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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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