Pasteur sometimes is said to have proved a “law of biogenesis” according to which life can only come from previous life. Creationists sometimes claim that this rules out a natural origin of life.
The short response
The law of biogenesis has a limited scope that does not cover the kinds of hypotheses that modern origin of life researchers examine. It rules out only antiquated notions of spontaneous generation, according to which adult organisms arose all at once from things like rotting meat and soiled underwear.
The longer response
I. Laws and limitations
The first thing to understand is that many scientific laws apply only a limited domain, yet tend to be phrased in a way that makes them sound as though they apply universally. Newton’s second law, which is summarized simply as F = ma, probably is the most familiar example: although it is called a law, and its form is universal, it does not apply at very high speeds and very small scales, where it undergoes relativistic and quantum mechanical corrections respectively.
II. The limited scope of the law of biogenesis
When one examines the context in which biologists speak of the law of biogenesis, one sees that this law, too, applies only in a limited domain—one which does not include the origin of life as understood by modern researchers. The law of biogenesis is intended merely as a denial of the old doctrine of spontaneous generation, according to which “smaller organisms could arise spontaneously from mud or organic matter” (Strickberger 1990: 10). Consider this (now amusing) example, by the seventeenth-century physician J. B. Van Helmont, as quoted by Monroe Strickberger:
If you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment, coming out of the underwear and penetrating through the husks of wheat into mice. But what is more remarkable is that mice of both sexes emerge, and these mice successfully reproduce with mice born naturally from parents…But what is even more remarkable is that the mice which come out of the wheat are not small mice, not even miniature adults or aborted mice, but adult mice emerge! (Strickberger 1990: 11)
It is this kind of thing that the law of biogenesis is meant to rule out. It should be readily apparent to everyone that modern origin-of-life hypotheses bear no analogy to ideas like adult mice somehow being generated from wheat and soiled underwear.
Perhaps after origin-of-life researchers have explored every avenue, it may turn out that the law of biogenesis should in fact be taken to be universal. But to take the law to have such a scope now, when origin-of-life studies are still relatively new and progressing just fine, would be an invalid extension of the law to a domain it never was intended to cover. Certainly, nothing in Pasteur’s experiments ruled out any hypothesis under investigation by modern origin of life researchers.
Strickberger MW. 2000. Evolution. Boston: Jones and Bartlett.