Small Monkey Gave Up Jumping

The first primates crawled about 65 million years ago, in the time of Tyrannosaurus rex, according to genetic studies of modern animals. But scientists have still not discovered the remains of the early primates, as the oldest known fossils only date back from the time after the disappearance of the dinosaurs.

No matter when the ancestor of the primates lived, scientists know that the family tree soon branched out in two. One branch led to strepsirrhini, including lemurs and lories, whereas the other branch, in which we belong, led to true monkeys. So far, scientists have not found many fossils which can help them understand this, but one single animal sheds light on the matter. Eosimias, which only weighted 100 g, was so small that it could sit in a human hand, but it still had a lot in common with us.

Eosimias’ heel shows signs of a transition from feet designed for long jumps (lemur behaviour) to feet which were primarily used to walk on branches. This very characteristic shows that Eosimias was presumably an early type of monkey. Consequently, the small animal probably had a relatively large brain and excellent colour vision just like its descendants. Colour vision is rare among mammals, and most likely used the new characteristic to find colourful fruit among the leaves of the trees.