Our ancestors: how primates spread across the earth

Primates were the world's experts in swinging, leaping and jumping from one tree to the next. Life was safest in the trees, far from carnivorous reptiles like crocodiles, which survived the fate of the dinosaurs and still oozed through the swamps below. Fruit and nuts were within easy reach. And up in the trees the view was much better. Getting around was also easier than on the ground, provided you had the skills of a leaping Tarzan.

Primates appeared on the island of Madagascar, which broke off the tip of Africa 165 million years ago. It now contains some of the most protected and endangered species of plants and animals in the world. Descendants of these primitive primates include lemurs whose name comes from the Latin word for "spirits of the night". Like many early primates they are mostly nocturnal, with excellent night vision. Lemurs have survived thanks to Madagascar's isolation protecting them from their descendants, monkeys and apes, which later overran them on the African mainland. Lemurs are an endangered species today because of illegal logging destroying their natural woodland habitat.

Bush babies are other nocturnal tree-dwellers. Big eyes, excellent hearing and long tails adapted them perfectly for a life in the shadows of the forest. But the biggest group of all was the monkeys. They evolved into three main varieties: "Old World" monkeys, "New World" monkeys, and apes. Modern science has found that the genes in monkeys and apes are very similar indeed. Although monkeys have tails and apes do not, biologically they are all part of one family.

Monkeys are placentals that first evolved in Africa, but they now live in both Asia and South America. About 25 million years ago one or more groups of African monkeys somehow found themselves on a raft bobbing across the Atlantic Ocean, eventually to be washed ashore on the coast of modern-day Brazil. This is when the first South American monkey fossils appear, providing evidence that by then they were thriving up in the trees of a newly adopted home.

These New World monkeys are distinctive for their flat noses and for using their tails to help them swing and balance in the trees. They can happily hang from a branch by their tailbone alone, using it like an extra hand.

As if one perilous transatlantic crossing wasn't risky enough, other African monkeys set off in the opposite direction trekking though Asia, across the land bridges of the Middle East and Arabia. Gibbons and orang-utans are found today in places such as India, Malaysia, China, Borneo, Indonesia and Java. Whether they lost their tails before they left, on the way, or later, isn't entirely clear.

Recent genetic evidence and fossil finds suggest that African great apes evolved from species such as gibbons and orang-utans which once lived only in Asia. This means that about 10 million years ago one of these groups of apes – either the orang-utans or the gibbons – made their way back across the Asian landmass into Africa. Here they evolved into today's gorillas and chimpanzees.

Gorillas are mild-mannered vegetarians that live on grasslands – not up in the trees. Only two species survive today. Both are endangered. Several hundred died in 2004 of the Ebola virus, for which there is currently no known vaccine or cure. Gorillas are highly intelligent. Koko, born in 1971, is a captive female gorilla living in California. She has been taught sign language from the age of one. Her trainer, Dr Penny Patterson, claims she can communicate using a vocabulary of up to 1,000 words. Something of a scientific debate has been raging for years since Koko first showed off her language skills. Does she really understand what she is saying? Or is she just prompted by the prospect of a reward if she says the right thing? In August 2004, Koko indicated that she had a toothache. According to her handlers she communicated that she was in pain. She could even indicate its level on a scale of one to 10.

Koko shows other human-like traits. She is one of the few animals ever to have been known to keep and care for a pet. In 1984, Koko asked for a cat. She selected a grey male kitten from an abandoned litter. She named him All Ball. Koko cared for the kitten as if he were a baby gorilla, until All Ball escaped from Koko's cage and was hit by a car. Koko cried for the next two days. Since then she has adopted a number of other pets, including two more kittens, Lipstick and Smoky.

Humans are apes. Until the 1960s it was thought that mankind split from apes about 20 million years ago – mostly because so few fossils had been discovered to prove, one way or another, what happened and when. There was also a strong feeling that the split had to be at least that far back or there could never have been enough time for us humans to have evolved into such apparently superior beings. We talk, we build things, we invent amazing machines, we are clean (generally), ingenious, and we appear to have mastered nature, tailoring it to our own ends.

But in the early 1990s, molecular biologists discovered that we humans share at least 96 per cent of our DNA with the other great apes (chimpanzees, gorillas and orang-utans). Their analysis showed that humans are descended from an ape which probably lived some time between 4 million and 7 million years ago – just 90 seconds before midnight on our 24-hour scale of all Earth history.

This ape's offspring divided into chimpanzees and their cousins, the bonobos, on the one hand; and early human beings on the other. Exactly who this ancestor was, and where he or she lived, is one of the greatest mysteries in all human knowledge – one that is still waiting to be solved.