Kadanuumuu is the name given to the fossilized remains of a male Australopithecus afarensis in Ethiopia, found in 2005 and dated to about 3.5 million years ago. The find has been both exciting and controversial because early analysis suggests Kadanuumuu (whose name is local indigenous Afar for "big man") was much taller than the later (and famous) A. afarensis specimen Lucy, and was bipedal, standing and walking on two legs, at a much earlier date than she was.
In 2005, a team including Yohannes Haile-Selassie of the Cleveland Museum of Natural History and Owen Lovejoy of Kent State University unearthed Kadanuumuu in Afar's famous Rift Valley, the same formation which yielded Lucy and several other early hominids. Only part of Kadanuumuu's skeleton survives, but what is there - including his pelvis and a scapula (shoulder blade) - is enough, in the minds of the research team, to conclude that he was definitely bipedal and that a number of other features, like long leg bones and arms unadapted for tree life, were also present from this early period. Until discoveries like Kadanuumuu, paleoanthropologists had often assumed that many of these features came much later in hominid evolution.
Until recently, the standard (and simplified) version of pre-human history stated that Australopithecus afarensis walked northern Africa between 2.9 and 3.7 million years ago, and marked the point at which early hominids began the difficult transition from walking on all fours, and living in trees, to walking bipedally and living on the ground. Other specimens than Kadanuumuu show that they still retained some tree-living features (particularly on their hands and feet), but Kadanuumuu, if his attribution to A. afarensis (and the dating) is correct, shows that the evolution of bipedalism may have occurred somewhat sooner than expected.
Indeed, other recent fossil records suggest that the transition from walking on all fours to walking on two legs was already underway even farther back. After Haile-Selassie and Lovejoy found Kadanuumuu, another team found "Ardi," a specimen of an earlier species now named Ardepithecus ramidus. Ardi is a 4.4. million-year-old female whose skeleton indicates that her species was in the midst of the transition to walking: she could do so bipedally, but still awkwardly, and probably still spent much or most of her time in the trees.
It is this middle range in human evolution - the time at which the ancestors of modern humans moved away from those of today's chimpanzees and other apes - that paleontologists are now focusing on. Ultimately, Kadanuumuu probably does little to overturn the better-established history of the Homo genus which emerged from Australopithecus, beginning with Homo habilis about 2.3 million years ago. He does help us understand, however, that the evolutionary steps which led to that point took much longer than originally assumed.
The Kadanuumuu find is not without its critics. Indeed, California anthropologist Zeresenay Alemseged points out that without dental or even skull remains (his entire head is missing), we still cannot be entirely sure that Kadanuumuu even belonged to the A. afarensis species, and not to one of its close relatives. Moreover, other A. afarensis specimens from a similar time period, like an infant given the name Selam, do seem to show the sort of tree ape-like features which the Haile-Selassie and Lovejoy team argue are not found in Kadanuumuu. And fingers and toes in other specimens (Kadanuumuu's are all lost) do suggest a continuing ability to climb trees quickly and easily. In a field where hard data points are frustratingly few and far between, the debate will likely not be fully settled for some years yet.