Why Did Early Humans Start Walking on Two Legs 6 to 7 Million Years Ago? — Here’s What Scientists Think

Have you ever walked and wondered why humans do it with two legs? Probably not, right? Because it has been the norm since we were all born, and some fossils even suggest that walking upright has been around for 7 million years.

We may take it for granted now; however, this skill, bipedalism, is actually one of the most significant milestones in our evolutionary journey. It not only helped us survive for all those years, but also made us who we are today, human beings. So, let’s explore why going from four legs to just two was such a big step, and why our ancestors started walking in this way.

When Did Our Ancestors Start Walking Upright?

Scientists think that human ancestors began walking with two legs as early as 6 to 7 million years ago. Some of the first hominins that went their separate way evolutionarily from the ape, chimpanzee, and gorilla lineage, Sahelanthropus and Orrorin, were thought to be the first ones to do it.

However, during this time, bipedalism or the skill to walk with two legs was not yet well-established. They did not walk the same way we do today. It was only about 3 to 4 million years ago when fossil evidence suggested they did.

That was around the time of Australopithecus afarensis, such as the fossil Lucy, as hominins’ bodies needed to change to support upright walking. The first human species to walk on two legs had not properly developed the hips, knees, and feet necessary for efficient and prolonged standing and bipedalism.

Additionally, some people might think that a hominin species is either bipedal or quadrupedal. But there was a transitional time where our early ancestors might have needed to be good at both climbing trees (arboreal) and upright walking (terrestrial). In other words, the shift from walking on four legs to two was not simple and quick. It was a gradual process that took years, influenced by the changing environment.

Read more: Sapiens

Why Did Two-Leg Walking Evolve?

The Seven-Million-Year-Long Journey - Why Humans Stood Upright ~ with JEREMY DESILVA

So, why did early humans start walking on two legs? Was anything wrong with four-legged locomotion? If so, then why do our apes, chimpanzees, and others still do it today? While there is no single answer that could explain these phenomena, scientists agree that it was because of a combination of factors.

The first reason was environmental and climate change. As climates shift, the dense forests of Africa began to shrink. This gives way to open grasslands, savanna, and woodlands. As stated by the open savanna hypothesis, human ancestors who were forced to travel long distances on the ground to find food and other resources began their evolutionary journey to bipedalism.

The second reason was energy savings. Early humans needed to travel longer distances on foot than they did in the forested environments. Bipedalism proved to be a more efficient way of doing this, and so it was favored by evolution and natural selection. The individuals who were better at this skill were more likely to survive and pass on their genes.

The third was thermoregulation and predator awareness. A vertical posture gave a better view of the surroundings, which is helpful for spotting both food and danger, especially over tall grasses. It also exposes less of the hominins’ bodies to the sun, helping early humans to regulate their temperatures more effectively.

The fourth and one of the most significant changes is the freeing of our hands. It allowed early humans to carry food and other resources with them on the journey in the open environments. Then this ability gave birth to our skill of manipulating objects with our hands, which set the stage for the eventual utilization of stone tools.

There could be a few more reasons or factors as to why humans began walking upright, but these are some of the most important. Ultimately, the point was that bipedalism did not happen fast, nor was it a perfect change. However, it helped our ancestors enough that natural selection favored it over millions of years.

How Did Our Bodies Change to Walk Upright?

The transition to walking on two legs or bipedalism also triggered an evolution or change to early humans’ bodies. This is because the bipedalism skill, as explained above, gave enough advantages that it was favored over time, as well as the traits that made it possible or more efficient.

This includes the pelvis and bone structures. Researchers found that the pelvis widened up to make it easier to balance the body while walking upright. Additionally, as humans were developing bigger brains later down the line, this characteristic also helped babies to be born safely.

It was also observed that our spine adapted and changed to have curves that can absorb the impact of walking on two legs, making it easier on our bodies over time. Knees and hips also improved to be stronger to be able to support one leg at a time while walking.

Lastly, the human feet, unlike apes, are not flat. It developed to have arches and strong heels, which helped us push off the ground and walk more efficiently. It conserves both energy and makes running faster.

Read more: Reuters

What Does This Mean for Human Evolution?

Hominids Walked Upright 7 Million Years Ago

Walking upright not only changes the way we move, but it also makes us who we are over a long period of time. When bipedalism freed our hands, early humans were able to gather resources more effectively, increasing their numbers.

Then, soon, the ability to use tools as we learn how to manipulate objects better with our hands made hunting larger animals safer and more successful. This created a positive feedback loop. Humans were able to gather more food and resources, their population increased, and then cooperation and collaboration became necessary.

Human brains adapted and changed to be bigger as social capability and communication became more important. Culture and tradition arose, and the strategies and techniques to survive were passed down to the next generation.

Over time, different human species evolved and were able to adapt to different environments until modern humans, or Homo sapiens, were born. Then, around 50,000 to 70,000 years ago, we migrated out of Africa, met several other human species, mated with them, and spread throughout the globe.

Of course, this was not the whole history of mankind; however, what we are trying to say is that bipedalism is a significant milestone in our evolutionary path, and one of the biggest reasons why we are who we are today.

Author's Final Thoughts

Walking on two legs may seem ordinary and normal today, but that was not always the case. It took millions of years for this skill to be refined. An evolutionary process that helped our early ancestors save energy, freed their hands, and helped them survive in changing habitats on the planet. Ultimately, it gives rise to the modern human species, to us.

Read next: Why Did Early Humans Leave Africa and Spread Across the World? — Here’s What Researchers Say Drove Them

References & Further Reading

Senevirathne, G., et al. (2025). The evolution of hominin bipedalism in two steps. Nature. https://www.nature.com/articles/s41586-025-09399-9

Harcourt-Smith, W. E. (2004). Fossils, feet and the evolution of human bipedal locomotion. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC1571304/

Crompton, R. H. (2023). Biomechanics and the origins of human bipedal walking. Journal of Biomechanics. https://www.sciencedirect.com/science/article/pii/S0021929023002701

Niemitz, C. (2010). The evolution of the upright posture and gait: a review. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC2819487/

Gruss, L. T., & Schmitt, D. (2015). The evolution of the human pelvis: changing adaptations to bipedality. Philosophical Transactions of the Royal Society B. https://royalsocietypublishing.org/doi/10.1098/rstb.2014.0063

Fernández, P. J., et al. (2018). Evolution and function of the hominin forefoot. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1800818115

Ko, K. H. (2015). Origins of bipedalism [Preprint]. arXiv. https://arxiv.org/abs/1508.02739