Our own outsized brain, on this account, traces back to ancestors that got more out of what they could see. The \u201cthinking\u201d frontal lobe never swelled on its own.<\/p>\n Brains do not fossilize<\/p>\n The neocortex \u2013 the outer, folded layer that handles sensory perception and complex thought \u2013 is greatly enlarged in primates compared with other mammals.<\/p>\n Exactly how it grew so large over the past 56 million years has been hard to pin down. Brain tissue does not fossilize, so the thing that changed left no direct record.<\/p>\n The team worked around that with skulls, many of them from the Duke Lemur Center<\/a> Museum of Natural History.<\/p>\n High resolution micro-CT scans at the Shared Materials Instrumentation Facility<\/a> captured the empty space inside each braincase. The models were then rebuilt digitally in 3D.<\/p>\n Those virtual casts, called endocasts, stand in for the brain itself. Researchers can measure their volume and surface area much as they would a living brain.<\/p>\n Comparing them let the researchers track how parts of the neocortex changed across living and extinct primates, from lemurs to monkeys to us.<\/p>\n Frontal lobe kept pace<\/p>\n The frontal lobe is the region popular accounts tie to reasoning and planning. It has long been assumed to have ballooned on its own, over and over, in separate primate lineages.<\/p>\n The endocasts say otherwise. Frontal lobe size grew gradually and predictably as overall brain size<\/a> rose.<\/p>\n One scaling pattern held across every major primate group. There was no sign of the dramatic, independent expansions earlier studies had proposed from the look of a fossil.<\/p>\n The study was led by Richard F. Kay, a professor of evolutionary anthropology at Duke. <\/p>\n \u201cEverything from humans down to tree shrews, they all fall on the same line: Relative to the size of the brain, the proportion of the frontal lobe is a constant,\u201d noted Professor Kay.<\/p>\n Vision shaped primate brain evolution<\/p>\n The real change sat behind the frontal lobe. The occipital, parietal and temporal regions carry most of the work of seeing, and they expanded fast and out of proportion.<\/p>\n That growth shows up in tarsiers and in anthropoids, the group that holds monkeys, apes and humans. It marks the branches where visual processing<\/a> took over the neocortex.<\/p>\n Those same branches are where the optic nerve grew larger. More visual information was flowing into the brain.<\/p>\n \u201cFossil brains have been frustratingly silent on this question for a long time,\u201d said Professor Kay. <\/p>\n \u201cWhen we let the fossils speak quantitatively rather than relying on impressions of their shape, the enlarged brains of monkeys, apes and humans turn out to be tied to vision much more than to the frontal lobe, which simply kept pace with overall brain size.\u201d<\/p>\n Optic nerve as a clue<\/p>\n Nerves do not fossilize either. The team used the optic foramen \u2013 the bony opening the optic nerve passes through \u2013 as a stand-in for how much visual traffic a skull once carried.<\/p>\n Tarsiers and anthropoids have the largest and most vision-dominated neocortices of any primates. They also had the largest optic foramina.<\/p>\n The visual regions of the brain appear to have grown even faster than the nerve feeding them. Small increases in visual input were magnified into large expansions of brain tissue.<\/p>\n That is a different picture of primate evolution<\/a> than the familiar one, in which a lineage gets smarter and the brain follows.<\/p>\n \u201cTypically, the story of primate (and human) evolution places emphasis on increased relative size of the frontal regions of the neocortex compared with other parts of the neocortex and brain size,\u201d Professor Kay told Earth.com.<\/p>\n \u201cThe received \u2018story\u2019 is that frontal lobe increases are associated with improvements in \u2018executive functions.\u2019 Our study suggests that any proportional increases in the size of the frontal lobe are a shared feature of increased brain size in all primates, not a novel feature of the human brain as distinct from the brains of other primates.\u201d<\/p>\n Why sharper eyes mattered<\/p>\n Why primates evolved to process so much more visual information remains an open question.<\/p>\n \u201cWhat are those visual signals for? One idea is the increased complexity of social communication. Another possibility is that their driving force is related to how efficiently these animals can forage for food,\u201d noted Professor Kay.<\/p>\n \u201cEach of those, I would imagine, could have a very strong impact on natural selection and primate evolution.\u201d<\/p>\n Either pressure would reward eyes that resolve fine detail. Anthropoids carry a retinal fovea, the pit of densely packed receptors behind sharp central vision.<\/p>\n They also carry a bony partition that shields the eye. Both features point to a lineage invested in high-acuity sight.<\/p>\n The pattern is old, too. Anthropoid brains have looked like this for at least 33 million years, long before anything resembling an ape.<\/p>\n Limitations of the study<\/p>\n An endocast records the outside of a brain, not its wiring. Boundaries between regions are read from impressions left on bone, and the soft tissue detail is long gone.<\/p>\n The optic foramen is a proxy as well. It tracks the room the nerve needed, not the number of fibers that ran through it.<\/p>\n \u201cHere is a caveat: As paleontologists, we have only the surfaces and shapes of fossil brains to work with. So many other changes in brain wiring must have occurred that fossil brain shape and size cannot capture,\u201d Professor Kay told Earth.com.<\/p>\n \u201cAll our inferences are contingent on what is known about the brains of living primates and their close relatives.\u201d<\/p>\n The two possible drivers, social life and foraging, remain hypotheses. Fossil skulls cannot reveal which pressure drove the change, or whether both played a role.<\/p>\n What the fossils do settle is where to look. Across 56 million years of primate<\/a> history, the brain grew mostly by seeing more.<\/p>\n The lobe we credit for thinking simply grew at the rate the rest of the brain set.<\/p>\n The study is published in the journal Science<\/a>.<\/p>\n Image Credit: Kay et al.\/MorphoSource<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Brains rot within days. The bony case that held them can last 33 million years, which is why…\n","protected":false},"author":2,"featured_media":848292,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[64,63,128],"class_list":["post-848291","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-au","tag-australia","tag-science"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/posts\/848291","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/comments?post=848291"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/posts\/848291\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/media\/848292"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/media?parent=848291"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/categories?post=848291"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/au\/wp-json\/wp\/v2\/tags?post=848291"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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