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

Abstract

Human straight-legged bipedalism represents one of the earliest events in the evolutionary split between humans (Homo spp.) and chimpanzees (Pan spp.), although its selective basis is a mystery. A carrying-related hypothesis has recently been proposed in which hair loss within the hominin lineage resulted in the inability of babies to cling to their mothers, requiring mothers to walk upright to carry their babies. However, a question remains for this model: what drove the hair loss that resulted in upright walking? Observers since Darwin have suggested that hair loss in humans may represent an evolutionary strategy for defence against ticks. The aim of this review is to propose and evaluate a novel tick-based evolutionary hypothesis wherein forest fragmentation in hominin paleoenvironments created conditions that were favourable for tick proliferation, selecting for hair loss in hominins and grooming behaviour in chimpanzees as divergent anti-tick strategies. It is argued that these divergent anti-tick strategies resulted in different methods for carrying babies, driving the locomotor divergence of humans and chimpanzees.

Splitting Hairs: Evaluating the Origin of Human Nakedness

“…It is clear, though, that there are certainly advantages to being hairless. Sweating efficiency (Wheeler, 1984) and the ability to see and minimize parasitic infection (Allen et al, 2013;Boutellis et al, 2014;Brown, 2021;Reed et al, 2004) are some of the most obvious advantages, though whether those advantages affect the evolution of hair loss is another matter. Still, Darwin's assertion is probably incorrect.…”

“…Fur can provide a habitable environment for pests like fleas, ticks, and lice, the coverage going so far as to protect their eggs, which could exacerbate an infestation (Dhugga et al, 2014). Some of these parasites are known to carry diseases, especially ticks, which could potentially serve as a massive force of natural selection on hominins (Brown, 2021). There are some problems with this hypothesis, however.…”

“…One variation of the ectoparasite hypothesis proposed by Brown (2021) has suggested that ticks, grounddwelling arachnids that reside around the edges of forested areas, might have been the primary catalyst for hair loss, as they provided a way for the last common ancestor (LCA) between humans and chimpanzees to more easily detect the presence of ticks on the body. The LCA, according to this hypothesis, would have been mainly arboreal and gibbon-like, walking bipedally on the ground when it was not in the trees.…”

“…This fragmentation increased forest edge habitats and, as a result, tick populations may have bloomed. The LCA would then have been forced to walk between these patches of forest in the search of food and forced to interact with tick populations (Brown, 2021). Tick-borne diseases may have put a significant pressure on LCA populations, favoring less hairy individuals who would better be able to see ticks and rid themselves of these parasites before disease transmission.…”

“…This evolutionary scenario would serve as the splitting point between chimpanzees-who would increase grooming habits to combat ticks, allowing them to feed from canopies and travel along the ground utilizing their independently evolved knuckle-walking-and hominins. This evolutionary situation, whereby ticks act as the impetus for hair loss which affected LCA's locomotor repertoire, forcing them to become obligate bipeds, positions the occurrence of hair loss around the same time as the emergence of the hominin clade 5 to 8 million years ago (Brown, 2021).…”