Human Evolution: Did Dexterous Hand or Flexible Foot Evolve First?

As humans evolved, their hands and feet changed. Hands became more dexterous as feet developed to accommodate bipedal locomotion. But which evolved first: hands or feet? Scientists may now have discovered the answer to that question.

In order to find out whether human hands or feet were first to evolve, the researchers employed functional magnetic resonance imaging in humans and electrical recording from monkeys in order to locate the brain areas responsible for touch awareness in individual fingers and toes, called somatotopic maps. These maps allowed the researchers to confirm previous studies showing that single digits in the hand and foot have discrete neural locations in both humans and monkeys.

So what else did they find? It turns out that monkey toes are combined into a single map while human toes are also fused into a single map--with the exception of the big toe. In humans, the big toe has its own map. This finding in particular seems to suggest that early hominids evolved dexterous fingers while they were still quadrupeds.

"In early quadruped hominids, finger control and tool use were feasible, while an independent adaptation involving the use of the big toe for functions like balance and walking occurred with bipedality," wrote the authors in a news release.

The researchers didn't only rely on modern evidence, though. They also examined the well-preserved hand and feet bones of a 4.4 million year-old skeleton of the quadruped hominid Ardipithecus ramidus. This species had hand dexterity that preceded the human-monkey split.

In the end, the researchers found that the parallel evolution of two-legged locomotion and manual dexterity in hands and fingers in the human lineage were a consequence of adaptive pressures on ancestral quadrupeds for balance control by foot digits while retaining the critical capability for fine finger specialization.

"Evolution is not usually thought of as being accessible to study in the laboratory," said Atsushi Iriki, one of the researchers, in a news release. "But our new method of using comparative brain physiology to decipher ancestral traces of adaptation may allow us to re-examine Darwin's theories."

The findings are published in the journal Philosophical Transactions of the Royal Society B.