So, I’m reading Life, Richard Fortey’s book on paleontology, and also looking on the web to expand my knowledge. Anyway, it seems that a fundamental assumption of evolutionary biology is that life only arose once and everything else proceeded from there forward. I can understand how the mechanics of evolutionary theory mean that everything must point back to a single starting state, but why is that so? In other words, what in the early days of our planet prevented multiple geneses? Wouldn’t it be possible for the organic chemical reactions that produced the first amino acids to have happened more than once, in multiple locations? Or, similarly, for the formation of RNA and other building blocks of life? Would we be able to tell the difference today if that is indeed what happened? Or am I just confused?
Here is an earlier thread on this subject:
http://boards.straightdope.com/sdmb/showthread.php?threadid=170352
No, I don’t think you’re confused. A multiple genesis scenario is possible, as is another your question prompted in my mind, that being life beginning and then dying out, and then starting again.
Thanks. I just read the linked thread, and it appears that there’s nothing to prevent multiple geneses from happening, but eventually one set of “organisms” won and wiped everyone else out. Makes sense. What gets me confused is passages in Fortey’s book such as (pp. 35-36):
Which sounds to me like he’s saying that it is only possible for life to arise once, not that it may have happened many times but only one form survived. It also sounds like he thinks that the creation of life on Earth is somehow miraculous, but perhaps that is just hyperbole for “against long odds.”
Let’s face it; there is no evidence one way or the other. What is known is that life arose pretty much as soon as the earth cooled enough for it to be viable, which suggests to me that the odds are not long and therefore that it likely happened only once. On the other hand there is one and only one genetic code and that points to the likelihood that all life around now goes back to one origin. The reason for that is that the genetic code is essentially arbitrary.
For an analogy, think of the fact that hominid apes seemed to evolve a number of times, maybe as many as a dozen, but they all died out, except one. So one can fairly conclude that starting from the great apes that existed in Africa 10 or 12 million years ago, it was quite likely that some sort of hominid would evolve, but not necessarily homo sap.