Anthropology 101: hominins
The last few Tech Corners have been very health, medical, and wearables focused, so I want to switch gears and talk about human evolution, and, more specifically, the rise of bipedalism in a niche group of primates we collectively call hominins, i.e. modern human ancestors. The origins of bipedalism are murky, but through the fossil record and genetic dating we’ve approximated the transition from habitual quadrupedalism (roaming around mostly on four legs) to habitual bipedalism (roaming around mostly on two legs) to have begun between 4 - 8 million years ago in East Africa. While that may seem like a wide swath of time, and not very precise at all, adjusting a complex skeletal and soft tissue system from four legs to two is a major renovation that required (to list a few highlights in just the skeletal realm) an almost complete revamping of the structure of the pelvis, a change in spinal curvature, a full overhaul of the structure of the foot, and even a shift in the location of the connection point between the spine and the skull. That level of renovation didn’t happen overnight! It was likely a slow process brought about by environmental changes that required those ancient critters to adapt or die out. There wasn’t an immediate transition from four to two legs, so we compare fossil fragments that may be in-between stages, where hominins moved around sometimes on four legs, sometimes knuckle-walking, sometimes on two.
Ardipithecus ramidus or ‘Ardi’, a shockingly complete fossil hominin, dated to 4.4 million years ago, and its proposed reconstruction. Source.
So what helps us trace the origins of bipedalism and why do we care about it today? The fossil record is tricky. It is rare that we find complete skeletons, especially as far back as 8+ million years ago, so it’s much more common to find partial skeletons or just fragments of bone that may or may not be informative (teeth aren’t super helpful when looking for indications of bipedal walking). If we’re lucky, using the fossil record we can use the shape, or ‘morphology’ of partial skeletons we find to check for indicators of bipedalism. As some of you know, bone is my favorite material because it is the smartest material we’re aware of; bone is force-sensitive, meaning you can use the shape of bone and its internal structure to help figure out what kind of loads were applied to it. Walking on four legs creates different loading patterns than walking on two legs, and that loading pattern shows up in the shape and structure of bone. Our bones carry our evolutionary history as hominins (as well as some baggage). If you understand the origin of the species, you’re better-placed to see the larger picture of longevity and the future of our species. 🧠
A - An adult chimpanzee proximal femur (the upper bone of your leg) and B - An adult human proximal femur. Chimps are knuckle-walkers, not habitually bipedal like humans, so you can see that the dense shell of bone around the femoral neck (the white ring cross-section) is relatively evenly distributed compared to the same spot on the human femur. Due to bipedal loads (shown in the third diagram), there is extra bone under the neck of the femur, reinforcing the compressive segment so it doesn’t snap, and less distributed on top, which is under tension.