Fossils thought to be oldest evidence to date of an essentially modern foot.
"Finding footprints in the early stage of human evolution is very rare. They're very fragile and they don't often preserve," explained study co-author John W.K. Harris, a professor of anthropology at Rutgers University in New Brunswick, N.J. "This is only the second finding in 30 years."
"This is not the first time that a footprint has been found that has shouted to us through time, but it certainly is one of the most important," added Jeffrey T. Laitman, distinguished professor and director of anatomy and functional morphology and of gross anatomy at Mount Sinai School of Medicine in New York City. Laitman was not involved with the study.
The features of the footprints indicate that they belonged to the hominid Homo ergaster, or early Homo erectus. These are the oldest footprints that can be linked to mankind's genus, Homo, according to an accompanying perspective article in the journal. This creature had the longer legs and shorter arms of modern man, rather than the longer arms and shorter legs of the more ape-like ancestors.
According to fossil records, the ability to walk on two feet -- called bipedalism -- seems to have emerged about 6 million years ago. But it's unclear when the more human form of bipedalism evolved.
Thirty years ago, in 1978, British archaeologist/anthropologist Mary Leakey discovered 3.75-million-year-old footprints at the Laetoli archaeological site in Tanzania.
But these prints seem to have belonged to a less "modern" bipedal creature, still with the big toe separated from the rest of the toes and with the more ape-like long arms with shorter legs.
And any skeletons of mankind's more modern-proportioned ancestors tend to have lacked foot bones.
"There's not much information regarding feet ... only a few foot bones have survived through time," Laitman said.
The new discovery, made at Ileret in Kenya, is notable for a number of reasons, Harris said.
"The size of the foot is conspicuous," he explained. "The foot is much larger than the bony remains we have of the feet of earlier hominids, and it's longer and more elongated. You or I could put our feet into the prints of what we've found on the landscape from 1.5 million years ago."
Harris realized the importance of the prints when a local tribesman started to put his foot into one of the prints. "He could almost put his modern foot into one of the prints," Harris recalled. "It's like the coloratura soprano hitting the high notes. It sends shivers up your spine."
The shape and other features of the foot also more closely resemble those of modern man.
"There's a well-defined heel, nice and round and large. It also shows the arch from where we transfer the weight from back to front and, then, the most conspicuous feature, is the big toe which is in line with other toes," Harris said. "That gives us the platform to step off. These are all features that define the modern foot, and they were there 1.5 million years ago."
The distance between the footprints also shows a much greater stride than had been evident before, indicating a change in landscape and in the hominids' ability to traverse this landscape.
"At this time, 1.5 to 1.7 million years ago, there was a change in the climate to more dire conditions, so patches of food were further apart," Harris said. "For a hominid to be successful on that landscape, he had to have a more efficient way of moving across the landscape... The creature was increasing its home range. We think that it was at this stage of human evolution that Homo erectus left the continent of Africa."
"You're uncovering a little window in time, a snapshot in time that nobody's looked at in 1.5 million years," he added. "Nobody's uncovered these prints since they were made by the hominid himself."
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