Klax
(The only thing that matters is faith expressed in love.)
41
My unreserved apologies. I’m seeing theism when it isn’t there. But… we are talking origins…
There is no doubt in my mind that the pre-LUCA, i.e. pre-genetic code “with ‘high fidelity replication’ and ‘and at least some rudimentary early form of molecular proofreading’”, Ur-organisms of late biogenesis engaged in gene transfer, just as all organisms do today. As per the hypotheses link.
My comment “Once we had Theobald’s (2010) and Saey’s (2010) LUCA there was no possibility of gene transfer from any other LUCAs” is based on the feeling that gene transfer from other LUCAs that all died out in the late Hadean-Eoarchean would have been prevented as as each LUCA will have had a different set of amino acids? Assuming different triplet coding bases? Which seeing that all four have been found found in meteorites would indicate not… so what other mechanisms would have blocked gene transfer from other LUCAs which died out? Perhaps none, but our LUCA dominated, out-competed them to extinction. How?
What would have differentiated competing LUCAs engaging in gene transfer? Or with sufficient gene transfer did they all homogenize to a single dominant LUCA with no more possible gene transfer from lesser LUCAs?
All the evidence is for LUCA. That is agreed. By science. Therefore how can there be any disagreement here?
Klax
(The only thing that matters is faith expressed in love.)
42
LUCA has nothing to do with the origin of life either. LUCA would have existed well after life first appeared on Earth.
The organisms that became chloroplasts and mitochondria would have already had informational and instructional nucleotides. Also, the endosymbiotic events that define modern eukaryotes would have happened well after LUCA and the origin of life.
I fully agree. LUCA doesn’t seem to be a good entry point for understanding how life started, only how different lineages coalesced into the trunk of the Tree of Life we see now. It could be that there were multiple origins of life where some initial life forms focused more on genetic copying and others focused more on metabolism.
Not particularly thinking of NASA but of the general phylogenetic analyses on protists and eukaryotes. The shared genetic code and other similarities across all known earth life makes a strong case that the life that we see on earth all has a single common ancestor. I don’t know of any particular biological reason to think that one versus more lineages of life are more likely. On the one hand, given that the early chemistry of earth seems to have been suitable for abiogenesis to happen once, it would seem credible that it could happen more than once. On the other hand, once one lineage got started, it might take over and gobble up relevant chemicals before anything else can get a start.
A single common ancestor of all current life on earth could represent the lone survivors of an extinction event, a lineage that gradually outcompeted its rivals, merely the random point where surviving lineages converge within a broader ancestral group, etc.
Interesting thoughts. But yes I think there are some pretty obvious reasons. Purely random variation is pretty much only found in viruses, i.e. the simplest of organism. Thus it seems to be the general rule that more complex organisms require greater controls on variation because too many of the possible variations are lethal.
However this does raise an interesting thought. Is it possible that there is a stage in the evolutionary process where these strategies have some overlap? Are there eukaryotes which engage in horizontal gene transfer? Apparently the discovery that this is the case is a recent development in biology which is shedding some new light on things. It suggests a time in our ancestry where all the evolutionary developments of single celled organisms could be shared. It means that any notions of a LUCA can only apply afterwards when such gene sharing was no longer a part of our ancestry, and before that any of the cells engaging in HGT with our ancestors would be included in our universal common ancestry.
2 Likes
Klax
(The only thing that matters is faith expressed in love.)
49
(a) then we agree. That’s what I meant, sorry for the ambiguity.
The evolution of genetic code with ‘high fidelity replication’ and ‘and at least some rudimentary early form of molecular proofreading’ = LUCA. Or = the nucleus organism involved in the endosymbiosis that became LUCA?
I assume you mean a single cell at one time as the parent of all living things today? Thus our lineage parallels (+/-) bacteria, fungi, pine trees. That has been my feeling as well, fwiw.
The best reason, logicall, to assume more than one is simply the diversity of all life. Many of us struggle to believe that our cousins are bacteria.
I suppose so. But the common theme for origins is always the incredibly unlikely sequences of events necessary for a cell to become “living”. It’s sort of like the probability issues of a monkey hammering out the Gettysburg Address on the typewriter.
Always on the table, as is seeding from alien visits. But in every case, the same question arises about the “original” parent, one time occurrence or the necessity for many.
That clearly could speed the process up a bit. Left unanswered (among the thousands of things such) is the Cambrian explosion, with 27 phyla exploding on the scene. Could HGT account for that, rather than believe in major mutations of the Hox genes without cellular death? And even accepting that possibility does little to address whether there were many “original cells” that began to exchange material, both developed and important, without the necessity for deriving it via parental lineage.
Perhaps, and if so we really have a problem defining what is meant by LUCA. Many have used terms such as Last Eukaryotic Common Ancestor or LECA to jump past the original, include much of the HGT/LGT, and move forward with evolution. Your question may require many L?CA’s to fill the empty places in the “tree of life”. I note that researchers have wide ranges for the amount of current material that could be represented by that process, and the article you linked suggests only 1%.
Klax
(The only thing that matters is faith expressed in love.)
52
The wood from all the trees we know tell of, including fossil (but not coal obviously…), is from the last universal common tree.
Note that part of that may be sampling bias–we have a couple of Lagerstaetten in the Ediacaran and Cambrian, but very few from a bit earlier. Hence, some of those phyla may have appeared a bit earlier. There is also the problem that without exceptional preservation, most phyla are simply generic worms. That also makes basal members very difficult to identify.
Klax
(The only thing that matters is faith expressed in love.)
54
Between the endosymbionts (organelle Ur-organisms?) making them more interdependent and compatible?
And
How many different Gettysburg Addresses could there have been? For the sharpshooter? The probability going forward was 1.
To this point, I am guided again by the forces that direct evolution, selective advantages. No matter how an organelle arrived, if it worked better that cell’s progeny had an advantage. I have found nothing to supplant basic Darwinism on that point.
And the monkeys at the typewriter, I’ve read so much written by mathematicians about the statistical odds against what we know happened actually happening (of course it did, so there is that ). It approaches one chance out of 10^90 which is like finding one atom in the universe (there are between 10^78 and 10^82, or some such absurdity). How are we to derive meaning from improbable occurrences when it actually turns out that it is 1?
Klax
(The only thing that matters is faith expressed in love.)
56
Aye, even excluding genetics and symbiogenesis, all is well for LUCA.
And we agree, there is no meaning in specious improbabilities.
The forward arrow of time guarantees that the highly improbable will occur. It is inescapable. As an example, the very act of shuffling a deck of cards will produce an order of cards with a probability of 1 in 8x10^67.
And yet the highly improbable will occur. In fact, all things will occur? You are operating under the axiom that infinite time demands that all events possible will occur. But if we are bound into a sequence that does not allow for a certain combination, then it shall not occur during infinity. I’m not saying that we are living in a sequence, but who knows?
Klax
(The only thing that matters is faith expressed in love.)
59
No 'e ain’t. Because reality does not. Many worlds is utter bunk for a start. Even if it were proved that some irrational numbers contain all number sequences. Including infinite ones… We don’t live in a number. How many possible cytochromes are there?
On this point, we are in total agreement. And we find T_aquaticus’ reasoning flawed. The “forward arrow of time” cannot be known, but we fully understand that time is finite in the past. So even if infinite in the future, not all possible outcomes will occur, just as the infinite of 1011011101111011111…will never include a 5.
we are talking origins…