Evolution and humans: big brain not explained (Evolution)

by David Turell @, Saturday, February 18, 2017, 01:28 (276 days ago) @ David Turell

This essay describes the dilemma of the big brain from a Darwinian standpoint. There are both an anatomical problem and an intelligent parent problem:


"Human newborns are helpless. Without adult care, human infants would not stand a chance. They are unable to locomote on their own, or to eat anything beyond the most limited of diets. There are species whose young exhibit adult behavioral characteristics virtually from birth. The horse is an example. If the horse is not helpless at birth, neither is he particularly smart—nor does he get very much smarter. With human infants, it is the other way around, a fact that requires an evolutionary explanation.

"The emergence of human intelligence is in part attributable to an increase in their brain volume, which is thought to have roughly tripled over the last 2.5 million years. At 1,300cc, human brains are enormous in comparison to those of chimpanzees and gorillas. Brain size relative to total body size has also increased. The human brain constitutes 2% of total adult body weight, and consumes 20–25% of basal metabolism. It is also three times larger than would otherwise be expected for a primate of human body size.

"For at least thirty years, the standard solution to the riddle of human neonate helplessness has been the obstetrical dilemma.


"Because our ancestors from the time of Australopithecus walked on two legs, their pelvises had to remain narrow to preserve mechanically efficient movement. But their larger brains required an increasingly wide birth canal. These competing selective pressures resulted in an evolutionary trade-off; the human pelvis became as wide as bipedalism permitted. Since intelligence was under selective pressure, the most straightforward trade-off required human beings to give birth when newborn crania were still relatively small, and their brains relatively underdeveloped.
Hence the helpless, or altricial, human newborn.


" the increasing helplessness of human newborns itself exerts a selective force because it requires intelligent parents.10 This requires that the adults have bigger brains, which was the reason for earlier birth in the first place.


"Piantadosi and Kidd’s theory relies on three assumptions: that infants do better with more intelligent parents; that intelligent parents require large brains; and that there is a connection between large brains in adults and helplessness in newborns. The third assumption is in question, if only because the second assumption is in question.


"Our ancestors certainly did not experience a reproductive advantage because of their ability to do algebra or play the piano.11 The Darwinian environment, in enlarging their brains, somehow granted our ancestors capacities that they would not require, and could not use, for millions of years. Stephen Jay Gould and Elisabeth Vrba have called such grafts exaptations.


"If we take science at face value, as scientists often do, we are again left with a strong evolutionary suggestion that selection should not have favored a species capable of creating such powerful theories as quantum mechanics or general relativity.


" Even if the theory applies to the evolutionary history of mankind, the most that can be said about our curious predicament is that it is a matter of historical accident. If these consequences of the obstetrical dilemma are ambiguous, its fundamental premises are implausible. It is by no means clear that brain size correlates with intelligence. Human brains are dwarfed by those of whales, dolphins, and elephants;


"The most reasonable cerebral measure seems to be the total number of neurons, which can differ between brains of the same size due to differing neuron-packing densities. Because of their relatively large cortices, small neurons, and high packing densities, primates have more neurons than expected given their absolute brain size.28 Human beings in particular have more neurons than any other species—about 15 billion cortical neurons and about 100 billion neurons overall.

"No measure of brain size quite explains the variations in primate intelligence."

Comment: The entire article struggles to explain by Darwin theories the chicken/egg problem of enlarging big brain and pelvic change coordinating in evolution and that is topped by the problem of developing intelligent parents so they can care for helpless kids! You can't have one or the other. All of it had to happen at once, by saltation, not Darwin.

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