Evolution theories with no universal common ancestor

[exchemist]

That seems to me like a better way of thinking for some purposes, including for research.

Except that the evidence of biochemistry does not support this idea.

While it is intuitively likely that rival biochemical systems may have been present at the start (and nobody I think challenges that), there is no evidence that more than one system is present in today's organisms.

That is what is meant by the universal common ancestor: only organisms based on one system have survived to the present day, or so it seems. Until such time as someone finds evidence of a second system, the universal common ancestor hypothesis will remain the assumption.

By the way this hypothesised LUCA may have been already complex enough to involve DNA. So it probably lived many millions of years after life first arose. There may well have been other competing organisms around when it lived that were based on rival systems. But from the biochemical evidence we have, none of them or their descendants persisted.

 

[Thief]

I’ve been discussing this in the “Evolution vs Creationism” forum, but it has nothing to do with creationism, so I’ve decided to post about it here. According to some current theories, some of the first lines of ancestry of living cells evolved separately from a pool or some pools of primitive protein factories sharing genes with each other. Eventually they divided up into three groups, which then evolved into the first species from which all other species evolved. There’s some disagreement between researchers about whether to call that pool or those pools an “ancestor.”

someone had to be first
every one else followed

 

[Jim]

While it is intuitively likely that rival biochemical systems may have been present at the start (and nobody I think challenges that), there is no evidence that more than one system is present in today's organisms.

What I’m thinking is that the chemistry that is common to all life today could have evolved separately and independently in many different places, or even in the same place many times.

 

[Thief]

What I’m thinking is that the chemistry that is common to all life today could have evolved separately and independently in many different places, or even in the same place many times.

so...more than one Adam

 

[Jim]

so...more than one Adam

I’ll answer that later, in the “Evolution vs Creationism” forum.

 

[exchemist]

What I’m thinking is that the chemistry that is common to all life today could have evolved separately and independently in many different places, or even in the same place many times.

Yes I agree, this is quite likely. However, I hope you can see this does NOT imply that the organisms alive today are descended from several of these. There is no evidence of that.

On the contrary, analyses of the common features of living organism have been made that result in an extensive list of things that would not be expected to be present if modern life were descended from more than one source. These range from common sections of DNA (genes), to common energy transmission chemistry (ADP<-> ATP) and the use of only 20 amino acids, out of hundreds that are possible. Significantly, these 20 are found exclusively in their L isomers.

This is evidence in favour of a universal common ancestor of all the lineages of life that have survived to the present day. The LUCA hypothesis has not just been made up, for some reason of evolutionary dogma. It is evidence-based.

 

[Jim]

What I mean by evolution theories with no universal common ancestor is that the researchers who have proposed those theories have said explicitly that in those theories there is no universal common ancestor.

 

[exchemist]

What I mean by evolution theories with no universal common ancestor is that the researchers who have proposed those theories have said explicitly that in those theories there is no universal common ancestor.

Can you provide references to these researchers and their work?

Bear in mind the references you have provided so far do not challenge the LUCA hypothesis, even though you originally thought that they did. My suspicion is that whatever you now have in mind will prove to be more of the same.

But I'd be really interested if there actually is credible evidence of multiple ancestry for life today.

 

[Jim]

Can you provide references to these researchers and their work?

I might have been wrong in thinking that there were more than one.

”Indeed, I too think that ‘LUCA was a population,’ but I argue that this in fact means that there actually was no LUCA” (Doolittle 2005: 125).

Universal common ancestry, LUCA, and the Tree of Life: three distinct hypotheses about the evolution of life

 

[exchemist]

I might have been wrong in thinking that there were more than one.

Universal common ancestry, LUCA, and the Tree of Life: three distinct hypotheses about the evolution of life

Aha. Thanks. You've posted this one before, but I re-read it more carefully this time in view of your comments above. It is quite interesting, I think. Several points strike me:

- This is not scientific research: there are no new observations reported here, so my hopes of exciting new biochemical findings remain unfulfilled, alas.

- It is not written by a biologist, but by a philosopher. Nothing wrong in that but we need to keep it in mind.

- It is an argument - as one would expect from a philosopher - to the effect that a distinction should be drawn between common ancestry and a common ancestor (i.e. a specific ancestor species).

This I thought was the real insight, as least as far as I was concerned. I and others have been belabouring the point that there is no evidence of different biochemistries at work in different forms of life alive today. This paper does not challenge that. What he is saying, if I understand correctly, is that at the stage of life we are talking about the very idea of a "species", i.e. a tribe of organisms which hand on their genetic material largely intact to their offspring, may not be appropriate (he quotes some sources). Instead there could, according to some of these models, have been a "pool" of proto-organisms, sharing the same biochemistry but among which genetic material was exchanged as much laterally as vertically. So, he says, these models suggest common ancestry, but without a single "ancestor species" as such.

Nothing in his paper argues that there could not have been a common ancestor, merely that it is not necessarily the case. However common ancestry, in the sense of a common pool of organisms with the same biochemistry, is something he regards as fairly certain.

So there we have it: this seems to make sense of both what I have been saying and what you have read.

 

[Jim]

]Indeed, I too think that “LUCA was a population,” but I argue that this in fact means that there actually was no LUCA.

- Doolittle WF (2005) If the Tree of Life fell, would we recognize the sound? In: Sapp J, Microbial phylogeny and evolution: concepts and controversies. Oxford University Press, Oxford, pp 119–133

 

[Jim]

As I understand it now, the universal common ancestor that most or all researchers agree on is some of the chemistry of life. Some examples are the amino acids that are used, and some of the mechanisms for coding and building proteins, which are identical in all forms of life today where they can be observed. That’s what most or all researchers agree on, and what some of them mean by “universal common ancestor.”

 

[Jim]

I see a possibility that life began in a multitude of places, and in some of the same places more than once. If so, then humans could have evolved separately from other forms of life, from the time that life began.

 

[exchemist]

I see a possibility that life began in a multitude of places, and in some of the same places more than once. If so, then humans could have evolved separately from other forms of life, from the time that life began.

So, the Jolly Roger is run up, at last, eh?

I had the feeling you have been angling towards this imbecile idea from the start, even though none of the discussion supports it in any way at all.

 

[Jim]

As I understand it, the idea of a tree of life is an analogy to a family tree that represents the descendants of one person. With that person at the bottom, it looks like a tree. Trees of life do the same thing with hypothesized species in the past, with one species branching off from another and then more species branching off from one or both of those. Various ways can be imagined of how that might have happened. One way is by some animals in a species being divided into two or more groups separated and isolated from each other. Then different groups evolve in different ways until no animal from one group can reproduce with any animal from any other group.

I think that most or all researchers have agreed on comparing genes between species as the best way to construct trees, but sometimes a tree using one gene has different branchings from a tree using other genes. That can be patched up by drawing lines across from some branches to others, like vines, but the farther back we look the more vines there are until there is nothing but vines. There’s some discussion among researchers whether the branches all merge into each other before they disappear into the vines, or whether two or three branches disappear into the vines without ever merging together at all, but I don’t think any of them would disagree that at some level the model they’re using looks more like a web or network than a tree. Some researchers have given up calling it a tree and started calling it a web or network. I haven’t seen anything in those discussions that excludes the possibility that life started out with a multitude of trunks or stems, at different times and in different places, with a part of the chemistry of a smaller multitude of those being the same chemistry that all life has in common today.

 

[Jim]

So, the Jolly Roger is run up, at last, eh?

I had the feeling you have been angling towards this imbecile idea from the start ...

For myself yes, but in the beginning I wasn’t thinking of actually posting it, and it wasn’t very clear in my mind. This discussion was only for my own education, to see if there was any reason to reject that possibility. One reason I’ve seen for rejecting it is for simplicity. Another is that it would be unlikely.

 

[Jim]

I’ve seen a few reports of probability calculations saying that a universal common ancestor model has a much higher probability than some models without a universal common ancestor, but I didn’t see specifically what those other models were. I don’t think there’s been any probability comparison with a model in which life began in a multitude of places with part of its chemistry being the same as the chemistry that’s common to all life today.

However that may be, I don’t see low probability as a reason for rejecting it. It looks to me like a lot of what happens in our universe has a probability that looks surprisingly low to some researchers.

 

[Jim]

Comparisons of probability between models with and without common ancestry, with a premise of only one place and time where life started with the same chemistry that is common to all life today, does nothing to prove that the premise is true.

What the comparisons say is that if there was only one place and time where life started with the same chemistry that is common to all life today, then the probability of universal common ancestry is much higher than the probability of no universal common ancestry. That says nothing about the probability that there really was only one place and time where life started that way.

 

[Jim]

I’m saying that the first life could have started in many different places, and more than once in the same place. As I understand it, no one is denying that. What I see people saying is that some parts of the chemistry of life are the same for all life today, and that means that if life started indifferent places and at different times, only one of those could be an ancestor of any life today, because if any life from any other time and place had survived, we would see some life with a part of its chemistry different from the chemistry that all life has in common today.

One possible explanation is that there might be some survival advantage in the chemistry that all life has in common today, and that’s why all the other life died out. If that’s true, if some kinds of life chemistry can exterminate all other kinds, then that could have happened at different times in different parts of the world, and even before life began. There could be a multitude of times and places where life began with the same chemistry that is common to all life today.

 

[Jim]

I’ll review what I’ve been learning about the chemistry of life, for context for what I’ll be saying.

Part of the chemistry of life, which makes it possible for it to do all things it does, in all its variety, is in the multitude of ways that proteins can be folded up into different shapes, like a Rubik’s snake. According to one author, with its 24 pieces a Rubik’s snake can be folded into at least 6,770,518,220,623 different shapes. Titin, a protein in muscles, has a chain of more than 30,000 pieces. Different shapes do different things. The pieces are amino acids, and they are strung together to make proteins, in sequences that are coded in genes.

One way that researchers have pictured the beginning of life is in a pocket of water with a multitude of teams of molecules building proteins following the codes in an ever-changing variety of genes, without any cellular structure. In that picture there are no cells creating copies of themselves with the same genes. The protein-building teams are tumbling around, bumping into each other, with genes passing back and forth between them. Some protein teams become self-perpetuating in different ways using different genes, and eventually the self-perpetuating teams divide up into two or three groups with three different sets of genes. Some time before or after that, some teams start creating copies of themselves with the same genes, and that’s where ancestry begins.