Early humans and chimps may have interbred to create hybrid man
- From: ano457@xxxxxxxxx
- Date: 17 May 2006 19:56:50 -0700
Early humans and chimps may have interbred to create hybrid man
By Steve Connor
Published: 18 May 2006
The close relationship between man and chimp has just got cosier,
according to a study which suggests that ancestors of the two species
interbred at some point in the distant past to form fertile hybrids.
It is well established that chimpanzees are the closest living relative
of humans but this is the first time that scientists have found
evidence for hybridisation through interbreeding.
The astonishing conclusion comes from an exhaustive analysis of the
genomes of humans, chimps, gorillas and monkeys published in the
journal Nature. The researchers were particularly interested in the
point at which the last common ancestor of man and chimp split into two
separate species - the process of speciation that gave rise to the
chimp and human lineages.
"The study gave unexpected results, about how we separated from our
closest relatives, the chimpanzees," David Reich, of the Broad
Institute of Harvard and the Massachusetts Institute of Technology,
said. "We found that the population structure that existed around the
time of human-chimpanzee speciation was unlike any modern ape
population," he said. "Something very unusual happened at the time of
speciation."
A comparison of the entire DNA of chimps and humans suggests that they
split apart no more than 6.3 million years ago,far more recently than
previously thought. The study found that an original split was probably
followed by a phase of interbreeding and hybridisation, possibly with
fertile female hybrids cross-breeding with male chimps, before the
final split occurred.
Hybridisation is a common feature of how plants split into different
species but scientists have not seen it as being so important in the
speciation of animals.
The suggestion could explain why an early human-like fossil known as
Toumai or Chad Man appears to date back to seven million years ago,
before the split between chimps and man, Nick Patterson, a member of
the research team, said. "The fact that the Toumai fossil has
human-like features suggests human-chimp speciation may have occurred
over a long period with episodes of hybridisation between the emerging
species," he said.
The human genome carries the genetic information for making a human
being but it also acts as a source of evolutionary history. Some parts
of the genome are known to be older than others, meaning that they
could date back to the common ancestor of chimps and man.
Dr Reich and his colleagues looked at the variation in evolutionary
history across the genomes of humans and chimps. They found that the
time from the beginning to the completion of divergence between the two
species ranges over more than four million years, much longer than
expected. In particular, the study found that the X chromosome - which
occurs twice in women and once in men - is the youngest part of the
genome and was still being modified just before the final split
occurred.
"A hybridisation event between human and chimpanzee ancestors could
help to explain both the wide range of divergence times seen across our
genomes as well as their relatively similar X chromosomes," Dr Reich
said. "That such evolutionary events have not been seen more often in
animal species may simply be due to the fact that we have not been
looking for them."
The close relationship between man and chimp has just got cosier,
according to a study which suggests that ancestors of the two species
interbred at some point in the distant past to form fertile hybrids.
It is well established that chimpanzees are the closest living relative
of humans but this is the first time that scientists have found
evidence for hybridisation through interbreeding.
The astonishing conclusion comes from an exhaustive analysis of the
genomes of humans, chimps, gorillas and monkeys published in the
journal Nature. The researchers were particularly interested in the
point at which the last common ancestor of man and chimp split into two
separate species - the process of speciation that gave rise to the
chimp and human lineages.
"The study gave unexpected results, about how we separated from our
closest relatives, the chimpanzees," David Reich, of the Broad
Institute of Harvard and the Massachusetts Institute of Technology,
said. "We found that the population structure that existed around the
time of human-chimpanzee speciation was unlike any modern ape
population," he said. "Something very unusual happened at the time of
speciation."
A comparison of the entire DNA of chimps and humans suggests that they
split apart no more than 6.3 million years ago,far more recently than
previously thought. The study found that an original split was probably
followed by a phase of interbreeding and hybridisation, possibly with
fertile female hybrids cross-breeding with male chimps, before the
final split occurred.
Hybridisation is a common feature of how plants split into different
species but scientists have not seen it as being so important in the
speciation of animals.
The suggestion could explain why an early human-like fossil known as
Toumai or Chad Man appears to date back to seven million years ago,
before the split between chimps and man, Nick Patterson, a member of
the research team, said. "The fact that the Toumai fossil has
human-like features suggests human-chimp speciation may have occurred
over a long period with episodes of hybridisation between the emerging
species," he said.
The human genome carries the genetic information for making a human
being but it also acts as a source of evolutionary history. Some parts
of the genome are known to be older than others, meaning that they
could date back to the common ancestor of chimps and man.
Dr Reich and his colleagues looked at the variation in evolutionary
history across the genomes of humans and chimps. They found that the
time from the beginning to the completion of divergence between the two
species ranges over more than four million years, much longer than
expected. In particular, the study found that the X chromosome - which
occurs twice in women and once in men - is the youngest part of the
genome and was still being modified just before the final split
occurred.
"A hybridisation event between human and chimpanzee ancestors could
help to explain both the wide range of divergence times seen across our
genomes as well as their relatively similar X chromosomes," Dr Reich
said. "That such evolutionary events have not been seen more often in
animal species may simply be due to the fact that we have not been
looking for them."
.
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