The story of a critical phase in human evolution may have to be rewritten after the discovery of two remarkable fossils in Kenya that have shed new light on the origins and behaviour of two ancient relatives of Homo sapiens.

One of the fossils found near Lake Turkana has shown that two early human species thought to have evolved one from the other actually lived side by side for almost half a million years, redrawing the most widely accepted version of humanity’s family tree.

The discovery means that only one of the two species, known as hominins, can be a direct ancestor of modern human beings, and not both as was previously proposed.

While scientists are still confident that Homo erectus, the younger of the two, ultimately gave rise to Homo sapiens, it is now suspected that the older, Homo habilis, was an evolutionary dead end.

Both are likely to have evolved from another, older common ancestor, a missing link that has not yet been found in the fossil record, which lived between two and three million years ago.

It was thought that Homo habilis, which means “handy man” in Latin and was the first hominin to make sophisticated stone tools, was likely to have been Homo erectus’s ancestor, and hence a more distant ancestor of modern humans as well.

The new Homo habilis fossil, a jawbone, makes this extremely unlikely. It has been dated to 1.44 million years ago, at least 200,000 years later than the oldest known example of the species, meaning it survived to live alongside Homo erectus for hundreds of thousands of years.

Homo erectus is thought to have emerged about 1.8 million years ago, and while a short period of overlap is consistent with the theory that it evolved from Homo habilis, the new dates virtually rule this out.

Meave Leakey, of the Koobi Fora Research Project, who led the discovery team with her daughter, Louise Leakey, said: “Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis. The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition.”

Frank Brown, of the University of Utah, another author of the research paper published in the journal Nature, said: “The most important conclusion is that there was more than one species of early man for an extended period of time in East Africa.”

The parallel existence of several species of hominin would have been normal through most of history: only since the extinction of the Neanderthals about 28,000 years ago has there been just one. “As you go through time, in general there was more than one hominin,” Dr Brown said. “Yet today Homo is represented by only a single species: us.”

The second fossil, which is described in the same paper, is significant for what it reveals about the probable sexual habits of Homo erectus. The exquisitely preserved skull has been dated to 1.55 million years ago.

Fred Spoor, of University College London, said: “It is the smallest Homo erectusfound thus far anywhere in the world.”

It belongs to an adult, almost certainly a female, and this suggests extreme variation in body size among the species, with males growing much larger than females. The disparity would have been almost as great as in modern gorillas. In modern monkeys and apes that are monogamous, such as gibbons, there is little variation in size between males and females. Among those where dominant males monopolise mating, such as gorillas and baboons, males are much larger.

Modern humans lie between the two, but the small size of the Homo erectus female suggests that our ancestors may have had a harem-like system, in which a single male who could see off rivals won the sexual favours of many females at once.

Susan Antón, of New York University, said: “In gorillas, males are much larger than females, and this sexual dimorphism is related to their strategy of having multiple mates. The new Kenyan fossil suggests that, contrary to common belief, this may have been true of Homo erectus as well.”

Times Recommends

• Thalidomide is story of pain and neglect
• Mum’n’dad vote will decide the next election
• Socialite wife in a class of her own