The University of the Witwatersrand – MSc
Two legs and walking upright by putting one foot in front of the other over long distances, make humans unique. We are the unrivalled elite of the long distance running and walking mammals. Yes, a cheetah would outrun Olympic champion Usain Bolt in a 100m sprint, but it would have to settle for second place against a Comrades Marathon runner over an 87km distance. Evolution has moulded our bodies to allow us to walk upright, and the story of this journey is written in our bones.
The science of evolution helps to find the answers to current problems by looking into the past. For example, scientists who are interested in hereditary diseases in people study the evolutionary history of the disease-causing genes in humans to improve their quality of life. Similarly, understanding how walking upright affects the structure of our bones helps us to identify where the weakest and strongest points are, how injuries and strain affect our daily lives, and how our bodies will compensate for that.
A hundred years ago, scientists thought our big brains made us unique. But the Taung child, a human relative discovered in South Africa in 1924, had a small brain. This species would become known as Australopithecus africanus, and the skull of that young infant would change our understanding of evolution for years to come. Walking upright would now become one of the defining characteristics of our species. That adaptation changed our anatomy, including our bones, and it is a combination of physics, mechanics, biology and evolution.
The story of our origin begins just outside of the bustling metropolis of Johannesburg, in a treasure trove of ancient fossils. Nestled in the Sterkfontein Valley, undisturbed for millions of years, lie the enigmatic fossils that could help solve our age-old questions: who are we and where do we come from?
Bone acts as a living diary and is often the only part of the body preserved in the fossil record. The daily stresses and strain that we subject our bodies to embed patterns in the internal structure of bone and may provide information on the daily life of people. This sponge-like internal structure, known as trabecular bone, changes throughout our lives, from the minute we start kicking in our mothers’ wombs, adapting as we crawl, stumble and eventually learn to walk. These changes in posture and movement leave distinct patterns in our bones that experts can interpret.
By studying bone, scientists can tell not only how someone moved, but also how active they were. For example, the introduction of handy apps like UberEats and Mr Delivery means that we no longer have to go foraging and hunting for food like our ancestors once did. This lack of activity – aka the couch-potato lifestyle – has resulted in modern humans developing a lower trabecular bone density. This isn’t necessarily bad for our health, but it does mean that our activity levels play a key role in the remodelling and strengthening of our bones.
Although we are accustomed to upright walking, our relatives (living and extinct) had to develop the posture associated with this form of locomotion. Early hominins descended from the trees 7-million years ago and began navigating the grass plains that lay beyond their forests. No two species did this the same way; multiple experiments by different species are now recognised within the science, and each species had its own unique way of adapting.
Walking upright has its advantages and the most important one is that it saved our energy so we could walk for longer. By only relying on two limbs to move, we freed our hands and were able to become inventors, using stone tools and reaching for fruit dangling from trees above our heads. Walking upright also helped us to survive – we were able to see predators camouflaged in the golden highveld outcrops before they could turn us into dinner.
Compared to other living primates (such as chimpanzees, orangutans and gorillas), humans have stiffer joints and a limited range of motion. Our distant cousins are comfortable walking on the ground with their knuckles and swinging from branch to branch (called brachiating), through lush tree canopies. That is not to say that other primates such as chimpanzees are unable to walk upright: they can, but only for short periods of time. However, when they do walk, their stance is different from ours, with bent hips and knees, they tend to resemble a person walking awkwardly on hot beach sand.
Unlike Tarzan, modern humans tend to walk for the better part of their lives as a means of getting around, so we swapped out our adaptations for trees for bones that favoured land. Modern humans walk with an extended leg, hitting the ground with our heel and pushing forward on our big toe. If Fitbit or any step-tracking software can be trusted, we take about 2,000 steps a day, repeating this cycle of “extend leg, heel strike, and toe off” as we vault our stiff bodies forward. Repeating a movement that often imprints a pattern in our bones, from the tip of our big toes, through to our ankles, shins, knees, and hips.
Analysing trabecular bone is a relatively new way to determine how extinct hominin species walked. Thanks to the advancement of technology such as microCT (Computed Tomography) scanning, researchers can prise the secrets out of ancient bones without damaging these precious and scarce relics of our past. My research at the University of Witwatersrand involves the tibia or shin bone, which is the primary load-bearing bone of the lower leg. This bone forms part of the ankle and is sensitive to changes in posture and loading.
I have examined the patterns within the bones of modern humans and a sample of primates with different locomotor repertoires, and used them as a base to compare the patterns in extinct fossil hominin species like Australopithecus africanus from the Sterkfontein Caves.
Studies to date conclude that the australopiths are unique in their locomotor ability. This species shows patterns in the bone that are somewhat ape-like in some aspects and somewhat human-like in others. So, although they probably walked upright, it was not as we do today. Instead this early hominin species experimented with the way in which they walked and probably had a greater range of movement than we do today.
The evolution of our species is often depicted as an ape knuckle-walking on all fours, then slowly transitioning, rising, extending the hip and knee, straightening the hunched back, and eventually standing upright like we do today, eyes gazing forward, the last step in evolution. If only the story was that simple. Ancient hominins were innovators and demonstrated ways of moving that are not observed in any living species today.
My research forms part of a greater effort to understand the trabecular bone in extinct fossil hominins to pinpoint when exactly we began walking upright and why. This field of research has wide reaching applications outside of palaeontology. If we understand the effects of our changing lifestyle on the structure of our bone, we can use this knowledge to create better prosthetics for amputees, study degenerative bone diseases like osteoporosis and arthritis, and better predict the consequences of people’s abnormal gaits on their bone diary.