image: Both monkey (Macaca fuscata, left) and human (Homo sapiens, right) have five physically independent fingers (top) and five toes (bottom), although the human foot is irregular in shape. Monkey hand, foot and human hand are similar in shape except for monkey heel (grey). In addition, human toe I is larger than the lesser toes, whereas monkey toes are similar in size. The fingers were represented independently (color coded) in the primate somatosensory cortex (SI): I, red; II, orange; III, yellow; IV, green and V, blue. By contrast, the representations of the toes were fused, with the exception of the great toe in humans. view more
Credit: RIKEN
Resolving a long-standing mystery in human evolution, new research from the RIKEN Brain Science Institute indicates that early hominids developed finger dexterity and tool use ability before the development of bipedal locomotion.
Combining monkey and human behavior, brain imaging, and fossil evidence, a research team led by neurobiologist Dr. Atsushi Iriki and including Dr. Gen Suwa, an anthropologist from the University of Tokyo Museum, have overturned the common assumption that manual dexterity evolved after the development of bipedal locomotion freed hominid hands to use fingers for tool manipulation.
In a study published today in Philosophical Transactions of the Royal Society, the researchers employed functional magnetic resonance imaging in humans and electrical recording from monkeys to locate the brain areas responsible for touch awareness in individual fingers and toes, called somatotopic maps. 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 found new evidence that monkey toes are combined into a single map, while human toes are also fused into a single map, but with the prominent exception of the big toe, 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 transitional bipedal hominin Ardipithecus ramidus, a species that retained 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", stated Dr. Iriki, "but our new method of using comparative brain physiology to decipher ancestral traces of adaptation may allow us to re-examine Darwin's theories".
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Juliette Savin
RIKEN
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Reference
T. Hashimoto et al. Hand Before Foot? cortical somatotopy suggests manual dexterity is primitive and evolved independently of bipedalism Philosophical Transactions of the Royal Society B, 2013 DOI:10.1098/rstb.2012.0417
About RIKEN
RIKEN is Japan's largest research institute for basic and applied research. Over 2500 papers by RIKEN researchers are published every year in leading scientific and technology journals covering a broad spectrum of disciplines including physics, chemistry, biology, engineering, and medical science. RIKEN's research environment and strong emphasis on interdisciplinary collaboration and globalization has earned a worldwide reputation for scientific excellence.
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About the RIKEN Brain Science Institute
The RIKEN Brain Science Institute (BSI) performs cutting-edge neuroscience research in the service of society and has earned an international reputation as an innovative center for research and training. Researchers at BSI seek to understand brain functions from molecules to neural circuits to cognition, using methods drawn from a wide range of disciplines. BSI is also leading efforts to provide career development for researchers in Japan and around the world.
Website: http://www.brain.riken.jp/en/
Journal
Philosophical Transactions of the Royal Society of London (B )