The quest for the original ape : unearthing the origins of apes and humans

Anthropoid primates today include New World monkeys, Old World monkeys, great apes and humans. Anthropoids differ substantially from other primates on anatomical, behavioral and ecological levels. Anthropoids have larger brains, generally live in larger groups and are more socially complex; all anthropoids, with the exception of the douroucouli, are diurnal, and all, with the exception of humans, move in forests or on the ground mainly by quadrupedal walking rather than by saluting from a vertical support. Humans differ from other anthropoids mainly in two particular adaptations: bipedalism and their large brains, but other fundamental aspects of our anatomy are inherited from our anthropoid ancestors.

The marked differences between anthropoids and other primates, both present and fossil, have made the search for the origin of anthropoids one of the oldest and most controversial issues in Paleoanthropology. Traditionally, paleontologists have placed the origin of the anthropoid lineage in Africa. Since anthropoids are usually considered more evolved than other primates, many who have studied them have assumed that they took a long time to evolve. Today's anthropoids are larger than lemurs and tarsiers, and many have suggested a trend in early anthropoids towards increased body mass. As a result, some scientists have written that anthropoids must have evolved from the largest of the Eocene primates, lemur-like adapiforms such as the recent (and largely discredited) German fossil Darwinius.

Over the past two decades, paleontological discoveries of tiny, ancient and highly primitive anthropoids in Asia (China, Myanmar, Thailand and Pakistan) have sparked a revolution in the study of anthropoid origins. Eosimias, a primate from China around 45 million years old, was the first of these important new fossils to be discovered, and remains one of the best known today. Eosimias was a very small primate, weighing around 150 grams. He possessed many primitive traits absent in other anthropoids, both present and fossil, as would be expected of such an old fossil. Transitional fossils, like Eosimias, are almost inherently controversial, because some scientists will point to the primitive (non-anthropoid) characters to claim that it is not an anthropoid at all. However, a detailed study ofEosimias shows that this fossil possesses many features found only in other anthropoids. These anatomical criteria indicate that ancient anthropoids were already adapted to a diurnal arboreal life, that they moved in trees as monkeys do today, and that they were specialized in eating hard, fibrous vegetation.

This lecture summarizes the long and delicate scientific struggle to reconstruct the ancient history of the anthropoid lineage. From our new knowledge of ancient Asian anthropoids like Eosimias, we can conclude that: (1) anthropoids appeared in Asia rather than Africa; (2) they were very small, as small as or smaller than the smallest primates of today; (3) that the anthropoid lineage separated from other primate lineages very early in the Cenozoic, at the latest at the beginning of the Eocene (55 million years ago); (4) and that the modalities and dates of their expansion from Asia to Africa and South America remain one of the great mysteries of our distant evolutionary history.