c. 1300, naturel, "of one's inborn character; hereditary, by birth;" early 14c. as "of the world of nature (especially as opposed to man)," from Old French naturel "of nature, conforming to nature; by birth," and directly from Latin naturalis "by birth, according to nature," from natura "nature" (see nature).
From late 15c. as "not miraculous, in conformity with nature." Meaning "easy, free from affectation" is attested from c. 1600. Of things, "not artificially created," c. 1600. As a euphemism for "illegitimate, bastard" (of children), it is first recorded c. 1400, on notion of blood kinship (but not legal status).
Natural science is from late 14c.; natural law is from early 15c. Natural order "apparent order in nature" is from 1690s. Natural childbirth first attested 1933. Natural life, usually in reference to the duration of life, is from late 15c. Natural history is from 1560s (see history). To die of natural causes is from 1570s.