[ Disclaimer – I don’t claim to be trained in the sciences, and I’m also not intending this as a religious question. ]
One view is that early in Earth’s existence, perhaps in the first half-billion years or so, some confluence of chemicals came together, and some surge of energy (lightning?) caused the first unit of ‘life’ to exist.
Any description I’ve ever seen (including Bill Bryson’s ‘Short history…’) implies this happened one time only, and that all life today evolved from that one bundle of life.
My question is ; Why just once? Wouldn’t it statistically have been likely to happen multiple times?
And, taking it further, why didn’t it keep happening? For that matter why doesn’t it happen sometimes today? Granted the makeup of the Earth’s atmosphere and oceans have changed, but all the basic elements and compounds, admittedly in different proportions, are still there.
In short, if it could happen once then why hasn’t there been many (potentially different?) starting points for ‘life’?
The usual answer is that it may have happened more than once but that our form of life was already in place and simply out-competed the new form. It then died off without any traces in the fossil record. (Likely considering how few fossils we have from those early conditions.)
I’m not a biologist, but my understanding is that all life that we find today has enough similarities that it is extremely unlikely that there is not a common ancestor.
But that doesn’t mean that life only started once. Remember that life not only had to start, but to spread and survive. Who knows how many times something started up just to be killed by the next flash of lightning, or a bad chemical reaction.
Also, if two different (and likely incompatible) primitive life forms came into contact, one would likely survive better and push the other out to die, much as when two competing species vie for resources and one survives and one doesn’t.
Of course if that did happen, it was long enough back that no traces remain.
So life as we know it may just be the last or most successful time it happened.
And that about sums it up. Any lifeform that is in the pre-celullar stage is just a big glob of insensate, immobile organic polymers. We call big globs of insensate, immobile organic polymers lunch. Any organims that already existed and had the ability to repsond to their environment and/or move would eat any such conglomeration of pre-biotic molecules as soon as it formed.
The question is really like asking why we can’t put baby rabbits into the lion house at the zoo. After all adult rabbits live alongside lions in the wild. You simply can’t put an undeveloped organism into an envrionment with a fully developed predator and expect it to survive.
I’m probably going to mess up this question, but here goes:
I thought I had read somewhere that certain cellular structures may be the result of two independent organisms merging? In that case, wouldn’t it be proper to say that life descended from a group of common ancestors rather than a single one?
There is certainly strong evidence that organelles like mitochondria and chloroplasts are ‘fixed’ symbiotic bacteria.
But that doesn’t mean that life is descended from multiple ancestors. Both the bacteria that gave rise to mitochondria and they organisms they invaded already shared a single common ancestor. As such it was simply a merging of two lines that were already interrelated. The last common ancestor remained exactly the same.
And, by the same token, there’s no reason to think that whatever gave rise to the original beginning protolife forms couldn’t be happening in some form now. But, as we have been told above, it’s not likely to get much of a toe hold against the rest of what’s already here. Competition for resources being the essential concept in the business about survival.
What are current guesses for how abiogenesis happened? Are there any theories considered fairly likely, or is it still all sort of a mystery? I know my high school biology textbook had an overview, but I don’t know how current that knowledge would be nowadays (or whether it was even correct then).
There are literally dozens, anything from variations on the old Urey-Miller experiment to RNA self-catalysis to lipid caps on micropores to ‘living’ clays to extraterrestrial origins. The probelm is that we have no real evidence to work with, so this really pushes the edge of what we can call ‘science’. Imaginations are free to run wild becuase it’s so hard to emphatically falsify most hypotheses.
It’s still a mystery. that;s why the hypotheses vary so wildly. Any guess is as good as any other when you have seentially no evience beyond "we know it must have happened. We can’t even conclude that it is more probable terrestrial life originated one Earth rather than somewhere else.
By the way, I’ve read that a new, similar abiogenesis is very unlikely today because of oxygen in our atmosphere which would tend to react with the fragile proto-life molecules. Billions of years ago the atmosphere didn’t have oxygen.
Th book still isn’t closed on exotic lifeforms on earth that aren’t related to those that we know. Life has been found in all kinds of harsh conditions like sea floor vents and deep ice. It is possible that we may have already found something that isn’t really related and we just don’t know it or, like life on another planet, we just don’t really know what we are looking at when we see it. There could be life very deep underground or in some harsh place that we haven’t tested thoroughly.