One has to be careful not to confuse a most recent common ancestor for some or most of our genes with the MRCA for all of them (except the genes which have evolved since the MRCA). Introgression of Neandertal and Denisovian genes pushes the MRCA for many human lineages several hundred thousand years further back than the traditional mtDNA and Y chromosomal lineage numbers.
To the question in the OP I would say that all of life came from the same type of molecules which gradually became “alive” for the various functions that are assembled to meet that definition but probably not in the sense of some single line of increasingly complex “organisms.”
What do you mean by “others”? If you mean mitochondrial lines and y-chromosome lines you’d be correct, but those are only a small part of our genetic material.
That’s the point. Somewhere in that primordial soup, replicating molecules formed. DNA is an example of this sort of molecule. DNA is an aggregation of amino acids (I think, never took organic chemistry). A simple collection of these will automatically attract the complement, forming the classic spiral. If this spiral starts to peel apart, the two sides will attract the required GATC so each repairs and you have two copies.
The classic Miller-Urey experiments showed that even a small sample of what was the likely early sea constituents formed organic compounds fairly quickly.
The thought was that with a sample the size of the entire world, and billions of years, the likelihood of even totally random collections of molecules forming the needed result is not impossible.
DNA is not made of amino acids; it’s made of nucleotides. Each set of three nucleotides (there are four of them to choose from) codes for an amino acid (there are twenty of those; some sets of three nucleotides code for the same amino acid). A string of many nucleotides therefore codes for a string of amino acids, which forms a protein molecule. Proteins include enzymes, which catalyze various chemical reactions in living things, which leads directly or indirectly to all of our various traits. DNA can’t replicate entirely by itself: Its structure does make it easy to replicate it, but you still need to have an energy source, the appropriate enzymes, and a supply of the four nucleotides to build from. Many viruses, for instance, have DNA (the others have RNA, a similar molecule, instead), but do not have their own energy source, enzymes, or supply of nucleotides, and thus rely on hijacking living cells to provide those.
Thanks. From what you’re saying and from the Miller experiments, there is a process by which the much more complex organic molecules can assemble from the very basic building blocks we observe in most of the universe. Earth, so far, is the only environment that came close to the ideal conditions for sustaining such reactions. General evidence seems to be that the gap between the earth cooling enough to allow this and the time when life definitely makes its mark would be about one to three billion years.
No. Quite apart from the error of saying that DNA consists of amino acids (which, frankly, shows that you have no idea of what you are blathering about), this is not correct. DNA molecules do not replicate by themselves. They only replicate in the right environment of other molecules, which, in the only cases where we have actually observed DNA to replicate, includes particular enzymes, which are large, complex protein molecules of very particular structure which arises only because it is coded for by DNA, and produced, according to that code, in the complex environment of a cell. Getting a self-replicating system going from scratch is a lot more complex than you seem to think. Almost certainly, life did not start with DNA, and certainly not with a bare DNA molecule.
I am not sure what this sentence is trying to say, but, as I pointed out in the post immediately above yours, there is no such thing as molecules (or anything else) “gradually” becoming alive (or even “alive”, let alone “'alive 'for various functions”). Either you have a self-replicating chemical system (which very likely requires more than one type of complex molecule to be present) or you do not. Once you do, evolution can get going. Without that, it can’t.
Well. actually, to get evolution going, you need a bit more than that. You need an imperfectly self replicating system, so that some of the “offspring” differ a bit from the parent system, but can still replicate. Without that sort of imperfection there will be no variation for natural selection to work on.
A simplified way of putting this is that transfer RNA has two sides. One side matches a codon. The other side matches an amino acid. It’s possible to connect just about any codon side with just about any amino acid side, so the code is arbitrary: you can make sets of artificial tRNA that could implement the code any way you want. Nature could have done that too.
To clarify: the other strains of mitochondria (the mitochondria from other ancestors) and the other strains of Y chromosomes (the Y chromosomes from other ancestors) all died out. But that’s a tiny bit of our genetic heritage, and we have lots of traits from lots of ancestors other than the MRCA for mitochondria and the MRCA for the Y chromosome!
Back to the OP’s question. There are at least two cases.
Either the first thing we’d all agree to call a cell was unique and is the ancestor of all living cells, or the conditions for the first cell to occur were “ripe” enough that multiple of them occurred around the same time, and we’re descended from the lot of them. My guess is that the first case is far more likely, and I believe that’s the common consensus, but it’s not a “fact”.
The truth is we can’t say a whole lot about abiogenesis because we really don’t know how it happened. Try the wikipedia article for lots of possibilities. So far, there are good arguments against any of the hypotheses being correct, and we have a lot to learn about how it might have happened. Until we know more, there’s not much we can say, other than “We’ve give that a lot of thought, and have a lot of good ideas, but so far no clear solution.”
Yeah, thanks, that’s why I said “I think” and “I did not study organic chemistry”. However, given a Miller experiment reaction vessel the size of the surface layer of our oceans and lakes, and running for two billion years IIRC, is it surprising that we ended up with what we have now or even that replicating molecules and then unicellular organisms eventually emerged?
Presumably any self-replicating molecule (or replication cycle of molecules) then proceeded to consume what it could of its environment; in a process analogous to evolution, the best-suited variations of that molecular structure survived, and any beneficial miscopies produced a new generation of more powerfully adapted copies.
Yes. There is an enormous distance between producing some amino acids in an reaction vessel and getting to anything close to self-replicating life. While there are plenty of theories, we still don’t have a very good idea of how this occurred.
We don’t know enough to say whether it’s surprising or not. Well, aside from the anthropic principle, which tells us that we shouldn’t be surprised by it no matter how rare or implausible it is.
What I was saying that the Miller-Urey experiment doesn’t lead immediately to the conclusion that the evolution of life was virtually inevitable given enough time, which is what md2000’s post implies.
The Miller-Urey experiment produced some of the basic building blocks of life. But understanding how some of the bricks could have been made doesn’t say much about how the Empire State Building came to be.
Those cases combine to give a third, most probable, case: Many different cells evolved, one of which was particularly successful at consumption and reproduction and whose progeny eventually starved out all the others. All existing life is descended from that particularly successful cell.
More like half a billion years at the very most (and probably half that). Still a long time. But I believe the most likely scenarios favored today aren’t the oceans, but slimy surfaces of clay and a few other possibilities.
Right: you can’t assess the probability for a process you can’t describe. However, I bet we can all agree that it’s “remarkable” and “amazing”.
If the universe is infinite, then we shouldn’t be the least bit surprised. If it’s finite, then we’d need to understand the process well enough to do the math. If so, the probability of abiogenesis might just be the best evidence we may have for how big the universe is likely to be – but only if we assume there’s only one universe like ours, which isn’t a very good assumption these days.
There’s the further complication that for all but eukaryotas (that is, for archea and prokaryotes), gene transfers happen between organisms without reproduction, and it’s common, so we might still have genes from more than one ancestor even in this case. (For bacteria, for example, it may make more sense to draw the evolutionary tree on a per-gene basis than per organism basis. Even that would gloss over a number of complications, since a gene could be created from the combination of multiple other genes, such as part of one gene being spliced into another.)
But that only works if the probability of abiogenesis is extremely low, and if we know what it is. The only way to know the probability is to look for life elsewhere and either find it or not. If we look and find it, then the probability of abiogenesis is high enough that it’s not useful to tell us anything about the size of the Universe. If we look many places and don’t find it, that just tells us that the Universe is larger than the set of places where we looked, which is something we already knew.