"In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there. ... There must have existed, at some time, and at some place or other, an artificer or artificers, who formed [the watch] for the purpose which we find it actually to answer; who comprehended its construction, and designed its use. ... Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all computation." -- William Paley, "Natural Theology" (1802)
To put it in simple terms ... If you find a rock in a field somewhere, you would just assume that it's been there for a long time and is the result of a "natural" process. If you find a watch in that same field, you'd be a damn fool to think it evolved as the result of a random, natural process. The complexity of the watch is evidence of a "purpose" that would distinguish it from a rock.
But we know for certain the origins of any watch ever found -- made by humans.
Every rock provides us with chemical, radiological, in situ & other clues as to its age & origins, often at the bottoms of lakes or seas, sometimes as meteors from outer space.
Every life-form also provides clues of its ancestry, ancient relationships and purposes in a physical sense.
"Purpose" in a metaphysical/teleological sense is outside the scope of science.