Lucy’s Baby was found in a block of sandstone by a team of palaeontologists led by Ethiopia’s Zeresenay Alemseged and Fred Spoor, of University College London.

Team leader Dr Zeresenay, who is based at the Max Planck Institute in Leipzig, Germany, said: "I see Australopithecus afarensis as foraging bipeds, walking on two feet but climbing trees when necessary, especially when they were little."

Other experts were amazed by the find. Dr Simon Underdown, of Oxford Brookes University, said: "This is a massively exciting discovery. For the first time we can start to understand how one of our distant evolutionary ancestors grew from child to adult nearly 3.5million years ago.

"The new infant skeleton provides evidence from the shoulder and ear canals (which help balance) that the first small-brained bipeds may have spent a lot of time in the trees."

Professor Maciej Henneberg, of Australia’s Adelaide University, said: "The new find adds support to the idea that Lucy and her tribe were bipedal creatures with very ape-like upper bodies.

"To my mind a better explanation is that of hand- supported walking in which creatures are upright but support themselves by holding on to lower branches of trees.

"We humans like to do it by holding a handrail when climbing the stairs or holding on to railings above our heads while standing on a bus or train."

Dr Charles Lockwood, of the anthropology department at University College London, said: "It is impossible to overstate the importance of this specimen.

"It is unique in being so complete - and because it is a child it will address some of the most pressing questions about the evolution of human development - how fast and in what ways the different parts of the human skeleton grew. There is a remarkable amount of new information here."

Remains of early hominids, including the Ape Men or Australopithecus afarensis, have almost only been found in the mountain valleys of east and north-east Africa.

This region was then well watered and wooded – ideal for primates living on a mainly non-meat diet.Much of lowland Ethiopia later turned to desert, the dry conditions preserving their bones as fossils.

A sideshoot in human evolution, Australopithecus garhi, was found in Bouri, Ethiopia, in 1996. They lived around 2.5million years ago and used stone tools.

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