Encephalization is defined as the amount of brain mass exceeding that 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. In fact, as early as 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 orangutan, is closely connected with his mental powers."
Williams has recently argued that the most accurate means for quantifying the encephalization of humans and other adult primate species requires the use of Lapicque's universal exponent of 0.28 in Snell's equation of simple allometry. Since Lapicque's slope was derived from various vertebrate groups, this equation may potentially be universally applicable for determining relative adult vertebrate encephalization and intelligence.
Encephalization may also refer to the tendency for a species toward larger brains through evolutionary 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 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.
- ^ See Figures 1 and 2 of "Molecular insights into human brain evolution." by J. Bradbury in PLoS Biology (2005) volume 3 page e50. (Full text online)
- ^ "Endurance running and the evolution of Homo" by D. M. Bramble and D. E. Lieberman in Nature (2004) volume 432 pages 345-352. Entrez PubMed 15549097
- Williams MF., Primate encephalization and intelligence. Med Hypotheses. 2002 Apr;58(4):284-90.
- Jerison H. J. Paleoneurology and the evolution of the mind. Scientific American 1976; 234: 90-101.
- Tobias P. V. The Brain in Hominid Evolution. New York and London: Columbia University Press, 1971.