Newly Discovered Hominin Footprints in Kenya

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March 04, 2009

Human footprints, whether encountered on a beach or after the snow has stopped falling, have an effable quality.  Bones and human artifacts, after examination by specialists, can tell us much about how life was lived in earlier times but footprints are full of wonder for us.  They may have been made yesterday or, as occurs so rarely, a million years ago and we feel a connection with the unknown person who made these seemingly indelible reminders of us.

Just such a remarkable occurrence has happened for only the third time.  In 1978, a team led by Mary Leakey found three and one half million year old footprints at Laetoli in Tanzania.  Later the same year in Kenya, Kay Behrensmeyer and Leo Laporte found a few footprints estimated to be one and one half million years old.  We learn in the February 27 issue of Science that a team working at Illeret, near Lake Turkana in Kenya, or many years the site of fossil finds, has uncovered two trails with footprints estimated to be 1.5 million years old and likely made by individuals assigned to Homo ergaster/erectus.

Here is a lengthy quote from the paper by Bennett et al., giving a context for the significance of the find:

“Bipedalism is a key human adaptation that appears in the fossil record by 6 million years ago (Ma). Considerable debate continues over when and in what context a modern human–like form of bipedalism evolved, because of a fragmentary record and disagreements over the functional interpretations of existing fossils and footprints. Modern human footprints reflect the specialized anatomy and function of the human foot, which is characterized by a fully adducted hallux [big toe in line with the other toes], a large and robust calcaneus [heel] and tarsal region, a pronounced medial longitudinal arch, and short toes. Footprints reflect the pressure distribution as the foot makes contact with the substrate [surface being walked upon], but also the sediment's geomechanical properties. During normal walking, the weight-bearing foot undergoes a highly stereotypical movement and pressure distribution pattern in which the heel contacts the ground first, making a relatively deep impression on the substrate. This is followed by contact with the lateral side of the foot and metatarsal heads, after which weight transfers to the ball of the foot with peak pressure under the medial metatarsal heads, and finally ending with toe-off pressure under the hallux.  As a consequence, the deepest part of a footprint often occurs beneath the first and second metatarsal heads, that along with a deep hallucal impression corresponds to the peak pressures at toe-off… This contrasts with the less stereotypical pattern of footfall observed in African apes during quadrupedal and bipedal locomotion. Here the heel and lateral mid-foot make contact with the ground first, followed by contact with the lateral toes that are often curled and with a hallux that is often widely abducted. Lift-off in the African apes is variable, but it usually involves relatively low pressure during final contact by both the lateral toes and widely abducted hallux, in stark contrast to modern human foot function.“

The authors summarize their findings: “Hominin footprints offer evidence about gait and foot shape, but their scarcity, combined with an inadequate hominin fossil record, hampers research on the evolution of the human gait. Here, we report hominin footprints in two sedimentary layers dated at 1.51 to 1.53 million years ago (Ma) at Ileret, Kenya, providing the oldest evidence of an essentially modern human–like foot anatomy, with a relatively adducted hallux, medial longitudinal arch, and medial weight transfer before push-off. The size of the Ileret footprints is consistent with stature and body mass estimates for Homo ergaster/erectus…”

The Behrensmeyer-Laporte investigation started in 1978 and excavations continued into 1979.  They were working at Koobi Foora, about 5 km south of Illeret, where the most recent hominin footprint trails were found.  These two sites lie on the eastern side of Lake Turkana and the prints were made in mud at both sites.  The Laetoli prints, two million years earlier, were made in volcanic ash on which light rainfall fell thereafter, hardening the ash into a concrete like material.  Volcanic ash yields reliable dates and the dating of the Koobi Foora and Illeret muds is no less reliable because they lay between reliably dated ash strata [layers].

Below is an illustration taken from the Science paper.   Here also are links to the two papers (Bennett et al. and Behrensmeyer-Laporte).  The New York Times carried a story on this discovery, as well.

Optical laser scan images color-rendered with 5-mm isopleths for footprints at both FwJj14E and GaJi10. (A) Isolated left foot (FUI1) on the upper footprint surface at FwJj14E. (B) Photograph of FUI8 on the upper footprint surface at FwJj14E, showing good definition of the toe pads; the second toe is partially obscured by the third toe. (C) Second trail on the upper footprint surface at FwJj14E, showing two left feet. (D) Third trail on the upper footprint surface at FwJj14E, showing a right and a left foot. (E) Print R3 from GaJi10 (22), re-excavated and scanned as part of this investigation. (F) Partial print (FUT1-2) on the upper footprint surface at FwJj14E; the heel area has been removed by a later bovid print. (G) Print FLI1 on the lower footprint surface at FwJj14E, rendered with 5-mm alternating black and white isopleths. (H) Inverted image of the toe area of print FUT1-1 with alternating 5-mm black and white isopleths. Note the locations of the pads of the small toes and the presence of a well-defined ball beneath the hallucial metatarsophalangeal joint. The first, third, and fifth toes are marked