Henry Gee

this internal skeleton is very, very unusual.

So the history goes back a long way.

And that's another episode, the origin of vertebrates.

Oh, yes, let's do that.

But our closest relatives are things called, believe it or not, sea squirts.

These are creatures that live in one place squirting seaweed.

They live in one place squirting seawater in an hour.

Yeah.

Well, these sea squirts have larvae, and these have little heads and tails like tadpoles.

And the tails have the notochord in.

The notochord is the precursor of the backbone.

And they have the muscles on either side.

And then what these things do, in their heads, they've just got one eye spot, not four, just one, and all it can do is detect light and dark.

And it's got a little gravity-sensing organ in it.

And all it does when it hatches, it goes on a very short journey with its tail just to find somewhere that's deep and dark.

And then it sticks onto the rock with its head end, and then it resorbs the tail and then balloons out into this giant pharynx that's this giant balloon mouth and stomach, which is the sea squirt.

So vertebrates are basically these two things in one, the mobile tail and the jaws and the viscera behind it.

So a famous paleontologist called Al Romer, one of his last papers in 1972, conceived of vertebrates as a mixture of two animals, the somatico-visceral animal, that we have the muscles and the brain and the backbone of the mobile animal, which is represented by the little tadpole larvae.

and the visceral part, which is the stomach, the intestines, all the squishy mixed grill parts, which are represented by the adult tunicate.

So it could be that the common ancestor of vertebrates and tunicates was an animal which was motile, and it had a segmented swishy tail, and it had a front end with jaws and a pharynx.