The Bonegarden

Carcinisation is fun, but it's just one example of an odd recurring pattern in parallel evolution. Humans probably will not become crabs; mammals have their own patterns which incongruously recur across unrelated clades. I believe the inheritors of humanity will evolve long, sticky tongues which enable them to feed on burrowing insects.

Up until the end of the Ediacaran period, animals reproduced either asexually (e.g. by budding, like hydrae) or by simply releasing eggs and sperm into water. But with the advent of vertebrates and arthropods and cephalopods, there we have actively mobile animals with complex sense organs and a central nervous system, able to perceive and act in the environment. This gave them the opportunity to approach partners to make fertilization more likely, and even to select a partner over another by its features. These were the first animals able to be horny. This is known as the Cumbrain Explosion