Apart from yeast, which is unicellular, most model organisms consist of millions or even trillions of cells. They have brains, guts, and more complex behavior, which implies that their cells need to communicate with one another. The cells also need to act unselfishly, for the good of the whole organism, as do human cells.

The microscopic worm Caenorhabditis elegans, for instance, gave scientists the opportunity to analyze the mechanisms of cell suicide, or apoptosis, a startling phenomenon that is essential to animal health. Cell suicide has also been studied in flies and humans.

The fruit fly Drosophila, which behaves surprisingly like humans when it gets drunk, turns out to be a good source of information about the activity of brain neurons and the genetics of addiction, as well as about cancer.

And the mouse, like all mammals, is genetically so close to humans that segments of its genome are sometimes hard to tell apart from ours. The mouse model will be invaluable in medical trials of experimental drugs.

Each model organism has its own set of advantages and disadvantages for researchers, who must carefully choose which model is best for a particular experiment. Increasingly, though, scientists can shuttle between species—including some new model organisms such as fish—to make the most of the special qualities of each one.

Despite all the obvious differences between, say, a fly and a person, and despite wide variations in the number of genes and chromosomes they contain, each organism has approximately the same number of core proteins.

"Complexity does not come from the number of genes but from the way in which they are used," explains Gerald M. Rubin, HHMI vice president for medical research, who heads the Berkeley Drosophila genome project. "Humans may have four copies of a gene where the fly has one, but if you look at the core proteome—the core set of parts—they're not that different."

Recently scientists examined a set of 289 genes that are known to be mutated or deleted in various human diseases. When they looked for similar genes in model organisms, they found a stunning number of such genes in the fruit fly: 177, or 61 percent of the total. When they limited their search to genes that are mutated in human cancers, they found an even higher proportion (68 percent) of parallel genes in the fruit fly. Worms and yeast also turned out to have large numbers of genes that resemble human disease genes.

Researchers hope they will soon learn enough about such genes and the proteins they encode to be able to short-circuit some of the faulty protein interactions that cause disease. They also hope their findings will bring new levels of precision to human physiology and medicine. If they succeed at both goals, as seems increasingly likely, it will be thanks to the genes we share with yeast, flies, worms, and mice—and to the towering evidence of the unity of life.

— Maya Pines

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