Scientific News  Hypotheses Historical hypotheses RAFTING RODENTS FROM AFRICA MAY HAVE BEEN ANCESTORS OF SOUTH AMERICAN SPECIES
RAFTING RODENTS
FROM AFRICA MAY HAVE BEEN ANCESTORS OF SOUTH AMERICAN SPECIES
 Forty million years
ago, rodents from Africa may
have colonized South America by rafting or swimming across the Atlantic, Texas
A&M University biologists theorize by studying the evolution of rodents,
looking at their genes instead of their fossils - an approach that promises to
revolutionize the field of evolutionary biology.
"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.
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Publishing date: October 25, 2001
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