'Lucy's baby' found in Ethiopia
The juvenile specimen is wonderfully preserved
The female Australopithecus afarensis bones are from the same species as an adult skeleton found in 1974 which was nicknamed "Lucy".
Scientists are thrilled with the find, reported in the journal Nature.
They believe the near-complete remains offer a remarkable opportunity to study growth and development in an important extinct human ancestor.
The juvenile Australopithecus afarensis remains vanishingly rare.
The skeleton was first identified in 2000, locked inside a block of sandstone. It has taken five years of painstaking work to free the bones.
"The Dikika fossil is now revealing many secrets about Australopithecus afarensis and other early hominins, because the fossil evidence was not there," said dig leader Zeresenay Alemseged, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
The find consists of the whole skull, the entire torso and important parts of the upper and lower limbs. CT scans reveal unerupted teeth still in the jaw, a detail that makes scientists think the individual may have been about three years old when she died.
"This puts afarensis in a special position to play a pivotal role in the story of what we are and where we come from"
Zeresenay Alemseged, Max Planck Institute
Judging by how well it was preserved, the skeleton may have come from a body that was quickly buried by sediment in a flood, the researchers said.
"In my opinion, afarensis is a very good transitional species for what was before four million years ago and what came after three million years," Dr Alemseged told BBC science correspondent Pallab Ghosh.
"[The species had] a mixture of ape-like and human-like features. This puts afarensis in a special position to play a pivotal role in the story of what we are and where we come from."
This early ancestor possessed primitive teeth and a small brain but it stood upright and walked on two feet.
There is considerable argument about whether the Dikika girl could also climb trees like an ape.
The lower limbs show the Dikika girl could walk upright
But the question is whether such features indicate climbing ability or are just "evolutionary baggage".
The Dikika girl had an estimated brain size of 330 cubic centimetres when she died, which is not very different from that of a similarly aged chimpanzee. However, when compared to the adult afarensis values, it forms 63 - 88% of the adult brain size.
This is lower than that of an adult chimp, where by the age of three, over 90% of the brain is formed. This relatively slow brain growth in the Dikika girl appears to be slightly closer to that of humans.
Slow, gradual development in an extended childhood is regarded as a very human trait - probably to enable our higher functions to develop.
Professor Fred Spoor of University College London said the find would give scientists a "detailed insight into how our distant relatives grew up and behaved... at a time of human evolution when they looked a good deal more like bipedal chimpanzees than like us."
Dr Jonathan Wynn of the University of St Andrews, UK, and colleagues at the University of South Florida dated the sediments surrounding the remains and came up with an age of 3.3 million years.
The "Lucy" skeleton, discovered in Hadar, Ethiopia, in 1974 belongs to the same species as the Dikika girl. For more than 20 years it was the oldest human ancestor known to science.