Why thumb was last part of foot to develop when we turned bipeds

When our ancestors began to walk, they still spent some time on the trees, using their feet to cling to the branches.

Scientists have recently discovered that our thumb was one of the last parts of the foot to evolve into its current form.

When our ancestors began to walk, they still spent some time on the trees, using their feet to cling to the branches. They walked on the ground in a different way than they would later, but they were able to move around with great agility.

It was the evolution of the big toe which allowed the foot to support itself better and that impulse more easily and that allowed a more efficient locomotion, shows a study of the American University of Marquette released last week in the Proceedings of the National Academy of Sciences, publication of the United States National Academy of Sciences.

Scientists have studied 3D images of fossil toe bones from primitive humans and current primates, such as chimpanzees. They then compared this information to current human bones and to the evolutionary tree of primates. The main finding is that the current shape of the big toe bones, the hallux, in medical terminology evolved later than the other bones investigated. The finding is important because "our ability to walk and run like bipeds is a crucial trait that has allowed humans to reach what they are today," says Peter Fernandez, a researcher at Marquette University in Milwaukee, and lead author of the study.

For everything to work together, the bones of the foot needed to evolve to accommodate the particular demands of the biomechanics of bipedalism, according to the scientist.

"Maybe it was the one that demanded the most difficult changes," says Fernandez.

"Humans have more stability in walking because of the orientation of the big toe, parallel to the other toes, but as a result, our feet are not as dexterous as a monkey's."The reason why at any given moment our ancestors ventured to stand and walk is still a mystery, although there are several hypotheses. One possibility is that walking allowed primitive humans to free their hands to carry and use tools.

Another hypothesis is that climate change led to the reduction of forests, which made it imperative to seek food elsewhere. A third hypothesis is that our ancestors already used their upper limbs holding onto twigs while using their feet for finer limbs.