New research suggests that our ancestors' feet took on modern features nearly 2 million years earlier than previously thought - literally laying the groundwork for hominin migration beyond Africa.
Scientists believe the prints were made by A. afarensis
Nearly four million years ago, our human relatives were very different from modern man. Australopithecus afarensis had a longer torso, a smaller brain and significantly stubbier legs - but we did have one thing in common: our feet.
After examining the ancient species' footprints using a new type of analysis, a team of British scientists concluded that the "human" gait emerged 3.7 million years ago. The study challenges previous research, which suggests that human-like walking did not develop in homonin species until nearly 2 million years later.
In the report, which was published online Wednesday by the Royal Society Journal Interface, scientists said the species presumed responsible for those tracks had feet that were strikingly similar to those of modern humans, and less like those belonging to chimpanzees or gorillas.
Since the footprints were found in Laetoli, Tanzania more than three decades ago, they have polarized the scientific community - pitting researchers who describe the footprints as more "ape-like" against those who see in them the origins of modern bipedal motion.
The footprints in Laetoli, Tanzania were discovered in the 1970s
The great debate
Robin Crompton of the University of Liverpool, the study's lead author, said his team's research could deal a decisive blow to that debate:
"They are more human prints than ape prints, and I don't think that can be debated anymore," he told Deutsche Welle.
The Liverpool scientists, who also collaborated with experts at Bournemouth University and the University of Manchester, arrived at that conclusion with the help of a very modern method: three-dimensional averaging.
Instead of analyzing each print separately, the team generated 3-D averages of all 11 and compared them to data sets generated in other studies.
This mapping technique had previously been used to analyze blood flow in the brain. Researchers argued that this method also served as an objective way to analyze the prints.
"In this paper, we simply substitute footprints for brains, and depth for blood flow," the study reads. The result is a statistical image.
"We've been very, very careful about how we've done this study," Crompton said.
The team used advanced technology to analyze the footprints
Nevertheless, William Sellers, one of the co-authors of the study from the University of Manchester's life sciences department, said the team was pleasantly surprised by the results.
"I think we were always on the side thinking they were more human than other people had thought," he told Deutsche Welle. "But we weren't expecting quite so much similarity with modern humans."
Those similarities include functional features, ranging from a medial arch to a gait driven by the toes. Taken together, the study said, those characteristics add up to a combination that is "very unusual, if not absent, in any living non-human ape."
Jeff Meldrum, an expert in evolutionary anthropology at Idaho State University who was not involved in the study, said hominin evolution involves a broader spectrum than the debate between "human-like" and 'ape-like" might suggest.
"It's falling back to this either-or," he told Deutsche Welle. "'If you don't look like a chimp, then you must be human.' That's not a fair statement."
Meldrum said that while there is no question that the Laetoli tracks don't look like those of a chimpanzee, there were plenty of things that distinguish them from footprints of modern humans.
He took exception with the study's assertion that an elevated region under the mid-foot could be evidence of the kind of medial longitudinal arch seen in modern man - as well the suggestion that a mid-tarsal break, something seen in apes, was absent in the prints.
Meldrum led a study published in January that asserted the footprints correlated with mid-foot flexibility, not a longitudinal arch. His team concluded that the Laetoli trackmakers "had not evolved the modern human foot form."
"My point is that you can walk effectively on two legs, without having to have evolved those features associated with modern forms of walking," he told Deutsche Welle.
Modern man's feet evolved as man transitioned to a life on the ground
Migration and colonization
Chimpanzees and gorillas exhibit big toes that are better suited for climbing than walking - a feature that our human ancestors lost as they transitioned from a life in the treetops to a life on the ground.
"If you look at primates as a whole, most primates have the sort of feet that you can peel bananas with," Sellers said. "They've got big, grasping toes that reach out sideways - because if you live in a tree, it's very, very useful to be able to grip with your feet."
The team's research suggests that the feet of mankind's ancient relative, Australopithecus afarensis, had developed to accommodate both an arborial lifestyle and bipedal motion.
Yet though the species' feet were made for walking, he couldn't get very far; from an evolutionary standpoint, A. afarensis was still waiting for his body to catch up.
The species had a long torso with short legs and small feet - a combination that was not well-suited for traveling long distances.
Had it been easier for A. afarensis to get from point A to B, he still wouldn't have been able to bring much with him. "It makes it very difficult to carry loads," Crompton said, referring to the species' anatomy.
Though the study suggests that our ancestors had very modern foot function 3.7 million years ago, Sellers said the size of their brains did not begin to increase until about 2 million years ago. By that time, their bodies had already grown much larger.
Without those advantages, it appears that the maker of the Laetoli footprints still had a while to wait before his descendants would make their way out of Africa and into parts of the world yet unknown.
Author: Amanda Price
Editor: Cyrus Farivar