Using multivariate calibration to evaluate hominin brain/body size relationships

Brain size evolution;body size evolution;multivariate calibration

Uhl, Natalie

关键词: Brain size evolution;    body size evolution;    multivariate calibration;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49577/Natalie_Uhl.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Modern humans are highly encephalized, having relatively large brains despite our already large bodies. The current assumption is that brains and bodies have both increased in size during the course of human evolution but the rate, timing, and evolutionary relationship of the two are still unclear. This study uses methods that increase the sample size of fossil hominins, provide more confidence in body mass estimates, and allow a comparison of the rate and timing of changes in each trait over the past 4+ million years (mya). Brains and bodies show different evolutionary patterns over time. Body size actually shows a decrease in the Pliocene while brains are essentially static. This would have resulted in more encephalized hominins. At the beginning of the Pleistocene (ca. 2.6 mya) body size began to increase and brains began a modest but discernible increase. Body size became static about 1.0 mya and brain size increased sharply around 0.4 mya (close to the appearance of the first Homo sapiens). These patterns indicate that both traits were subject to differing selection (directional or stabilizing) or genetic drift at different times. Their genetic covariation is low; this trait is also subject to selection so the amount of covariation can change throughout time. Taken together, these results suggest a complicated relationship between brain size and body size. Both traits were probably experiencing some direct selection, while also susceptible to indirect selection from the other based on their covariation. The dissimilarities in the change of body size and brain size indicate that their covariation was actively changing during the course of human evolution. Other research indicates that humans have less phenotypic integration than great apes. For example, the transition to bipedalism may have been eased by lowering covariation within the hip, thus reducing evolutionary constraint. This pattern of ``evolvability" could extend to more general modules or groups of traits, like the skull and brain size, or skeletal and body size. Changes in selection on brain size and body size would have been rooted in environmental and life history changes; many life history changes accompany brain size and body size changes. If we examine the analyses of the entire sample, Australopithecines were experiencing decreasing body size and relative stasis in brain size, producing an overall result of encephalization. As brains got relatively larger compared to bodies it was inevitable that at some point either brains would have to get smaller along with bodies or bodies would have to start getting larger to birth large-brained offspring. After 2.6 mya the selection for bigger brains could be responsible for driving larger bodies, but brains show positive allometry relative to bodies. These changing phenotypic relationships suggest changing genetic relationships, indicating a different pattern in integration that began to emerge.There is still much to learn and understand about the evolution of human brains and bodies. These analyses lend some clarity to the rate and timing of evolution of both brain size and body size over time, but the trends are not directly comparable because body size changes linearly over time while brain size shows an exponential increase. Results support previous findings in many ways, including probable changes in energy allocation to support a large brain, an increased rate of evolution in more recent hominins, and changing phenotypic integration. New results include body size decreasing and brain size stasis in the Pliocene, as well as static body size and increasing brains in the Pleistocene.