Small Bone - Big Find

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February 10, 2011

A small, three million year old bone found ten years ago and recently analyzed is helping to resolve an important question and illustrates how exacting science is done.

A controversy surrounding the three million year old fossil Australopithecus afarensis from its discovery nearly forty years ago is well on its way to being resolved.  Discovered by Donald Johanson at Hadar in Ethiopia in 1974 and nicknamed “Lucy”  this fossil was the most complete skeleton and oldest member of what was then known of the human lineage but numerous scientists disputed she was truly bipedal, stating this species practiced a form of locomotion intermediate between the quadrupedal tree climbing of chimpanzees and human terrestrial bipedality.

A fossilized bone, the fourth metatarsal of the left foot, recovered from Hadar shows that by 3.2 million years ago human ancestors walked bipedally with a modern human-like foot, a report that appears Feb. 11 in the journal Science, concludes. The fossil indicates that a permanently arched foot was present in the species Australopithecus afarensis, according to the authors, Carol Ward, William Kimbel and Donald Johanson.  The question of whether Au. afarensis had fully developed pedal, or foot, arches has been part of this debate. The fourth metatarsal described in the Science report provides strong evidence for the arches and, the authors argue,support a modern-human style of locomotion for this species. The specimen was recovered from the Hadar locality 333, popularly known as the "First Family Site," the richest source of Au. afarensis fossils in eastern Africa, with more than 250 specimens, representing at least 17 individuals, so far known.

"This fourth metatarsal is the only one known of Au. afarensis and is a key piece of evidence for the early evolution of the uniquely human way of walking," says Kimbel. "The ongoing work at Hadar is producing rare parts of the skeleton that are absolutely critical for understanding how our species evolved."

Humans, uniquely among primates, have two arches in their feet, longitudinal and transverse, which are composed of the mid foot bones and supported by muscles in the sole of the foot. During bipedal locomotion, these arches perform two critical 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. Ape feet lack permanent arches, are more flexible than human feet and have a highly mobile large toe, important attributes for climbing and grasping in the trees. None of these apelike features are present in the foot of Au. 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," observes Ward. "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."

This species, whose best-known specimen is "Lucy," lived in eastern Africa 3.0 to 3.9 million years ago. Prior to Au. afarensis, the species Au. anamensis was present in Kenya and Ethiopia from 4.2 to 4.0 million years ago, but 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. Although Ardipithecus appears to have been a part-time terrestrial biped, its foot retains many features of tree-dwelling primates, including a divergent, mobile first toe. The foot of Au. 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 onthe trees for refuge or resources.

The Hadar project is the longest running paleoanthropology field program in the Ethiopian rift valley, now spanning more than 38 years. Since 1973, the fieldwork at Hadar has produced more than 370 fossil specimens of Australopithecus afarensis between 3.4 and 3.0 million years ago – one of the largest collections of a single fossil hominin species in Africa – as well as one of the earliest known fossils of Homo and abundant Oldowan stone tools (ca. 2.3 million).

Carol Ward teaches at the University of Missouri and fellow authors Kimbel and Johanson are faculty members of the Institute of Human Origins at Arizona State University.  IHO sponsors this website.

Click here to read the full article in Science

Here are three views of this important find: the first shows the location of the 4th metatarsal in the foot; the second is an image of the fossilized bone itself; and the third is that of four, rotated views of the bone, illustrating the torsioning of the fossil.