When did our ancestors start to walk on two legs?
Based on skeletal remains, it was believed that bipedalism -- the ability to walk on two legs, developed sometime between 6-7 million years ago. Scientists who study human origin from fossils (paleoanthropologists) believe this was when the human species separated out from the great apes.
However, a new discovery is challenging our current understanding. Researchers in Germany have unearthed the remains of an ancient ape in a clay pit in Bavaria. The new species, named Danuvius guggenmosi, lived about 11.6 million years ago!
Why Walk On Two Feet?
There are several theories, some of which have been debunked. Scientists no longer believe that the four-legged apes stood up on their hind legs to look out for predators on open grasslands, or that they were trying to collect food by wading into lakes and rivers.
The most likely explanation is that the first hominids evolved in forests where they walked upright on branches to access fruits, and it was the development of muscle structure to support this that led to bipedalism.
Either way, the transition from four legs to two took place over millions of years. The pelvis had to evolve from a tall and flat structure (in apes) to being more rounded and bowl-shaped to support the muscles. The thigh bones developed to point inwards to bring the legs closer and under the pelvis, and the spinal column changed into an upright, S-shape. Longer limbs and shorter toes helped our ancestors move easily on the ground as they no longer had to grip tree branches.
The Recent Discovery
In 1974, researchers discovered the largest skeletal remains from a single hominid, nicknamed Lucy, in Ethiopia. Lucy had all the features of bipedalism, a defining characteristic of hominids, and was dated to be 3.2 million years old. Since then, excavations have revealed skeletal fragments of other species, including Ardi -- a 4.4 million-year-old hominid that lived in a woodland environment.
The recently discovered fossils include the thigh, shin, and lower arm bones, as well as several vertebrae and hand and foot bones. D. guggenmosi was likely a small ape; the vertebrae and thigh bones suggest the ability to walk upright. However, the ape also had strong arms, hands, and feet that suggest the ability to climb, swing and grip onto branches. Researchers have coined a new term "extended limb clambering" for this behavior -- a combination of walking on two feet and hanging by the arms.
What makes the discovery even more curious is that D.guggenmosi was found in Europe, far from Africa. Also, this discovery pushes the timeline for the origin of bipedalism much earlier than when hominids and apes separated into distinct species. There is also a big gap between D.guggenmosi and the appearance of Ardi in Africa -- so while apes roamed around in Europe, what happened to them after is unclear.
Researchers will certainly be conducting more analysis and until then, each discovery is yielding one more piece of the puzzle. Here is a video on the discovery of Ardi.
Sources: The Scientist, Smithsonian, Nature, ASU.edu