“The hox genes are, more or less, the standard library for the structural assembly of animal bodies. It’s a DNA-modifying type of gene, one that activates specific other regions of DNA during early development. Need a leg? Activate the ‘leg’ hox gene, and that will start a huge cascade of related processes that make a torso segment that contains a couple legs plus all the necessary hip-joints and such in a somewhat standardized way.** This makes it easier than you’d think to adapt animal body plans on the fly; rather than reinventing legs from the ground up every time you want to adapt from quadruped to hexaped, you can just have a mutation that calls the hox gene two more times. It is also a primary mechanism of directionality, in which animal bodies have a clear orientation with a front and back.

As you might imagine, animals almost never survive a mutation to the hox genes proper, let alone thrive and speciate. It tends to mean that you get born without a head or something. So, both humans and house flies tend to have a very similar set. That holds true across the entire animal kingdom, with only a few exceptions- hox genes are frozen in time, proportionate with their importance. Care to guess which types of animal lack fully formed hox genes?

That’s right: sponges and jellyfish. Even those have a rough proto-hox thing going on, but it’s a far cry short of ours.

It goes without saying that this will have implications for species diversity in the animal kingdom. Adaptation isn’t as easy as building an animal out of legos, but it has at least gotten a lot easier. Animals can experiment with body shapes in more radical ways, and be successful more often when they do so—they’re now exploring a much larger space of possibilities.

We’re starting to piece together a rough framework here, in which the rise of oxygen and the slow development of meta-genomic advantages work together to provide space for a more dynamic ecosystem, but is that enough to make sense of something as dramatic and surprising as the Cambrain Explosion? It’s a start!”

Anonymous. “Notes From an Apocalypse.” Lesserwrong.com. September 23, 2017.