Human 'missing link' fossils may be jumble of species

ONE of our closest long-lost relatives may never have existed. The fossils of Australopithecus sediba, which promised to rewrite the story of human evolution, may actually be the remains of two species jumbled together.

The first fossils of A. sediba were found at Malapa, South Africa, in 2008. At 2 million years old, they show a mix of features, some similar to the ape-like australopithecines, others more like our genus, Homo. To its discoverers, this hotchpotch means A. sediba was becoming human, and that the Homo genus first evolved in South Africa, not east Africa as is generally thought.

“To its discoverers, A. sediba was becoming human and is evidence that we evolved in South Africa”

But a new analysis suggests A. sediba didn’t exist. “I think there are two different hominin genera represented at Malapa,” says Ella Been at Tel Aviv University in Israel. One is an Australopithecus and one an early Homo. We can’t yet tell if the australopithecine remains are distinct enough to call them a new species, Been says.

Been studies the spinal columns of ancient hominins, so she was curious when a paper was published last year focusing on the spine of A. sediba (Science, doi.org/r7k). There are fragments from two skeletons at Malapa, a juvenile male and an adult female. Looking at photographs of the vertebrae, she noticed familiar features on the young male.

“I realised they looked a lot like the vertebrae of the Nariokotome Boy,” she says. Also known as Turkana Boy, this is a 1.5-million-year-old skeleton of Homo erectus, a widespread species that may be our direct ancestor. Its vertebrae, like ours, are much wider than they are tall.

In contrast, the adult female’s vertebrae are taller, says Been, a classic Australopithecus feature. She concludes that the spines belong to two different species.

When Been shared her findings with Yoel Rak, also at Tel Aviv University, she found an ally. “He sees the same in the [lower jawbone]: an australopithecine and an early Homo,” says Been. But here the species are switched: a notch in the young male jaw looks like Australopithecus, while the same notch in the adult female jaw looks human.

The pair conclude that there are not two but four individuals in the remains from Malapa: an adult and a juvenile of both Homo and Australopithecus. They presented their findings at a meeting of the Paleoanthropology Society in Calgary, Canada, this week.

Unsurprisingly, A. sediba‘s discoverer, Lee Berger of the University of Witwatersrand in South Africa, doesn’t agree. For one thing, he says the positioning of the adult skeleton’s bones in the ground makes it likely they came from a single individual.

Berger admits that the vertebrae of the young A. sediba look like those of H. erectus, but he says vertebrae grow taller throughout childhood. If the young A. sediba had grown up, his vertebrae may have become more Australopithecus-like.

Been isn’t convinced. Fossils of other australopithecine children had tall vertebrae, she says.

Regardless, Berger says that Been and Rak’s observations make sense if A. sediba really was a transitional species between Australopithecus and Homo. “A central tenet of evolutionary theory is that variation within taxa becomes variation between taxa as species diverge,” he says. With anatomy in flux, it is possible that one A. sediba had an Australopithecus-like spine and Homo-like jaw, while another had a Homo-like spine and Australopithecus-like jaw.

There are other features of the A. sediba vertebrae that might explain the differences Been found. Berger’s latest work hints that the young male’s vertebrae may show signs of disease. If so, they are not representative of the species.

This article appeared in print under the headline “Missing link fossils may be a jumble of species”