Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

Nails grow rapidly and constantly, such that without constant artificial trimming they would reach lengths difficult to manage. How did this benefit early humans, say 200kya? Were they used like claws? Did physical activity keep them at a short length?

share|improve this question
1  
My guess is that they would work very much like the claws of other animals. Dogs for example will trim their nails by walking around outside. Dogs that live in houses need to have them cut. –  terdon Dec 31 '13 at 16:08
add comment

2 Answers

It didn't have to benefit them, evolution has no intent and not all traits are advantageous. Not every thing we have in our bodies is there "for a reason", some things are just there, others evolved, and some are vestigial (inherited from our ancestors).

A whale can move his tail up-down only because he's a mammal and can't ever be anything else, while fish move their tails left-right because they're fish and that's their heritage.

Constantly growing nails never helped a single human, but they sure did help our ancestors which had claws and needed them for survival. We lost the claws but didn't lose the constantly growing trait. Basically, next time you trim your nails remember they (in shape of claws) saved your ancestor's life a long time ago and that's why you have them although you don't need them. He needed them.

share|improve this answer
1  
I don't think this is a very good answer. It sounds like you're arguing that the constant growth of human nails is just a useless vestigial trait. Yet the change from claws to flat nails in our evolutionary history is a non-trivial one, and must surely have involved some considerable selection pressure. Had it been advantageous at the time for our nails not to grow during adulthood (or at all!), that, being a comparatively minor change, surely could've evolved as well. –  Ilmari Karonen Mar 12 at 0:55
    
The change from claws to nails happened when we became monogamous (bipedalism evolved around this time as well) and could therefore expend resources into something else than fighting each other for females. That's how we lost the jaw muscles and allowed for brain growth, and that's when we lost canine teeth as well. There was no selective pressure to lose those traits, but the absence of one. Rival males. That's why we don't have claws, but still have evidence of having them a long time ago. –  Dan Horvat Mar 12 at 9:35
    
Also, "advantageous" or "deleterious" is only important when it affects your ability to survive long enough to reproduce. If it doesn't, there is no selective pressure on that trait even if it's deleterious. The reason for the change from claws to nails is energy - it was better used elsewhere. Bear in mind that hunger is prevalent in the wild, and organisms which had smaller claws were more likely to not die of starvation. Or/and were better in tool use. In any case, when claws got small enough the selective pressure disappeared and that's why we didn't lose them completely. –  Dan Horvat Mar 12 at 9:41
1  
An interesting theory, but I'm not sure how it can explain the fact that all great apes have essentially human-type nails, as do most other primates. While some primates are indeed more or less monogamous, many are not; this includes our closest evolutionary relatives, the chimpanzees and bonobos, which nonetheless have very human-like nail morphology. –  Ilmari Karonen Mar 12 at 15:13
    
You're right, I overlooked that fact. The development of nails must have taken place much earlier. The most recent common ancestor of us and great apes must have had nails already, otherwise we'd be saying nails evolved separately in each species. No fossils of that ancestor have been found and we're down to speculation about the time when it lived and it's physical traits. –  Dan Horvat Mar 12 at 15:33
add comment

I can't say anything for sure (and, with questions like this, it's rare that anyone can), but my guess would be that human nails grow constantly for the same reason as the (analogous) claws of most mammals do: to keep them from accumulating damage.

Your bones, if they suffer mild to moderate damage, are capable of healing through the concerted action of several specialized cell types in your body. This healing process, however, is only possible because the bones are inside the body, surrounded and permeated by living tissue.

Your nails, however, are simple sheets of keratin, and contain no living cells that could heal them, nor any blood vessels that could keep such cells alive. Being exposed to the outside of the body, outside the skin barrier, they cannot really support living tissue that could heal them.

Instead, the way human nails (and animal claws, more generally) "heal" is by growing. Since the nail itself cannot contain cells that would rebuild it, those cells are instead located in the nail matrix, which is located under the nail and from which the exposed part of the nail continually grows out of.

(Indeed, the same is true of most of the exposed parts of your body: your hair and even the surface of your skin itself are also composed of non-living keratin constantly growing from an underlying matrix. This is necessary to maintain the skin barrier between the interior and the exterior of the body.)

The reason human nails must keep growing all the time is that they do continually wear down and suffer damage from various causes (and would surely have worn down much faster yet for our primitive ancestors, who had to sustain themselves through hard manual labor with few or no tools). If your nails just stopped growing after you reached adulthood, they'd be in horrible shape even just a few years down the road, not to mention decades later.

While the rate at which human nails do grow is pretty slow — it takes several months for a damaged fingernail to fully regrow, and up to a year for a toenail (a fact I can confirm from personal experience, having broken my toenail last summer) — it's still enough to ensure that your nails will only bear less than a year's cumulative damage at any moment, rather than every nick and scratch and cut they've suffered during your entire lifetime so far.

In a relatively sheltered modern lifestyle, it's quite likely that even this moderate growth rate may exceed normal wear on your nails, requiring them to be clipped regularly. Still, the fact that you can clip your nails (or, in a pre-modern society, just bite or scrape or cut them down) reduces the selection pressure against excessive nail growth. On the other hand, not having your nails grow back as fast as you wear them down through hard manual work could be quite bad for your fingers, and could easily reduce your working ability or even lead to harmful infections. Thus, it makes sense that evolution has optimized out nails to grow at a rate that, on average, tends to somewhat exceed the typical rate of wear.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.