Scientist Traces Rat Migration Using Genetics

Rodney Honeycutt set out to solve a scientific mystery, but he soon found himself up to his eyeballs in rats with a very strange story to tell.

Honeycutt, a professor of biology at Texas A&M, has been studying the evolution of rodents for the past eight years, and he has been particularly interested in the giant rats of South America, some of which can tip the scales at nearly 200 pounds. Genetic evidence shows that these strange rats shared a common ancestor with rats now living in Africa.

For years now scientists have postulated and bickered over how two species of closely related rodents could have found themselves separated by an ocean. Fossil evidence indicates that the rats arrived in South America between 36 million and 40 million years ago.

The Atlantic would have been a bit smaller then, because Africa and South America have been separating for about 100 million years at about the rate of the growth of a human fingernail. It would have still been an ocean, although perhaps about half its current width.

So how did the rats travel across the ocean from Africa to South America?

A Prehistoric Boat Trip?

They could have done it only one of two ways, as far as anybody knows. Either they swam across, which seems preposterous. Or they climbed aboard a raft and followed the currents across the Atlantic to the new world.

"That seems highly improbable," Honeycutt says. But it's the best answer anyone has been able to come up with. The thought of a bunch of rats, manning a raft and sailing across the sea, falls a bit short of an adequate scientific explanation, and it helped set Honeycutt off on a personal expedition several years ago. Perhaps, he reasoned, the South American rats split off from their African cousins much earlier than had been thought, and they were able to cross into South America when the two continents were joined together.

Honeycutt turned to one of the newest tools in the evolutionary sciences, called "molecular evolutionary biology."

When animals or plants adapt to a changing environment they are said to have evolved when the adaptation becomes "fixed" in a DNA molecule, allowing that change to be passed on to succeeding generations. And the rate at which mutations occur remains fairly constant.

"There's a pretty standard rate in which DNA mutates, and it's pretty similar across fairly diverse groups of organisms," Honeycutt says. "Within a group like rodents there's a more or less standard rate at which mutations become fixed."

DNA Timeline May Match Fossil Record

By determining the "rate of fixation" for the rodents on both sides of the Atlantic, Honeycutt came up with a "molecular yardstick, or clock," that allowed him to measure the time since the rats' ancestor diverged into two distinct groups. At that time, all the rats were in Africa, so if he could push that date back quite a bit, it would have been possible for some of the rats to scurry from Africa to South America before the two continents drifted too far apart.

No such luck.

Honeycutt and his graduate student, Diane Rowe, and fellow researcher Ron Adkins of the University of Massachusetts, haven't come up with a precise date, but so far it looks like the evidence will support the fossil record.

Divergence probably occurred around 45 million years ago, long after the two continents drifted far apart. And to that, one expletive comes to mind.

Rats.

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