The nervous systems of modern animals are amazingly diverse. A few hundred nerve cells are all a lowly nematode needs to find food and a mate. With about 100,000 neurons, a fruit fly can perform aerial acrobatics, dance to woo a mate, and throw kicks and punches to repel a rival. The sperm whale’s 8-kilogram brain, the largest on the planet, is the navigation system for cross-ocean travel and 1000-meter dives and enables these highly social creatures to communicate. The human brain—one-sixth that size—is the wellspring of art, literature, and scientific inquiry.
In this month's Origins essay, Greg Miller takes a look at how nervous systems got started. He investigates what the first neurons might have looked like and what advantages they conferred on the animals that possessed them. These were questions the father of evolution, Charles Darwin, was ill-equipped to address. Only around the time Darwin died in 1882 were scientists beginning to develop stains to label individual cells for detailed postmortem neuroanatomical studies. Methods for investigating the electrical properties of individual neurons in living brain tissue were still decades away, to say nothing of techniques for investigating genes and genomes. Using such modern tools, scientists have recently begun to gain some tantalizing clues about the evolutionary origins of nervous systems. By looking down the tree of life, they are concluding that assembling these components into a cell a modern neuroscientist would recognize as a neuron probably happened very early in animal evolution, more than 600 million years ago. Most scientists agree that circuits of interconnected neurons probably arose soon thereafter, first as diffuse webs and later as a centralized brain and nerves. But the resolution on this picture is fuzzy. The order in which early branches split off the animal tree of life is controversial. And Miller takes a look at the different story lines implied by these different arrangements.
Illustration: Katharine Sutliff