Q&A: Why Walk on Two Legs?


Brian Richmond, curator of human origins in the Division of Anthropology, spends a lot of his day thinking about how humans walk and what it says about us as a species. Our two-legged gait is a defining human trait—but, millions of years after we made the move from four legs to two, our bodies still aren’t optimized for it. 

Next Wednesday, April 1, Dr. Richmond will be joined by Boston University’s Jeremy DeSilva at SciCafe to discuss the pros and cons of walking on two legs, how we evolved bipedalism, and why it may be responsible for your sore back.

Brian Richmond is the new curator of human origins in the Museum's Division of Anthropology.  ©AMNH/D.Finnin

Brian Richmond is the new curator of human origins in the Museum's Division of Anthropology. 


What drew you to studying walking in the first place?

In graduate school, I learned that there are a whole lot of questions we don’t have the answers to about how and why and in what context we evolved our anatomy. The transition from to four legs had a profound impact on our anatomy, our skeletons, and our physiology from head to toe, but that important transition is not fully understood yet. So that really piqued my interest.

What are some things we do understand about the transition from four legs to two?

We know that as far back as about four million years ago, our early ancestors were walking on two legs. We can see that in a 3.7-million-year-old trail of fossil footprints from early humans, a cast of which is on display in the Spitzer Hall of Human Origins. But this was a transitional period. Between four million years ago and two million years ago, our ancestors likely walked on the ground, but their apelike upper arms suggest they also spent a lot of time climbing trees, perhaps to find food or rest at night away from predators.

Human bodies are still working from an evolutionary plan with four legs.  © Roberto Aiuto

Human bodies are still working from an evolutionary plan with four legs. 

© Roberto Aiuto

From an evolutionary point of view, why did humans evolve bipedalism? 

Walking on two legs is way more efficient than walking on four. We use about half of the energy to go a given distance than a chimp does. Walking on two legs also means we can put our hands to work for other tasks, like carrying things and using tools.

What can the way we walk tell us about humans?

It’s one of the fundamental characteristics that make us human. It’s a prerequisite for developing big brains, as well as manipulative hands that let us make tools—lots of animals use tools and some make them, but bipedalism has let us take it to a whole new extreme. 

This illustration from the Spitzer Hall of Human Origins contrasts the skeletons of chimpanzees, humans, and Neanderthals.


What are some of the disadvantages of walking on two legs?

We have a whole host of ailments from walking on two legs, because we’re imperfectly adapted for it. Evolution only works on existing structures to fine-tune them, and for millions of years, our body plan was one that walked on four limbs. The shift to bipedalism involved changes that can cause problems. Chronic knee pain is one example, because two limbs, not four, now support the whole weight of the upper body. It also explains back problems, as that same weight falls on just a few vertebrae in the lower back. The body is a compromise solution that works pretty well, but not perfectly.

How does doing research into walking change your day-to-day life? 

When people walk down the street, I can’t help but notice their walking style, because everyone walks a little differently—you can pick out a friend from someone you don’t know at a great distance just by their walk. And as a runner, I do pay a lot more attention to form now. 

Click here for more details about the April 1 SciCafeincluding links to papers about the evolution of bipedalism.

Tags: SciCafe, Q&A