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 land
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