Linking Man To a Monkey

By Alice Park

They weighed no more than a hand-ful of peanuts, had large, saucer-like eyes and flitted about the treetops of humid Asian rain forests on feet no bigger than rice grains. Shy nocturnal creatures, they snapped up insects and nectar as quickly as their tiny bodies could digest them, all the while trying to avoid becoming a meal themselves for nightly predators such as owls. Meet Eosimias, the "dawn monkey"--what some experts are calling the primate equivalent of the missing link.

Eosimias, which lived 45 million years ago, made news last week when paleontologists disclosed new fossil evidence placing the little animal at a critical juncture in the history of life. According to their report in Nature, it was the species that directly followed a significant branching of the evolutionary tree. One limb led to the prosimians, or lower primates, such as lemurs and bush babies, and the other to the anthropoids, or higher primates, such as monkeys, apes and humans. Eosimias may be the first species on the anthropoid branch of the tree.

The evidence consists of tiny fossilized ankle and foot bones uncovered in central China beginning in 1995. Painstakingly analyzing their find under a microscope, Daniel Gebo of Northern Illinois University and his colleagues identified features common to both anthropoids and their more primitive forebears. "There's always been a big hole between the earliest fossil anthropoids and prosimians," says Gebo. "The fossils were either true anthropoid or true prosimian; we've never found anything in between until now."

Gebo and his colleagues speculate that the anthropoid-like structures of the foot bones helped Eosimians shift from the leaping and clinging typical of tree-dwelling lower primates to walking on all fours, a characteristic that evolved later.

He acknowledges, however, that it will take more than a few foot bones to establish Eosimias as the common ancestor of monkeys, apes and man. Some experts, in fact, say that Eosimias is probably closer to the tarsiers, which split off earlier from the monkey-to-human branch. "So far, we don't have the smoking gun," argues anthropologist Eric Delson of the City University of New York's Lehman College. He would be convinced, he says, by a full skull showing fused eye sockets and forehead. (By contrast, lemurs have gaps in both places.)

Continuing their excavations, Gebo's team hopes that prize may yet turn up at the fossil-rich site in China. But even without it, dawn monkey's tiny feet have given us the best glimpse so far of what our primate forebear might have looked like.