Age of Man
Primates -- the mammals from which humans evolved -- emerged on Earth much earlier than had been thought, originating perhaps 85 million years ago during the age of the dinosaurs, according to a new analysis.
The findings should add fuel to the debate between paleontologists, who place the origin of primates at 55 million years ago, and molecular biologists, who use DNA sequencing to suggest they may be as old as 90 million years.
Paleontologist Robert D. Martin, vice president of academic affairs at Chicago's Field Museum, acknowledged that his team's new research supported the views of the "molecular clock" school, even though the method used by his team did not involve DNA analysis.
Instead, the researchers developed a statistical model that builds an evolutionary tree based on the number of primate species alive today (235) and the number of recorded fossil species (396) and their ages.
By assuming that each primate species would live approximately 2.5 million years, the team was able to estimate the length of time that elapsed between the oldest known fossil primate, which is 55 million years old, and a hypothetical "last common ancestor" of all primates, 80 million to 85 million years ago. The findings were reported in today's issue of the journal Nature.
"I've been arguing for years that there's so many gaps in the fossil record that [primates are] probably much older than we thought," Martin said. "You look at how many species there are, and you can estimate the time of the original."
The implications of these findings, if shown to be accurate, could be profound. A primate ancestor 85 million years old would have shared the world with the dinosaurs, which went extinct 65 million years ago, probably in an ecological disaster caused by a meteor hitting the Earth.
The model also would mean that humans diverged from chimpanzees about 8 million years ago, rather than 5 million years ago, as is currently thought. Martin said the earliest primate was probably a lemur-like tree dweller that weighed perhaps two pounds and dined on insects and fruit.
Finally, an earlier date for primates would mean that continental drift (in which huge, ancient land masses broke up to form what are today's continents) had a significant effect in creating different primate species.
The new findings, however, drew criticism from several paleontologists, who noted that there is little fossil evidence from the dinosaur period that points to primates or indicates that mammals in general were flourishing.
"The primates are a successful group of animals," said K. Christopher Beard, curator of vertebrate paleontology at Pittsburgh's Carnegie Museum of Natural History, and the dinosaur era "should have been a good time to evolve, diversify and take over the planet.
"If primates were around and doing well, I suspect we would have found their fossils," Beard continued. "What I would do is issue a challenge and ask them to say where these animals are hiding."
What the fossil record shows is that before the dinosaur extinction, mammals were neither particularly diverse, nor particularly large. Most were rodent-like creatures, and "the biggest was about the size of a beaver," said Johns Hopkins University paleontologist Kenneth Rose.
"There was an explosive radiation of mammals after extinction," Rose continued. "It took very little time before we get an animal the size of a steer, and soon we had some truly impressive creatures. With no dinosaurs in the way, there was a lot of open ecological space."
But while everyone agrees that the first unequivocal fossil primate did not appear until 55 million years ago, Martin argues that the record is too skimpy to conclude that the order began only a few million years prior to that.
"Primate paleontologists read the fossil record as if it told us everything," Martin said. "That's reasonable if you have a dense record, but our calculations show we have fossil evidence for only about 5 percent of the extinct primates."
By contrast, molecular biologists analyze DNA to calculate the amount of genetic difference in related species, creating an evolutionary tree that can be projected backward in time to the earliest common ancestor.
"Our results actually fit the molecular trees, and they're always much earlier," Martin said. "Several people have generated the tree for mammals," and have come up with dates "around 90 million years."
But "there are no fossils that say that," said Michael Novacek, provost and curator of paleontology at New York's American Museum of Natural History, even though "it could be that the record is incomplete.
"So what's the truth?" Novacek asked. "I don't know."