by Mary Caperton Morton Monday, December 11, 2017
A curious set of 5.7-million-year-old bipedal footprints found in western Crete — far from the cradle of humanity in Africa and dating to the Late Miocene, long before hominins are thought to have walked upright — has paleoanthropologists scratching their heads.
During the Late Miocene, Crete was not yet an island. It was attached to what is now the Greek mainland, and savanna-like ecosystems extended from North Africa all the way to the eastern Mediterranean. So it’s possible that early hominins ranged from Africa all the way into southeastern Europe; and fossils of Late Miocene-aged primates — some resembling hominins — have been found in Europe. But could one such species have walked upright? That seems to be the story left by at least two mystery animals that milled around a sandy river delta, leaving an assortment of distinctly bipedal tracks, according to a new study published in Proceedings of the Geologists' Association. But not everybody is ready to redraw the timeline, or the map, of early hominin evolution.
The Crete footprints were discovered in Trachilos in western Crete in 2002, and in 2012, researchers got permission from the Greek government to conduct a detailed study of the trackway. Laser scans and three-dimensional imaging of a roughly 400-square-meter area containing about 50 individual prints allowed the researchers to analyze the shapes of the prints. In the team’s estimation, the tracks display various human-like features, including a big toe in line with the other toes. Even when chimps and other apes occasionally walk upright for short periods, they leave very different footprints, showing an opposable big toe used for grasping branches. Dating of the sedimentary rock in which the prints were found was tightly constrained by analyses of foraminifera preserved in sedimentary layers above and below the track-bearing layer, and by a distinctive layer — lying just above the track-bearing layer — laid down 5.6 million years ago when the Mediterranean Sea briefly dried up during the Messinian Salinity Crisis.
“All of this adds up to something very interesting,” says Per Ahlberg, a paleontologist at Uppsala University in Sweden and a co-author of the new study. Ahlberg and his colleagues concluded that the prints were made either by an early hominin or by an unknown nonhominin primate that coincidentally evolved human-like feet. Despite having experts on fossil trackways and hominid footprints on the team, however, “we had a devil of a time getting this data published,” he says. For years, “we submitted it and got fiercely critical reviews, which typically asserted that these aren’t footprints at all, just random holes in the ground, which they most certainly are not.” Since the work has been published, though, “nobody has said they aren’t footprints, and we are mainly fielding arguments about whose footprints they are, which is a very valid, rational debate,” he says.
The footprints are curiously hominin-like, but that doesn’t mean they were made by one of our distant ancestors, says David Begun, a paleoanthropologist at the University of Toronto in Canada who was not involved in the new study. “The tracks lack several really important human characteristics that we find in the oldest-known, unambiguous [ancestral] human footprints,” which are found at Laetoli in Tanzania, date to 3.7 million years ago, and are attributed to Australopithecus afarensis, one of the earliest human ancestors. For example, the study finds that the tracks don’t show a deep heel impression from heel-strike locomotion, or a clear impression of the arch. As humans walk, the heel strikes first, then the arch of the foot rolls along the ground, leaving a convexity in the middle. “These prints [from Crete] are kind of flat,” Begun says.
“The bottom line is I don’t know what to think about these tracks,” Begun says. It’s possible they were made by a primate, he says, “something that had a form of bipedalism that is different from humans.” Fragmentary fossils from large, possibly bipedal primates have been found throughout southern Europe, including from the 7.2-million-year-old Graecopithecus from Greece, which Begun and his colleagues recently described in PLOS ONE.
The footprints need further analysis, says Robin Crompton, a biological anthropologist at the University of Liverpool in England who was not involved in the new study. “This is a very important find; there’s no doubt about that.” But characterizing footprints is complicated: “One of the issues … is that you need a lot of them to say for sure what you’re looking at,” Crompton says. “I’m pretty sure they are bipedal footprints, but I’m not convinced they were made by human ancestors,” he says. They were more likely “made by a member of the great ape clade, of which there are many candidates from this part of the world during this time period.”
Ahlberg’s team plans to make its data publicly available as it continues to analyze the footprints, as well as to push for more protection for the site, which is located near a major road in Crete. In September, 10 of the tracks were cut from the ground and stolen from the site, though they were quickly recovered. “The actual print-bearing surface is very thin, only about 1.5 centimeters thick, making them very vulnerable,” Ahlberg says. “Mercifully, due to very efficient work by the Greek police, the stolen prints were recovered, but they have been taken out of context and the thief has trimmed them so bits of the surface are gone.”
Fortunately, existing laser scans of the site preserve a record of the surface of the site, so any missing parts can be filled in. “For now,” Ahlberg says, the site is “under emergency cover and will hopefully be better protected for future study.
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