Editorial: “Heading south for the new origin of our species“
THE pair lay at the bottom of a deep hole for 2 million years, buried in oxygen-starved mud that fossilised their bones. The conditions were so unusual that an additional treasure may have been preserved: the oldest known human skin.
Since the two Australopithecus sediba skeletons were unearthed at the Malapa cave site in South Africa in 2008, researchers studying them have dropped tantalising hints that they could have remnants of skin. The latest signs suggest an announcement is imminent. Meanwhile, a set of studies published last week has confirmed that A. sediba is a pivotal species that may bridge the gap between the ape-like australopiths and the first members of our own genus (see diagram).
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The skeletal analyses confirm that A. sediba has a mosaic of ancient australopith and modern Homo features. Its teeth, for instance, are remarkably human-like. Whereas most australopiths have large, prominent canines, A. sediba‘s are small, like ours, according to at Texas A&M University in College Station and his colleagues ().
The skeletons also suggest that A. sediba had the early makings of a tapering human waist. Peter Schmid at the University of Zurich, Switzerland, led a team that found that its lower ribs sweep inwards, as ours do (Science, ). This allowed abdominal muscles to be arranged in a way that makes walking more efficient. Other australopiths are thought to have lacked a waist, says Schmid.
In other ways, A. sediba was very unlike early humans. at Boston University in Massachusetts and his colleagues say its legs and feet were those you would expect of a tree climber. Humans – like most other australopiths – have a rigid foot. A. sediba‘s foot was much more flexible, making it perfect for gripping tree trunks and branches (). “In my opinion, A. sediba was perhaps the most tree-dwelling of the australopiths,” says DeSilva.
This poses a puzzle. If australopiths spent more time walking the savannah and less time in the trees than their ancestors, why was A. sediba, the most human-like of all australopiths, so well adapted to tree living?
“This is the question we are struggling with right now,” says DeSilva. “Is sediba evidence for a return to life in the trees? Or is this body plan evidence for a deeper [tree-dwelling] lineage in South Africa?”
The second option is a real possibility, according to a study led by at Liverpool John Moores University, UK. By comparing the teeth of A. sediba with those of other hominins, Irish and his colleagues found that there may have been two distinct, ancient groups of australopiths: one in East Africa that included the famous Lucy (A. afarensis), and one in South Africa that may have been better adapted to climbing – and that ultimately became human ().
Together, the new studies cement A. sediba‘s position as a critical species for understanding why australopiths underwent a dramatic evolutionary surge that gave rise to modern humans, after 2 million years of relatively subtle evolution.
We could have more clues to how this transition happened if a thin layer of material that coats the bones turns out to be mummified skin. Such a find would be a first for the ancient human fossil record: the oldest confirmed human skin samples are from the last 10,000 years, from natural and human-made mummies.
“If a thin layer of material on the bones is mummified skin, it would be a first for the human fossil record”
, who discovered A. sediba, and Rachelle Keeling – both at the University of the Witwatersrand in Johannesburg, South Africa – discussed the possibility of A. sediba skin at a , last week. They have used a raft of techniques to analyse samples of a thin layer of material covering a skull and a lower jaw belonging to A. sediba. In their presentation, they stopped short of confirming the presence of skin but said there was a “provocative body of evidence that the specimens were indeed organic in origin”.
Robert Reisz at the University of Toronto, Canada, has recently analysed what is thought to be soft tissue in dinosaur fossils, and says the tests are the right ones to detect ancient skin. But even if confirmed, this would be such an unexpected addition to our hoard of hominin finds that it’s unclear exactly how much it could tell us about our forerunners. Its age makes DNA preservation unlikely – the oldest DNA we know of is only 800,000 years old.
Even so, Berger says skin could provide a remarkable insight into A. sediba‘s appearance. In particular, we could find out how hairy our ancestors were at this stage in our evolution, and how long their hair was. We might also learn something about their sweat glands, and perhaps even about the colour of their skin and hair.
As changes go, losing body hair and gaining sweat glands are literally skin deep, but they may have been critical hominin adaptations, says at Pennsylvania State University in University Park. She told the American Association for the Advancement of Science meeting in Boston in February that they could have helped our ancestors cope with the extra heat generated by their large brains, which were three times the size of australopiths’ brains.
There is no consensus about exactly when hominins became smooth and sweaty. Jablonski thinks we were well on the way to nakedness by 1.6 million years ago, around the time hominin brains began growing rapidly. But at the Florida Museum of Natural History in Gainesville thinks it happened much earlier. He says that differences in the DNA of head and pubic lice suggest the two insects have been isolated by vast expanses of naked skin for 3.3 million years (). If so, australopiths were naked long before brains began to get bigger. A. sediba is a good age to settle the matter. “Fossil skin would indeed be very interesting,” Reed says.
“Ancient skin could settle the issue of exactly when our ancestors became smooth and sweaty”
Fossil skin might also tell us about A. sediba‘s health. In 2009, , also at the University of the Witwatersrand, identified 200,000-year-old human hairs in ancient hyena faeces (). “91ɫƬiness is reflected in the scales,” she says, referring to the cells forming the outer layer of a hair shaft. Diabetes, for instance, leads to very distinctive scales.
With such a rich insight into our distant past potentially at our fingertips, many are awaiting more announcements from Berger and Keeling. “I assure you that you will not have to wait too long,” Berger says.
This article appeared in print under the headline “Fossil skin could be a bridge to our past”
