species and even build a tree based on our findings that will look one last time like this. So reason number 6 is Endogenous retroviruses serve as markers for when changes in DNA occurred and thus can be used to map out the tree of life. So whether we chart out species by where their fossils are found, or by how similar their body plans are, or by how much DNA they share in common, everything leads to the same conclusion. There's a pattern to life on Earth that can be best described as looking like a tree. In which, from the top down, each branch eventually merges with other branches, which then merge with other branches, and so on, until we get to one single trunk. Going the other way, from the bottom up, life started small, but over time mutations that turned out to be useful led to permanent changes in some species. And as the process repeated itself, not only did more and more species come to exist, but more and more categories of species. Hence all the branches of the tree. Again, I'll admit it's a pretty difficult thing to wrap one's mind around, but that's because our human brain did not evolve to think in terms of hundreds of millions of years. Logically, we can understand what the number 100 million means, but it's really hard for us to truly grasp just how long 100 million years really is. And how, given that much time, how a sequence of tiny little changes can in fact add up to a massive amount of change by the end. Now before I go, I want to say 2 things. First, keep in mind that accepting evolution does not mean giving up one's belief in God. For some people it does, but for many others it does not. That's because accepting evolution and believing in God can go hand in hand. Secondly, understanding evolution takes time. If you're on the fence, this one video is probably not going to be enough to convince you. Therefore, I'm going to leave you with a few book recommendations that will help you explore further. If you're going to read just one book, it should be either Why Evolution Is True by Jerry Coyne or The Greatest Show on Earth by Richard Dawkins. Both cover a lot of the same ground that I covered in this video, but in much greater detail. Another great book is Your Inner Fish by Neil Shubin. It demonstrates how our own bodies serve as a testament to all the various changes that have occurred throughout evolutionary history. Now, for those of you who are religious and might be interested in reading something from an author who shares that in common with you, I'll give 2 final recommendations: Finding Darwin's God by Kenneth Miller, and Life's Solution: Inevitable Humans in a Lonely Universe by Simon Conway Morris. Thanks for watching. Thanks for watching.