Brain and body size tendencies in hominid evolution

A logistic equation is proposed to describe the brain size, body mass, and encephalization coefficient rates and tendencies throughout hominid evolution. This model gives acceptable results for body mass, better for encephalization coefficient, and very good for brain size. Two factors seem to explain this different correlation. First, brain volume is a measured value, whereas body size are an estimated data. Second, hominids and pongids have different evolutionary tendencies in body mass, but similar tendencies in brain size. The equations show three phases throughout human evolution.: firstly a slow encephalization rate for early hominids; secondly, a strong increase from Australopithecus until Homo erectus; and thirdly, a tendency to minimize the rate in Homo neanderthalensis and H. Sapiens. During these three phases, according to the model, brain volume began to inccrease clearly about 4 m.y. Ago, but body mass did not. In turn, body mass inccrease slowly throughout human evolution. On the other hand, the equation provides an estimation of the values of brain volume, body mass, and encephalization coefficient for the early hominids from five to six million years ago. It indicates a brain volume of about 208 cm3, a body mass of about 37 kg, and an encephalization coefficient of about 5.6 cm3/kg. These results are low because we used a specimen of Oreopithecus bambolii an early pogidean of low brain volume. Alternatively, Dryopithecus, with a higher brain volume, would be a more suitable candidate, but available data on this form are scarce.