The human body typically has two hands and two feet since long before human language developed. All four are of generally equal importance. From this start I'd be inclined to think that the English words for hands and feet would share a similar etymology. So my question is, "Why isn't there a distinct plural for 'hand' (hand, hands) but there is for 'foot' (foot, feet) or, equivalently, why isn't the plural for foot, foots?" I've read on this site that some variant of 'foots' existed in predecessor languages of Modern English but was apparently crowded out by "feet". Was there never a rival plural of 'hand' to crowd out hands?
The plural form of 'foot' became 'feet' due to our fore-fathers' laziness; otherwise, it should have been 'foots'. This laziness can etymologically be termed as i-mutation.
I-MUTATION (also known as "i-umlaut") is the raising and fronting of a root vowel in anticipation of "i" or "y" sound in a suffix.
Think of the difference between the -o sound in the do of "How do you do?" and that of the last word in "How are you doing?" The last word of that sentence might be written *diwin if it were spelled phonetically the way the average modern American pronounces it. When that -o- shifts up to an -i-, that's i-mutation.
I-mutation is caused by the very human habit of laziness: taking the shortest distance between two points. The plural of man in ancient West Germanic, the ancestor of Old English, used to be a word something like *manniz. The speakers "cheated" on the first vowel in the word to be in position for the second vowel. It's the same thing you do with doing. It doesn't change the meaning of the word to do so. So after hundreds of years of this, the plural came out as *menniz, or something similar, when people said it. Eventually, the shifted vowel itself comes to stand for the plural, and since laziness dislikes doing the same job twice, the syllable at the end of the word slowly shriveled and dropped off.
Noun plurals in -iz: man-men, foot-feet, tooth-teeth, goose-geese, louse-lice, mouse-mice. Along with woman-women (derived from wif-man) these are the only survivors of this class, which was numerous in Old English and included such words as the ancestors of modern book, goat, and friend, which now have gone over to the -s plural.
(from ONLINE ETYMOLOGY DICTIONARY)