These were much less complex than the macroscopic life forms that exist today and that have existed for around a billion years. Even the oldest life forms that we can find in the fossil record, however, dating back to 3.5 b.y.a., are very complex--they are very sophisticated organisms that relied on DNA and RNA to transfer genetic information, on ATP to store energy in a usable and accessible form, and, for some of them, on photosynthesis to get access to energy. These organisms are much more complicated than we imagine the first life forms would have been. Life did not exist, however, for one-half to one billion years following
the formation of the Earth. The earliest half-billion years on the Earth were marked by the continued influx of impacting objects left over from the formation of the planets. Even today, these objects are capable of dramatically affecting the terrestrial environment when they collide with the Earth. Back then, when larger objects were present, they were capable of sterilizing the Earth completely. There were an estimated five or so impacts subsequent to about 4.5 b.y.a. that were capable of completely sterilizing the Earth's surface. The early environment on the Earth, therefore, was not conducive to the continuing existence of life, and the earliest life may not have been able to grab a foothold until sometime after around 4.2 or 4.0 b.y.a. Thus, life may have taken as little as a hundred million years, and no more than about a half-billion years, to form once the environment became sufficiently clement to allow it. The rapidity with which life originated on the Earth has important implications for the process of forming life. It tells us that the formation of life is not a difficult event, but, rather, it is a relatively straightforward consequence of natural events on the planet. This is consistent with our current view that life originated through chemical and geochemical processes, starting with organic molecules in a wet surface or near-surface environment and using energy from some chemical source to build them into more complicated molecules. Even though we do not yet understand the specific processes that led to the origin of life, we can easily see that simple chemical processes can lead to more complicated molecules and, eventually, to life. The ingredients for life On Earth, we imagine that li
fe really needed only a few key ingredients to get started. Liquid water is one such substance. It is difficult for us to imagine that life could exist without liquid water. Even with this requirement, we still see that life could be widespread throughout the universe; if we allow for the possibility that a different liquid also could hold the key to life, then life could be even more widely distributed. The second ingredient is access to the necessary biogenic elements, such as C, H, O, N, and so on. This is not a very limiting factor, though, since we expect these elements to be very widespread throughout the universe, to be incorporated into planets during their formation, and to be readily available at the surfaces of geologically active planets. A source of organic molecules was required for the origin of life on Earth, and presumably would be required for life elsewhere as well. On Earth, organics could have come from one or more of several different sources. These include the Earth's atmosphere, where they could form from energetic processes such as lightning in a slightly reducing atmosphere (the so-called Urey-Miller process); hydrothermal vents at the bottoms of the early oceans, heated by the extremely active volcanism that would have been present then, where organics could form by a chemical slide toward equilibrium as very hot water cools off once injected into the oceans; or from organic molecules that were present in dust and planetesimals accreting onto the Earth. Most likely, all of these sources contributed to the prebiotic supply of organic molecules.