The evolution of head lice is closely tied to human evolution. Around 13 million years ago, specific species of lice began to evolve to affect only humans. About 42,000 to 72,000 years ago, human lice separated into head and body lice *. This separation is believed to coincide with the time when humans began wearing clothing [1]. While head lice remained on the scalp, body lice mutated into parasites with claws that can grab onto the smoother fibers of clothing rather than needle-thin hair shafts [1]. * Special note about human body lice: Pubic lice are another form of human body lice. We will include them in discussing all types of human lice, but not in this article as they are believed to have originated from gorilla lice more than 3 million years ago acquired through direct contact with gorillas, and have always just affected the human pubic area. They are not part of the evolution of the types of lice living only on human heads and in their clothing.

Head lice are obligate parasites, meaning they cannot survive without a human host [2]. They are specifically adapted to live on human hosts and cannot be contracted from pets or other animals [2][3]. This is because head lice have evolved alongside humans for millions of years [4][5][7].

The genetic diversity of lice also mirrors human migration patterns. For instance, a genetic analysis of lice from around the world revealed a clear split between lice originating from Asia and those from Europe [4][5][6]. This suggests that lice traveled with humans to the Americas at least twice — once during the first wave of human migration across the Bering Strait, and again during European colonization [4][5].

This is the reason why head lice are only found in humans and not in pets is due to their long co-evolutionary history with humans, which has resulted in them being specifically adapted to live on human hosts. This co-evolution has also led to the genetic diversity of lice mirroring human migration and evolution.

Head Lice Lack of Mobility Links them to Humans as Evolutionary Partners

Head lice cannot fly or jump [9][10]. Head lice spread by crawling from person to person during close contact [9]. Their short, stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces [10]. So, the transmission of head lice primarily occurs through direct head-to-head contact with an infested individual. They can also spread by sharing personal items like combs, brushes, or towels with the infested person [8]. This limitation in their mobility has tied them closely to their human hosts, contributing to a shared co-evolution. When humans move, they carry these ectoparasites with them. This close relationship has led to the evolution of lice in tandem with humans and our hominid relatives over millennia [20].

Evidence of this co-evolution can be found in the genetic analysis of lice. Studies have shown that the genes of head lice mirror waves of human migration. For instance, a clear genetic split has been observed between lice originating from Asia and those from Europe. In the Americas, hybrids of Asian and European lice populate North and South America, while lice that first evolved in Asia dominate in Central America. These patterns reflect the influence of human migrations on the evolution of lice [20].

Other Global Archaeological Head Lice Evidence

The use of fine-tooth combs to remove head lice and nits has a long history, likely much earlier in other parts of the world than in the Americas. For example, an ancient comb dating back some 3,700 years ago was found in Israel bearing an inscription encouraging people to comb their hair and beards to rid themselves of lice [21]. The researchers found microscopic evidence of head lice on the comb, suggesting that combs were used as a tool to remove lice from hair, indicating a development path for these combs. [21] Another example includes the discovery of lice and eggs in three combs, one of which was recovered from archaeological excavations in the Hatzeva area of the Judean desert, and two of which were found in Moa, in the Arava region, close to the Dead Sea [22]. These findings further demonstrate the long-standing relationship between humans and lice, and the early measures humans have taken to control lice infestations.

Another ancient comb with lice evidence was discovered in Israel and dates back around 3,700 years. It bears an inscription in the Canaanite language, which is likely the oldest known full sentence in Canaanite alphabetical script. The inscription reads: “May this tusk root out the lice of the hair and the beard” [24]. This suggests that the comb was used to remove lice from hair. Microscopic evidence of head lice was also found on the comb [25]. The comb was first excavated in 2016 at Tel Lachish, an archaeological site in southern Israel [24].

Development of Head Lice Treatments

Historically, humans have used a variety of methods to treat head lice infestations. The first lice treatment took place around 1550 B.C. in Egypt, where ancient priests were encouraged to shave their heads to not give the lice a place to live. In the 1600s, humans started trying different concoctions to rid themselves of lice, including mixtures of tomato juice, vinegar, cheese whey, sneezing powder, ginger, and melted butter. In the 1700s, people started wearing powdered wigs (perukes) to avoid lice [26]. The wigs were boiled to treat lice infestations. Treatments varied from a concoction from vinegar and lard to leaving the responsibility to the women to nit pick themselves and their families and friends [26][27].

Treatments for head lice have evolved significantly over the centuries (and Fort Crawford). The use of sulfur and lard, as well as mercuric oxide powder, are historical methods that are no longer in use today due to their potential harmful effects [12][13]. Mercuric oxide powder, also known as “red precipitate,” was also used in the past to treat various ailments, including lice infestations [15]. Mercury compounds were common in the early 19th century for various applications, including medical treatments [17]. However, the toxic properties of mercury were not fully understood at the time, resulting in significant health risks for those who were exposed to it [16][19]. By the late 19th and early 20th centuries, the harmful effects of mercury were becoming more widely recognized, leading to a decline in its use in medical treatments [18].

Today, the use of mercury in medical treatments is highly regulated due to its toxicity, and it is no longer used in treatments for head lice [13][14]. Modern treatments for head lice are much safer and more effective, focusing on the use of specialized combs and medicated shampoos, creams, or lotion [12][14].

Resources