title: 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.
authors.affiliation: Biochemistry and Human Nutrition, Faculty of Agriculture, The Hebrew University, 76100 Rehovot, Israel.
authors.forename: Yosef
authors.initials: Y
authors.lastname: Dror
authors.affiliation: Department of Geography, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Michael
authors.initials: M
authors.lastname: Hopp
keywords: Brain; Encephalization; Hair; Keratin composition; Methylation; Sialic acid hydroxylation