The remains of the earliest known child from humanity’s family tree have been discovered in Ethiopia, in an unprecedented find that fills in a critical missing link in human evolution.

The almost complete skeleton belongs to a baby girl of the species Australopithecus afarensis — a probable human ancestor that was among the first to walk on two legs — who died at the age of three about 3.3 million years ago.

The girl, which has been named "Selam" after the word for peace in several Ethiopian languages, is by far the oldest fossil of a hominin child yet unearthed, and blurs the line between apes and humans.

She has also been nicknamed "little Lucy", after the famous specimen of the same species discovered just 4km (2.5 miles) away in 1974, which was named Lucy as the scientists who found her were listening to the Beatles song Lucy in the Sky with Diamonds.

Early analysis of Selam has already started to transform understanding of a pivotal stage in the evolutionary process that led ultimately to Homo sapiens. Her anatomical features lie squarely in between those of humans and other apes, showing adaptations both for walking upright on two legs and for climbing and swinging from trees.

This suggests that the species lived on the cusp of the human family’s transition to a bipedal, ground-based existence, which is generally accepted as one of the most crucial events in the emergence of the modern anatomy.

Selam’s brain case also suggests that while her intellect was more similar to a chimp’s than a person’s, her species’s brain had already started to evolve in the direction that would produce modern human intelligence. Details of the fossil are published today in the journal Nature.

Zeresenay Alemseged, an Ethiopian scientist at the Max Planck Institute in Leipzig, Germany, who led the team that found and examined Selam, said she ranks as one of the most important hominin fossils on record.

"Her completeness, antiquity and age at death combined make this find unprecedented in the history of palaeoanthropology, and open many new research avenues to investigate the childhood of early human ancestors," he said.

The find was also hailed by independent experts in the field, including Donald Johanson, who led the group that found Lucy 32 years ago.

"It’s a remarkable and rewarding discovery," Dr Johanson told The Times. "The completeness is extraordinary, and what is very gratifying for me is that the find was just 4km directly south from where Lucy was found all those years ago. This is going to throw a whole new light on the growth and development of afarensis.

Professor Chris Stringer, of the Natural History Museum in London, said: "This is a very special find. It’s not until we come to intentional burials by Neanderthals and modern humans in the last 100,000 years that we otherwise find such well-preserved infant remains, because of their fragility."

Bernard Wood of George Washington University in Washington, DC, said: "It is a veritable mine of information about a crucial stage in human evolutionary history."

Selam was found in the Dikika region of northeastern Ethiopia on December 10, 2000, by Tilahun Gebreselassie, a member of Dr Zeresenay’s team. It has since taken the scientists almost six years to extract her remains from the sandstone in which they were set, often using dental tools to remove rock grain by grain, to identify and date them, and to examine her anatomy.

The entire skull and torso and most of the upper and lower limbs are present, providing an unprecedented look at a hominin child in the process of development. While other juvenile fossils exist, such as the 2.5 million year old Taung child of the species Australopithecus africanus, they are known only from fragments of skull, bone or teeth.

Selam’s leg and foot bones show her to have been already adept at walking upright even at the age of three, showing conclusively that A. afarensis was an accomplished biped.

Her upper body, however, is more enigmatic: her shoulder blades are similar to those of a modern gorilla, while he fingers are long and curved, like those of a chimpanzee. The canals of her inner ear — important for balance — are also quite chimp-like.

All this suggests that A. afarensis divided its time between walking upright on the ground, and climbing trees. It is possible that, like modern gorillas, females and infants spent more time in the trees, where they would have been safer from predators, while heavier males were more exclusively ground-based.

"I’d say it was effectively and fully bipedal, but it might have spent some time in trees," Dr Zeresenay said.

"We can see primitive features in the upper body, but it is hard to know whether these are left from a common ancestor with apes, or whether they had important functions."

Another interesting feature is the hyoid or tongue bone, which has never been found before in a species older than Neanderthal man. It influences the voice box and is important to the debate about the origins of human speech. Selam’s hyoid is much more similar to that of modern apes than humans, suggesting that A. afarensis was not capable of language.

Her brain had a volume of 330 cubic centimetres, which is similar to that of a chimp of the same age. It had only, however, reached between 63 and 88 per cent of adult brain size, while chimps’ brains are more than 90 per cent grown at the age of three.

"This is slightly closer to the average in humans than to chimps, which may be a hint that important aspects of behaviour had already started to change 3.5 million years ago," Dr Zeresenay said.

Selam’s skull has also enabled a team from National Geographic magazine to produce an artist’s impression of what the girl might have looked like. Only parts of Lucy’s skull were found, so scientists have never been able to reconstruct her face.

"This is something you find once in a lifetime," Dr Zeresenay said. "Unlike Lucy, the baby has fingers, a foot and a torso. But the most impressive difference between them is that this baby has a face."

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