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• W3136356230 abstract "Modelling a muscle key to flexing the thumb, a new study suggests that the powerful opposability that characterises the dextrous human hand evolved in some of our fossil relatives about two million years ago — a time when tool use became more important. Modelling a muscle key to flexing the thumb, a new study suggests that the powerful opposability that characterises the dextrous human hand evolved in some of our fossil relatives about two million years ago — a time when tool use became more important. If you have learned anything about the evolution of the human hand or what makes it special, it is likely something about our opposable thumbs. Our thumb is indeed distinct in its functional abilities, but its opposability is not unique to humans. All other apes and most African and Asian monkeys also have an opposable thumb, which facilitates the enhanced grasping abilities that characterise all primates1Napier J.R. Prehensility and opposability in the hands of primates.Symp. Zool. Soc. Lond. 1961; 5: 115-132Google Scholar. Thumb opposability is enabled by a curved joint between the base of the thumb (first metacarpal) and the wrist (the trapezium) and several muscles that allow the thumb to both flex and abduct towards the fingers. What makes the human thumb special is the greater number and size of the muscles compared with that of other apes, as well as its length, making it easy for the pads of the thumb and fingers to connect2Napier J.R. The prehensile movements of the human hand.J. Bone Joint Surg. 1956; 38: 902-913Crossref Google Scholar,3Marzke M.W. Precision grips, hand morphology, and tools.Am. J. Phys. Anthropol. 1997; 102: 91-110Crossref PubMed Scopus (269) Google Scholar. Together, this anatomy allows the human hand to do forceful precision grips, which are considered critical to our remarkable manipulative abilities, now and in the past3Marzke M.W. Precision grips, hand morphology, and tools.Am. J. Phys. Anthropol. 1997; 102: 91-110Crossref PubMed Scopus (269) Google Scholar. When and why this forceful opposition arose during our evolution and its relationship to tool use have been long-standing questions in the study of human evolution, in part due to a fragmentary fossil record that preserves little to no information about soft tissues. A new study in this issue of Current Biology by Fotios Alexandros Karakostis, Katerina Harvati and colleagues4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar suggests that a powerful thumb arose in some, but not all, fossil human relatives (hominins) around two million years ago, at a time when archaeological evidence suggests a greater reliance on tool use and butchery of animals5Thompson J. Carvalho S. Marean C. Alemseged Z. Origins of the human predatory pattern: The transition to large animal exploitation by early hominins.Curr. Anthropol. 2019; 60: 1-23Crossref Scopus (38) Google Scholar. The evolutionary process is wonderful in its ability to produce multiple solutions to the same functional problem6Wainright P.C. Functional morphology as a tool for ecological research.in: Wainright P.C. Reilly S.M. Ecological Morphology: Integrative Organismal Biology. University of Chicago Press, Chicago1994: 42-59Google Scholar. Previous research aiming to reconstruct the manipulative abilities of fossil hominins has primarily focused on bones, using the human hand skeleton as the ‘gold standard’ for dexterity, the assumption being that the closer the morphology is to that of recent humans, the greater the manipulative ability7Tocheri M.W. Orr C.M. Jacofsky M.C. Marzke M.W. The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo.J. Anat. 2008; 212: 544-562Crossref PubMed Scopus (156) Google Scholar,8Kivell T.L. Deane A.S. Tocheri M.W. Orr C.M. Schmid P. Hawks J. Berger L.R. Churchill S.E. The hand of Homo naledi.Nat. Commun. 2015; 6: 8431Crossref PubMed Scopus (79) Google Scholar. Karakostis and colleagues4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar argue that this assumption ignores alternative biomechanical solutions to evolving enhanced dexterity, including soft tissue solutions. Indeed, when we look at the fossil record of hominin hand bones, we see a variety of different shapes and sizes just in the bones of the thumb (Figure 1). Australopiths — early hominins between 4–2 million years ago — tend to have a gracile first metacarpal with limited muscle markings (entheses) and a small, curved joint with the wrist compared with that of humans or Neandertals7Tocheri M.W. Orr C.M. Jacofsky M.C. Marzke M.W. The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo.J. Anat. 2008; 212: 544-562Crossref PubMed Scopus (156) Google Scholar. In contrast, two isolated first metacarpals (2–1.8 million years old) from Swartkans, South Africa, which may belong to either Australopithecus or early Homo, have more robust metacarpal shafts and well-developed muscle markings but vary greatly in size and joint shape7Tocheri M.W. Orr C.M. Jacofsky M.C. Marzke M.W. The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo.J. Anat. 2008; 212: 544-562Crossref PubMed Scopus (156) Google Scholar. Fast forward 1.5 million years and the thumb of Homo naledi (∼250,000 years old) contrasts a robust metacarpal shaft and a flared muscle marking, with a remarkably small, curved joint with the wrist8Kivell T.L. Deane A.S. Tocheri M.W. Orr C.M. Schmid P. Hawks J. Berger L.R. Churchill S.E. The hand of Homo naledi.Nat. Commun. 2015; 6: 8431Crossref PubMed Scopus (79) Google Scholar. Karakostis and colleagues4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar aim to understand the functional significance of this morphological variation and how it relates to manipulative abilities by quantifying not just the bony morphology, but also by modelling one of the key muscles for thumb opposition, the opponens pollicis muscle. Using musculoskeletal modelling, their analysis simulates contraction of the opponens pollicis muscle to flex the thumb, taking into account the size and robustness of the muscle marking, to estimate the amount of force (torque) or ‘biomechanical efficiency’ each fossil hominin specimen might have produced when opposing their thumb4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar. In the absence of muscle preservation in the fossil record, they estimate the size of hominin muscles using a human model (large muscle, large force) and a chimpanzee model (small muscle, small force), offering two extremes of potential biomechanical efficiency. Karakostis and colleagues4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar find that, predictably, humans and Neandertals show powerful thumb opposability (i.e. high biomechanical efficiency) due to their (presumably) large opponens pollicis muscles and flared muscle markings. However, the unidentified Swartkrans specimens and, to a lesser extent, Homo naledi, also show a biomechanical efficiency of the thumb that is similar to or, in some analyses, greater than that of humans, despite having quite distinct bony morphology. In contrast, all australopiths had a less powerful thumb, being more similar to chimpanzees or at least intermediate between them and humans, suggesting they did not have the biomechanical efficiency to produce enhanced, human-like manipulative abilities, despite at least some australopith species being associated with tool behaviours9Backwell L.R. d'Errico F. Evidence of termite foraging by Swartkrans early hominids.Proc. Natl. Acad. Sci. USA. 2001; 98: 1358-1363Crossref PubMed Scopus (165) Google Scholar, 10McPherron S.P. Alemseged Z. Marean C.W. Wynn J.G. Reed D. Geraads D. Bobe R. Béarat H.A. Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia.Nature. 2010; 466: 857-860Crossref PubMed Scopus (392) Google Scholar, 11Harmand S. Lewis J.E. Feibel C.S. Lepre C.J. Sandrine P. Lenoble A. Boěs X. Quinn R.L. Brenet M. Arroyo A. et al.3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya.Nature. 2015; 521: 310-315Crossref PubMed Scopus (430) Google Scholar. Thus, powerful thumb opposition appears around two million years ago in some (the Swartkrans individuals) but not all (Australopithecus sediba) hominins. This correlates in time with archaeological evidence for an increased reliance on stone and bone tool use and animal butchery5Thompson J. Carvalho S. Marean C. Alemseged Z. Origins of the human predatory pattern: The transition to large animal exploitation by early hominins.Curr. Anthropol. 2019; 60: 1-23Crossref Scopus (38) Google Scholar. Why is this interesting? In some ways, those of us (albeit there are not many) who have spent a long time staring at fossil hominin hand bones might say that these results come as no surprise: australopiths have gracile metacarpals suggesting that they had a more limited capacity to produce forceful thumb opposition than the robust metacarpals of Homo species. However, if we look at the variation in hand bone shape and size across both Swartkrans specimens, H. naledi, Neandertals and humans (Figure 1), and then consider that this hand variation comes within the broader context of (when preserved) distinctly different skeletons, body and brain sizes, and ecological and geological contexts, we might question how many times powerful thumb opposability evolved and for which reasons. Much experimental and behavioural evidence makes clear the importance of powerful thumb opposition during tool-making and tool-use12Marzke M.W. Shackley M.S. Hominid hand use in the Pliocene and Pleistocene: evidence from experimental archaeology and comparative morphology.J. Hum. Evol. 1986; 15: 439-460Crossref Scopus (110) Google Scholar, 13Key A.J. Dunmore C.J. The evolution of the hominin thumb and the influence exerted by the non-dominant hand during stone tool production.J. Hum. Evol. 2015; 78: 60-69Crossref PubMed Scopus (38) Google Scholar, 14Williams-Hatala E.M. Hatala K.G. Gordon M. Key A.J. Kasper M. Kivell T.L. The manual pressures of stone tool behaviors and their implications for the evolution of the human hand.J. Hum. Evol. 2018; 119: 14-26Crossref PubMed Scopus (28) Google Scholar. However, if we look at the variation in type, frequency and complexity of tool use in our living primate relatives, it is clear that tool use is likely to have evolved multiple times in different species, subspecies and populations15Rolian C. Carvalho S. Tool use and manufacture in the last common ancestor of Pan and Homo.in: Muller M.N. Wrangham R.W. Pilbeam D.R. Chimpanzees and Human Evolution. Harvard University Press, Cambridge2017: 602-644Crossref Google Scholar. There is no reason to assume that enhanced manipulative ability, including powerful thumb opposition, is limited to our own genus and could not have evolved, perhaps in parallel, among other hominins as well. Indeed, the Swartkrans specimens are equally likely to belong to Homo as to a robust australopith, and both species are equally likely to have been proficient tool users two million years ago. As Karakostis and colleagues4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar highlight, there can be different combinations of both bony and soft tissue morphology to produce similar biomechanical outcomes. They have demonstrated that, when it comes to the opponens pollicis muscle, somewhat distinct hominin morphologies may have produced similarly powerful thumbs. However, the opponens pollicis is only one of ten muscles that move the human thumb, and muscles do not act in isolation16Marzke M.W. Toth N. Schick K. Reece S. Steinberg B. Hunt K. Linscheid R.L. An K.-N. EMG study of hand muscle recruitment during hard hammer percussion manufacture of Oldowan tools.Am. J. Phys. Anthropol. 1998; 105: 315-332Crossref PubMed Scopus (99) Google Scholar. Moreover, the thumb is but one anatomical region of the hand. Hominins with a seemingly less efficient opponens pollicis may have generated powerful opposition by increasing the size of other muscles within the thumb, or through compensatory bony or soft tissue adaptions in the fingers or palm. The modern human hand form is not necessarily the be-all and end-all — a critical take home message from this study4Karakostis F.A. Haeufle D. Anastopoulou I. Moraitis K. Hotz G. Tourloukis V. Harvati K. Biomechanics of the human thumb and the evolution of dexterity.Curr. Biol. 2021; 31: 1317-1325Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar. Musculoskeletal modelling may be considered risky business when half of the data required — the muscles — are not preserved and there is a far from direct relationship between muscle marking size and the size of the muscle itself17Williams-Hatala E.M. Hatala K.G. Hiles S. Rabey K.N. Morphology of muscle attachment sites in the modern human hand does not reflect muscle architecture.Sci. Rep. 2016; 6: 28353Crossref PubMed Scopus (23) Google Scholar. However, this does not mean such endeavours are doomed to failure. Historically palaeontologists have put muscles on the back burner due to the lack of direct evidence, but muscles are obviously integral to the function of the skeleton and to reconstructing behaviour in the past. As we build complexity into these models, adding more muscles, improving our understanding of the relationship between soft tissues and their bony landmarks, and incorporating bone shape, we gain a more holistic understanding of the range of potential biomechanical solutions for hominin dexterity." @default.
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• W3136356230 title "Human evolution: Thumbs up for efficiency" @default.
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