In a proposal that is already starting to make the evolutionary fur fly, an international team of scientists has pushed back the origin of primates by some 20 million years, to when dinosaurs still inhabited Earth.
The new statistical model, which shakes up the human evolutionary tree among others, pushes back the last common ancestor to around 85 million years instead of the widely accepted 65 million and suggests the fossil record accounts for no more than 7 percent of the primates that ever existed. The far-reaching proposed revisions take on conventional methods of dating and retracing lives of eons ago. The report will be published Thursday in the British journal Nature.
The research, which suggests humans diverged from chimpanzees 8 million rather than 5 million years ago, addresses a long-standing discrepancy between calculations computed by molecular biologists and physical evidence unearthed by paleontologists. Modern genetic comparisons estimate the lineage leading to primates diverged from other placental mammals about 90 million years ago, some 35 million years before recognizable primates appear in the fossil record.
In a project initiated a decade ago, Simon Tavare of the University of Southern California in Los Angeles and his team of computational biologists, geologists and anthropologists from Los Angeles, Chicago, Boston and London proposed a way to fill in the gap.
"The field of phylogenetic tree reconstruction uses molecular data to estimate the tree linking the species, and to date the splits in the tree. They use the fossil record only to calibrate the height of the tree -- hence in part the importance of getting the height right," Tavare told United Press International. "Part of our interest was to see why molecular dates of divergence times appeared to differ so much from the molecular estimates -- we show that in fact (for the primate split) there is not any real difference."
Using their way to interpret nature's diary, the team concluded primates emerged on the scene some 85 million years ago. The view discards the widely held theory that not until the dinosaurs' demise in the aftermath of an asteroid's collision with Earth 65 million years ago did the way clear for primates to establish a foothold.
"Current interpretations of primate and human evolution are flawed because paleontologists have relied too heavily on direct interpretation of the known fossil record," said study co-author Robert Martin, vice president of academic affairs at The Field Museum in Chicago.
"Our calculations indicate that we have fossil evidence for only about 5 percent of all extinct primates, so it's as if paleontologists have been trying to reconstruct a 1,000-piece jigsaw puzzle using just 50 pieces," Martin said in a telephone interview. "It's certainly going to be controversial."
That may be an understatement.
"Claiming that the fossil record is 'inadequate overall' is a perfectly meaningless statement. The fossil record is what it is, and it provides tangible evidence that primates existed from the present back 55 million years in time," said Philip Gingerich, professor of geological sciences at the Museum of Paleontology at the University of Michigan in Ann Arbor and a long-time critic of the views first proposed by Martin in 1986.
"The authors then want us to believe that primates existed for another 30 million years farther back in time ... with no evidence preserved. It is like walking 55 miles to reach a coastline and then being told that the land really extends another 30 miles," Gingerich said.
"Over and over again in the popular literature we are bombarded with the dogma that the evolutionary radiation of modern groups of mammals did not -- could not have -- taken place until the extinction of the dinosaurs some 65 million years ago. Although I am definitely not convinced by the arguments of the authors ... the article presents an important message," William Clemens, professor of integrative biology at the University of California, Berkeley, and faculty curator of the UC Berkeley Museum of Paleontology, told UPI.
While questioning some of their conclusions, Clemens said he agreed with the authors' assessment of the scarcity of primate fossils.
"The available collections of mammalian fossils contain representatives of only a very few of the species that have existed; probably 7 percent is in the right order of magnitude," Clemens told UPI.
A number of factors can explain the scarcity.
"Small animals are less likely to be fossilized than big ones, and you have to die in exactly the right circumstances to leave fossils behind," said Tavare, who is the George and Louise Kawamoto Chair in Biological Sciences. "Thus many species will never have left a good fossil record."
The earliest unquestionable primate fossils date from 55 million years ago. In keeping with the common practice of adding a few million years when dating the origin of a group, most paleontologists interpret the fossil record to mean that primates made their appearance no more than 65 million years ago.
While reviewing the scientific literature, the investigators found 474 recorded fossil species. Then, applying their method, they estimated there were 8,000 to 9,000 extinct primate species in all.
"We hope our research will help reconcile the discrepancies between the various dates suggested by paleontologists and molecular biologists, not just for primates but for other groups of organisms, too," Martin said.
His image of the earliest common ancestor of lemurs, lorises, tarsiers, New World monkeys, Old World monkeys, apes and humans is of a furry, 1-to-2-pound nocturnal, tree-loving creature, with grasping hands and feet, large forward-facing eyes and a shortened snout. It probably favored tropical forests and enjoyed a diet of fruit and insects.
"I hope that other groups will try to model this type of problem, and using these different approaches we might come to some consensus about what is likely to have happened," Tavare said.
Martin summed up the potential controversy over the research by saying, "Rather than making the most of what we do know, we're trying to estimate how much we don't know. I don't believe in the 'X Files,' but I do believe in earlier origins."
