06-09-2020 - The importance of the lumbar vertebrae in bipedalism

    Walking on two legs is what makes a human a human, right? Well, according to biological anthropology, bipedalism is one of the requirements to categorize a primate into the hominin category, and generally is considered an even more important factor than brain encephalization, or the adaptation of a bigger brain. Bipedalism is not only evident in humans, but there is evidence of bipedal pre-humans and possibly even earlier than that in the last known common ancestor to humans (Homo sapiens) and chimpanzees (Pan troglodytes).

    You might be thinking, what makes bipedalism is humans unique? Don't other animals like kangaroos, birds, and some lizards walk bipedally? What makes human bipedalism unique is there is no balancing limb in a human, like how kangaroos have their tails and birds have their wings. Humans experience obligate bipedalism, which is "the ability to walk habitually upright," meaning it is their main mode of movement (Warren et. al 2019). The majority of the weight is stacked around the center of gravity, allowing for less energy expenditure. For apes, the center of gravity is in their torso.

http://efossils.org/book/lumbar-vertebra#:~:text=The%20number%20and%20size%20of,smaller%20than%20human%20lumbar%20vertebrae.

Moving bipedally was likely adopted for a plethora of reasons, perhaps related to energy, thermoregulation (there is less surface area exposed to sunlight), or allowing the use of hands to carry food or tools.  

Image: http://efossils.org/book/anatomical-evidence-bipedalism

    General trends that show that early hominins were beginning to walk upright include a larger hallux (big toe), a more compact foot with an arch, a more robust, long femur that is angled at the knee, a robust tibia, a bowl-shaped pelvis, and a more anterior foramen magnum, which is the angle at which the spine connects to the skull (more straight in humans). 

        Evidence of bipedalism in the fossil record goes back as far as seven million years ago with the discovery of Sahelanthropus tchadensis, which had smaller canines, a short cranial base, and a foramen magnum more similar to humans. However, it is debated whether this is the actual first bipedal hominin, since only a cranium was found. 

    The lumbar vertebra is critical to bipedalism - to balancing on two legs. Humans have 5 larger lumbar vertebrae, while apes have 4 smaller vertebrae. The number of vertebrae in humans allows for more flexibility in the lower back, permitting the hips and butt to swing forward. Apes are less flexible, and the hips must move forward more with each step.