A man is an adult male human. Prior to adulthood, a male human is referred to as a boy.
Like most other male mammals, a man's genome usually inherits an X chromosome from the mother and a Y chromosome from the father. Sex differentiation of the male fetus is governed by the SRY gene on the Y chromosome. During puberty, hormones which stimulate androgen production result in the development of secondary sexual characteristics, thus exhibiting greater differences between the sexes. These include greater muscle mass, the growth of facial hair and a lower body fat composition.
Male anatomy is distinguished from female anatomy by the male reproductive system, which includes the penis, testicles, sperm duct, prostate gland and the epididymis, as well as secondary sex characteristics.
The word derives from Old English mann. The Old English form primarily meant "person" or "human being" and referred to men, women, and children alike. Man only came to mean adult male human in Middle English however both meanings remained in English in to the late 20th century. This can be seen in English words such as mankind, chairman and layman which are today perceived as historically sexist but were gender neutral when in widespread use.
The Old English term for man was wer, which survives today only in the compound werewolf (from Old English werwulf, literally "man-wolf"). This, of course, implies that all werewolves were meant to be male in the original meaning.
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