The evolution of monkeys remains a mystery

Why monkeys and apes took separate evolutionary paths has long been a mystery. One widely held theory is that environmental changes that led to more open habitats drove a wedge between these animals, leading the ancestors of monkeys to make do with a less nutritious diet of leaves and those of modern apes to thrive upon fruits and seeds. A study led by John Kappelman of the University of Texas and the late David Rasmussen of Washington University, published this week in PNAS, suggests that this idea is wrong.

There are few vertebrate groups that have a worse fossil record than monkeys. Fossils form best when animals die in places where sediment is constantly being deposited to cover up their bones, like streams, river deltas, coastlines and sand dunes. Because monkeys typically live in lush forests where sediment is rarely deposited, they rarely fossilise. Indeed, while genetic analysis of modern species makes it clear that they diverged from apes 30m years ago, evidence of their first 12m years of existence has until now been composed of just two molars that are too worn to show much detail.

A new fossil discovered in Nakwai, Kenya by a team of Kenyan and American scientists has now been dated as being 22m years old. Composed of several jaw fragments with well-preserved teeth still stuck in their sockets, the fossil clearly belonged to a monkey. Yet the specimen has raised more questions than it has answered because it lacks an important dental trait known as bilophodonty.
Best described as teeth that have crests running between their cusps, bilophodont molars are found in all members of the old world monkey family and play a pivotal part in helping these animals to chew leaves efficiently. Because the Kenyan fossil does not have these crests, Dr Kappelman and his colleagues believe it was much more likely to have fed on fruits and seeds. That goes against the prevailing theory that leaves became a major part of the monkey diet after their split from apes 30m years ago.

Although Alophia, as the researchers have named the fossil, may just be an odd early monkey lineage that broke from its kin and later started eating fruit, it is also possible that this animal had teeth that were typical for monkeys of the time. If so, the monkey puzzle deepens: something other than a taste for leaves must have led them away from apes.

This article appeared in the Science and technology section of the print edition under the headline "Monkey puzzle"