Footprints left by eight ancient hominins 1.43 million years ago beside a Kenyan lake have puzzled researchers because they don鈥檛 look quite as expected for species known to have lived there at that time.
Altogether, 21 hominin footprints have been found at the Lake Turkana site, known to researchers as GaJi10, part of which was first discovered in 1978.
The tracks were made on what would have been mud in shallow water, between 2 and 10 centimetres deep, at the edge of the lake.
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Fossil remains and other footprints discovered nearby and studied over many decades suggest there were two hominin species co-existing in the area: Homo erectus and Paranthropus boisei.
P. boisei, nicknamed 鈥渘utcracker man鈥�, was a more ape-like hominin with a small body but large jaws and teeth.
The shape of the footprints look similar to others made by P. boisei, but they are considerably larger. Previous estimates have suggested the largest members of the species were just over 160 centimetres tall, but analysis of the new footprints indicates that if they were made by P. boisei, they must have been over 180 centimetres tall.
While the tracks fit within the size range of the other species known from the area, H. erectus, the shape doesn鈥檛.
鈥淚f these footprints are Paranthropus boisei, then they really increase the upper limits of body size in that species,鈥� says at Chatham University in Pittsburgh, Pennsylvania, who worked on the study.
鈥淚f they are H. erectus, then they would imply that species has a range of variation in footprint morphology that far exceeds what we鈥檝e observed in large samples of modern humans.鈥�
The footprints have flatter arches and a big toe that is more angled away from the other toes, characteristics similar to other sets of prints in the area identified as P. boisei.
鈥淏oth of these features differ from what we usually see in modern humans, and that led us to hypothesise that footprints with that shape were more likely made by P. boisei,鈥� he says. 鈥淲e鈥檝e also seen other footprints with human-like arches and human-like big toes, and we believe those were more likely made by H. erectus.鈥�
The third possibility is that the footprints represent a different species. But that seems unlikely, says Hatala, since many fossils are known from this area and time period and have all been attributed to either P. boisei or H. erectus.
Whichever species left the tracks, the height of the group means that it probably consisted of males travelling together.
At around the same time as the hominins passed through, numerous other animals also walked across the same stretch of mudflats. They included two species of hippopotamus, as well as bovids, birds and equids 鈥� a scene not unlike what a visitor would see today in a modern African lakeshore environment.
鈥淭he presence of many hippos also indicates that this environment included some real dangers to the hominins, and that whatever brought them to the lakeshore 鈥� maybe they were seeking water or food 鈥� must have been worth the risk,鈥� says Hatala.
PNAS
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