When and why did we learn to stand on our own two feet?
We may never know for exactly how long humans have walked on two legs, and the
debate about why we do it continues, but evidence and research give us plenty of clues.
For many years, scientists and anthropologists disagreed about whether early humans
started walking on two legs before or after their brain had increased. The predominant
view was that brain size was important, and until our brains had reached a particular
size and mass, bipedal movement would have been impossible. Then, in 1974, two
scientists, Donald Johanson and Tom Gray, were mapping a remote area of Ethiopia
when by chance they came across some fossilised bones which appeared to be from an
early human, Australopithecus afarensis. Excavation of the site led to the discovery of
several hundred more bones and bone fragments, all from a single skeleton. Scientific
research of the bones later that year showed that they were 3.2 million years old and
belonged to a young female hominid who the scientists nicknamed ‘Lucy’. Most
remarkably, however, the research showed that while Lucy had a very small brain
compared with humans today, she was also bipedal.
How did the scientists know this? Lucy’s leg bones were angled relative to the condyles
(knee joint surfaces), which allow bipeds to balance on one leg at a time when walking.
There was also a prominent lip in the knee to prevent the patella (the knee cap) from
dislocating due to this angle. Evidence was also found in Lucy’s pelvis, which was able
to accommodate an upright stance and the need to balance on only one limb with each
stride. The shape of her ankle also showed that her big toes would have aligned with
her other toes, which would have sacrificed manipulative abilities for efficiency in
bipedal locomotion. Her feet, therefore, would have been used almost exclusively for
getting around rather than for holding things. Finally, her backbone showed evidence of
the spinal curvature necessitated by a permanent upright stance.
Lucy’s discovery was exciting for two reasons. Not only was she bipedal in spite of her
brain size, but she was also believed to be our oldest ancestor. However, a discovery
over thirty years later changed all that. In 2005, Professor Yohannes Haile-Selassie,
head of Physical Anthropology at Cleveland Museum of Natural History, led an
international team that discovered and analysed a 3.6-million-year-old fossilised partial
male skeleton. It was found in the Woranso-Mille are of Ethiopia’s Afar region, and it
took Professor Haile-Selassie’s team over five years to excavate. The team recovered
the most complete shoulder blades ever found in the human fossil record. A significant
portion of the rib cage was also found.
It was a significant find because this early hominid, also a member of Australopithecus
afarensis, is 400,000 years older than Lucy, and significantly larger in size. Research on
the new specimen revealed that advanced, human-like bipedalism occurred much
earlier than previously thought. The specimen was nicknamed 'Kadanuumuu', which
means 'big man' in the Afar language and reflects its large size. The male hominid stood
between 1.5 and 1.7 metres tall, while Lucy stood only 1.1 metres tall. This individual
was fully bipedal and had the ability to walk almost exactly like modern humans.
Kadanuumuu's discovery was important for another reason. Despite all the research,
there were still some in the scientific world who felt there was insufficient proof that Lucy
walked fully upright. 'AS a result of our discovery,' said Haile-Selassie, 'we can now
confidently say that Lucy and her relatives were almost as proficient walking on two legs
as we are, and that the elongation of our legs came earlier in our evolution than
previously thought. Until now, all of our understanding of Australopithecus afarensis'
locomotion has been dependent on Lucy. Unfortunately, because she was an
exceptionally small female with very short legs, this gave some researchers the
impression that she was not fully adapted to upright walking. This new skeleton falsifies
that impression because if Lucy's frame had been as large as this specimen, her legs
would also have been proportionally longer.'
Professor Haile-Selassie's research goes a long way to explain when humans began
walking upright. However, one tantalising question remains: why did we start walking
upright? There are several schools of thought, but two are particularly compelling. One
is that bipedal activity is linked to the need to carry as much as possible. 'Something as
simple as carrying, an activity we engage in every day, might have, under the right
conditions, led to upright walking,' says Dr Brian Richmond, who carried out research on
bipedal movement in apes. 'Standing on two legs allowed early humans to carry more at
one time because it freed their hands.' It is possible to observe this in apes. While many
are capable of short bursts of bipedal movement, they only choose to do it when they
need to carry something. And, interestingly, the more valuable the object is to them, the
more they are prepared to walk on just two legs in order to carry it.
However, another group of researchers working at the University of Arizona has
conducted a study which suggests that walking upright is more beneficial because it
saves energy. 'For decades now researchers have debated the role and evolution of
bipedalism,' said David Raichlen, Assistant Professor of Anthropology. 'However, the
big problem in the study of bipedalism was that there was little data out there.' Under his
guidance, a group of researchers at the University trained five chimpanzees to walk on
an exercise machine while wearing masks that allowed measurement of their oxygen
consumption. The chimps were measured both while walking upright and while moving
on their legs and knuckles. That measurement of the energy needed to move around
was analysed alongside results from similar tests on humans. Raichlen discovered that
humans walking on two legs use only one-quarter of the energy that chimpanzees use
while knuckle-walking on four limbs. And of course using less energy means you need
to eat less, which leaves more time for other things.
