The answer? As a baby, a human goes about on all fours ("four legs in the morning"; morning = childhood), until he learns to walk, which he does so well into adulthood ("two legs in the afternoon"; afternoon = adulthood), until old age requires him to use a cane to support himself ("three legs in the evening", evening = old age), finally he dies ("no legs at night", night = death). So the answer is "a man".
