Why Early Human Ancestors Took to Two Feet


A new study by archaeologists at the University of York challenges evolutionary theories behind the development of our earliest ancestors from tree dwelling quadrupeds to upright bipeds capable of walking and scrambling.

The researchers say our upright gait may have its origins in the rugged landscape of East and South Africa which was shaped during the Pliocene epoch by volcanoes and shifting tectonic plates.

Hominins, our early forebears, would have been attracted to the terrain of rocky outcrops and gorges because it offered shelter and opportunities to trap prey. But it also required more upright scrambling and climbing gaits, prompting the emergence of bipedalism.

The York research challenges traditional hypotheses which suggest our early forebears were forced out of the trees and onto two feet when climate change reduced tree cover.

continue to source article at sciencedaily.com


  1. This seems to me a very poorly thought out hypothesis.Have the authors looked at any other animals?. For example, goats.It certainly does not explain all the key processes in hominin evolution as is claimed.

  2. It’s interesting that a rocky environment might provide both hunting opportunities and a little increased security from predators. It might explain why this kind of terrain is aesthetically attractive to people. Like why modern humans like to put crushed lava and rockeries in their gardens.

    The climate change theory also sounds superficially plausible but also doesn’t really explain much. Climate change might have indirectly opened up niche opportunities vacated by other, much less flexible species, rather than have more directly impacted on humans existing lifestyle.

    And bipedalism and intelligence are already very closely related and provide a much simpler explanation. Intelligence enables / is the accumulated capabilities of social cooperation, language, empathy, planning, division of labour, fat metabolism etc. which enables long range foraging and scavenging on behalf of less mobile group members, and which also requires the ability to transport food resources externally. i.e. without ingesting and regurgitating like the albatross, or why male humans didn’t evolve to also lactate like females.

    There’s many potential solutions to the evolutionary ‘problem’ of long range scavenging– at least as it appears in hindsight to be a problem that is being solved. But natural selection is path dependent and is always based on what has already happened. Maybe it’s just an accident of evolution that humans are fully bipedal instead of being compulsive regurgitators.

    The freeing of hands for climbing as a driver of very efficient bipedalism doesn’t make much sense as tree-dwelling apes already have that ability. And it doesn’t explain all those other adaptations to long range scavenging. What needs to be explained is not a shift from four-legged locomotion like dogs but a shift from semi-bipedal to very efficient bipedal endurance, very long range locomotion of the 100km+ ultramarathon kind. Most apes are already bipedal to a reasonable extent. They just aren’t very efficient at it, and maybe couldn’t cooperate and plan sufficiently to exploit that ability even if they had it.

  3. “The broken, disrupted terrain offered benefits for hominins in terms of security and food, but it also proved a motivation to improve their locomotor skills by climbing, balancing, scrambling and moving swiftly over broken ground — types of movement encouraging a more upright gait.”

    Did monkeys have to improve climbing balancing and scrambling skills?

    What are the different time scales involved in climate change and changes in terrain?

    Even as a non scientist this seems to me to be a likely srory.

  4. In reply to #2 by Pete H: “or why male humans didn’t evolve to also lactate like females.”

    An interesting aside: I have read, somewhere or other, that old men in traditional Australian Aboriginal communities, develop the ability to lactate. Does any one know anything about this?

  5. In what way is this any better than the currently unpopular Aquatic Ape Hypothesis? Some evidence, perhaps?

  6. My theory is that they just liked waving their arms about continuously while walking. Or that they needed their arms to carry their stuff long distances. Or that it made for a clearer view over the savannah. I can come up with a few theories about as wacky as this tbh. It’s just all suppositions and hypothesis.

  7. Our species must have become ground dwellers first and perhaps walked among tall grasses which would be a more likely habitat to make apes adapted to walking upright – so that they had clear views across the savanna. Certain baboons live in rocky outcrops and gorges but they are not bipedal – theres still questions about wether the human brain/skull increased in size first – causing the pelvic widening of females which altered the skeleton to be more upright and perhaps facilitated bidepality….or did bipediality come first and big brain/skull followed ?

  8. My thought is that bipedalism was probably multifactorial; many different influences impacted early hominids and favored those who could stand upright and walk for longer periods. It might have been a combination of terrain and climate and a bigger brain that could conceive of greater opportunities in that environment. There may have been an element of sexual selection; those who were better at walking may have been seen as more attractive. Maybe they could mate more often or more successfully. Chimps today show varying levels of proficiency at standing and walking. Whatever brought it about, some ancient ancestor was better than average at standing up and was more successful at passing on his or her genes.

  9. In reply to #8 by Sue Blue:

    My thought is that bipedalism was probably multifactorial.

    Yeah, something like that. An adaptation that was brought on slowly, and provided increasing benefits across several areas (migratory social apes, bipedalism sounds like a good advantage). But I don’t really know, I’m not an anthropologist. 🙂

  10. Another example of the multiple factors being papa lazaru’s comments above about waiving the arms. There’s a theory that increasingly complex verbal language enabling sophisticated planning and cooperation may have followed as an optimisation of gesturing. Something that is still crucial for communicating when teaching language to kids and when there are language barriers. Plus gestures are a major aspect of public speaking.

    In reply to #8 by Sue Blue:

    My thought is that bipedalism was probably multifactorial; many different influences impacted early hominids and favored those who could stand upright and walk for longer periods. It might have been a combination of terrain and climate and a bigger brain that could conceive of greater opportunities…

  11. Changing from upright to four-footed seems more likely to me on such terrain. When I scramble up rocky terrain I have a tendency to use all fours. Why would this terrain be more attractive for our forebears? Why leave the forest with food and shelter for a more difficult habitat? Such rugged areas are also generally lacking in water.

  12. So bipedalism is an advantage in rocky terrain? More traditional theories would have us understand bipedalism is an advantage in flat terrain (savannah). AAT would have us believe bipedalism is an advantage in riverine and watery environments. Is there a terrestrial environment where bipedalism isn’t considered advantageous? Oh yeah, forests. I wonder, if the other apes had gone extinct by 1 mya, would some scientists be theorising that bipedalism was a natural progression from brachiation and an obvious advantage in forested environments?

    Sexual selection of arbitrary characteristics (such as a preference for bipedalism while still mainly quadrupedal) as an explanation for bipedalism is no explanation at all. Human traits distinguishing us from apes need be explained as a package, or as a natural progression. Why would “hairlessness” and bipedalism be shared by humans alone amongst the great apes, indeed all primates? Are we to understand that sexual selection selected both (apparently unrelated) traits independently at much the same time?

    The AAT looks particularly good if you contrast it with the “mountain goat theory” !

  13. Surely, explaining the improved capacity for bipedal balance might be the primary point?

    In all assertions such as this, the hard things to explain are always how the systems followed the behaviour so as to allow for it to happen.

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