Scientists often study mice and fish and fruit flies — in the hopes that doing so will reveal insights about human health.
This seems a little weird and counterintuitive. But the truth is that humans are far more closely related to many species than you might think. Jishai Evers created this handy bar chart at Dadaviz.com that puts it all in perspective. (It's based on data from an interactive accompanying a Carl Zimmer story at National Geographic.)
We have a lot of genes in common with all kinds of other creatures — which is why learning about them can be so handy. For example, 73 percent of zebrafish genes are also found in humans. (That's a greater overlap than we have with chickens.) If scientists can find out that a certain gene in zebrafish is related to a particular health problem, then they have a better sense of what to look for in people, too. Combine that with zebrafish being inexpensive to house and quick to breed, and zebrafish have become one of the hottest animals in scientific research.