Academics believe they have made a major breakthrough in understanding the evolution of humans.
Working with fossils found in South Africa, Professor Noel Cameron and Professor Barry Bogin, of Loughborough University, have linked Australopithecus afarensis, which died out 2.1 million years ago, with Homo erectus, the first of the "modern human" species, which emerged about 1.9 million years ago.
It is the first time a fossil has been found to fill the void between the two species. It has been named Australopithecus sediba.
The academics have been studying the remains of a juvenile male found in 2008 with an adult female, a second adult and an infant – possibly all related – in a cave in Malapa, 25 miles from Johannesburg.
Prof Cameron said: "We can't say for sure whether sediba was the one that led to modern humans, but it's certainly the filled the gap before Homo erectus, which is part of the lineage which ended up as modern humans."
The find is the largest known post-cranial haul of bones of early human and closely-related species – hominins – that evolved after the split from the line that led to chimpanzees.
Prof Cameron said: "Usually, very few fossil fragments, other than the skull, are found because animals will have taken the rest away and eaten them.
"The chances of finding a skeleton from the neck down are fairly small."
Profs Cameron and Bogin were asked to determine whether the remains displayed the growth characteristics of modern humans or of primate ancestors.
Prof Cameron said: "We were able to look at a variety of bones to allow us to determine whether or not the pattern of growth reflected that of an ape or something more modern.
"The pattern of growth is fundamentally important.
"The only reason you look like you do is because of the way in which you grew.
"So if you understand the pattern of growth, you can determine how far they were from apes and how close they were to modern humans."
Prof Cameron said he could not give more information about the study because the work was "clearly ground-breaking and the results are embargoed until publication in a major scientific journal next year".
Our oldest known ancestor, Sahelanthropus tchadensis, is thought to have lived about six to seven million years ago – although some scientists argue it does not belong to the hominin group.
There are 19 known species in the evolutionary chain, but finding how they link to each other is a difficult task because of a lack of fossils.
