Parasitic Lice Help to Fill in the Gaps of Early Hominid History

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

“…A divergence date of 5–7 Ma for the human head louse and chimpanzee body louse and a divergence date of 3–4 Ma for the human pubic louse and gorilla body louse support the conclusion that hominin body hair was lost between 5–7 Ma and 3–4 Ma [ 151 ]. This early time frame is consistent with the view that hair loss occurred when hominins still occupied fragmented forests (also the most likely location for contact with gorillas [ 151 ]) [ 28 ] and supports the tick-based hypothesis.…”

“…Hair is an epidermal appendage that has several important functions, including insulating the body from heat loss and protecting the skin from ultraviolet radiation [ 150 ]. Given these important benefits, humans are unique among primates due to their relative lack of body hair [ 40 , 151 ]. Humans and chimpanzees have a similar number of hair follicles [ 152 ]; however, while chimpanzee body hairs are long, thick and pigmented (i.e., terminal hairs), human body hairs are short, thin and transparent (i.e., vellus hairs) [ 153 ], providing the human body with little insulation or protection [ 151 ].…”

“…Currently, indirect evidence from a variety of sources provides only broad, general estimates for the timing of hair loss [ 151 , 170 ]. For example, following the loss of hair, skin pigmentation likely increased to protect the skin against ultraviolet radiation [ 167 ].…”

“…Taxonomists have long recognized that humans serve as hosts to two different genera of lice ( Pediculus and Pthirus ) [ 172 ], with the human head louse ( Pediculus humanus capitis ) related to the chimpanzee body louse ( Pediculus schaeffi ) and the human pubic louse ( Pthirus pubis ) related to the gorilla body louse ( Pthirus gorillae ), although no explanation was offered for this interesting observation [ 151 ] [the human clothing louse, Pediculus humanus humanus , diverged from P. humanus capitis much more recently (between 170,000 to 83,000 years ago) and is not part of the story [ 168 ]]. Recently, Reed , et al [ 173 ] recognized that divergence dates for the different species of lice could be used to date the evolution of hominin hair loss based on the presumption that the LCA body louse became confined to the hominin head due to hair loss over the rest of the body and that the subsequent evolution of a visual patch of pubic hair for sexual signalling created a novel ecological niche that allowed the gorilla body louse to move to hominins [ 151 ]. A divergence date of 5–7 Ma for the human head louse and chimpanzee body louse and a divergence date of 3–4 Ma for the human pubic louse and gorilla body louse support the conclusion that hominin body hair was lost between 5–7 Ma and 3–4 Ma [ 151 ].…”

“…Recently, Reed , et al [ 173 ] recognized that divergence dates for the different species of lice could be used to date the evolution of hominin hair loss based on the presumption that the LCA body louse became confined to the hominin head due to hair loss over the rest of the body and that the subsequent evolution of a visual patch of pubic hair for sexual signalling created a novel ecological niche that allowed the gorilla body louse to move to hominins [ 151 ]. A divergence date of 5–7 Ma for the human head louse and chimpanzee body louse and a divergence date of 3–4 Ma for the human pubic louse and gorilla body louse support the conclusion that hominin body hair was lost between 5–7 Ma and 3–4 Ma [ 151 ]. This early time frame is consistent with the view that hair loss occurred when hominins still occupied fragmented forests (also the most likely location for contact with gorillas [ 151 ]) [ 28 ] and supports the tick-based hypothesis.…”