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David Begun of the University of Toronto in Canada reanalysed fossils of Dryopithecus apes, which lived in what is now Europe as early as about 12.5 million years ago. He says that the characteristics of the skull suggest that rather than evolving earlier than the great apes, as was previously thought, Dryopithecus was actually a great ape itself.
Orang-utans are the earliest of the apes to have split from the human lineage, thought to be followed byDryopithecus, then gorillas, then chimps. But if Dryopithecus is in fact a gorilla, that puts the species closer to humans and chimps.
The features [angles at which bones in the skull connect, and the way the brain case is connected to the face] suggest Dryopithecus split from the human lineage about 14 million years ago, Begun says. From that, he says, we can extrapolate that the human lineage split from chimps about 10 million years ago.
That’s more than 2 million years earlier than the previous estimate based on the fossil record, but is actually close to recent estimates based on genetic analysis.
Genetic comparisons can also indicate when species diverged from a common ancestor. They are based on the number of genetic differences between two species, which is proportional to the time that has passed since their last common ancestor was alive. Because of this they are also known as molecular clocks.
Current molecular clocks date the split between humans and chimps to at least 7 million years ago, matching the age of the oldest fossil thought to be in the human line, Sahelanthropus. But some reports quote molecular dates up to 13 million years.
Calculating the amount of time elapsed between the last common ancestor and the appearance of two separate and co-existing species depends on the population of the original species: the more individuals, the more distant the last common ancestor.
If the initial population was large, as Scally says the genomes of humans and chimps show was the case with their last common ancestor, several million years could elapse before the two species finally separated, says Scally.
Interbreeding between subgroups, as is thought to have happened between modern humans and Neanderthals, also could delay speciation.
The real test will depend on finding fossils from 9 to 10 million years ago. But that won’t be easy. So far, finds have been few and very incomplete, and with early chimp and human ancestors likely to be very similar, Begun says, it’s going to be difficult to decide which line they fall on.
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