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Mouse-Sized Primates Shed Light on Human Origins

27 October 2010 1:08 pm
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Illustration by Mark A. Klinger; (inset) MPFL

Diverse roots. Humans, apes, and monkeys can trace their origins back 39 million years in Africa, where several types of tiny anthropoids already existed in what is now the Sahara Desert of Libya (inset).

Tiny primate teeth discovered in the Sahara Desert may illuminate our own humble beginnings as creatures the size of mice. The 39-million-year-old fossils belong to a subgroup of primates known as anthropoids, which includes monkeys, apes, and humans. Researchers say the find indicates that our ancient ancestors were entrenched in Africa earlier than expected and that they didn't begin to get larger until well after they had moved to Africa and adapted to new environments there.

Researchers once thought that the first anthropoids arose in Africa. That's because for many years the earliest fossils of universally accepted anthropoids came from 37-million-year-old fossil beds in the Fayum region of Egypt. But over the past 16 years, scientists have discovered tiny primates in Asia that many think are the earliest known anthropoids, such as the 45-million-year-old Eosimias from Myanmar, says paleontologist Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

Now, in three field seasons in central Libya, Beard and an international team of paleontologists have learned more about when these higher order primates reached Africa, possibly from Asia, and diversified. They discovered teeth from three completely different groups of anthropoids: Afrotarsiidae (which now includes Old World monkeys such as macaques), Parapithecidae (extinct primates also known from the Fayum region of Egypt), and Oligopithecidae (primates from the Fayum that gave rise to some Old World and New World monkeys). Other kinds of animal fossils suggest that the site is between 38 million and 39 million years old, as does paleomagnetic dating, which relies on a pattern of well-dated reversals in Earth's magnetic field recorded in sediments.

If the date is correct, it would suggest that anthropoids left Asia soon after they arose there about 45 million years ago and dispersed to Africa and other parts of the globe much earlier than expected, says team leader and paleontologist Jean-Jacques Jaeger of the University of Poitiers in France. "Our goal is to nail down when these anthropoids got into Africa," adds Beard. An alternate view is that these tiny anthropoids arose in Africa instead of Asia, allowing enough time for them to evolve the diversity seen in these fossils from Libya. The team reports its find online today in Nature.

Although the identification of several of the fossils as early anthropoids is solid, not everyone is convinced that their age is accurate. Paleontologist Erik Seiffert of Stony Brook University in New York thinks that several of the fossils are so similar to those he and others have found in Egypt that they might be roughly the same age of 37 million years or younger—and thus not evidence for an earlier presence of anthropoids in Africa.

Paleontologist Richard Kay of Duke University in Durham, North Carolina, notes, however, that the Libyan fossils are smaller than those in Egypt, suggesting that they may indeed be older and more primitive. Their small size and other features, Jaeger says, also link the Libyan fossils to the earliest anthropoids in Asia—but not to Ida, a 47-million-year-old fossil primate from Germany whose discoverers controversially proposed her as an ancestor of anthropoids.

Only after the first wee primates migrated out of Asia and scooted rapidly to new habitats in Africa did some anthropoids begin to get larger and start evolving down the path toward becoming apes—and, eventually, humans, he thinks. "If this migration to Africa had not occurred, the anthropoid might have become extinct in Asia and we would not be here," Jaeger says.

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