It came from the Vendian lagoon
One of the most interesting things to me about the process of evolution is how weird things could have been. Biological evolution isn’t a ladder of progress, it’s a frantic Rube Goldberg race to try and make something useful from randomly-generated bits of gene code. Gaining a new mutation is like having someone from the city recycling center drop by your workshop/garage and throw you a random machine part. “Hey, here’s something from an old washing machine,” or “Think you can use this old carburetor?” You just get stuff, and your task is to use everything you have to try and build a car/food harvester/weapon system, and your life depends on it. You can’t go buy more parts from the store, you’re only allowed to use the random crap thrown your way, and everything you get takes up valuable garage space. It’s either use it or toss it out. What can you make?
Some of the things you can make, given 4 billion years of garage tinkering, include all the life forms ever to have lived on Earth. But… what about all the life forms that didn’t live? As Richard Dawkins says in The Blind Watchmaker…
However many ways there are of being alive, it is certain that there are vastly more ways of being dead, or rather not alive.
There are also vastly many more ways of being alive than the few we’ve been privileged to witness on our one little planet. Chordates, which include all vertebrate life, trace back to a single group of animals that split off from the echinoderms (including sand dollars and sea stars) about a billion years ago, give or take. If chordates hadn’t shown up, there would have been no dinosaurs, no birds, no frogs, no mammals. What else would be here? Just more insects, or something completely different?
Stephen Jay Gould makes this point eloquently in his book Wonderful Life, written about the wild menagerie of fossil animals that evolved during the Cambrian explosion of animal diversity, about 540 million years ago. Gould uses the most alien-ish fossils of the early Cambrian to make the point that evolution is contingent on happenstance. Why did Anomalocaris, the predatory ruler of Cambrian seas, die out while the lowly Pikaia, a tiny slip of a thing, survive? Pikaia and its cousins went on to found the vertebrates, while Anomalocaris, which doesn’t really belong in any living group of animals, vanished forever from the Earth.
If you go back to before the Cambrian, you see even weirder animals… and fossils of things that we can’t even tell for sure were animals. In the Vendian period, just before the Cambrian explosion, animals were very experimental. Back then animals had only recently (i.e. in just the previous few million years) begun to evolve beyond worms, blobs and spheres, into a plethora of more complicated body plans. The first homeobox controler genes had evolved. Homeobox, or hox, genes orchistrate how bodies are laid out. A result of early hox diversity was an ocean of wild, weird life forms. A single mutation in such simple genomes could trigger substantial remapping of the body plan, so lots of different animal designs appeared more or less around the same time. Most limped along for a while… until the more elegent mutants killed them all or ate all their food.
What did these mutants look like? The latest Vendian weirdo, called Eoandromeda octobrachiata, looked like a sea star but had eight limbs in a pretty little spiral arrangement. Why is that a big deal? Because normal echinoderms – including sea stars – have pentameral symmetry, meaning they’re five-sided. It’s a basic feature of the group. Having eight limbs means having at least bilateral symmetry (left side – right side), or at most some kind of four- or eight-sided radial symmetry. Octopi have eight limbs, but they don’t have eight-sided bodies. This Eoandromeda thing is unique, and not a clear member of any living, established animal group.
So what, you still say. Well, echinoderms (five-sided) and chordates (left-right sided) are cousins, and this new fossil may be an intermediate between the two. It probably isn’t a direct ancestor to vertebrates, but it’s likely one of many alternate experiments of the Vendian world. Why didn’t this new creature survive to found a major group? No one knows. Maybe no one will ever know. Fossils like Eoandromeda show us how life might have been, had evolution taken a slightly different course. To me, these kinds of “what if?” questions – to which no one will probably ever have an answer – are some of the most fun questions in science.