A. Encephalization is defined as the amount of brain mass related to an animal's total body mass. Quantifying an animal's encephalization has been argued to be directly related to that animal's level of intelligence. Aristotle wrote in 335 B.C. "Of all the animals, man has the brain largest in proportion to his size." Also, in 1871, Charles Darwin wrote in his book The Descent of Man: "No one, I presume, doubts that the large proportion which the size of man's brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his mental powers."
In 2004, Dennis Bramble and Daniel Lieberman proposed that early Homo were scavengers that used stone tools to harvest meat off carcasses and to open bones. They proposed that humans specialized in long-distance running to compete with other scavengers in reaching carcasses. It has been suggested that such an adaptation ensured a food supply that made large brains possible.
More encephalized species tend to have longer spinal shock duration.
B. Encephalization may also refer to the tendency for a species to evolve larger brains through time. Anthropological studies indicate that bipedalism preceded encephalization in the human evolutionary lineage after divergence from the chimpanzee lineage. Compared to the chimpanzee brain, the human brain is larger and certain brain regions have been particularly altered during human evolution. Most brain growth of chimpanzees happens before birth while most human brain growth happens after birth.
In Snell's equation of simple allometry "E" is the weight of the brain, "C" is the cephalization factor and "S" is body weight and "r" is the exponential constant. The exponential constant for primates is 0.28 and either 0.56 or 0.66 for mammals in general.
The "Encephalization Quotient" (EQ) is the ratio of "C" over the expected value for "C" of an animal of given weight "S".
Evolution of the EQ
The evolution of the EQ shows a close correlation with the evolution of the diversity of life generally. During the Paleozoic the EQ generally increased throughout the period, peaking in the late Carboniferous and early Permian. However, the Permian - Triassic mega-extinction events 251 million years ago reversed this trend. Occurring through the probable release of oceanic methane clathrates, and the burning of coal from the Siberian volcanic basalt traps, it saw the elimination of 96% of species, and slowed the rate of EQ development, such that only by the end of the Cretaceous Period had the EQ recovered to its earlier level, with the appearance of the dromaeosaurids and troodontids. A second K-Pg (Cretaceous-Paleogene) extinction event of 66 million years ago, with the extinction of the non avian dinosaurs, ammonites and many other creatures this time saw 78% of species become extinct. These events had a huge effect in setting back the further evolution of the EQ. In the case of humans, the rise in encephalisation from Australopithecus to Homo neanderthalensis, with a brain capacity of 480cc to 1450cc, over 1.5 million years, is an equivalent to an increase of 125,000 additional brain cells per generation. Such an increase without a great increase in technology suggests that it was fueled by a social and not a technological imperative.
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