CHICAGO - New research that accounts for gaps in the fossil record challenges traditional methods of interpreting fossils and constructing evolutionary trees. Applying a new statistical approach to primates demonstrates that this group-from which humans developed-originated 85 million years ago (Mya) rather than 65 Mya, as is widely accepted.
This revision has implications throughout the evolutionary tree of primates, including the time of origin of the human lineage. Key findings from the new approach to interpreting the fossil record include:
* Primates originated while dinosaurs still roamed the earth. This challenges the widely accepted theory that primates could not establish a foothold until the end of the Cretaceous (65 Mya) when an asteroid cleared the way by hitting the earth and wiping out dinosaurs.
* If times of divergence within the primate tree are revised accordingly, it is likely that the divergence of humans from chimps occurredabout 8 Mya rather than 5 Mya.
* An earlier origin for primates makes it very likely that continental drift played an important part in initial geographical subdivisions within primates.
* The new approach supports previously disputed findings from several molecular evolutionary trees calibrated with fossil dates from better-known parts of the mammalian tree. Calibrations outside the primates include mammal-like reptiles, horses and cetaceans (whales, dolphins and porpoises), where the fossil record is much more complete.
* Using the fossil record to date the origin of any group for which the fossil record is sparse (including certain other mammals, such as bats) is unreliable.
"Current interpretations of primate and human evolution are flawed because paleontologists have relied too heavily on direct interpretation of the known fossil record," says Robert D. Martin, PhD, vice president of academic affairs at The Field Museum and co-author of the research to be published in Nature April 18, 2002. "Our calculations indicate that we have fossil evidence for only about 5% 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."
New statistical approach fills in fossil record
The earliest unequivocal primate fossils date from 55 Mya. Most paleontologists interpret this to mean that primates originated no earlier than 65 Mya.
"This view reflects the common procedure of dating the origin of a group according to the estimated stratigraphic age of the first fossil representative, and then adding a few million years," Dr. Martin explains. "This doesn't work well for primates because so few fossils have been found, many of the fossils we do have amount to a few teeth or bone fragments, and many species are known from only one fossil specimen."
The new statistical approach, however, estimates the length of time between the oldest known fossil and the earliest common ancestor of a given group. It also estimates the likely number of extinct fossil species in that group. It is based on an assumed species lifetime of 2.5 million years, the number of fossil species known in each stratigraphic interval, and the number of species alive today (taken as 235 for primates, now thought to be a minimum).
A painstaking review of the scientific literature revealed a total of 474 recorded fossil primate species. Applying the new approach to the data divided into stratigraphic intervals indicated that there were 8,000-9,000 extinct primate species.
These conclusions have ramifications throughout paleontology, anthropology, primatology and other disciplines. They require a rewriting of the story of primate evolution. For example, if primates originated 85 Mya, then continental drift that broke up Gondwanaland during the Cretaceous probably contributed to primate divergence.
Also, the earlier date of origin indicates that primates probably originated in southern tropical/subtropical regions and then expanded northward, rather than originating in northern regions, as is currently thought.
A complete lack of undoubted primate fossils from tropical and subtropical regions of the southern continents during the late Cretaceous (98-65 Mya) and Paleocene (65-55 Mya) has traditionally been taken as evidence that primates did not exist there during the Cretaceous. The first, abrupt appearance of primate fossils in the northern continents about 55 Mya is often taken as evidence for a northern continental origin during the Paleocene.
Contrary to this accepted theory, the authors attribute the dearth of primate fossils during the Cretaceous and Paleocene to the fact that conditions in southern latitudes did not favor fossil preservation from those times. The earliest primates were presumably quite small, which would greatly reduce the probability of fossilization and discovery.
New dates for calibrating trees
Many scientists use inferred dates of origin provided by paleontologists as temporal anchors for their work. In particular, molecular biologists have relied heavily on these derived dates when constructing a timescale for evolutionary trees of animals.
Molecular biologists estimate the length of time along branches between related species on these trees by estimating the number of changes in DNA sequences. However, there is no known way of deriving a timescale from molecular data alone.
In order to attach a timescale to a molecular tree, the standard practice has been to calibrate it using usually only one date derived from the fossil record. If the date of origin of a group derived from the fossil record is seriously underestimated, the same must be true for any molecular tree calibrated using that date.
"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," Dr. Martin says.
Earliest common ancestor of all primates
Existing primates can be divided into six subgroups: lemurs, lorises, tarsiers, New World monkeys, Old World monkeys, and apes and humans. Their 85-million-year-old earliest common ancestor probably looked like a primitive, small-brained version of today's dwarf lemur, according to Dr. Martin, who has studied primate evolution from many different perspectives for the past 30 years.
That animal would probably have been a nocturnal, tree-living creature weighing about 1-2 pounds, with grasping hands and feet, also used by the infant to cling to the mother's fur. It probably had large forward-facing eyes for stereovision and a shortened snout (reflecting a reduction of the anterior dentition). It would have inhabited tropical/subtropical forests, feeding on a mixed diet composed mainly of fruit and insects. Like humans, it probably had a slow pace of breeding characterized by heavy investment in a relatively small number of offspring.
The research to be published in Nature represents an unusual combination of mathematicians' statistical expertise with biologists' knowledge of primate evolution. In addition to Dr. Martin, the authors are Dr. Simon Tavarι and Dr. Oliver Will (University of Southern California in Los Angeles), Dr. Charles Marshall (Harvard University), and Dr. Christophe Soligo (Natural History Museum in London).
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