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Sometimes it's hard to identify exactly what a fossil is from—but wouldn’t it be cool to be able to point at a fossil and know that it’s the first plant? Join Michael Aranda for a fun new episode of SciShow where we've put together a list of the oldest fossils of their kind that we know of so far! Let's go!

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[SciShow intro plays]   Michael: Wouldn't it be cool to be able to point at a fossil and know that it's the first, say, plant, or four-legged creature, or insect? But identifying the first anything in the fossil record is hard for a few reasons.

First, the fossil record is patchy, because things only fossilize by chance, and we haven't found all the fossils there are to find; we could find a new oldest mammal or oldest insect tomorrow. Second, scientists like to group animals by their evolutionary relationships to each other. It's especially important for these groups to have a single ancestor.

So, for example, the group "mammals" includes me, platypuses, and the ancestor we have in common, plus everything in between. When we find an ancient creature, it's hard to know for absolute certain if we've found that mysterious ancestor, or just an offshoot of the ancestor's lineage, a cousin. So what we think is the first reptile could be an animal that was related to reptiles, but wasn't actually their ancestor at all.

Finally, there's no person in charge of declaring one fossil or another to be the official first thing of its kind. That's left for scientists to debate, and they don't always agree, because it's hard to tell. Evolution is a gradual process, and there's no easy way to distinguish the last thing that is not a bird from the first thing that is a bird. But, with all that said, here are some of the first fossils of their kind that we know of.

Number one: first evidence of life. The oldest fossils are really, really old; to find them, you have to find the oldest rocks in the world. The oldest rocks in the world are in Australia, and the oldest fossils are 3.5 billion year-old bacterial colonies known as stromatolites. A stromatolite is a structure similar to the ones made by some of today's Australian bacteria.

It looks like an unassuming lump of rock. But because the bacteria grow layer by layer and trap the minerals as they do, stromatolites have a characteristic spiked pattern. These are the oldest fossils we've ever found, but there are traces of what could be life that go back even earlier.

Rocks don't last forever; they're always being melted down, or smashed, or some combination of the two. But tiny crystals, called zircons, are much tougher than regular rock, and can get incorporated into new rocks when old ones are destroyed. Scientists have identified zircons from 4.1 billion years ago that contain traces of carbon, and the chemistry of this carbon is consistent with biological activity.

Which is weird, because for a long time, scientists thought that life couldn't have existed more than 3.8 billion years ago, because there were too many asteroids crashing into Earth all the time. So, if researchers confirm that these 4.1 billion year-old zircons contain traces of life, we might have to revise our view of early Earth.

Number two: first vertebrate I'd really like to show you the first vertebrate known to science – you know, an animal with a backbone – but scientists actually have no idea what the first vertebrate was.

When it comes to early vertebrates, the fossil record is especially spotty. The earliest vertebrates didn't have mineralized bones, so there were no hard parts to fossilize. What few soft tissue fossils we have tend to look like squashed blobs, and blobs are difficult to interpret, even for paleontologists with 3D modelling systems.

We do have a few milestone blobs: Pikaia was long thought to be the ancestor of all vertebrates, but its status isn't universally accepted. Haikouichthys, Myllokunmingia, and Metaspriggina, all from 520 million years ago, seem to be either early vertebrates or something very close. Based on these fossils, we can guess what the first vertebrate would've looked like: it would've had eyes that faced forward and up and a long fin along the midline of its body, it had no jaw, it was a mere few centimetres long. Pretty humble start for the diverse vertebrates we know today.

Number three: first crustacean. Crustaceans are animals made up of three segments with a hard exoskeleton and eyes on stocks. These crustaceans include crabs, lobsters, and shrimp. But the earliest crustacean fossils come from a famous deposit called the Burgess Shale, which represents the huge boom of animal diversity from way back in the Cambrian period more than 505 million years ago, known as the Cambrian explosion.

From it, we know that many types of animals that are still around, like crustaceans and the early ancestors of vertebrates had already evolved by half a billion years ago! That's a long time for a group of animals to last.

Of the crustaceans in the Burgess Shale, the best known is Canadaspis; we have literally thousands of specimens. This is unusually good luck as fossils go, because more fossils mean that all the interesting features are more likely to be preserved. So, scientists are quite sure this one's a crustacean based on the structure of its head; it had eyes on stalks and a hard shell just like modern crustaceans.

Number four: first land plant. The earliest known plants on land came after the earliest crustaceans, and had no stems or roots. They looked like modern liverworts, and they lived 472 million years ago. Plants didn't always fossilize well, but the spores of these early plants were nearly indestructible.

Scientists in Argentina found fossilized spores in local sediment, and when they dissolved the sediment to recover them, they found that the spores represented multiple genera, or groups of species. That means these liverworts had already had some time to evolve and diversify; they probably first appeared on land 10 or 20 million years before these spores were made, maybe even earlier.

472 million years is a surprisingly long time ago. The next oldest plant spores ever found are similar, but they're 10 million years younger and found far from Argentina, in the Czech Republic and Saudi Arabia.

Number five: first jawed fish. Jaws were a major evolutionary innovation, but both heads and backbones came first. The earliest fish with jaws were called Placoderms, and they ruled the seas between 430 and 360 million years ago. They had huge, bony plates around the outsides of their heads, making them look like armoured submarines from the front. But the armour only extended part of the way down their bodies, and their tails were bare or only lightly covered with scales.

Most Placoderms had simple jaws, but there's one that looked a little more advanced. Entelognathus, which lived 419 million years ago, evolved a jaw structure that looks like the complex arrangement found in bony fish and all their descendants. Biologists aren't sure whether this means that Entelognathus is the ancestor of all bony fish, or if they evolved the pattern independantly from bony fish, but it does seem to be the first fish with a modern face.

Number six: first insect. The oldest fossil of a creature we can be sure is an insect comes from Scotland. It's called Rhyniognatha. Rhyniognatha lived between 407 and 396 million years ago, and it can be identified as a true insect based on the shape of its jaws.

In fact, its jaws specifically resemble those of a winged insect, so it's possible that flight had already evolved by 400 million years ago. But since the only Rhyniognatha fossil we have is partial and doesn't include wings, we can't be sure if it could fly. But this does mean insects probably evolved well before Rhyniognatha. We know that plants had already begun to colonize the land by the Silurian period, about 438 million years ago, and it's likely that insects were a part of these earliest land-based ecosystems.

Number seven: First tetrapod. Everything that walks on four legs or later learned to walk onto has a common ancestor. This group of animals is called tetrapods or four-footed creatures. Many early tetrapods look like something that's halfway between an amphibian and a fish which isn't too surprising because that's pretty much what they are. Two of the earliest tetrapods are closely related specimens that go back 375 million years: Elginerpeton and Obuchivictis.

Obuchivictis was initially thought to be a fish, but analyzing its head together with Elginerpeton showed them both to be early tetrapod. But there's evidence that tetrapods maybe 20 million years older than these two. A set of fossilized footprints in Poland is 395 million years old and they were made by a four-footed creature. We don't know much about this animal but it may have been surprisingly big and stocky at 2.5 meters in length, which probably means it's been walking for a while and the later fishy forms of four footed life weren't the first to come out of the ocean

Number eight: First reptile. The first reptile was also the first amniote, a group of creatures that evolved eggs that didn't need to be kept in water. This allowed animals to finally cut the tie binding them to the water and live on land completely. Reptiles, birds and mammals are all amniotes.

The earliest reptile ever found is called Hylonomus. It lived 315 million years ago in what is now North America. It was small, only about 20 centimeters in length and looked more or less like a modern lizard It probably ate insects because plant-eating tetrapods didn't exist yet. Amniotes have probably been around since about 35 million years before Hylonomus what is the earliest one we can be fairly confident about.

Number nine: First mammal. The earliest placental mammal ever found was a tiny shrew like creature called Juramaia. It lived alongside the dinosaurs in the Jurassic period, a hundred sixty million years ago. Its name means Jurassic mother.

Juramaia is eutherian mammal, meaning it's more closely related to modern mammals with fully functional placentas than other mammals such as monotremes like the platypus or marsupials like the Koala. Eutherians give birth to live young which mature inside the mother's body and get the nutrition from the placenta. Marsupials give birth much earlier and keep their young in pouches and monotremes lay eggs.

Despite its name, Juramaia is probably not our direct ancestor but a close relative. Still, the discovery of Juramaia did turn conventional wisdom about mammal evolution on its head. Before Juramaia, the earliest known placental mammal was only 125 million years old but Juramaia came along 35 million years earlier than that. That means the split between eutherian mammals and marsupials probably happened about a 165 million years ago That's much earlier than we thought and shows mammals were already evolving and diversifying in the age of the dinosaurs.

Number ten: First bird. You might already know this one: it's Archaeopteryx. Well, it's probably, Archaeopteryx. We've known about this 145 million year old animal since Darwin's time and it became a poster child for evolution. Its fossils had clear impressions of wing feathers like the ones Birds used to fly but it also had teeth and a bony tail It looked like the perfect transitional form between dinosaurs and birds.

But there is another animal that has a pretty strong claim. It's called Aurornis and has many of the same transitional features Archaeopteryx does. It also has wings and a tail like Archaeopteryx. Except, Aurornis was first, by 10 million years. The more dinosaurs with feathers we discover, and there are a lot, the less unique Archaeopteryx seems.

The transition between dinosaurs and birds was messy and gradual. Technically, all birds are dinosaurs. So, it's impossible to find a dividing line where they started being birds because they never stopped being dinosaurs.

Really, it's hard to identify the first of anything but it's worth trying to do it anyway because it gives us insight into when and how life evolved on Earth. And if you want to learn more about the history of life on Earth, be on the lookout for upcoming mini-series.

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