Do they have any evolutionary purpose?
First, let’s get one thing clear - not everything has an “evolutionary purpose”. Some things are features of a critter or plant simply because there’s no penalty for continuing to have it. It might have been useful to a long-ago ancestor, and now no longer serves any purpose. It might be just a random thing that was never eliminated. Whatever. But not everything has a purpose.
On to the question: Short answer - we have toenails because our primate ancestors did.
Finger and toenails are a feature of primates. For some reason, they seem to work better than claws when climbing trees - at least for primates since things like squirrels do just fine with claws. Anyhow, early primates found nails advantageous and evolved to have them on the digits of all four limbs.
At some point, the ancestors of human beings started to differentiate from other primates, and differentiate their fore and hind limbs because the “fore” and “hind” became “upper” and “lower” and were called upon to specialize. Instead of all four limbs being able to function as hands/feet/arms/legs we wound up with the upper limbs specializing in manipulation and holding and the lower limbs specializing in locomtion. This called for structural differences in these limbs, which are easily seen today.
However, embryo development tends to be conservative. There is also a connection between the development of arms and hands and legs and feet - that’s why if a birth defect affects the hands you frequently have affected feet and vice versa. So evolution of distinct hands and feet required altering the programing for development without altering certain parts of it too much*
The end result is that, while hands and feet look quite different they also retain much in common. For instance, while the digits look considerably different, there are five in total on a normal hand and foot. Four of them have three phlanges, one had two. Although different in appearance, toes can be taught to manipulate objects, you can learn to walk on your hands, and surgeons have transplanted toes from a person’s foot to replace missing fingers with good results.
It’s as if the body has a plan for “basic digit” that it is compelled to complete first, then it varies that 'basic digit" for its final role. The 'basic digit" plan includes (among other things) a nail (it also includes skin ridges on the bottom, which we call “prints”, an abundance of sensory nerves, and sweat glands, but those aren’t as visually obvious)
So the answer is - we have toenails because its in the “recipe” for human digits.
I mean, you might as well ask why human toes are so sensitive to touch. Anyone who has stubbed a toe has certainly asked that while hopping around in pain - why on earth do we need so many potential pain sensors in such a small place? Well, in a sense we don’t - we have them because that’s part of the “basic digit” plan and so far there hasn’t been enough Darwinian penalty in sensitive toes to elimiate the feature.
Most likely, as long as we continue to have fingernails we will have toenails, although it would not be surprising if toenails became thinner, smaller, and more vestigal. But they couldn’t disappear unless you alter the “basic digit” recipe, and that’s hard to do without messing up important things like hands.
- Nitpickers - yes, I know I’m symplifying. If you want to explain things like homeobox genes go right ahead. I understand it, but not well enough to explain it clearly. I’m going for the basics here, someone else can elaborate
I read somewhere that we have fingernails to anchor the skin/tissue at the ends of our fingers. If that’s true, it would apply to our toes as well.
Thanks for the explanation. I recognized that humans have seemingly useless body part, but I was looking specifically for what they were used for. You answered my question, so im happy. 
thanks,
culov
So we can paint them?
Seriously, I always thought it was to protect our toes, but just like we have nails to protect our fingertips, but then, that sounds a bit silly, I guess.
It’s silly only in a society where people wear shoes all the time.
The probably do serve a somewhat protective function in people who are barefoot
Useless body part? It’d be difficult to do some tasks like open a soda can or pull out a splinter without fingernails. Toenails might very well be useless.
They are a leftover. In a time when we ran around naked, it was probably useful to hold on to something/someone and kick and scratch at it/them at the same time, there just hasn’t been/will not be reason for them to evolve away. Tonsils and appendexes probably served a useful purpose once, also, but now they do more harm than good.
To my understanding, the tonsils and appendix are part of the lymphatic system, which has some sort of immune role. We have enough lymph tissue in the body that their removal doesn’t have any major negative longterm effect, but there is likely some advantage to having more of it than less. I don’t know enough to give a more detailed answer.
Evolution only subtracts things that are fatal before reproduction.
Since toenails do not inhibit procreation, they will never disappear.
Well, earlier today, I learned the purpose of a big toenail is to be slowly pried out after ten days of mild infection and swelling following some guy stomping on my foot during a game of floor hockey.
You want a painfull foot experance for hocky? back in the middle school We had a pride day. The theme was pajamas and slippers. Well there was extra credit in math class for anyone that did. (shows how much pride we had) So I wore them. Anyway in gym we had a choice day that day, I went up and played floor hocky. I always play goaly, and I block the puck with my body, usually my foot. I kick the puck alot too. 6 years later my foot is still sore.