Species Suspected Of Being Our Last Non-Human Ancestor Is Unlikely To Be It

Australopithecus afarensis (left), Homo habilis (center), and Australopithecus sediba (right) are the main candidates for our last non-human ancestor, but sediba's claim looks weak. Matt Wood UChicago

The popularity of genealogical databases demonstrates that humans have a great desire to learn where we come from. Going back a few million years, this translates to a debate about which Australopithecus species was the direct ancestor of Homo, the genus to which we belong. New research shows it is unlikely southern Africa's A. sediba could have evolved into early humans.

A. sediba was discovered in 2008 at Malapa near Johannesburg. The upright nature of its walk led its finders to argue it made a better candidate for the ancestor of humanity than other Australopithecus species.

All known A. sediba fossils are around 2 million years old. The oldest Homo fossil is 2.8 million years old, from thousands of kilometers away in Ethiopia. To non-anthropologists, these facts might seem to be enough to conclude A. sediba was an evolutionary dead end, plugging away while the real action was elsewhere.

However, Dr Andrew Du of the University of Chicago explained the claims of A. sediba's discoverers to have found our great-great...grandparent cannot be dismissed that easily. "It is definitely possible for an ancestor's fossil to postdate a descendant's by a large amount of time,” Du said in a statement. Human fossils degrade so easily, and so much of Africa remains unstudied for these purposes, that it is certainly possible A. sediba was around far earlier, and the Homo genus broke away from it early on.

However, possible and likely are very different things. For his doctorate, Du explored the chance of this having occurred. In Science Advances, he calculates the chance of A. sediba being so much older without us knowing about it at 0.09 percent. Du looked at 28 cases of species broadly related to humans and what are believed to be their ancestors. In only one case the first evidence we found of the descendant was older than the equivalent example we have of the ancestor.

Moreover, the anomaly, Homo antecessor, preceded its likely ancestor by a relatively believable 100,000 years in the fossil record, requiring Homo erectus to have appeared somewhat earlier than fossils indicated. By comparison, Du said, “800,000 years is quite a long time.”

So if we don't come from A. sediba, then who are our ancestors? Du thinks the most likely candidate is A. afarensisis, widely known as “Lucy” after the name given to the first specimen to be discovered. We have examples going back 3.9 million years, placing them before any known Homo representative. She may have been more tree-dwelling than A. sediba, but Lucy and all her kin lived in Ethiopia, overlapping with where the earliest humans were found.

Other anthropologists continue to make the case for other twigs on the family bush, such as A. africanus, so the debate is unlikely to end soon.

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