Yes, there’s some of that, and that makes sense. So there’s sort of a relative pattern in the code, but no absolute pattern, if that makes sense. You could pick up that chunk of related codon/amino acid pairs and move the amino acids to a different chunk of similarly related codons, and it would still work correctly, even in the long term.
If you are willing to believe it occurred a billion times, why not willing to believe that a couple of times were identical, and that those also survived as indistinguishable, just more of the genome? We know a virus can invade a cell and if the cell is simply dividing, then the virus can go with it, spreading throughout the population, and spreading outside it as well.
Because the relation of a billion to 21[sup]64[/sup] is for all practical purposes the same as 1 to 21[sup]64[/sup].
Even if it happens a billion times the odds against any two being identical is zero or so close to zero that it could happen a billion times a second every day for a billion years and still be impossible. (About 10[sup]59[/sup] to 1 if I got my math straight.)
60k years ago, maybe 4 species of Homo, if we assume some remnant Erectus populations existed in Asia. The last non-sapien hominid to die out was H. floresiensis, which can be found up to 12k year ago. Neanderthals went extinct about 28k years ago.
I don’t think this is true - it may be very rare for something to get started, but there’s still only a small range of ways that it can feasibly work.
Over 4 billion years? Quite good odds, really, since it did happen.
That’s probably not true. Read “At Home in the Universe” by Stuart Kauffman for more insight on this.
I guess it’s not impossible that it happened a billion times and two of those examples were similar enough to combine at some early stage.
It’s also possible that it happened multiple times and despite their inherent differences, several different cases merged together to form the ancestor of all living things. At the really early stages, things would have been simpler and the possibility of merger less astronomically unlikely.
It’s possible that it happened more than once and more than one lineage descended today, but that we just haven’t encountered any members of the other ones yet - maybe they’re tucked away in some closed subterranean system, or something.
What is unlikely beyond belief is the notion that abiogenesis happened more than once and two similar and parallel, but separate lineages descended all the way down to the organisms we know about today. Because that’s not only two similar and independent starts, but two similar sets of everything else that has happened since (and that’s a lot of everythings)
As Chronos stated, there are many, many possible combinations of the 64 codons. There’s no reason to think that the 20 that wound up being used are the only way it could be work, or that only a few exist. And as Mangetout said, the notion that a variety of starts could all lead to this same DNA is beyond belief.
What is more likely is that once any version of DNA got established, it could outcompete any new start or less complete version of life or a replicator. But that original version of a successful DNA just happened to be the one we have now rather than a version that must have occurred for structural reasons.
Could several different versions make it all the way to a complete DNA and then one outcompeted the other? Logically possible but again exceptionally unlikely.
I’d like to offer the chirality of DNA as evidence that we all share a common ancestor. Life may have arose multiple times, but our common ancestor beat the snot out of those alterna-forms.
Or just ate them.
Or quite possibly just got lucky.
Keep in mind that we have little idea what abiogenesis is, how it happened, or, actually, anything. Multiple independent lineages could have existed before RNA (and the genetic code) came into being. But we have no idea what those lineages were, what they looked like, or how to think of them in an evolutionary way (during a period when evolution, as we know it, did not exist… no, i mean it, we have no frikkin clue how non-genetic life worked!). So you can’t go too far back.
Then there are nanobes and nanobacteria, which might be extant samples of life from that epoch, and which we are having trouble analyzing. Here is some food for thought: (courtesy wiki’s Nanobacterium article)
Interpret that as you will… (and frankly I’ll have to do some more reading to get a picture of it myself).
But here is the problem. The same as someone already mentioned. We have great trouble growing them, and we only tend to study things we can grow. Plus, these things were found in rocks deep underground. We haven’t studied that biosphere all that much, even though it may be the largest on earth, may be the predominant one on other planets, may have been the origin of life on earth, and may be the best place for all the misfits who got out-competed by prokaryotes to still hang out.
So I think we’ve turned over a lot fewer rocks than some here have argued.
Well, it’s the beauty of science, not the flaw that gets tossed around on creationist sites, that an incontrovertible piece of confounding evidence will either force a reappraisal of existing small-t theories or lead to new pathways and understandings.
A find that is evidence of a second pathway to life would be about the greatest discovery of the ages. In this 40th anniversary of the moon landing we saw lots of hype from the time saying that the day was the greatest achievement of human history. Non-DNA life would be a better candidate for that, IMO. Extrapolating from two points is far more interesting than from just one. Look at how all understanding of solar system formation changed almost overnight with the discovery of extra-solar planets.
The only hitch is that nobody has yet found this second form of life. It’s a matter of if we do, and not when. It my never happen. Unless and until it does, science, a consensus body, has to put speculation off to one side for lighter moments and concentrate on what exists. What exists is unitary and all descriptions of the history of what exists has to assume that single pathway.
Having good notions of where and how to look for additional evidence is one of the hallmarks of good science, as opposed to amateur speculation. Scientists keep looking at odder and previously less accessible sites and finding odder forms that no one had previously suspected. Thus far they are all been related. More new and odd forms will be found in the future. No one can say if they all will be related. That jump simply can’t be made until there is something to jump to.
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Well, it’s certainly a better system than resistance to incontrovertible evidence. But why do you (and others like you) celebrate the fact that it has to be incontrovertible? Do you not think it is a sign of stubborness (not changing one’s mind until uncontrovertible evidence shows up, that is, as opposed to merely convincing)?
Consider it this way: If incontrovertible evidence shows up and proves a standing presumption false, does that not mean the old evidence was of a standard rather less than ‘incontrovertible’? Ergo, you advocate having a higher standard evidence for new ideas than for old ones.
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No, it means the old evidence was the best we had at the time. Of all the things science is, it’s not oracular. We can’t make hypotheses and theories with evidence that we can’t measure or haven’t found yet. This doesn’t denigrate the standard, it only notes that science is very much an empirical method, hostage to the fineness and scope of our detection methods. And it never makes any kind of argument otherwise.
Incontrovertible in science is only incontrovertible as far as the evidence goes. New evidence can throw out currently indisputable theories. But if the theory has been shown to hold for a 100 years or so with all the intermediate evidence, then the new evidence better be solid. ETA: see the amount of evidence Darwin supplied for his theory as a good example of the amount of work it can take to make a convincing case for a “paradigm shift”.
Anyway, evidence that shows those nanobacteria for example would not have a common ancestor with us would be pretty amazing, but it won’t really throw out the notion that all the studied organisms so far do have a common ancestor. That would require a lot more evidence - though it would make the idea that there could be more organisms that don’t share our family tree a lot more probable.
We’re using differing definitions.
Check any issue of, say, New Scientist magazine. It will be stuffed full of reports about new research and findings. Although the headlines often make it appear that these findings change everything - they want to sell copies - reading the articles to the end reveals that while some scientists accept the results, others are skeptical or reject it entirely. And they are often, perhaps usually, quite right to do so, because the next issue will have another round of new research and findings, sometimes on the same topic, with other scientists making other claims to be evaluated. And most of these breakthroughs quietly go away, because they can’t be replicated or because the correlations were too weak or because better research provides better information to work with.
Science is almost entirely a field of tiny incremental findings adding up to a greater whole. Game changers are incredibly rare. And game changers do require better than normal evidence. Other scientists will challenge it in a thousand ways, because that’s the only way to know if it stands up.
Again, science is a consensus body of information. Even those researchers that New Scientist pours loving attention on are using standard and accepted and consensus science for 99.9% of their work and understanding. Even those tiny incremental steps where they are in new territory are studied and torn apart by everybody else in the field. If it looks good then others in the field will try to use that bit and see how it attaches to the field and what other questions it can answer or what other findings it can lead to.
If it happens with the small in a tiny specialized field, the need for a game changer to be more than merely convincing is many orders of magnitude higher, because it will affect the work and understanding of orders of magnitude more scientists. They will all quite properly tear into it. It changes everything they do everyday on every subject. It changes the way they think about everything they do in the future. If I were them I’d require mountains of proof. You can decide what word is strong enough to use for this. Do they have to be convinced? Does the evidence have to be absolute? Is the proof incontrovertible? Beyond a reasonable doubt? Good to the 0.001 confidence level?
You can find very good scientists convinced of all sorts of things. That the speed of light is not a barrier. That gravity changes over large distances. That an infinite numbers of universes exist that are adjacent to ours that we can’t see. Scientists are always battering against the certainties of the past. Does the mountain of evidence exist for these things sufficient to add them to the consensus of science? Not yet.
But if good science by good scientists convinced of its rightness isn’t yet enough to sway opinion, how is bad science by non-scientists going to make any mark at all? It doesn’t and it shouldn’t.
If a piece of evidence comes along for a second kind of life, it will be properly doubted and attacked and studied and restudied and confirmed and debunked and reconfirmed and redebunked by every expert in the field. When will it be accepted? When the mountain grows high enough. A hillock won’t work. Scientists already sit on top of mountains, built by tens of thousands of scientists over centuries. You can’t tear down a mountain or shove it aside. You can only add another peak to the range.
While DNA chirality is a piece of evidence for all life having a common ancestor, it’s a pretty weak one by itself, since there are only two possible chiralities. It’s inconceivable that two independent life-origin events would end up with the same genetic code, but it’s not at all implausible that two independent life-origin events might end up with the same chirality by chance.
I disagree that it is incontrovertible that different lineages could come up with the same genetic code. It could very well be simply the most likely one to succeed. It is not proven that all other genetic codes have equal probability or any probability at all.
Considering the chirality issue. Not only is it a complete non-issue, but there is no need for life at all in order to propagate complete conversion of organic compounds to one chirality. It is likely that the process that develops the chemistry of life was autocatalytic. Since it is by necessity chiral, the process falls into the category of asymmetric autocatalytic enhancement. In such a system, a purely racemic mixture pulls to one chirality at an exponential rate because the first chiral products go on to catalyze the formation of the second. Even an immeasurable excess of one chirality will quickly wash out the others.
You don’t seem to appreciate the enormity of the problem. Let’s say, for the sake of argument, that the genetic code we have is better than 99.99999% of all possible genetic codes, and that life arose a billion separate times, and each separate lineage evolved a genetic code that was just as good as the one we have. The odds of a match between any two of them would still be about 1 in 10[sup]58[/sup] against.
And of course, that’s even assuming that the other life forms would have the same basic structure we have, with DNA encoding information in codons of three base-pairs, each of which codes to a different amino acid. Life could presumably also have ladder-shaped molecules made out of some completely different set of building blocks, or maybe even have a foundation that isn’t based on ladder-shaped molecules at all. The only reason I’m leaving those possibilities out of the calculation is that I have no idea how to numerically account for them.