Rafting Rodents From Africa May Have Been Ancestors Of South American Species

"We have good evidence that suggests that South America was founded by a single ancestral stock of caviomorph relatives from Africa," says Rodney Honeycutt, a Texas A&M professor of biology who has been studying the evolution of rodents for the last eight years.

"The radiations of these rodents to South America are too young for continental drift to have played a role in their colonization by African ancestors. This presents a dilemma, because either caviomorph ancestors dispersed to South America from Africa over water (e.g., by rafting) or the caviomorph radiations are considerably older than suggested by the paleontological evidence. We're seeking answers by using molecular data to address these questions."

Honeycutt, his Ph.D. student, Diane Rowe, and a former student, Ron Adkins from the University of Massachusetts, are using genetics to study the evolution of South American and African rodents. The approach has been to sequence specific genes from several diverse rodent species and then use changes in these genes to reconstruct the evolutionary history of these rodents.

Scientists sequenced genes of several rodents and looked at the changes - or mutations - among the genes. Then they compared the number of changes between the genetic sequences of the rodents and constructed the rodents' evolutionary tree.

"We found that the South American and African radiations were unique but they do share a common ancestry," Honeycutt says, adding that the time when the two groups diverged may be considerably older (45 million years) than what the fossils are suggesting (36 million years).

"This radiation was unusual because these rodents became the grazers of South America," he continued. "Many caviomorphs were extremely large; indeed, today some species are the largest rodents in the world. In fact, unlike most rodents, many species of caviomorphs are highly social and have a metabolism and reproductive strategy more like large mammals."

The idea that the South American group shares a common ancestry with the African group has been controversial for years. Scientists have indeed suggested that caviomorph rodents in South America had multiple origins.

This new research by Honeycutt and his colleagues suggests that the ancestors of all South America caviomorph rodents colonized South America from Africa by rafting along oceanic currents connecting the two continents, Honeycutt says.

The novelty of this research pertains to the use of molecular techniques to address evolutionary questions posed decades ago primarily by paleontologists.

"Recent discoveries in molecular evolutionary biology are changing significantly the traditional ideas about mammalian evolution," Honeycutt says.

"For example, in the late 1960s and early 1970s Wilson and Saich, two scientists from the University of California at Berkeley, proposed that the human and chimpanzee shared a common ancestry to the exclusion of the gorilla, and the separation between humans and chimpanzees was no greater than six million years. That finding sent the entire palentological community in a tailspin because it was so counter to what they had been proposing.

Though molecular evolutionary biology is providing unprecedented insight into animal evolution, it cannot provide information on extinct animals.

"The problem is that we cannot sequence genes on some of the fossils, because they are too old," Honeycutt says. "The big questions are: 'How many fossils have anything to do with a direct ancestry of the lineages we see living today? And how do we fit those fossils to the genetic evolutionary tree?'"

So scientists are trying to combine data from fossils and living forms, genetic data and morphological characteristics.

"For a long time, the study of microevolutionary processes and macroevolutionary processes were two separate things," Honeycutt says, "but what we are finding with genetics and molecular evolutionary biology will allow us to merge those two fields."

Talking about the recent results of his group about the evolution of rodents, Honeycutt says they will help answer more general questions about the origin of mammals and how they relate to each other.

"This work will help reconstruct more accurately the tree of life, which is a really important goal for the future," he says.