Why did human ancestors start walking upright?

Researchers do not agree on a single cause. The fossil record shows a gradual shift from climbing trees to regular upright walking between about 6 and 3 million years ago, but the reason for the shift is still debated. Many proposed explanations are not mutually exclusive, so a combination of pressures may have been responsible. [1] [2]

The traditional account holds that as the climate changed, forests gave way to tree-and-shrub savanna, and upright walking brought survival advantages such as carrying food or tools and standing to see over tall grass. Teaching materials from the Max Planck Institute's OpenEvo project present this as selection pressure favoring individuals who walked upright. [3] [2]

Fossil finds weakened the simple version. Ardipithecus ramidus, Orrorin tugenensis and Sahelanthropus tchadensis were upright hominins that apparently lived in wooded settings between 7 and 4 million years ago. Ar. ramidus fossils were found with wood, seeds, woodland monkeys, parrots and snails. Science reported that this ended the savanna hypothesis in its original form. Geochemist Thure Cerling argues that grasses were abundant in all hominin habitats, and that a revised version concerning food is alive. Carol Ward says there are still more questions than answers. Yale's Andrew Hill, meanwhile, calls most stories of environmental change shaping evolution unproven, and Peter deMenocal describes the climate case as circumstantial. [4]

Older hypotheses include carrying food or other items over longer distances, freeing the hands for foraging, tool use or protection, moving more energy-efficiently than other primates, and long-distance running. Another proposal is thermoregulation: in hot, open habitats a tall, lean upright body exposes less surface to overhead sun and more to cooling air. [2]

Owen Lovejoy's explanation, described by PBS NOVA, is social. If males brought high-quality food to mates and offspring and helped protect them, their genes were more likely to continue. The NOVA author adds a different view: that ancestors before Lucy became upright in forests, where dangers were fewer, and that this locomotion then let them move into savannas, where the other benefits followed. Natural selection cannot create a behavior, the article notes, but it can favor it once it appears. [5]

A University of York study published in Antiquity argues that bipedalism may have been a response to rugged terrain rather than to climate-driven vegetation change. The authors suggest that rocky outcrops and gorges in East and South Africa, shaped by volcanoes and shifting tectonic plates, offered shelter and chances to trap prey. Moving across such ground required climbing, balancing and scrambling, which encouraged a more upright gait. Free hands could then develop greater dexterity and tool use. [6]

The search results do not show how widely specialists have accepted this proposal compared with the others.