Evolutionary and Developmental Perspectives on Human Toe Reduction
Location
Philadelphia, PA
Start Date
8-5-2019 1:00 PM
End Date
8-5-2019 4:00 PM
Description
Digit lengths are highly variable among primates, and humans are distinguished by short fingers and particularly short toes. The dramatic reduction in toe length has been hypothesized to limit the excursion and mechanical work of digital flexors during toe-off. Finger reduction has been alternatively proposed to be a correlated response for selection on toe length due to shared gene expression during development or the direct effect of selection for tool use and manipulative behaviors. If the former hypothesis is correct, then human fingers and toes should show similar patterns of digit reduction. Lengths of the proximal (PP) and intermediate (IP) phalanges of the fingers and toes were collected for a large sample of extant and extinct anthropoids. Proximal articular widths and metrics from the carpals and tarsals were used for size normalization. Despite great variation in PP and IP lengths of both the hands and feet, the IP/PP proportion within each digit is remarkably consistent across anthropoids. Human toes are an exception. The IP becomes progressively and disproportionately shorter from digit 2 to digit 5. In fact, lateral IPs are frequently fused to the terminal phalanges or missing altogether. IPs are also short in Neandertals and Homo naledi, but typical anthropoid proportions are retained in Ardipithecus ramidus and Australopithecus afarensis. In light of recent work that demonstrates the remarkable specificity of gene regulatory elements to independently target fore- and hind limb development, it is likely that human fingers and toes evolved independently in response to distinct selection pressures.
Embargo Period
5-24-2019
Evolutionary and Developmental Perspectives on Human Toe Reduction
Philadelphia, PA
Digit lengths are highly variable among primates, and humans are distinguished by short fingers and particularly short toes. The dramatic reduction in toe length has been hypothesized to limit the excursion and mechanical work of digital flexors during toe-off. Finger reduction has been alternatively proposed to be a correlated response for selection on toe length due to shared gene expression during development or the direct effect of selection for tool use and manipulative behaviors. If the former hypothesis is correct, then human fingers and toes should show similar patterns of digit reduction. Lengths of the proximal (PP) and intermediate (IP) phalanges of the fingers and toes were collected for a large sample of extant and extinct anthropoids. Proximal articular widths and metrics from the carpals and tarsals were used for size normalization. Despite great variation in PP and IP lengths of both the hands and feet, the IP/PP proportion within each digit is remarkably consistent across anthropoids. Human toes are an exception. The IP becomes progressively and disproportionately shorter from digit 2 to digit 5. In fact, lateral IPs are frequently fused to the terminal phalanges or missing altogether. IPs are also short in Neandertals and Homo naledi, but typical anthropoid proportions are retained in Ardipithecus ramidus and Australopithecus afarensis. In light of recent work that demonstrates the remarkable specificity of gene regulatory elements to independently target fore- and hind limb development, it is likely that human fingers and toes evolved independently in response to distinct selection pressures.
Comments
Presented at American Association of Physical Anthropologists Annual Conference 2019.