When our ancestors that lived out on open land began to diverge from our arboreal cousins, bipedality became an incredible advantage. It is much more efficient to walk upright instead of moving on all fours. Recent analysis of a 6-million-year-old femur has given clues about how this trait came about. The results were announced by lead author Sergio Almécija from Stony Brook University and were published in Nature Communications.

The bone belongs to a member of the species Orrorin tugenensis. The species lived approximately 6 million years ago at the end of the Miocene. There have only been about 20 fossils recovered since its initial discovery in 2000. The nickname “Millennium Man” has been given to the species, based on the year of discovery.

Millennium Man’s femur was subjected to cutting-edge technology that analyzes the 3D morphology of the bone. The information was compared to that of a variety of apes and other hominins, using over 400 individuals. The femur was deemed as having an intermediate morphology between fully bipedal primates and those who were strictly arboreal and quadrupedal. There were also striking similarities to Australopithecus, the most famous of which is Lucy.

It is also dated to be intermediate to quadrupedal apes and bipedal hominins. This new information brings about new hypotheses about how bipedality could have originated. Some suspect that it could have aided apes who walked upright on branches. 

Some scientists aren’t certain that Millennium Man was truly a hominin. If it is, it was likely one of the very first. Millennium Man does not have features that are between current humans and current chimps, but instead between the earliest hominins of the Pliocene and ancestral apes of the Miocene. This also opens the door to the idea that not all apes from the Miocene looked and moved just like modern-day chimps, which shouldn’t be too surprising, given the fact that we don’t look and move just like our ancestors.

Further study of other apes from the Miocene will need to occur in order to better understand where and when bipedality originated and how it came to be such an important and advantageous trait.