Humans Evolved Dexterous Hands Before Agile Feet

A long-standing mystery in human evolution has been solved: humans evolved dextrous hands before developing the ability to walk on two legs.

Asian Scientist (Oct. 8, 2013) – Resolving a long-standing mystery in human evolution, researchers in Japan have found that early hominids developed finger dexterity and the ability to use tools before they developed agile feet.

Combining monkey and human behavior, brain imaging, and fossil evidence, a research team has overturned the common assumption that manual dexterity evolved after the development of bipedal locomotion freed hominid hands to use fingers for tool manipulation.

In their study, published in Philosophical Transactions of the Royal Society B, the researchers combined functional magnetic resonance imaging (fMRI) in humans and electrical recording from monkeys to construct somatotopic maps that pinpoint the brain areas responsible for touch awareness in individual fingers and toes.

With these maps, the researchers confirmed previous studies showing that single digits in the hand and foot have discrete neural locations in both humans and monkeys.

However the researchers discovered that while monkey toes are combined into a single map, only four of the toes found on the human feet are fused into a single map.The prominent exception is the big toe in humans, which has its own map not seen in monkeys.

These findings suggest that early hominids evolved dexterous fingers when they were still quadrupeds. Manual dexterity was not further expanded in monkeys, but humans gained fine finger control and a big toe to aid bipedal locomotion.

“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,” the authors explained.

The brain study was supported by analysis of the well-preserved hand and feet bones of a 4.4 million year-old skeleton of the quadruped hominid Ardipithecus ramidus, a species with hand dexterity that preceded the human-monkey lineage split.

The findings suggest 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 Dr. Atsuki Iriki, leader of the study, “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 article can be found at: Hashimoto et al. (2013) Hand Before Foot? Cortical Somatotopy Suggests Manual Dexterity Is Primitive And Evolved Independently Of Bipedalism.

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Source: RIKEN.
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