Open your mouth.

Close it.

This simple movement is one of the oldest in nature that apes like you still perform today almost unmodified.

This movement evolved well over 400 million years ago.

That's 50 million years before trees existed.

That's 100 million years before vertebrates took their first steps on land.

300 million years before flowers, and over 400 million years before creatures like you even existed.

Which is to say that the entire history of the human species gets caught up in rounding the number when we're talking about time at this scale.

Every vertebrate we've spoken about on this show descends from the first animals that could perform this movement, the first creatures with jaws.

Vertebrates are, of course, the animals with backbones or spinal columns. When we talk about vertebrates, we're talking about all fish,

all reptiles, all birds, all amphibians, all mammals, we're talking about most of the

species we speak about on this show.

Unless you're an entomologist, we're talking about most of the animals you can think of off the top of your head.

Now, I want you to imagine the vertebrate lineage as one of those trees that split right near the base.

You know the kind.

They basically got two trunks that connect into one trunk right near the ground.

They split off before radiating out into their own branches.

While we have climbed other trees to talk about various invertebrate lineages,

every episode of this show we've spent in this tree,

every episode where we've spoken about a vertebrate,

has been spent on one of the two trunks.

For years now, we've climbed only one side of this vertebrate tree,

...