25 October 2012

Ancient humans walked, but climbed trees like apes

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An extinct human species that includes the famous fossil ‘Lucy’, walked on two feet like Homo sapiens, but climbed trees more like modern apes, according to new research.
Selam shoulder blades

Selam's skeleton, view of the upper back including left and right ribs either side of the vertebrae and complete right and fragmentary left shoulder blades. Credit: Courtesy of Zeray Alemseged / Dikika Research Project

Selam cranium

Selam's cranium, face and jawbone, including upper and lower 'deciduous', or baby, teeth. Credit: Courtesy of Zeray Alemseged / Dikika Research Project

SYDNEY: An extinct human species that includes the famous fossil ‘Lucy’, walked on two feet like Homo sapiens, but climbed trees more like modern apes, according to new research.

The results, published in the U.S. journal Science, come from an analysis of two complete shoulder blades of Selam – a three-year-old female Australopithecus afarensis who lived more than three million years ago.

The researchers took detailed measurements of Selam’s shoulder blades, digitised them, and compared them to other early human species, such as Homo floresiensis (‘the hobbit’), as well as modern humans, chimpanzees, orang-utans and gorillas.

Ape-like humans

In doing so, they not only found that the structure of the shoulder blades were more ape-like than human-like – suggesting A. afarensis was more of an adept tree-climber than Homo sapiens – but also that there was little difference between the shoulder blades of a child and adult of the ancient hominid species.

“This is just what we see in juvenile and adult apes, as opposed to modern humans,” explained David Green, a professor of anatomy at Midwestern University in Illinois, and co-author on the paper. Human shoulder blades, unlike apes, change shape notably from childhood to adulthood.

“We hope that our results will help move forward a longstanding debate in our field, as there has been much disagreement about how to interpret the functional relevance of traits that may have been retained from our common ancestor with chimpanzees,” he added.

“Pivotal place in human evolution”

Selam was discovered encased in sandstone by the second paper co-author, Zeresenay Alemseged, in Dikika, Ethiopia in 2000. The find represents the most complete skeleton discovery of Selam’s species, which includes ‘Lucy’ – an adult female whose remains were uncovered in another part of Ethiopia in 1974.

Previous fossil studies have already shown A. afarensis to have walked on two feet (bipedalism) and use tools, but this latest research adds more ape-like characteristics – retained from our shared ancestor with apes – to our understanding of the species.

“Bipedalism is the hallmark of our lineage. It’s how we’ve grown to define what is a hominin or hominid, versus a fossil or individual that would be more closely related to living apes,” said Green in a Science press podcast.

“This new find confirms the pivotal place that Lucy and Selam’s species occupied in human evolution,” Alemseged said in a statement.

Natural selection, or evolutionary by-product?

Palaeoanthropologist Carol Ward of the University of Missouri in the U.S. co-authored a 2011 study on a fossilised A. afarensis foot bone demonstrating that the early human species walked on two feet.

Commenting on this latest research, which she wasn’t involved with, Ward said there is “no simple interpretation” of the findings, and that it remained difficult to determine whether the ancient human’s ape-like shoulder blades were simply a ‘neutral’ evolutionary by-product, or retained for a function, such as tree climbing.

“Primitive traits may be retained either actively by selection, or because there was no selective reason to change them,” she explained, adding: “This makes the significance of the Dikika scapulae [shoulder blades] difficult to interpret.”

Its own, unique species

Ward also said that it seems more likely that A. afarensis is neither more ape-like nor human-like, but somewhere in between. “It seems that A. afarensis had morphology [biological form] unlike that of any living animal, but I’m not sure that it is most like any single living [species],” she said.

The separation of Selam’s shoulder blades from the rest of the skeleton and their subsequent removal from the sandstone they were encased in was a painstaking, 11-year process.

“In 2011, when I was able to hold both complete scapulae of this individual, I couldn’t believe my great fortune,” said Green, who added that he hopes next to perform a similar study on Selam’s clavicles, or collar bones, which were also preserved in the sandstone.

“The fact that these scapulae come from a partial skeleton means they can be compared with the rest of the skeleton to develop a fairly complete picture of the suite of anatomies that made up A. afarensis, and so provide an important insight into their behaviour, life history and adaptations,” Ward said.

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