In the newly published study, the authors report that they found \u201ca significant relationship between relatively longer thumbs\u2014a key feature of precision grasping\u2014and larger brains across 95 fossil and extant primates\u2026\u201d Furthermore,\u00a0<\/p>\n
Most hominins [modern humans and their evolutionary ancestors], including\u00a0Homo sapiens, have uniquely long thumbs, yet they and other tool-using primates conform to the broader primate relationship with brain size. Within the brain, we surprisingly find no link with cerebellum size, but a strong relationship with neocortex size, perhaps reflecting the role of motor and parietal cortices in sensorimotor skills associated with fine manipulation.<\/p>\n
Enhanced manual dexterity is key to the refined control necessary for stone tool production, a key technological adaptation by the human lineage. Although the authors recognize that numerous evolutionary modifications of the human hand contribute to increased dexterity, \u201can increased ability to manipulate small objects is enhanced by long thumbs\u2014particularly relative to the index finger. Longer relative thumbs facilitate greater opposability\u2014and\u00a0Homo sapiens\u00a0is noted to have both longer thumbs compared with other apes as well as enhanced manipulative ability.\u201d Opposability refers to the ability to bring the tip of the thumb into contact with the tips of the other fingers of the hand, allowing a high degree of control over objects being held.\u00a0<\/p>\n
However, although other primates possess varying degrees of thumb opposability and exhibit some degree of\u00a0tool use<\/a>, only humans have the highly refined precision grip which allows them to perform the uniquely refined and delicate manipulations necessary for stone tool production. The authors propose that this is only possible based on a highly developed sensorimotor control originating in the brain, the result of co-evolution between the two. They hypothesize that this difference \u201cmay explain some of the marked variation in relative brain size among primates and the trend for this to increase through time.\u201d\u00a0<\/p>\n The authors propose to test this along with two alternative possible explanations regarding the observed variation in thumb length verses brain size. If long thumbs are specifically the result of hominin tool manufacture, then they would expect the relationship between thumb length and brain size only among that group (i.e., not among other primates). On the other hand, if there is in fact no correlation between thumb length and brain size, then some other factors must be considered as responsible for each independently.\u00a0<\/p>\n These alternate hypotheses were tested by statistical analyses using data from 95 fossil and contemporary primate species spanning the full range of primate diversity. <\/p>\n \u00b7 \u00a0The first hypothesis posited that \u201c[i]f fine manipulative abilities require enhanced sensorimotor control with an associated neural processing cost [enlarged brain], then we would expect to see a general co-evolutionary relationship between thumb length and brain size across the primate order.\u201d In other words, there would be a consistent degree of evolutionary correlation between brain size and thumb length across primates in general.\u00a0<\/p>\n \u00b7 \u00a0The second hypothesis was that a correlation between brain size and thumb length exists only among hominins and\/or tool users and not in other primates.\u00a0<\/p>\n \u00b7 \u00a0Thirdly, \u201c\u2026it is possible that brain size and thumb length coevolved across all primates, but there is a shift in the intercept [a qualitatively greater degree] of the relationship between hominins and\/or other tool-using species, which might be the case if there was some reorganization of the neuro-behavioural basis of manipulation.\u201d<\/p>\n Based on statistical analysis it was found that:<\/p>\n 1. \u00a0\u201cAs expected, hominin thumbs are significantly longer than those of other primates. The general primate-wide relationship predicts hominin species to have much shorter thumbs than are actually observed.\u201d<\/p>\n 2. \u00a0The researchers found that there is a \u201csignificantly positive relationship between relative thumb length and brain size \u2026 as well as thumb length and finger length \u2026 across all non-hominin primates.\u201d In other words, the correlation between thumb length and brain size is not exclusive to hominins.\u00a0<\/p>\n 3. \u00a0A significant correlation was also found between thumb length and documented tool use in both hominin and non-hominin primates (e.g., chimpanzees).\u00a0<\/p>\n Among the results, the researchers concluded that there is a strong association between brain size and manual dexterity. However, they go on to caution that although thumb length is only one factor in dexterity, other features also contribute. These include \u201c(among others) relative proportions and morphology of other digits, bone shape and structure and bone traits associated with soft tissues such as muscle attachment sites.\u201d\u00a0<\/p>\n An additional, analysis was conducted examining manual \u201cworkspace,\u201d \u201c\u2026 the range of motion a small object can be freely moved between the thumb and index finger\u2014and thumb length.\u201d The study found that, \u201c\u2026thumb length significantly predicts peak manipulation workspace.\u201d Furthermore, analysis found that, \u201c\u2026brain size is a significant predictor of peak workspace.\u201d\u00a0<\/p>\n The researchers concluded that, \u201con the basis of both our thumb length and workspace analyses, we interpret our results to indicate sustained historical coevolution between brain size and dexterity across the primate order, reflecting significant neural costs of manipulation behaviours and helping to explain the rapid increases in brain size observed in hominins.\u201d Enhanced workspace would be a notable advantage in both tool manufacture and use.\u00a0<\/p>\n Finally, \u201c\u2026our results demonstrate coordinated change in both hands and brains and therefore confirm\u2026\u201d the initial hypothesis.\u00a0<\/p>\n The authors then refer to \u201c[r]ecent evidence [that] has even demonstrated possible functional and anatomical overlap in brain activation patterns involved in both tool-use and language processing.\u201d Other researchers, including Charles Darwin, have explored this\u00a0correlation<\/a>.<\/p>\n This study provides an important contribution to our understanding of the complex, dialectical process of human evolution. The proposed relationship between tool-use and language supports the insight proposed by Engels a century and a half ago that the \u201cexplanation of the origin of language from and in the process of labor is the only correct one.\u201d\u00a0<\/p>\n Sign up for the WSWS email newsletter<\/p>\n","protected":false},"excerpt":{"rendered":"A century and a half ago, Frederick Engels, in an unpublished 1876 essay titled,\u00a0The Part Played by Labor…\n","protected":false},"author":2,"featured_media":36264,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[164,29821,29822,5351,29823,85,46,26783,141],"class_list":{"0":"post-36263","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-brain","9":"tag-dexterity","10":"tag-engels","11":"tag-evolution","12":"tag-hominins","13":"tag-il","14":"tag-israel","15":"tag-primates","16":"tag-science"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/36263","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/comments?post=36263"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/36263\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media\/36264"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media?parent=36263"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/categories?post=36263"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/tags?post=36263"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Example of hand held stone tool production [Photo by Jos\u00e9-Manuel Benito \u00c1lvarez \/ CC BY-SA 2.5<\/a>]<\/p>\n
<\/a><\/p>\n