Hair for brain trade-off, a metabolic bypass for encephalization

Abstract

Hair loss in humans is perplexing and raises many hypothetical explanations. This paper suggests that hair loss in humans is metabolically related to encephalization; and that hair covered hominids would have been unable to evolve large brains because of a dietary restriction of several amino acids which are essential for hair and brain development. We use simulations to imply that hair loss must have preceded increase in brain size & volume. In this respect we see hair loss as a major force in human evolution. We assume that hair reduction required favorable climatic conditions and must have been quick. Using evolutionary and ecological time scales, we pinpoint hair loss to a period around 2.2-2.4 million years ago. The dating is further supported by a rapid selection at that time of the sialic acid deletion mutation which may have protected growing human brains against calcium ion flux. In summary we view encephalization, in part, as a metabolic trade-off between hair and brain. Other biochemical changes may have intervened in the process too; and the deletion mutation of sialic acid hydroxylation may have been involved as well.

Ticks, Hair Loss, and Non-Clinging Babies: A Novel Tick-Based Hypothesis for the Evolutionary Divergence of Humans and Chimpanzees

“…Alternatively, studies of hominin thermoregulation suggest that hair loss must have occurred prior to or at the time of living in more open environments (~2.0 Ma) to enable heat loss by sweating [ 16 ]. Finally, research indicates that hair loss must have preceded hominin encephalization (~2.2–2.4 Ma) based on the dietary restriction of several amino acids that are required for both hair and brain development [ 171 ]. These time ranges are not helpful in determining the date of hominin hair loss, however, since a date prior to ~2.4 Ma is still consistent with either an early (~5–8 Ma) or late (~2.4 Ma) date.…”

The Evolution of Mammalian Adipose Tissues

“…The extensive vasculature and its neural control combined with reduced pigmentation enable the skin to acquire roles in social and sexual signalling, as it has in other higher primates (Street et al 2016). From about 2.3 My ago, visibility of such indicators of social and sexual status was enhanced by reduction in body hair (Dror and Hopp 2014), contrasted against denser hair on the head and external genitalia. Exposed skin enabled sex and ethnic differences and age-related changes in adipose tissue distribution to evolve under sexual as well as natural selection (Pond 1998).…”