Why did Homo sapiens develop a large Brain?

Published in Human Evolution – Vol. 31 – n.4 – 2016

DOI: 10.14673/HE201641024
KEY WORDS: brain size, bipedalism, gravity, blood supply, nutrition.

Abstract
Bipedalism and a high encephalization quotient are unique characteristics of the human species. It is plausible that these characteristics are connected through the evolutionary process of the homo genus and may have influenced each other’s de-velopment. The connection between bipedalism and a high encephalization quotient may have been conferred through to gravity’s effect on the blood supply to the brain, both in utero and in the first year of life.
The enlarged human brain initiates at birth whereby the neona-tal brain weighs 350-400g compared to P. troclodytes (chimpanzee) neonatal brain weight of 155g. After a progressive reduction in breech presentation throughout pregnancy, more than 97% of human fetuses present cephalically at the end of pregnancy. Adverse outcomes for the fetus are known to occur for breech presentation, prematurity and post-dates delivery. The appropriately adjusted gestational age in the homo genus, possibly under evolutionary pressures, encouraged cephalic presentation. Gravity would have assisted blood supply, nutri-tion and cerebral metabolism of the growing brain. Another ob-stetric surrogate is that both body weight and brain volume in multiple pregnancies are significantly larger in the lower, first born twin, compared to the higher second born twin.
The gravitational effect of brain blood supply persists beyond birth. Human babies only become fully bipedal at the age of 1-1.3 years. During the first year the greatest growth in brain weight is registered when it increases to 900g-1kg. The combi-nation of Obstetric and Paediatric surrogates suggest that grav-ity’s influence, through the evolution of human bipedalism, on blood supply may be responsible for the high encephalization quotient in the Homo sapiens species.

 

Muscat Baron, Y.
MD FRCOG, FRCPI, PhD Consultant Obstetrician and Gynaecologist
Department of Obstetrics and Gynaecology, Mater Dei Hospital, University of Malta, Malta.
E-mail: yambaron@go.net.mt