A decade after the discovery of two astonishingly well-preserved fossils of a species linking humans to apes, a special issue of the journal PaleoAnthropology has been dedicated to all we have learned about these specimens. The papers confirm Australopithecus sediba (Au. sediba) really was a distinct species and reinforce how strange and complex human evolution was.
Au. sediba was found at Malapa, South Africa, by Professor Lee Berger of the University of Witwatersrand. At 1.977 million years old the pair could be dated remarkably precisely for such old fossils. The find challenged the view that east African fossils such as Lucy represented humanity's direct ancestors. Berger's subsequent discovery of Homo naledi, another southern African species whose place in the human family tree has also been hard to identify, has confused things further.
An overview paper argues that even though Au. sediba probably overlapped in time and place with Au. africanus and the first members of our own Homo genus, it was neither. Instead, Au. sediba has certain characteristics in common with each of its fellow early hominin, suggesting this may be the long-sought breakaway from which the first Homo sprang.
On the other hand, some scientists have questioned whether differences in the vertebrae of the two reconstructed Malapa fossils make them separate species (some other bones from the same site have yet to be allocated). Co-editor Dr Scott Williams of New York University expressed confidence in a statement. “The differences in these vertebrae can simply be attributed to their developmental age differences: the juvenile individual’s vertebrae have not yet completed growth, whereas the adult’s vertebra growth is complete.”
The relationship to us is more tricky. “Our findings challenge a traditional, linear view of evolution,” said co-editor Dartmouth College's Dr Jeremy deSilva. “It was once thought that a fossil species a million years younger than Lucy would surely look more human-like. For some anatomies of Australopithecus sediba, like the knee, that is true. But, for others, like the foot, it is not.”
Six of the papers focus on specific body parts, namely the skull, upper, arm, hand, pelvis, and lower limbs. Each offers insight into Au. sediba's lifestyle. Like Lucy, Au. sediba's skeleton indicates it was used to walking upright, but still spent time in the trees, possibly to escape predators, or because fruit was still a big part of its diet. The hands were like no other species we have seen, with the strong grip well-suited to climbing, but a capacity for precise manipulation.
The final papers cover body size and proportions, suggesting each of the specimens we have weighed around 35 kilograms (77 pounds), with a computer animation reconstructing how we think Au. sediba walked.