Modern comparative neuroanatomy linear chain of being

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Modern Comparative Neuroanatomy linear chain of being does not represent the evolutionary process. Larger brains (or brain regions) have fitness costs (brain tissue is energetically costly) and benefits (more/faster computation). Trade-offs between costs and benefits determine how brain evolve. Reduction in size and/or complexity of brain/sensory structures is possible (and has happened many times), depending on selective pressures experienced by the lineage. ex) deuterostomes w/ secondarily evolved radial symmetry – and loss of cephalized brain Brain composition is not tied directly to brain size (mosaic brain evolution is possible) Brain scaling Body size explains much of the variation in brain size in animals positive correlation – bigger body size = bigger brain size But brain size does not increase proportionately with body size Thus larger animals tend to have proportionally smaller brains Brain composition also evolves October 10, 2012: Primate and Human Brain Evolution Primates have larger brains, with more neurons per unit size, than similarly sized mammals Humans are the largest-brained primates – by a lot When did human brains get so big? Hominid cranial capacity can be estimated from fossils Are human brains unusually large? Human brains are very big for an animal of our size Humans do not have the biggest brain as a proportion of body weight Humans have the biggest brains in terms of what would be predicted from our body size Why did big brains and big neocortices evolve in primates, and especially in humans? (ultimate causation) Big brains are good for planning, dealing with uncertainty, and coping with changing conditions – behavioral plasticity Ecological (climate, resources/hunting, predators) Social (interactions with other group members) Ecological hypotheses - Fruit eating primates have bigger brains than leaf eating primates – big brains help remember locations and timing of ephemerally ripening fruits. Primates with larger home ranges have larger brains – big brains improve spatial memory generally Primates that have “extractive foragers” (e.g. cracking nuts, opening clams, etc.) have bigger brains – big brains improve ability to perform complex, multi-step behaviors Most of these trends are actually more strongly related to body size or group size. There are lots of animals that exploit large territories and ephemeral or difficult to harvest food resources; why are brains so bing in primates in particular Social brain hypothesis (SBH) – brains facilitate social interactions – larger groups select for larger brains. The SBH seems to hold for some taxa but not others (carnivores), suggesting increasing group size alone does not select for large brains/larger neocortices.
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