The Origin of LifeDuring the first

The Origin of LifeDuring the first billion years after the formation of the Earth, the conditions for the emergence of life gradually fell into place. The primeval fireball was large enough to hold an atmosphere and contained the basic chemical elements out of which the building blocks of life were to be formed. Its distance from the sun was just right-far enough away for a slow process of cooling and conden­ sation to begin and yet close enough to prevent its gases from being permanently frozen.After half a billion years of gradual cooling, the steam filling the atmosphere finally condensed; torrential rains fell for thou­ sands of years, and water gathered to form shallow oceans. During this long period of cooling, carbon, the chemical backbone of life, combined rapidly with hydrogen, oxygen, nitrogen, sulfur, and phosphorus to generate an enormous variety of chemical com­ pounds. Those six elements-C, H, 0, N, S, P-are now the main chemical ingredients in all living organisms.For many years scientists debated the likelihood of life emerg­ ing from the "chemical soup" that formed as the planet cooled off and the oceans expanded. Several hypotheses of sudden triggering events competed with one another-a dramatic flash of lightning or even a seeding of the Earth with macromolecules by meteorites. Other scientists argued that the odds of any such event having happened are vanishingly small. However, the recent research on self-organizing systems indicates strongly that there is no need to postulate any sudden event.As Margulis points out, "Chemicals do not combine randomly, but in ordered, patterned ways."22 The environment on the early Earth favored the formation of complex molecules, some of which became catalysts for a variety of chemical reactions. Gradually different catalytic reactions interlocked to form complex catalytic webs involving closed loops-first cycles, then "hypercycles"­ with a strong tendency for self-organization and even self-replica­ tion.2 3 Once this stage was reached, the direction for prebiotic evolution was set. The catalytic cycles evolved into dissipative structures and, by passing through successive instabilities (bifurca­ tion points), generated chemical systems of increasing richness and diversity.Eventually these dissipative structures began to form mem­ branes-first, perhaps, from fatty acids without proteins, like the micelles recently produced in the laboratory.2 4 Margulis speculates that many different types of membrane-enclosed replicating chem­ ical systems may have arisen, evolved for a while, and then disap­ peared again before the first cells emerged: "Many dissipative structures, long chains of different chemical reactions, must have evolved, reacted, and broken down before the elegant double helix of our ultimate ancestor formed and replicated with high fidel­ ity."2 5 At that moment, about 3.5 billion years ago, the first auto­ poietic bacterial cells were born, and the evolution of life began.