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THUMBS UP! AND OVER

AN OPPOSABLE THUMB optimizes interaction with the other four digits of a hand (or foot—gorillas have opposable big toes too). Humans have opposable thumbs, as do most other primates. Cats, dogs, horses, and raccoons don’t. 

But when did ancient hominins first grasp things in this now routine way? 

Pamela J. Hines’ item in AAAS Science, April 29, 2021, addresses this: “Hand bones of australopithecine hominid fossils dated to over 2 million years ago have a long, slender thumb that may have added dexterity.”

Musculature of the thumb includes the m. opponens pollicis (Latin: opposed thumb). This muscle may have evolved to facilitate tool use. This and other images by F.A. Karakostis, et al, Curr. Bio. 31, 1318 (2021). 

Full details are reported in cell.com.“Biomechanics of the Human Thumb,” by F.A. Karakostis, et al, Current Biology 31, March 22, 2021. Represented among the international researchers are Eberhard Karls University, Tübingen, Germany; University of Stuttgart, Stuttgart, Germany; National and Kapodistrian University of Athens, Athens, Greece; and Natural History Museum of Basel, Basel, Switzerland. 

Here are tidbits gleaned from the AAAS Science item and the Current Biology paper.  

Dexterity and Tools. Karakostis and colleagues write in their Abstract, “Systematic tool production and use is one of humanity’s defining characteristics, possibly originating as early as >3 million years ago. Although heightened manual dexterity is considered to be intrinsically intertwined with tool use and manufacture, and critical for human evolution, its role in the emergence of early culture remains unclear.” 

Most previous research, they note, assumed that an earlier species’ dexterity depended upon its similarity to the modern thumb. Instead, they focused on thumb fundamentals by studying the efficiency of opposition with several species of fossil hominins.

Methodology. Karakostics and colleagues studied specimens of the trapezia (the trapezium is the wrist structure at the base of the thumb). These included those from modern Homo sapiens (humans) and Pan troglodytes (chimpanzees), to early Homo sapiens and Neanderthals dating from 23 ka–130 ka (thousands of years ago), to earlier hominins dated to as much as 3.85 mya (millions of years ago).  

The researchers focused on a particular muscle, m. opponens pollicis (Latin: opposed thumb), one of short thenar muscles of the trapezium. 

Joint torque is a fundamental aspect of biomechanical efficiency.

“Essentially,” researchers note, “the objective of the present study is not to reconstruct habitual physical activity patterns in early hominins, but to employ an integrative biomechanical approach for detecting key functional adaptations for increased manipulatory skills in the fossil record.”

They continue, “Through the integration of muscle modeling in 3D and geometric morphometric shape analysis, our methodology considers the crucial effects of muscle parameters (i.e., force-producing capacities) and bone morphology at the sites where muscles attach in life.” 

Biomechanical Findings. The researchers write, “In summary, our results provide biomechanical evidence that, approximately 2 mya, certain hominins developed greatly increased thumb opposition efficiency (joint torque) relying on m. opponens pollicis.”

They note, “This crucial evolutionary advantage [of particular musculature], which is shared with all later species of Homo, was found to be less pronounced in the earliest proposed stone-tool-making hominins (i.e., Australopithecus species, including the late Australopithecus sediba). The increased thumb opposition efficiency shown by all Pleistocene Homo species investigated here highlights the significance of this functional feature in the bio-cultural evolution of our genus.” 

It also separated the “thumbs up” from other distinctive single-digital expressions. ds 

© Dennis Simanaitis, SimanaitisSays.com, 2021

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