You may be wondering why we evolved to have ten fingers and ten toes. Why not eight, or only four?
The vast majority of terrestrial vertebrates have five digits. Most of the ones that don't have five digits as adults actually have five digits in early embryonic stages; their digits fuse together before they're born. A larger number of digits seems to have been present only in early aquatic vertebrates.
These pioneers had lots of slender ’toes’ on all four feet too many and too slender for control and power. There must have been strong selection for a more definite joint structure and more strength in each digit. By the time the first true amphibians appeared, toes had thickened and been reduced to eight or so on each foot.
Long before the first reptiles evolved, five toes had become pretty much standard issue. Mammals continued the pattern, which seems to be so robust and versatile that it has persisted among most non-specialised groups and a good few specialists as well, such as tree climbers and their descendants, including humans.
Specialisation tends to reduce the number of toes. Creatures that run, for example, need light feet more than they need versatile bone architecture, so their toes reduce in number — down to one in horses — and in size — two main toes and a couple of vestigial ones in artiodactyls (cattle, deer and suchlike). Some creatures, such as snakes, have even lost entire limbs. The only example of an added toe that I can think of is the giant panda’s ’thumb’, which is, of course, not really a toe. How the Panda’s thumb evolved twice.
Special thumbs
Some other mammals, like koalas and possums, also have opposable digits, which they use to keep a firm grip on tree branches. But these ‘thumbs’ evolved independently from ours, and ours are a lot more versatile – a koala couldn’t text nearly as fast as you can.