Go west, young primate

The earliest known primate fossils occur in 55-million-year-old geological deposits ranging across Asia, Europe, and North America. That almost simultaneous appearance has long posed a riddle to paleontologists: where did primates originate, and how did they subsequently disperse? Shortly before those early primate fossils were laid down, the Earth began a 100,000-year period of global warming. A new study now shows that the timing of the primates' rise may have been no coincidence: environmental changes caused by the warming probably enabled them to disperse fast enough to account for their rapid emergence on three continents.

Like today's global warming, the ancient warming was caused by massive releases of carbon-bearing greenhouse gases. Those releases altered the ratios of various carbon isotopes in the atmosphere, notably decreasing the fraction of carbon-13. Rock layers that trapped air during that period now help paleontologists calibrate the dating of geologic formations worldwide.

The relative dip in carbon-13 enabled Thierry Smith, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels, and two colleagues to date early primate fossils more precisely than ever before. With their new dates, the investigators determined that a small, big eyed tree-dweller of the genus Teilhardina, and possibly other early primates, originated in Asia, spread to Europe, and then continued on to North America. The entire journey took less than 25,000 years. Smith and his team theorize that the warm temperatures enabled the primates to cross the Atlantic at high latitudes, over a land bridge then linking the two continents via Greenland. Because Teilhardina was strictly arboreal, the investigators think evergreen forest must have covered a wide swath of the north. (PNAS 103:11223-7)

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