The discovery of a partial foot fossil in Ethiopia suggests that our human ancestors were possibly an occasional tree-climber and an occasional upright walker.
In a search for additional clues on how and when our ancestors stopped climbing trees and started walking on two feet, scientists went to the central Afar region in Ethiopia. It’s home to some of the world's richest fossil and artifact sites, including the famous Hadar site. “Lucy,” the partial ape-human skeleton, was excavated at Hadar in 1974.
About 30 miles north of Hadar in 2009, scientists excavated a surprising set of foot bones at the Burtele palaeontological site. Scientists spent the next three years analyzing their findings before reaching a moment of eureka.
“For the first time, we have good evidence that there is indeed another hominin lineage that lived at the same time as Lucy’s species,” study co-author Bruce Latimer said in a scientific news briefing. He is an anthropologist at Case Western Reserve University in Cleveland.
The fossils of the partial foot of the early hominin include eight bones, all from the right forefoot. They were discovered in fossil-rich deposits dated to 3.4 million years ago. That makes it roughly the same age as Lucy and her species, Australopithecus afarensis.
“Scientists have long-argued about whether there was only one species or more than one species, particularly between the time three to four million years ago," said Yohannes Haile-Selassie, lead author of the study and curator and head of Physical Anthropology at Case Western.
"Once walking on two legs – bipedality - evolved, that was how our limit was characterized. We never expected another related species running around with capabilities to climb up trees at the same time,” said Haile-Selassie.
The fossils’ discoverers say Lucy's species did not climb trees. They had feet much like modern humans and had a big toe that was curved at the top, allowing the toes to hyperextend to "help push off" at the end of each step.
Markings on the new Burtele foot indicate that like Lucy's species, it had the ability to walk upright but did not have much joint mobility. The key difference is in the big toe. The Burtele foot lacks the large spherical ends at the big toe and instead has a short, divergent big toe, like a chimp, also making it a part-time grasper and climber.
“What we have now is a creature that has a different mode of locomotion from Lucy,” said Haile-Selassie. “This clearly demonstrates that our evolution was not characterized by a single linage evolving through time. There were also others living in the Afar region at the same time.”
Their findings of the study were released Wednesday in the journal Nature.
Scientists say analysis of the physical and sedimentary chemical characteristics of the rock strata show that the environment was aquatic, surrounded by woods along the shores.
“This is just another window into solving the problem of how we got from a primitive foot to the modern human foot,” said Latimer.
To shine more light into that window, scientists are putting one foot in front of the other, saying more fossil evidence is needed before they can draw definitive conclusions on whether the Burtele foot bones represent a new species of hominin.
“This is the first time we’ve had these particular elements that we can start looking at and comparing them. This allows us to tease out what the sequence is in creating a hominin foot,” said Latimer.
For now, this ancient foot only adds more evidence to the understanding that scientists hope will bring us one step closer to unlocking the mysteries of our ancestral past.