Expert: John Locke - 1/11/2007

Question
I understand that life is thought to have begun in water approx. 4 billion years ago.  What do scientists know about the actual mechanism involved?  Is it known whether this process repeated itself indefinitely and is perhaps occurring even today?

Answer
Ralph, thank you for using AllExperts; I'll give answering your question a shot, with the caveat that much of the field in question is still emerging today (the field is abiogenesis, the science of determining the origin of life). To that point, I should also note that this is fundamentally a historical science: we will not ever be able to exactly recreate the conditions that led to the development of life, though we can learn quite a bit about them and speculate as to what probably happened.

Let me summarize, more or less, what we currently know about abiogenesis:

Life began about 4 billion years ago, as you note, in an aquatic environment. This form of life, whatever it was exactly, was extremely primitive and probably preceded by self-replicating structures that did not have the full characteristics of life (that is, proteins and DNA that could replicate themselves, but were not distinct living things). How exactly these self-replicating structures came about is also the topic of contention, but perhaps the most common theory is that they were aided by some simple catalytic environment: some kind of environment (submerged clay is often suggested, or deep-sea vents) favored simple reactions that made organic molecules. These organic molecules were starting materials; "organic" in this context means that a molecule has carbon in it. All living creatures today are "organic" in this sense, suggesting that their common ancestor organized from carbon-containing compounds.

These self-replicating structures grouped or clustered together in some kind of membrane, which kept them apart from the environment as a whole and allowed them to function by themselves. Indeed, all cells today have this kind of membrane that separates them from the outside environment. These proto-cells showed some very simple responses to their environment and were able to carry out at least some of the other functions of life (movement, replication, adaptation); this almost certainly took place multiple times (by "multiple," I mean millions to billions of times), with the majority never surviving very long. By far the most important characteristic of these proto-creatures was their ability to replicate (the term "reproduce" is probably a bit inaccurate when applied here, as reproduction involves a fairly complicated setup that these early creatures did not have). If they could replicate, then evolutionary selection could take place, as detailed next.

This is perhaps the first situation where evolutionary theory really has an effect--since there were many, many distinct organisms all fighting for limited resources, any that had a particular advantage would be more likely to survive and replicate themselves.

This process did repeat many, many times during the early history of life, but among the scientific community, there is confidence that the process has stopped. The conditions that have existed for perhaps a billion years after the origin of life did not support the further organization of self-replicating cells from non-living materials.

There's far more known about this subject than I can write here, so let me direct you to a website that effectively summarizes the most common views on abiogenesis; as an area of emerging science, though, the conclusions reached in the article are likely to change as more information is discovered:

http://www.talkorigins.org/faqs/abioprob/spontaneous-generation.html