I can see where fingernails would be useful to our forebearers (cavegirl fight!!!) and for digging, or picking out sabretooth bones from ones teeth, but are toenails any good for anything? They just sit there, occassionally getting dirty and infected. Why do we still have them?
I don’t know about toenails, but last Christmas there was a chap on TV who had lost all his toes to frostbite and didn’t seem impaired at all. (This was on the Royal Institution Christmas lecture. His blackened, frostbitten toes were passed around the audience of kids, in a perspex box. Made my day!) So toenails and even toes might be an evolutionary hangover.
On the other hand, people who do without shoes throughout their lifetime may have a contrary opinion.
Indeed toes are pretty useful when trying to balance on a ledge, stand up and start to lean forwards, in order to regain your balance you will instinctively go up on your toes like a ballet dancer and use your toes to push yourself backwards.
As for toenails I imagine (and this is pretty much a WAG) that they are simply there for protection. If you drop something on your toes then the tough shield may to some extent prevent damage to the otherwise soft and flimsy digits.
Having had a fingernail pulled out (it grew back), I know it is impossible to pick something up or grip with the ends of your fingers without fingernails. I would guess that tip-toeing or any other nifty foot work would be similarly difficult without toenails.
Say what? Could your difficulty have had anything to do with the fact that your finger was injured? Or are you saying the nail gives the fingertip more strength?
I’m picturing more-evolved humans in the future, born without fingernails, and they can pick things up just fine.
Ever try to pick up a penny lying flat on the table? Fingernails come in handy there because of their thin, curved surface area. The area of your fingertip is simply too bulky to do it as easily.
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It is not the case that every trait possessed by every organism necessarily has an adaptive function.
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If anything, toenails are a byproduct of development, not evolution. The same sets of genes control the growth of fingers and toes (thus the many similarities between them). Fingernails, being possibly advantageous, might well be selectively maintained, but that means bringing toenails along for the metaphorical ride even if they serve no obvious purpose. Similar to the reason male humans have nipples, etc.
As someone who has an ongoing issue with ingrown toenails and has had the entire toenail removed more than once, I can tell you that mental or otherwise, the lack of them does effect your sense of balance.
Others who have had the same procedure have reported the same effect.
And as an example of that: You can have all toes except for the big toe removed with little loss of balance function. Losing the big toe, however, makes it much more difficult to maintain balance. And yet, we have five toes? Why?
For the evolutionary advantage of playing “Five Little Piggies”, of course! 
Toes were, of course, at one point, a second set of grasping appendages, as in chimpanzees, in which five digits were (presumably) more effective than one or two, and therefore are a byproduct or hangover from past adapation, as with the coccyx or veriform appendix. As fingenails, they do provide a significant amount of stiffness to the fingertip. I used to be in the habit of cutting my nails right down to the skin, but have found that when typing, cutting them too short makes my fingertips hurt. I now leave a little band of free nail, which helps support the tip in compression. Try this out for style: take the ball of your left index finger and smush it up against the bone with your thumb. You’ll notice that the skin tries to flow around the nail, but is supported by it. The toenail (at least, the big one) provides similiar support when you are perched on your toes.
Now here’s a question for you; why do we have these big, honking, unsupported breathing snorkels on the front of our faces made of cartilage and attached to the skull at only one point? What kind of intelligent design is that?
Stranger
I managed to rip of the entire nail from my big toe on my right foot about a year and a half ago. Aside from the pain, it didn’t affect my balance (or my ability to walk at all, for that matter) in the slightest.
How could they even affect someting like balance?
I don’t think anybody has suggested that toenails effect balance in anyway, I made reference to balance in response to matt’s post where he suggested that someone who had lost all their toes, (not toenails), remained unaffected.
The question of the nose is an interesting one, our closest relative the chimpanzee has a very flat nose rather than the monstrous protrusions some of us sport.
Having no toes will affect your balance. You might be able to compensate but you’ll always have less balance without your toes.
I’d imagine that without shoes, and having long enough toe nails, it might offer a little grip when climping dirt/grass hills at speed. I know when I’m running through sand with bare feet my toes “sweep” the sand. With longer toe nails (no longer possible with shoes) it might offer an advantage.
My impression was that nails, finger and toe, were not primarily selected for for their value in manipulation, balance, or whatever, but as protective elements. I’m not sure that there’s a “substance” term for what they’re composed of, but it’s a close variant on horn (the substance, not the pointy things made of it). And the nails tend to provide a relatively hard covering that is an impact attenuator and spreader over the “top” surfaces of the extremities of the appendages that usually go somewhere first as your body moves. Protecting the extremities with them has to have some sort of selective advantage.
[url=http://en.wikipedia.org/wiki/Keratin]Keratin**. It’s also one of the main proteins in your hair.
Neat stuff. I’m not sure how to make genetically engineered keratin structures that can withstand “concentrated molecular acid” though.
Stranger
They are sexual signalling devices. The bigger the snozel, the bigger the … well you get the picture
from a proud owner of big snorkel
The short answer is that the human nose is particularly suited to preventing moisture loss. It’s unfortunate that I sent most of my textbooks back home after graduation, otherwise I could give you a more thorough answer.
If I stub my toe hard and the toe itself takes the hit, it hurts like mad. If I stub my toe hard but the toenail takes the hit, it hardly hurts at all.
Had a teacher in high school who lost the front half of his left [iirc] foot, ended his career as a minor league ballplayer, and he limped a bit. I do know that when I had a horse stomp my foot [more than one time it had happened] I couldn’t balance well for a while even after the pain stopped.
And noses keep the rain from filling the sinus cavity? Serve to refine down an area we can gather scent from [so we scent what is immediately before us and not all the way around us?]
Yes, but one or two toes are sufficient for balance. The rest are, from a design standpoint, extraneous. As Darwin’s Finch points out, however, the same genotype controls the development of the hand and the foot, and because it is (presumably) advantageous, or at least not significantly disadvantageous, to keep five digits on the hand, we also get five toes. Splitting the development instructions for hands and toes into different genotypes would require a major and deliberate restructuring of the genotype controlling appendage development.
Note also that aquatic cetacean mammals have fingers/toes/whathaveyou complete with vestigal articulating joints as well even though they have no use for them, and they’re all bound together with flipper webbing. There’s no major disadvantage to it, and so except for random genetic drift, there’s no tendancy to evolve these structures out of existance. I do think, however, that toenails do serve some protective value.
Sure, but the design is suboptimal, to say the least. The H. sapiens nose is poorly protected and supported compared to, say, the flat nose of a gorilla or a chimp, or the snout-wrapping nose of the canine or ursine. I was actually making a joke of the common notion that evolved structures are “designed” or “optimized”, and that humans are at the top of the evolutionary tree. While in some venues extreme specialization has resulted in an ideal design (we see this often in insects), the usual result is that the characteristics are just good enough to be better than any competition, and no so bad that they are a hinderence; although, in some cases, competing pressures–say, sexual selection in opposition to physical prowess–can result in phenotypes that are exaggerated and seriously compromised…like the peacock’s tail.[sup]*[/sup]
Stranger
*I’m just using this example to rile up Darwin’s Finch, who’ll disagree with me, and perhaps correctly, on the specific example and the rationale behind it.