A fossilized arched foot bone recovered from Ethiopia shows that our human ancestors walked upright 3.2 million years ago, according to a study published Thursday in the journal Science.
The fossil, a fourth metatarsal, or midfoot bone, belongs to a group of the famed hominid Lucy, and indicates that a permanently arched foot was present in the species Australopithecus afarensis. The findings are the first evidence to address the question of how this species moved around.
“This fourth metatarsal is the only one known of A. afarensis and is a key piece of evidence for the early evolution of the uniquely human way of walking,” said study co-author William Kimbel of Arizona State University.
The research helps resolve a long-standing debate between paleoanthropologists who believe A. afarensis walked essentially the same as modern-day humans and those who believe the species practiced a form of locomotion intermediate between the quadrupedal tree climbing of chimpanzees and human terrestrial bipedalism.
The question of whether A. afarensis had fully developed pedal, or foot, arches has also been part of this debate. The fourth metatarsal described in the study provides strong evidence for the arches and supports a modern-human style of locomotion for this species.
The fossil was recovered from the Hadar locality 333, often referred to as the “First Family Site”. The area is the richest source of A. afarensis fossils in eastern Africa, with more than 250 specimens representing at least 17 individuals.
“This fourth metatarsal is the only one known of A. afarensis and is a key piece of evidence for the early evolution of the uniquely human way of walking,” said Kimbel.
“The ongoing work at Hadar is producing rare parts of the skeleton that are absolutely critical for understanding how our species evolved.”
Humans are unique among primates in having two arches in their feet — longitudinal and transverse. These arches are composed of the midfoot bones and supported by muscles in the sole of the foot.
During bipedal locomotion, these arches perform two vital functions: leverage when the foot pushes off the ground and shock absorption when the sole of the foot meets the ground at the completion of the stride.
By contrast, ape feet lack permanent arches, are more flexible than human feet and have a highly mobile large toe. These attributes facilitate climbing and grasping in the trees. However, none of these apelike features are present in the foot of A. afarensis.
“Understanding that the foot arches appeared very early in our evolution shows that the unique structure of our feet is fundamental to human locomotion,” said Carol Ward of the University of Missouri, a co-author of the report.
“If we can understand what we were designed to do and how natural selection shaped the human skeleton, we can gain insight into how our skeletons work today. Arches in our feet were just as important for our ancestors as they are for us.”
A.afarensis lived in eastern Africa between 3.7 and 2.9 million years ago, and its most famous specimen is “Lucy.”
Prior to A. afarensis, the species A. anamensis was present in Kenya and Ethiopia from 4.2 to 4.0 million years ago, although its skeleton is not well known. At 4.4 million years ago, Ethiopia’s Ardipithecus ramidus is the earliest human ancestor well represented by skeletal remains. However, it appears to have been a part-time terrestrial biped, whose foot retained many features of tree-dwelling primates, including a divergent, mobile first toe.
The foot of A. afarensis, as with other parts of its skeleton, is much more like that of living humans, implying that by the time of Lucy, our ancestors no longer depended on the trees for refuge or resources.
Commenting on the study, Professor Chris Stringer, a paleoanthropologist at London’s Natural History Museum, said scientists were slowly filling in the detail of Lucy’s place in human history.
“Bipedalism in Lucy is established, but there has been an issue about how much like our own that bipedalism was,” he told BBC News.
“Was it a more waddling gait or something more developed?
“And certainly there’s evidence in the upper body that the Australopithecines still seemed to have climbing adaptations – so, the hand bones are still quite strongly curved and their arms suggest they’re still spending time in the trees.
“If you are on the ground all the time, you need to find shelter at night and you are in a position to move out into open countryside, which has implications for new resources – scavenging and meat-eating, for example.
“If the Australopithecines were on that road, they were only at the very, very beginning of it.”
Image 1: This is the fossilized foot bone — fourth metatarsal of Australopithecus afarensis (AL 333-160) — recovered from Hadar, Ethiopia. Credit: Elizabeth Harmon/Arizona State University
Image 2: This image shows the position of the fourth metatarsal Australopithecus afarensis (AL 333-160) recovered from Hadar, Ethiopia, in a foot skeleton. Credit: Carol Ward/University of Missouri
Image 3: Paleoanthropologists report in the Feb. 10 edition of Science on the recovery of a fossilized foot bone recovered from Hadar, Ethiopia, locality 333, popularly known as the “First Family Site,” the richest source of Australopithecus afarensis fossils in eastern Africa. Credit: Donald Johanson/Institute of Human Origins/Arizona State University
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