The Origins Of Vertebrates Part 3: Baby Got Back
If you’re looking at the featured image for this blog you might be turning your nose up, and I wouldn’t blame you, but before you leave in disgust I’d like to introduce you to the acorn worm, a strange little creature that’s anatomically very similar to the ancestor of all vertebrates. They possess ‘primitive’ versions of distinctive vertebrate characteristics (such as branchial slits for feeding rather than breathing like the gills we know today), a threefold division of the body (proboscis, collar, trunk, what will in vertebrates be head, trunk, tail) and most importantly, a stiff but flexible hollow tube. It might not be a nerve cord but it did provide the structure and movement we take for granted. Ladies and gentlemen, baby got back.
Now, this creature isn’t a chordate, it’s a hemichordate (not quite a vertebrate), but there are a few key elements to the chordate body plan that will let you know what you’re dealing with.
1. A dorsal nerve cord: nerves running down the back from the brain.
2. A notochord: a support for the nerve cord made from cartilage.
3. A post-anal tail: I’m certain I don’t need to explain this.
4. Pharyngeal slits: connecting the inside of the throat to the outside of the neck, not necessarily gills, but more often than not.
If it has all four of these, it’s probably a vertebrate, and if it doesn’t but you know that it’s a vertebrate, take a look at its fetal form, a stage where humans have rudimentary gills and a tail, so in a way our body plan is clearer before we are born than after it, although you’d have a hard time arguing that any tetrapod is an insect.
One of the early split offs from this initial group of proto-vertebrates was the ancestor of the sea squirts, which by their adult stage have become basically sedentary, this animal isn’t incredibly important in terms of our history, but I find the fact that a mobile larvae attaches to a rock and then proceeds to dissolve its own brain until its just a pulsating remnant pretty interesting if not morbid.
As our ancestors developed there was more divergence that has extant descendants. The lancets with a divided notochord for directional movement for example, these worm-like creaturees bury themselves in sediment and lack true vertebrae and well-developed sense organs. An artifact of the development of heads is the hagfish, a slimy, jawless member of Craniata that has scales, paired and dorsal fins, gills and a cartilage skeleton. The development of the head was important for protecting the brain, and although the hagfish doesn’t have a complete skull, there is protection there, as there also is in the jawless lampreys, that possess our next devleopment, the rudimentary vertebrae that tell us we’re getting closer to the true backbone we know and love.
The vertebrae is actually a relatively complex structure, so it’s no mystery as to why it took so long to develop into the powerful, connecting repeating spinal unit we see in most of the animals we generally think of as vertebrates, but even they had come a long way before the jaw developed the gill arches we see in lampreys. It’s thought that these bony gill inbetweeners fused to the skull, with the bop and bottom becoming different halves of the jaw. There’s actually pretty good evidence for this in the gene sequences of sharks, which are a fine demonstration of how jaws can help turn the grazers of the sea into active, brooding hunters.
We know from placoderms (huge, armored fish) that color vision had developed by around this time, its external skeleton (the internal skeleton could not be preserved) having a distinctive color, but another thing that’s interesting about placoderms is their sharp tusks, a development which paved the way for the development of teeth, and that’s where we’ll start next week, with the sharks and the rays, leading up to a true bony skeleton and the emergence of amphibians as the chordates conquered the land.