How long can hair grow? - Maksim Plikus

Scalp hair is one of the most defining features of human appearance, and one that we have a bit of an obsession with. Indeed, a good hair day can be a self-esteem booster, while more-than-usual hair strands in the shower drain can cause a panic attack. Yet, despite all of the attention that hair gets, we do not know exactly when our ancestors first started growing luscious curls. Reconstructed faces of extinct hominids, from Australopithecus to Homo Erectus, vary from having chimpanzee-like “buzz cuts” to very modern hair styles. These are, of course, artistic liberties and what they truly reveal is that we don’t really know their true appearance, and this is because soft tissue remains are missing from the available archeological finds of the “missing links.”

Yet, hair preserves rather well in frigid conditions, such as in the Ice Age permafrost. Indeed, there are numerous exquisite specimens of the Wooly Mammoth fur found frozen in Siberia. It is from these remains that scientists definitively know that mammoths were indeed wooly, with dirty-blond hair reaching up to a meter in length. There is probably one in a million chance that an extinct human, such as a Neanderthal, is laying somewhere buried in deep-freeze along with the answers to ancient hair styles; but finding it would require extraordinary luck. Yet, it is not a complete impossibility! The chance find of the naturally-frozen mummy of Ötzi the Iceman in the Alpine mountains revealed that at the time of his death some 5,300 years ago, he had darkly-pigmented skin and was likely balding, although most of Ötzi’s hair was lost to postmortem decay.

Scientists think they have better understanding of why humans evolved to have very long scalp hair. Or at least they have a very compelling theory. Our ancestors evolved to walk upright and thrived in the rather scorching weather conditions of open savannas in equatorial Africa. Naturally, effective strategies for reducing body overheating became key to their survival. First, humans evolved to cool themselves through sweating and got really good at it. Indeed, a modern human can sweat as much as 4 liters of water per hour, but this is on an extreme end of what is possible, and it surely can result in life-threatening dehydration. It is thought that very long and, importantly, tightly curled scalp hair evolved along-side or soon after our sweating superpower.

Tens of thousands of continuously growing spiral hair strands intertwine with one another into a naturally voluminous hair style, much like in modern-day people of African ancestry, effectively shielding skin against direct sunlight. Moreover, unlike long straight hair, which is common in modern people of non-African ancestries, long tightly curled hair is more effective at resisting wetting and so it would not easily lose its volume and sun-blocking effect during profuse sweating.

Yet, despite being able to tell why our ancestors were in need of long scalp hair, scientists are mostly left guessing as for the genetics of this human trait. Detailed comparison between genomes of modern human and chimpanzee has yielded numerous differences in the so-called regulatory regions of the DNA. The raw data is available but remains largely unexplored in the context of the many hair differences between us and our closest living relative.

Slow progress on untangling genetic clues to human hair is not entirely surprising. When scientists examined the genomes of modern humans comparing hirsute and balding people, they uncovered no less than 380 DNA sequence differences that link to hair loss. One can imagine this number to be significantly larger for all other aspects of hair biology, including its ability to grow for years at a time.

But one can say, “Wait, we have many breeds of pet animals with long fur. Can’t we compare their genetics to understand how to make a long hair?” And, indeed, angora rabbits look like real-life fuzzballs. Similarly hairy breeds are found among cats, dogs, mice and guinea pigs. When geneticists compared all of these animals to their less ridiculously hairy counterparts, in almost every instance they linked the long fur trait to a defect in just one gene, the Fibroblast Growth Factor 5 (FGF5). Yet almost all humans have a perfectly well-functioning FGF5. There are FGF5 mutations in humans too, but these are exceedingly rare, and they mostly result in very long eyelashes.

One can also consider looking for secrets to hair growth in a hairy nevus, a type of pigmented skin mole featuring long hair. Hairiness of such moles was shown to be dependent on a molecule called Osteopontin, that becomes abundant within their pigment-making cells. But something as complex as hair growth never comes down to one thing, and so the “hunt” for hair lengthening molecules is very much open.