It is a growing mystery that has baffled scientists and delighted barbers.
But a new hypothesis to explain why human head hair keeps getting longer suggests the haircut itself may be responsible.
The most popular explanation for this unique feature of humanity was that it protected our ancestors' heads from the heat of the sun on the African savannah.
But Bernard Thierry, a research director who works with primates at the Centre National de la Recherche Scientifique's Centre of Ecology, Physiology and Ethology in Strasbourg, France, believes that the advent of human culture may have nurtured the growth of human hair.
In the journal Evolutionary Anthropology , Dr Thierry notes that hairstyles provide a signal that can be read at a distance. "Hair features are signs that herald the cultural identity of their bearer, providing cues about tribal belonging, rank in the community, marital status and value or history of an individual," he says.
"Continuous growth is the critical character that gives humans control of their hair. It provides abundant and renewed matter for the shaping of hairstyle."
According to the hypothesis, individuals with flowing locks that could be combed, cut, coloured and braided might gain a reproductive advantage over the less hirsute. Dr Thierry suggested this may have coincided with an intense period of interacting biological and cultural evolution that shaped modern Homo sapiens some 100,000 years ago.
His hypothesis places growing hair and the hairdresser alongside the growth of the brain and the development of language at the dawn of human culture.
Alison Jolly, senior visiting scientist in biology and environmental science at Sussex University, wrote in the same issue of Evolutionary Anthropology : "Coiffure-demanding genes could be at least as old as Homo sapiens ."
Chris Stringer, director of the human origins programme at the Natural History Museum, said: "Signalling is so important to modern humans and this idea could be part of that story."