Even for the most complicated organs, such as the brain, the lessons of Drosophila have proved to translate directly to mammals. Corey Goodman, for instance, has been using Drosophila to study the wiring of the brain and nervous system. Although flies are not the most intelligent of creatures, his work has already led to a better understanding of how the human brain develops.

"When we started 20 years ago," he says, "we knew almost nothing about how you wire up a brain, especially a human brain, which has trillions of nerve cells, each making hundreds of thousands of connections. Somewhere in the genome is the information that gets read out and somehow allows those cells to find each other.

"I began with the assumption that whatever the fly has, with its brain of 100,000 nerve cells, the human must have as well. If a fruit fly has one cell X hooking up with one cell Y, humans might have 10,000 Xs and 10,000 Ys. If you want to know how to hook them all up in the first place, a good place to start is the fly. I can find out which of the fly's 15,000 genes control wiring in the nervous system. And then, later, I found out that humans did have many of the same genes, used in the same way."

Goodman and his colleagues identified a family of proteins in flies that controls the wiring of the nervous system by chemically repelling the growing nerves away from one cell and nudging them toward another. "Cell X grows toward cell Y," he says, "not just because Y is attracting it but maybe because cells W and Z are sitting there repelling it. So we found this family of repellents, called semaphorins, of which there are 5 members in the fruit fly and maybe 25 in humans, all highly homologous to the ones in the fly."

— Gary A. Taubes

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