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Our Closest Ape-Like Ancestor Is Reshuffling Thinking About Human Evolution

Australopithecus sediba included a strange mix of both modern Homo and ape-like australopith features

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An A. sediba skull. Photo: Brett Eloff

Around 2 million years ago, the first humans evolved from australopiths, our smaller-brained ape-like ancestors. Back in 2008, researchers found two skeletons in South Africa from the ape-like Australopithecus sediba. A male and female skeleton, called MH1 and MH2, were buried together, and further excavations revealed an infant and another partial adult skeleton nearby. All of the remains dated back to around 1.8 to 1.9 million years old. These skeletons began to raise questions about what we really know about human evolution and Homo origins.

The researchers published their results in the journal Science in 2010, writing:

Despite a rich African Plio-Pleistocene hominin fossil record, the ancestry of Homo and its relation to earlier australopithecines remain unresolved. Here we report on two partial skeletons with an age of 1.95 to 1.78 million years. The fossils were encased in cave deposits at the Malapa site in South Africa. The skeletons were found close together and are directly associated with craniodental remains. Together they represent a new species of Australopithecus that is probably descended from Australopithecus africanus. Combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than any other australopith species and thus might help reveal the ancestor of that genus.

Until this discovery, researchers had assumed that Lucy, the remains, more than 3 million years old, of an Australopithecus afarensis female found in 1974, represented either our direct evolutionary ancestor or else a very close ancestor. But Lucy’s skeleton was found in Ethiopia, about 4,000 miles away from the A. sediba remains uncovered in South Africa. 

Immediately,  i09 explains, researchers began to second guess whether Homo emerged from East Africa after all. Our origins instead may be more southerly. Now, a new slew of studies published by the same research team in Science answers some questions about what our ancestor was like while also opening up some new mysteries. The New Scientist gives a run down of the “bizarre mosaic” of qualities resembling both Homo and Australopithecus africanus (another South African species that lived around 2 to 3 million years ago) that a closer examination of the A. sediba specimens revealed.

The Homo-like traits included:

  • Same number of vertebrae
  • Human-like waist
  • Bottom of the ribcage narrows
  • Walked upright
  • Small canine teeth.

And the ape-like traits were:

  • Top of the ribcage tapered towards the shoulders, preventing the arms from swinging when walking  
  • Arms and legs appear well equipped to swing and balance on branches
  • When walking, rather than planting its heel first like Homo, A. sediba’s gait was more twisty and hoppy thanks to a flexible midfoot.

Where A. sediba fits into the evolutionary tree is still under debate. Based upon studied of the specimens’ teeth, it does not appear that A. sediba evolved from A. afarensis (Lucy) in East Africa. Instead, the New Scientist writes, A. africanus seems to be the most likely ancestral candidate.

That suggests the roots of both lineages of australopiths – from East and South Africa – are even older. “It appears that there may be a ‘ghost lineage’ of unrecognised hominins that goes back deeper in time than afarensis,” says Lee Berger at the University of Witwatersrand in Johannesburg, South Africa, who discovered A. sediba.

National Geographic points out that the questions surrounding A. sediba, such as why it seemed to return to the trees after it first evolved to walk on the ground and where it fits in on the human evolution puzzle, are far from resolved.

Are the ways that Australopithecus sediba resembles early Homo species true indicators of a close evolutionary relationship—or are they traits that evolved independently in both lineages?

Few scientists believe this question has even begun to be settled.

But A. sediba will likely leave a significant mark on science, in any case:

Regardless of what Australopithecus sediba turns out to be, however, the fossils offer an important caution about interpreting more fragmentary human remains found elsewhere.

The hominin “is so curious in its totality,” Potts says, “it might lead to some rethinking of how we classify fossil humans and place them in our evolutionary tree.”

More from Smithsonian.com:

The Human Evolution World Tour 
A Human Evolution Summer Reading List 

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