The strength required to access the high calorie content of bone marrow may have played a key role in the evolution of the human hand and explain why primates hands are not like ours, research at the University of Kent has found.

In an article in The Journal of Human Evolution, a team lead by Professor Tracy Kivell of Kent's School of Anthropology and Conservation concludes that although stone tool making has always been considered a key influence on the evolution of the human hand, accessing bone marrow generally has not.

It is widely accepted that the unique dexterity of the human hand evolved, at least in part, in response to stone tool use during our evolutionary history.

Archaeological evidence suggests that early hominins participated in a variety of tool-related activities, such as nut-cracking, cutting flesh, smashing bone to access marrow, as well as making stone tools. However, it is unlikely that all these behaviours equally influenced modern human hand anatomy.

To understand the impact these different actions may have had on the evolution of human hands, researchers measured the force experienced by the hand of 39 individuals during different stone tool behaviours—nut-cracking, marrow acquisition with a hammerstone, flake production with a hammerstone, and handaxe and stone tool (i.e. a flake) - to see which digits were most important for manipulating the tool.

They found that the pressures varied across the different behaviours, with nut-cracking generally requiring the lowest pressure while making the flake and accessing marrow required the greatest pressures. Across all of the different behaviours, the thumb, index finger and middle finger were always most important.

Professor Kivell says this suggests that nut-cracking force may not be high enough to elicit changes in the formation of the human hand, which may be why other primates are adept nut-crackers without having a human-like hand.

In contrast, making stone flakes and accessing marrow may have been key influences on our hand anatomy due to the high stress they cause on our hands. The researchers concluded that eating marrow, given its additional benefit of high calorific value, may have also played a key role in evolution of human dexterity.

The manual pressures of stone tool behaviors and their implications for the evolution of the human hand by Erin Marie Williams-Hatala, Kevin G. Hatala, McKenzie Gordon and Margaret Kasper, all Chatham University, Pittsburgh, USA and Alastair Key and Tracy Kivell, University of Kent is published in the Journal of Human Evolution.



Read more at: https://phys.org/news/2018-07-bone-m...ution.html#jCp


It is widely agreed that biomechanical stresses imposed by stone tool behaviors influenced the evolution of the human hand. Though archaeological evidence suggests that early hominins participated in a variety of tool behaviors, it is unlikely that all behaviors equally influenced modern human hand anatomy. It is more probable that a behavior's likelihood of exerting a selective pressure was a weighted function of the magnitude of stresses associated with that behavior, the benefits received from it, and the amount of time spent performing it. Based on this premise, we focused on the first part of that equation and evaluated magnitudes of stresses associated with stone tool behaviors thought to have been commonly practiced by early hominins, to determine which placed the greatest loads on the digits. Manual pressure data were gathered from 39 human subjects using a Novel Pliance®manual pressure system while they participated in multiple Plio-Pleistocene tool behaviors: nut-cracking, marrow acquisition with a hammerstone, flake production with a hammerstone, and handaxe and flake use. Manual pressure distributions varied significantly according to behavior, though there was a tendency for regions of the hand subject to the lowest pressures (e.g., proximal phalanges) to be affected less by behavior type. Hammerstone use during marrow acquisition and flake production consistently placed the greatest loads on the digits collectively, on each digit and on each phalanx. Our results suggest that, based solely on the magnitudes of stresses, hammerstone use during marrow acquisition and flake production are the most likely of the assessed behaviors to have influenced the anatomical and functional evolution of the human hand.
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