90 million years ago, a stealthy predator
slipped in and out of burrows that it dug in the shadows of the dinosaurs, in what would
later become Argentina. Today this creature is known as Najash -- named
after a monstrous biblical snake. And, for the most part, it had all of the
classic traits that you expect a snake to have, like a long, sinuous body and ribs for
days. But this ancient snake also had … legs. Which sounds weird, right? Like a snake with limbs? But what’s even stranger about this creature
is: We’re not at all sure where it came from. OK, first, let me answer the question that’s
probably on your mind already: Yes! Najash is considered a true snake, even though
it had legs -- two of them, toward the back of its body. It’s one of the earliest known snakes in
the fossil record found with limbs intact. But it was by no means the only one. Snakes-with-legs have been described in such
disparate places as Great Britain, Morocco, Romania, and Wyoming. Some of these reptiles, like Najash, had two
legs. Others might have had four. But most of them are known from only a few
bones -- a piece of a jaw here or a vertebra there. And because of this, the evolutionary history
of snakes is among the most mysterious of all the vertebrates. Biologists know that snakes diverged from
lizards, probably as early as the Jurassic Period, about 200 million years ago, and eventually
radiated into the 3,000 species that we have today. But we don’t really know which group of
lizards gave rise to the snakes -- or when or why they lost their legs in the first place. We can’t answer these questions, because
we don’t have enough data. Most of the very oldest snake fossils that
have been found are just fragments. But, we do have a couple of good, if competing,
theories about where snakes came from. Some scientists think that the first snakes
descended from burrowing lizards. Others think they might have come from from
lizards that swam in the open ocean. That’s because the way that snakes move
seems to work best for either burrowing through soil, like a worm does, or propelling through
water, as eels do. So, let’s start with that. The consensus is that snakes came from lizards. But … where does one animal stop and the
other one start? Like what is the difference between a
snake with legs and a … lizard? Well, most of us would identify a snake by
its long, slinky body. But herpetologists actually define snakes
not by their slinkiness but by their mouthy-ness. Specifically, snakes are identified by adaptations
in their skulls that allow them to unhinge their jaws. For example, several parts of a snake’s
skull are smaller than they are in other reptiles. And its two lower jaw bones aren’t fused
together, like they are in other vertebrates. Instead, snakes just have a little cartilage
there. These adaptations work together to provide
maximum flexibility, so snakes can swallow things bigger than their own heads. Which we do not recommend that you try at home. Because you can’t do that. Snakes are also unique in that they don’t
have … ears. No ear holes, no ear drums, and no inner ear
bone that other reptiles have. Instead, what was once their ear bone became
fused to their jaw, where it allows snakes to detect vibrations in the ground. So, you’d think that knowing all this could
help us figure out where snakes came from: We just have to find out when these adaptations
first appeared. But it turns out, that’s not easy. Because there’s evidence that seems to support
each hypothesis -- that snakes evolved from terrestrial lizards, and that they came from
aquatic lizards. Advocates of the aquatic lizard hypothesis,
for example, think that snakes may actually be distant cousins of the fiercest predators
of the Cretaceous seas -- the mosasaurs. Mosasaurs were themselves descended from land-dwelling lizards. And like snakes, they developed long bodies
and jaw adaptations that allowed them to open their mouths wide for a crushing bite. And for decades, researchers looked for an
evolutionary link between snakes and mosasaurs in one of the most-studied snakes of the Cretaceous
-- a two-legged snake known as Pachyrhachis Pachyrhachis was found in the West Bank, near
Ramallah -- a region that was underwater when the snake lived there 95 million years ago. And studies of its anatomy have found that
it probably moved the way an eel does, by whipping its tail from side-to-side, to get
short bursts of speed. Some paleontologists have argued that snakes
like Pachyrhachis descended from mosasaurs, because they share a lot of similarities in
the shape of their skulls, especially around the jaw. Plus, Pachyrhachis and other aquatic snakes
that have been found in Ramallah were, for a long time, thought to be the oldest snake
fossils ever found. And if the oldest snakes were marine reptiles,
then snakes must have come from the sea, right? Well, in the last 10 years or so, a bunch
of new discoveries have shaken up this idea -- as well as the rest of the snake family
tree. Paleontologists have recently described at
least half a dozen previously unknown species of ancient snakes -- many of which are even
older than Pachyrhachis, and most of them lived on dry land. Among these new species is Najash, which lived
around the same time as Pachyrhachis, but many are even older. Some go back as much as 167 million years,
like Eophis, which was described from Great Britain in 2015. It’s now considered the oldest snake ever
found, and it was terrestrial. So, to some experts, the notion of snakes
originating on land makes a lot more sense. For one thing, many of the classic traits
that snakes have, like jawbones that pick up vibrations, seem better suited for life
on the ground. And not having legs can offer a lot of advantages
if you’re a reptile that burrows through the dirt. After all, the skinnier you are, the skinnier
your burrow can be, which makes it easier to evade predators. But what about genetics? Can molecular evidence settle the Great Snake
Debate? Well, in 2013, scientists used gene data from
more than four thousand living snake and lizard species to construct a new family tree for
these groups of reptiles. And the results suggested that snakes’ closest
relatives are probably the Varanid lizards, a group that includes monitor lizards and
Komodo dragons. That might sound like score one for the terrestrial
hypothesis. But the problem is, Komodo dragons and their
kin are also thought to be the closest living relatives of the mosasaurs. So, depending on how you read them, these
new genetic data could support EITHER the terrestrial hypothesis OR the aquatic hypothesis. Which is kind of frustrating! But also fascinating! Ultimately, all we know is that snakes came
from lizards, but we don’t know the details of their remarkable transformation. And we know that there were snakes before
the likes of Najash -- the leggy reptile from Argentina -- but we don’t understand which
lineage of lizard they came from. So in the end, snakes pose one of the greatest
evolutionary mysteries in the history of animal life. Not even modern genomics has been able to
tell us how and when they became a thing. At least, not yet. Thank you for watching this particularly mystifying
episode of Eons. And thank you to Dr. Allison Hsiang, evolutionary
biologist at the Swedish Museum of Natural History, who consulted with our editorial
team on this one Unsurprisingly, in the entire history of life There are some pretty big mysteries. If you have questions We would love to know some of your great paleo mysteries. Let us know in the comments if you want us to take on some topics And you can keep watching by going to youtube.com/eons to subscribe. Now, have you ever wanted to get a really
close look at the natural world? Then visit Deep Look, from PBS Digital Studios,
where you can explore big scientific mysteries by going incredibly small!
Oh wow, that was very good and surprisingly entertaining. Love the mix between biology and anthropology. I guess it would be evolutionary biology?
PBS eons is the shit. I watch all their videos
Doesn't the fact that some have arms and legs (but not flippers) strengthen the case for a land based evolution?
Makes me wonder if this is where the "Long" type dragon (Or Asian dragon) came from.
PBS eons is the bomb
Well this is OBVIOUSLY proof that SATAN put snakes on Earth to TEMPT EVE to commit ORIGINAL SIN!
/badparodyofacreationist
Snakes had legs at one point eh? Score another one for the Bible.
there are some lizards currently existing to evolve into a more snake like animals, some already lost thier limbs, some still have tiny ones.
Those tiny legs are adorable.