Two fossil discoveries from the East African Rift reveal new information about the evolution of primates, according to a study published online in Nature this week led by Ohio University scientists. The team's findings document the oldest fossils of two major groups of primates: the group that today includes apes and humans (hominoids), and the group that includes Old World monkeys such as baboons and macaques (cercopithecoids). Geological analyses of the study site indicate that the finds are 25 million years old, significantly older than fossils previously documented for either of the two groups. Both primates are new to science, and were collected from a single fossil site in the Rukwa Rift Basin of Tanzania.
Rukwapithecus fleaglei is an early hominoid represented by a mandible preserving several teeth. Nsungwepithecus gunnelli is an early cercopithecoid represented by a tooth and jaw fragment. The primates lived during the Oligocene epoch, which lasted from 34 to 23 million years ago. For the first time, the study documents that the two lineages were already evolving separately during this geological period. "The late Oligocene is among the least sampled intervals in primate evolutionary history, and the Rukwa field area provides a first glimpse of the animals that were alive at that time from Africa south of the equator," said Nancy Stevens, an associate professor of paleontology.
Documenting the early evolutionary history of these groups has been elusive, as there are few fossil-bearing deposits of the appropriate age, Stevens explained. Using an approach that dated multiple minerals contained within the rocks, team geologists could determine a precise age for the specimens. Prior to these finds, the oldest fossil representatives of the hominoid and cercopithecoid lineages were recorded from the early Miocene, at sites dating millions of years younger. The new discoveries are particularly important for helping to reconcile a long-standing disagreement between divergence time estimates derived from analyses of DNA sequences from living primates and those suggested by the primate fossil record, Stevens said.
Studies of clock-like mutations in primate DNA have indicated that the split between apes and Old World monkeys occurred between 30 million and 25 million years ago.
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