Putting meat on the bones of Au. sediba, our oldest ancestor

We now know the exact age of a species that confounds scientists. Lee Berger/University of the Witwaterstrand

Since its discovery in August 2008, the site of Malapa in Johannesburg has yielded more than 220 bones of early hominins representing at least six individuals, including the remains of babies, juveniles and adults.

And now there’s something else, something very significant.

As lead geologist on the team that found MH1 and MH2 in caves at the Cradle of Humankind World Heritage Site, I’m pleased to announce we have now accurately dated these individuals, and can reveal several findings that cast doubt on long-held theories about human evolution.

What we’ve discovered

Our findings (through detailed descriptions of the brain, hand, pelvis and foot of the fossils) place the fossils as an intermediate form between earlier Australopithicines (hominid family originating between 10 and 5 million years ago) and later species of the genus Homo, to which modern humans belong.

Evidence for this, published today in Science, is based on the remains of two individuals from the site – MH1 (a juvenile male) and MH2 (an adult female), who we think could possibly be mother and son. These superbly preserved fossils were first announced to the world in April last year.

At the time, their age was estimated at between 1.78 and 1.95 million years old. But we can now say with some certainty they are 1.977 million years old.

Courtesy Duke University.

This date places the occurrence of Au. sediba at Malapa briefly after the last occurrence of Au. africanus (represented by Mrs. Ples, a skull found in South Africa in 1947) at about 2.03 Ma – a unit of time equal to one million years ), and well before (approximately 0.2 Ma) the first definite occurrence of members of the genus Homo, in the shape of H. erectus.

Unfortunate endings

The site where the fossils were discovered is the infill of a de-roofed cave that was about 30-50 metres deep 2 million years ago.

The individuals appear to have fallen, along with other animals, into a deep cavern, landing up on the floor for a few days or weeks.

The bodies were then washed into an underground lake or pool, probably pushed there by a large rainstorm. They didn’t travel far, maybe a few metres, where they were solidified into the rock, as if thrown into quick-setting concrete.

There is evidence in the deposits that the cave experienced a major collapse shortly after the fossils were buried, and it’s possible the hominins wandered into the cave for shelter and fell into an unexpected shaft that had formed after the cave floor had collapsed.

The hominin skeletons were found with bones either in partial articulation or in close anatomical association, which suggests the bodies were only partially decomposed at the time of deposition in the lower chamber.

This, in turn, suggests they died very close in time to each other, either at the same time, or hours, days or weeks apart.

The hominins are thought to have fallen into a deep cavern.

Juvenile

The juvenile MH1 is around 10–13 years old in human developmental terms. He was probably a bit younger in actual age (perhaps as young as eight or nine) as he is likely to have matured faster than humans.

Mrs Ples? That’s Australopithecus africanus to you. mharrsch