A two-million-year-old fossil could change what we thought we knew about one of our ancient human relatives. A few vertebrae from the lower back of an Australopithecus sediba reveal that the hominin was surprisingly well adapted to walking upright like a human, in addition to climbing like an ape.
Whether or not A. sediba was a direct ancestor of modern humans remains up for debate, but either way it’s a fascinating creature. It lived about two million years ago, stood around 1.5 m (5 ft) tall, and had a strange mix of modern and ancient facial features.
The species was described from two specimens, a juvenile male and an adult female, uncovered in Malapa, South Africa. And now, scientists have discovered and described extra bones belonging to the female specimen, designated MH2 and nicknamed Issa. These new bones are lumbar vertebrae from the animal’s lower back, a part of the body that can reveal much about how a creature got around.
“The lumbar region is critical to understanding the nature of bipedalism in our earliest ancestors and to understanding how well adapted they were to walking on two legs,” says Professor Scott Williams, lead author of the study. “Associated series of lumbar vertebrae are extraordinarily rare in the hominin fossil record, with really only three comparable lower spines being known from the whole of the early African record.”
Previous research had assumed that A. sediba would have had a relatively straight spine, as seen in modern apes and more ancient hominins, like other australopithecines. It’s usually an indicator of powerful trunk muscles, perfect for climbing. But surprisingly, the newly discovered vertebrae showed an inward curve, a feature known as lordosis. That makes it rather human-like, and suggests an adaptation for walking upright like we do.
Most intriguingly though, it seems that Issa hadn’t yet completely abandoned life in the trees – the team says that other features still indicate strong climbing adaptations. The creature walked like a human, but climbed like an ape.
“While the presence of lordosis and other features of the spine represent clear adaptations to walking on two legs, there are other features, such as the large and upward oriented transverse processes, that suggest powerful trunk musculature, perhaps for arboreal behaviors,” says Professor Gabrielle Russo, an author of the study.
This paints A. sediba as a kind of transitional species on the path from tree climbing to ground walking. Even if it wasn’t a “missing link” in our own evolutionary chain, it demonstrates how the transition can take place, which may also have occurred in one of its contemporary species, from which we’re descended.
The research was published in the journal eLife.
Source: New York University
