Analysis of a 7.2-million-year-old thigh bone recovered from the Azmaka fossil deposit in Bulgaria suggests that the capacity to walk upright on two legs - a distinctly human trait known as bipedalism - existed in pre-human ancestors at least one million years earlier than previously thought.
The analysis by an international team of researchers, including University of Toronto paleoanthropologist David Begun, a professor in the department of anthropology in the Faculty of Arts & Science, adds to the theory that human ancestors first evolved in Europe rather than Africa, as has long been believed.
The findings are published in the journal Palaeobiodiversity and Palaeoenvironments .
Bipedalism is considered a fundamental threshold in human evolution. The oldest known fossil remains of humans were found in Africa, and researchers have long believed that bipedalism evolved there between six and seven million years ago. The new femur from the site of Azmaka in southern Bulgaria, however, has attributes of a biped, suggesting a human ancestor there was already walking on its hind legs.
"At 7.2 million years old, this ancestor, which we classify as belonging to the genus Graecopithecus, could be the oldest known human," says Begun.
The first Graecopithecus specimen, a fragment of a lower jaw, was discovered at a site near Athens, Greece. A team of researchers, including Begun, reanalyzed this finding in 2017 and concluded that the shape of the tooth roots suggested that Graecopithecus might be an early human ancestor.
"The lower jaw could not provide evidence on how the creature moved, but this newly discovered femur from the Bulgarian site of Azmaka provides valuable new information about its locomotion," says Begun. "Graecopithecus probably needed to move bipedally on the ground to see across the horizon to scan for both food and predators, and to carry food, tools and offspring."
The researchers suggest the thigh bone likely belonged to a female weighing about 24 kilograms who lived beside a river in what was then a savanna landscape similar to that of present-day eastern Africa. Their analysis shows several external and internal morphological similarities with bipedal fossil human ancestors and modern humans. These include an elongated, upward-pointing neck between the femur shaft and head, special attachment points for the gluteal muscles and the thickness of the outer bone layer.
Begun and his colleagues note that the creature was not exactly human in the way it moved. The Azmaka femur combines attributes of terrestrial quadrupeds such as monkeys, knuckle-walking African apes and bipeds. "It represents a stage in human evolution between our four-legged and two-legged ancestors that can fairly be called a missing link," says Begun.
The researchers believe Graecopithecus descends from older apes from Greece and Türkiye, Ouranopithecus and Anadoluvius respectively, which evolved from ancestors in western and central Europe. Begun notes that today's African savanna fauna largely originates from the Balkans and western Asia, particularly from Greece, Bulgaria and North Macedonia to Türkiye and Iran. He suggests that Graecopithecus also moved into Africa, which led to the origins of early human bipeds such as Ardipithecus and Australopithecus afarensis, whose most famous representative is the fossil known as Lucy.
"Whether the ancestors of chimps, gorillas and humans had already separated in Europe or whether these splits happened in Africa remains to be determined by future discoveries," says Begun.
"But we do know that extensive movements of mammals to Africa from Eurasia between eight and six million years ago were caused by large-scale climate changes in the eastern Mediterranean and western Asia, which led to the emergence of desert regions, including the Arabian Desert."
The team hopes that ongoing work at Azmaka and other sites in the Balkans, particularly in North Macedonia, will deliver more evidence of Graecopithecus and provide more knowledge about the ecology and evolution of this early biped and possible human ancestor.
