Current theories on the origin of life.

Abstract

To modern scientists, the origin of life seems one of the most difficult of all problems. This was not always so. From classic Greek times until the middle of the 19th century it was generally accepted that living organisms could originate spontaneously, without parents, from nonliving material. Thus, for centuries it was believed that insects, frogs, worms, etc. were generated spontaneously in mud and decaying matter. This notion was experimentally disproved in 1668 by Redi, who showed that larvae did not develop in meat if adult insects were prevented from laying their eggs on it; but it was revived again following the discovery of microorganisms by Leeuwenhoek in 1675. Since bacteria, yeasts and protozoa were much smaller and apparently simpler than any previously known living things, Redi’s disproof did not seem to apply to them, and the possibility of their spontaneous origin became a matter of controversy for nearly 200 years. We know today that these organisms, despite their small size, are enormously complex—as complex as the cells of higher organisms—and the possibility that they could originate spontaneously from non-living material is as remote as it is for any other cells. In a series of brilliant experiments, Pasteur (82) in 1861 finally overcame the technical difficulties that had prevented solution of the problem and demonstrated, by logically the same argument that Redi had used, that microorganisms arise only from pre-existing microorganisms. The genetic continuity of living organisms was thus established for the first time.