Walk Don’t Run: Ambling Along With 4.5 Million Year Old Flat Feet

by PRIBUT on OCTOBER 1, 2009

Ambling Along With 4.5 Million Year Old Flat Feet

A hominid species predating Lucy, (Australopithecus africanus) has been more fully described. Journalists have had first crack at the issue of Science in which the updated description of the species,  Ardipithecus ramidus (from 4.5 million years ago), appears. The rest of us could only see this issue late in the day.  The recently studied Ardipithecus specimens include the feet, which were clearly missing on Lucy (from 3.2 million years ago), but present among a limited set of other Australopithecus specimens. Ardipithecus was originally discovered in 1994 in Ethiopia.

The treasure trove of the day are the eleven fresh papers detailing  Ardipithecus and it’s (with a delay in publication for years) environment including botanical and other specimens found locally, the anatomy, and evolutionary conjectures all published in Science magazine. Details on excavation, locale, personnel, CT scans, three-dimensional reconstruction, dimensions and sizes of the specimens, were included in this comprehensive set of articles.

Ardipithecus was thought to live and spend time in trees, but would carefully climb rather than swing from the branches. Ardipithecus was also believed to spend time foraging for food, primarily plant based, on the ground while moving in a bipedal manner. Australopithecus was not a runner, nor is it likely with feet less well adapted  was Ardipithecus. Among other lower extremity differences between Homo erectus and Ardipithecus were flatter feet and an opposable big toe (metatarsus primus varus – actually  metatarsus primus adductus – a large angle in stance between the first and second metatarsal bones – but I’ll have to check out more photos and study the  articles in detail in Science magazine). If Ardipithecus twiddled her thumbs she could likely also twiddle her opposable big toes. Speaking of toes, one of the changes that is thought to make running possible for the later hominids was a shortening of the length of toes, in addition to an increase in arch height, and a host of other biomechanical changes.

An opposable big toe with a non-functional first ray makes for a decidedly different bipedal gait than even Lucy had. The tight grouping of the cuneiforms present in later hominids allowed the first ray to function effectively in weight transfer and propulsion rather than the little it could do as an opposable digit without stability in ground based bipedal gait. Of course a humanoid great toe does not offer much assistance in the trees. An important feature to note is that Ardipithecus did not knuckle walk, as can be determined from the wrist and hand structure, during bipedal gait, although the upper extremities were long. Nor did Ardipithecus appear to brachiate or swing through the tree branches.

Below are images of the upcoming cover of Science magazine with Ardipithecus on the cover and an image of Lucy missing her feet.

Update: Freely available articles at Science









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Additional References:

Fossils, feet and the evolution of human bipedal locomotion W E H Harcourt-Smith and L C Aiello. J Anat. 2004 May; 204(5) : 403–416.
  Walk Don’t Run