The Bilaterian Split represents the most significant “hardware upgrade” in the history of animal life. It marks the evolutionary divergence where organisms moved away from radial symmetry (like jellyfish or sponges) and developed bilateral symmetry—a distinct front, back, left, and right side. This structural shift was the “Idea” that made complex life possible, transforming animals from passive drifters into active, goal-oriented hunters and foragers. This is when life transitioned from a passive state of “being” to an active state of “doing,” which accelerated evolution.

Your agency has deep evolutionary roots, and in a real sense, this is the start of it. With a forward driven shape in a forward time-moving world, came a body aimed into the world. Earlier animals such as jellyfish already show coordinated behavior, but philosophically, what jellyfish do is probably better understood as proto-agency.

Directional movement requires a “command center.” The concentration of nerve cells into a head is called cephalization. Yes, the Bilaterian Split is considered the “Origin of the Brain” because these concentrated sense organs eventually evolve into brains. From here, agency becomes increasingly organized as animals develop a through-gut (a mouth-to-anus digestive tract). 

In 2020, scientists identified a tiny, worm-like fossil in Australia named Ikaria wariootia. About the size of a grain of rice, it lacked legs or eyes but possessed the fundamental bilateral blueprint: it could burrow through the seafloor with intent, using muscles to move toward organic matter. This species, or one like it, is the common ancestor of complex animals alive today, from the tiniest tardigrade to the human being.

This is when life began to navigate. If the history of life is a book, this is the start of the chapter on “Will.”