Before our evolutionary ancestors had a brain—before they had any organs—18 different cell types got together to make a sea sponge. Remarkably, some of these cells had many of the genes needed to make a brain, even though the sponge has neither neurons nor a brain.
In my neuroscience lab at the University of California, Santa Barbara, my colleagues and collaborators discovered this large repository of brain genes in the sponge. Ever since, we have asked ourselves why this ancient, porous blob of cells would contain a set of neural genes in the absence of a nervous system? What was evolution up to?
The sea sponge first shows up in the fossil record about 600 million years ago. They live at the bottom of the ocean and are immobile, passive feeders. In fact, early biologists thought they were plants. Often encased by a hard exterior, a row of cells borders a watery center. Each cell has a tiny cilium that gently circulates a rich flow of microorganisms on which they feed.
This seemingly simple organization belies a giant step in evolution. For the previous 3 billion years, single-celled creatures inhabited the planet. In one of evolution’s most creative acts, independent cells joined together, first into a colony and later into a truly inseparable multicellular organism.
Why did this porous blob of cells contain neural genes? What was evolution up to?
Colonies of single cells offered the first inkling that not every cell in the colony had to be identical. Cells in the interior might differ subtly from those on the periphery that are subject to the whims of the environment. Colonies offered the advantages of cooperation among many nearly identical cells.
The next evolutionary innovation, multicellularity, broke radically from the past. With multicellularity, cells became highly specialized; they relinquished their individual identities and independence for the greater good of the organism. This innovation was one of the greatest acts of altruism in the history of life.
Although these cells were now no longer able to survive on their own, the organism gained by the collective functioning of different cell types. As organisms added specialized cells, integrated modules called organs conferred novel functions and abilities.
Nowhere have specialized cells within an organ become more diversified than in the nervous system. The evolution of the nervous system in animals was critical to the wildly successful kingdom of life. But it did not happen all at once. As ancient creatures crossed the long-vanished boundary from single-celled organisms to animals, a complex nervous system emerged.
The early evolution of animals is full of puzzles, and the origin of the nervous system lies at the core of the maze. From a common ancestor of all animals, for which no trace exists today, sprang two lineages called sister groups. The more recent sister is the sponge. The older sister is the comb jelly, which is just a few millimeters in size or as large as five feet, consisting of a jelly-like body with a layer of cells above and below that creates a cav
