Anna Salleh - ABC Science Online
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| Settled: Fruit bats are not more closely related to primates
than to other bats. |
Humans are
closer to rats than they are to any other distant relatives, according
to new research which claims to settle a number of hot debates
on the origin of mammals.
Two independent
international research efforts reported in this week's Nature
have analysed the differences and similarities in DNA sequences
of a large number of placental mammals (the group of species which
includes humans). Despite using different data, both studies have
come up with the same patterns of relationships between species.
"It's
very impressive work," says Dr Steve Donnellan from the Museum
of South Australia. "It's the first time that the power of
genomics has been used to look at a series of long standing problems
surrounding the origin of mammals."
The evolutionary
history of the placental mammals has provided grounds for scientific
debate as acrimonious as you would find anywhere. Over the past
decade, evolutionary biologists seeking to reconstruct the evolutionary
relationships of placental mammals using DNA and molecular evidence
have often locked horns with scientists using an approach based
on careful study of bones, teeth and anatomy.
The DNA studies
have often resulted in glaring discrepancies, however, the two
new studies in Nature overcome many of the inadequacies of the
earlier studies by analysing data from a large number of genes
from a large number of species.
Donnellan
says one of the debates settled by the research is the relationship
of flying foxes to other bats. Scientists looking at the anatomy
of the eye have recently proposed that these large fruit-eating
bats are more closely related to primates than to other smaller
insect-eating bats.
"This research has put that hypothesis to bed," says
Donnellan. "It clearly shows that all bats are more closely
related to each other than anything else." In fact bats were
found to be closer to pigs and cows than to rodents and primates.
The molecular
research also favours a closer relationship between primates and
rodents, and between whales and hippopotamuses than previously
thought.
Another interesting
consequence of the research is that many adaptations seen in placental
mammals, ranging from aquatic habit to flight, evolved many times
independently.
"Up until
now, we've really only had mickey mouse approaches to studying
these questions," says Donnellan. "Now, we are able
to use the data waterfall that's coming out of genome research".
Gondwanan
origins
The new molecular
research also gives new life to a 19th Century idea that a very
eclectic group of animals which includes elephants, dugongs and
aardvarks are all closely related and probably originated in Africa.
According
to Professor Tim Flannery of the Museum of South Australia the
findings also support a controversial theory advanced by himself
and Dr Tom Rich of the Museum of Victoria that placental mammals
originated in the Southern Hemisphere.
He says that
changes in DNA sequences occur at a rate that can be used to calculate
the divergence of various evolutionary pathways. Applying this
"molecular clock" to this most recent data, he says,
supports the idea that placental mammals originated 120 million
years ago during the age of the dinosaurs.
Flannery and
Rich have reported the discovery of placental mammal fossils of
this age in Southern Victoria. Their theory challenges the idea
that placental mammals orginated in the Northern Hemisphere much
later - around 65 million years ago.
Dr Don Colgan
from the Australian Museum in Sydney, however, is not so convinced
suggesting the "molecular clock is too sloppy" to draw
such conclusions about the age of species. He instead prefers
to focus on the ability of DNA to describe the relationships between
species.
"The
research demonstrates the maturity of molecular evolutionary biology,"
says Colgan. "The degree of agreement between the two different
data set is outstanding. You couldn't get the same sort of agreement
using two sets of morphological data."
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