Out on a limb

2012 SCIENCE NEWS TOP 25: 16

More than 3 million years ago, a chimp-sized creature climbed a tree to take a nap. Later, she clambered back down, stood up on two legs and strolled off to find some food, or maybe a mate. Or maybe another tree to scale.

That’s a controversial view of how the famous human ancestor Lucy and her kind, Australopithecus afarensis, maneuvered about the landscape. Scientists have debated whether the early hominid climbed trees. New fossil evidence announced this year suggests the species did spend some time in the treetops even though it mainly walked upright on the ground. This fossil find and others from 2012 challenge the idea that human evolution involved ever more humanlike species replacing more primitive brutes. Instead, the hominid family tree was a tangled bush of motley species.

“This last year has really solidified the idea of diversity in anatomy and behavior in our earliest ancestors,” says biological anthropologist Brian Richmond of George Washington University in Washington, D.C.

The insight into Lucy’s travel habits came from an A. afarensis child that lived in Ethiopia 3.3 million years ago. After comparing the child’s shoulder blades with those of other ancient hominids, modern humans and apes, researchers determined that A. afarensis’ shoulders (one shown) underwent the same developmental changes seen in ape shoulders, with size and shape affected by climbing during childhood. The similarities imply that A. afarensis spent at least some time scaling trees, too (SN: 12/1/12, p. 16).

Lucy wasn’t alone in the canopy. Scientists dug up a 3.4-million-year-old hominid foot in Ethiopia that, unlike that of A. afarensis, still had a grasping big toe like a chimpanzee’s (SN: 5/5/12, p. 18). This suggests the foot belonged to a different species that had superior climbing abilities.

“The experiment of bipedalism played out in a number of different ways,” says Jeremy DeSilva, a biological anthropologist at Boston University. Some species were better bipedal walkers, while others spent more time climbing, he says. One anatomical oddity was Australopithecus sediba, a nearly 2-million-year-old species from South Africa with a flexible foot that twisted inward (SN: 5/19/12, p. 14).

Experiments in hominid diversity weren’t limited to the ankles. Some anthropologists think that at least two separate species of Homo lived some 2 million years ago after the genus emerged in Africa; others say there was just one species with males much bigger than females. Fossils unearthed in Kenya from this time — the partial face of a child and two jaws — are too different from some other early Homo fossils for them all to be one species (SN: 9/8/12, p. 8), researchers say, providing the best evidence yet of multiple species of early Homo.

All of the discoveries from this year, DeSilva says, are “forcing us to really think about variation.” 

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