Geneticists can tell how recently two species diverged from a common ancestor from information etched in the organisms' genes: The fewer differences researchers find between two genetic codes, the more recently the species parted evolutionary company.
Using this method, geneticists have concluded that about 90 million years have elapsed since all living primates shared their last common ancestor.
Based on such studies, evolutionary biologists such as Pennsylvania State University's Blair Hedges believe that early primates lived alongside the great reptiles.
"[Primates and other] major groups of mammals evolved for tens of millions of years before the dinosaurs became extinct," Hedges said. That mass extinction occurred about 65 million years ago.
Fleshing out the Fossil Record
To reconcile the genetics-based date with the comparatively young estimates of paleontologists, Tavaré and his colleagues fashioned an evolutionary tree of the primates as it's known from fossils.
Not wanting to depend entirely on what paleontologists have turned up, the researchers then fleshed out the rudimentary tree with educated guesswork. They used mathematical equations to predict how many species of primates are not represented in the fossil record, and to predict when and for how long those species may have lived.
The resulting model—based loosely on hard evidence but expanded to take into account species that lived and died out but remain unknown to science—suggests that the earliest primate lived about 81.5 million years ago, long before the age of the oldest fossils uncovered by paleontologists.
"Naturally," said Hedges, "I am pleased with this result because it shows agreement with our molecular-clock studies."
Tavaré's team suggested that the earliest primates might have been small, nocturnal creatures that inhabited tropical forests. But, assuming they did exist that long ago, numerous forms could have evolved prior to the fateful cataclysm that wiped out the dinosaurs 65 million years ago.
That event, presumably caused by a giant impact from outerspace, didn't wipe out all primates living at the time, but probably spared relatively few of them, Tavaré theorized. Those primates that survived would have subsequently evolved into myriad species.
"Of course, this is all speculation," Tavaré acknowledged. "We have not found any fossils in that bin yet."
In fact, scientists may never know how these proposed lost kin of ours looked—or be certain they existed at all—unless paleontologists someday recover fossilized remains.
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