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Why do fingernails and hair keep growing? Wouldn't they be a limitation at a certain point, evolutionarily?
It turns out that for some reason or another, life and evolution have decided that hair is necessary for more than just Pantene shampoo commercials and nails useful for more than just a good scratching session.
Based on the fossil record, primates likely developed the earliest fingernails between 55 and 58 million years ago. Fingernails would have helped maintain a wide shape for fingers and increased the surface area for finger pads, improving grip for climbing through trees. Nails also have a direct protective function, acting as mini-shields that protect thousands of nerves located within our fingers: a service that is still highly beneficial and crucial today.
Similarly, hair is excellent for protection and particularly evaporative cooling. Hair on our heads shields from the direct heat of the sun and even leaves an area for sweat evaporation between your scalp and the surface of the hair (curly-haired people, this is your time to flex -- curly hair wins the gold medal in performing this cooling task). Vellus hair, on the other hand, could be considered an evolutionary relic, though. The light, downy hair that is barely visible, vellus hair technically is not necessary anymore. However, human bodies still have a lot less hair than the full-fur suits our ancestors once sported, a change that has taken place over time from evolutionary development.
Like any other part of your body, your fingernails and hair are composed of cells that are constantly growing, duplicating, and dying. Cells in both nails and hair are made of the same tough, dead substance, keratin. In nails, the root -- also known as the germinal matrix -- produces the keratin cells that form the nail. As more cells form, they’re gradually pushed forwards by the constant production of cells behind them, and the older cells are jostled out to form the hard nail plate. This results in a growth rate of about 0.1 inches per month for fingernails. Head hair follows a similar process at about 0.5 inches a month on average thanks to the follicles remaining in an anagen (active growth) stage for a relatively longer time. This constant replenishing of old with new allows for replacement relief for potentially damaged or injured areas, or even more modern problems like a bad dye job.
All in all, the fact that we’ve been growing hair and fingernails for millennia despite constant resource and energy expenditure in maintaining these traits indicates that there is still a positive selection for them. So the next time you’re at the barber or nail salon (or having a mental breakdown and giving yourself quarantine bangs), have a moment of gratitude for these constantly-growing keratin deposits -- they’ve survived thousands of years of evolution to make humans into what we are today.
Fingernail and/or hair length is merely another avenue for achieving the pinnacle award of a lifetime: holding a Guiness World Record. If you manage to grow your fingernails beyond 8.65 meters -- yes, that’s 28 feet and 4.5 inches -- you thereby have the privilege of carrying the entire fingernail beauty and hygiene industries on your shoulders. Good luck typing!
For hair, the commitment is no less. So far, the longest documented hair comes in around 5.627 meters, or 18 feet and 5.54 inches. Disclaimer though: not sure if folklore indicates Rapunzel’s is longer ??