In the spirit of current biology discussions, it always seems anomalous that all the evidence we have suggests that life arose on earth one time only.
Why is this? Did life arise multiple times when the circumstances were right and vanish without evidence, save for that life which did survive? Or did it in fact arise only once, even though the circumstances were right.
What is the significance of the evidence?
I’m inclined to believe that it arose only once. I suspect that the earliest forms of self-replicating “things” would be hard to consider life. The threshold from non-living to living is fuzzy at best, so defining when life actually arose is tough, but probably when it did, the conditions were right for it to quickly dominate.
My reasoning here may also seem a bit fuzzy, as it went through several revisions, but essentially, it is a hard question to answer without a more rigorous definition of what “life” we are talking about.
What I like about the field is that the uncertainty is so widespread. What indeed is life, what could “quickly dominate” mean in context?
However, it would perhaps be better to ask, why did only one life-creating event survive?
IIRC for most of the biological history, life was in simple forms, that would be unlikely to leave any direct evidence. So I surmise, that on the evidence it is possible multiple life events survived until pre-fossil/evidence-producing times. Is that possible?
What evidence do we have?
It hardly seems surprising that the first self-replicating proto-life would, well, self-replicate and spread over the world in a short (comparison with the time it would take for another bit of self-replicating proto-life to arise independently) time. Once that happened, a second independent origin of life would be unlikely, as some of the building blocks would be food for existing life forms.
Well, as others noted, it depends on what you call life. If you think single cell organisms, then we already have proof, that there is not a single origin of life but at least three of them: bacteria, eucaryotes and (another one I currently can’t remember, and are too lazy to google for you, maybe procaryotes or archae).
However, we also have evidence, that these single cell organisms are still compund organisms and that there must have been proto-organisms, that no longer exists for themselves. Mitochondriae are just one of the more famous examples.
Actually it is more likely, that many different proto-life molecule systems existed for a long time and developed independently, and a few of them might still be identifiable in nowadays cells, while the majority will be extinct by now due to the vast environmental changes that life brought upon our planet.
Through a casual google, I found that The Scripps Research Insititute did a few studies on the topic of proto-lif peptide systems, that are able to replicate. They “invented” a four peptide system that already has the major replication capabilities of RNA and DNA. So further googling in that direction might help you with your question.
Just my 2 cents, as this isn’t a topic I’m very much familiar with.
cu
This question is more complex than you might expect. Some answers:
Life COULD have arisen independantly on our planet, many times over. We can’t really know. It could have been eaten by the life we know today. It could have gone extinct because of error catastrophe or more conventional reasons. Or, more intriguingly, several forms of life could have arisen and then mixed and matched into the sort of life we know today! Indeed, the tree of common ancestry gets pretty messy as you approach the origin of life. Early lifeforms probably shared genetic material between “species.” They absorbed each other in symbiotic relationships like modern day mitochondira. So figuring out lines of descent is, well, tricky, and not simply because of a lack of knowledge.
However, most scientists would agree that any proto-life appearing since bacteria became king would quickly be devoured. Chemically, the modern earth is probably a LOT less conducive to life than early earth environments.
That doesn’t mean that life couldnt arise again here in other ways though.
One possibility I’d really like to hear more about is prions. Prions clearly have not always existed: they appeared instead in the chemically fertile world of living bodies. They are complex proteins that seem to reproduce via “recruitment.” Could they be potential fruit borne on the original tree of life?
It is suspected, for instance, that the sequence of 120 letters found in our chromosome I that is key to the operation of ribosomes is one of the most ancestral sequences around: no in the least because if you try to evolve raw RNA towards catalysis, you often end up with a sequence that it remarkably similar to it (the 5S gene). This has led many to suspect that the first organisms were sort of like free-living, self-sufficient RNA ribosomes. DNA was a development of these things that allowed them to live much longer and in more diverse environments.