Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics

TLDR

Debates over hominin bipedalism focus on whether early bipeds walked like humans with extended hind limbs or like apes with flexed hind limbs. The study aims to determine Laetoli hominin kinematics to assess whether selection reduced energy costs of bipedalism by 3.6 Ma. Using experimental trials on a sand trackway, the authors compared extended‑limb and flexed‑limb walking, showing Laetoli hominins’ weight transfer resembled human extended‑limb bipedalism. The 3.6‑Ma Laetoli footprints provide the earliest direct evidence of kinematically human‑like bipedalism, matching extended‑limb weight distribution and indicating that energetically efficient extended‑limb bipedalism evolved before Homo and likely drove selection by 3.6 Ma.

Abstract

Debates over the evolution of hominin bipedalism, a defining human characteristic, revolve around whether early bipeds walked more like humans, with energetically efficient extended hind limbs, or more like apes with flexed hind limbs. The 3.6 million year old hominin footprints at Laetoli, Tanzania represent the earliest direct evidence of hominin bipedalism. Determining the kinematics of Laetoli hominins will allow us to understand whether selection acted to decrease energy costs of bipedalism by 3.6 Ma.Using an experimental design, we show that the Laetoli hominins walked with weight transfer most similar to the economical extended limb bipedalism of humans. Humans walked through a sand trackway using both extended limb bipedalism, and more flexed limb bipedalism. Footprint morphology from extended limb trials matches weight distribution patterns found in the Laetoli footprints.These results provide us with the earliest direct evidence of kinematically human-like bipedalism currently known, and show that extended limb bipedalism evolved long before the appearance of the genus Homo. Since extended-limb bipedalism is more energetically economical than ape-like bipedalism, energy expenditure was likely an important selection pressure on hominin bipeds by 3.6 Ma.