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ONE common ancestor??

SkepticalScience

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S
Joined
Jul 2, 2003
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I have trouble understanding something that I hope someone here can clear up.

I have no problem in accepting that life arose from some chemical soup. It makes sense to me on a lot of levels.

What I don’t have a clear picture of, is if science contends that ALL life is the byproduct of the FIRST self replicating molecule.

I can imagine a bunch of primordial soup puddles, and self replicating molecules being created in each of them. Couldn’t some of the life we see around us today be the descendants of Puddle 1 and other life forms have their ancestors in Puddle 2? If it can't, I'd like to know, how we know this.

Hope that made sense! I don’t want this thread to turn into a “there is no evidence for evolution” thread or something. I am just concerned with what current science says about this.

Thanks!
 
I've always wondered this, myself. If the conditions on the primordial Earth were adequate, I can only expect life to emerge several times, at different places. I find it hard, but not impossible, to believe that only one population survived to form today's entire ecosystem.
 
SkepticalScience said:
What I don’t have a clear picture of, is if science contends that ALL life is the byproduct of the FIRST self replicating molecule.

I can imagine a bunch of primordial soup puddles, and self replicating molecules being created in each of them. Couldn’t some of the life we see around us today be the descendants of Puddle 1 and other life forms have their ancestors in Puddle 2? If it can't, I'd like to know, how we know this.
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Well, there's too much biochemical similarity among all known forms of life for there to be too many puddles involved. Every life form above the virus (which really aren't free-standing "life forms"), for example, contains DNA, RNA, proteins composed of amino acids, and uses phospholipids to form a cell membrane.

Furthermore, every known lifeform uses the same genetic code. For example, RNA strands are always decoded in triplets, and the code itself is uniform (so CGG always codes for arginine, while UAA is a stop codon).

At this point, we don't really have much evidence beyond Occam's razor. But which do you think is more likely -- that several puddles independently decided to use the same set of molecules, and even to give them the same meaning, or that the decision was taken once and then became widespread?
 
SkepticalScience said:
I can imagine a bunch of primordial soup puddles, and self replicating molecules being created in each of them. Couldn’t some of the life we see around us today be the descendants of Puddle 1 and other life forms have their ancestors in Puddle 2? If it can't, I'd like to know, how we know this.
Click to expand...

The replication sequence - the sequence that makes one cell become two - is exactly the same in all life forms save for one bacterium, where the change is a single point mutation that doesn't affect the protein being coded.

Since there are a number of potential sequences that are still genetically viable, it would appear that the first sequence to appear simply took over the planet in a rather short period of time.
 
Xeriar said:
Since there are a number of potential sequences that are still genetically viable, it would appear that the first sequence to appear simply took over the planet in a rather short period of time.
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That's exactly the piece of science I'm curious about in this matter. Has it been demonstrated (theoretically, of course -- I'm aware that even the "normal" way hasn't been replicated artificially) that there could be self-replicating molecules that use different sequences or that code up in quads, for example? My admittedly non-trained application of Occam would have it that it's more likely that there is a way for life to form which is chemically far easier than other ways and that that particular way developed independently in several locations and times than it is that there was only one master life appearance or that a single appearance outcompeted all the others.
 
We might not be monoancestral, but we'll never really know. It might have been two competing replicators early on, with parts of each eventually subsumed. Personally, I think it was an arms race for resources until whichever one finally gained the ability to use light. Once all the soup is sopped up, where do you get your nutrients? They have to be made from other chemicals, not the free lunch of the early primordial soup, because predation will eventually wear thin. Additionally, eukaryotes (us) aren't really the product of one branch anyway- given some interpretations of mitochondria as endoparasites converted to symbiotic organisms and then obligitory symbiotes.
 
drkitten said:
Well, there's too much biochemical similarity among all known forms of life for there to be too many puddles involved. Every life form above the virus (which really aren't free-standing "life forms"), for example, contains DNA, RNA, proteins composed of amino acids, and uses phospholipids to form a cell membrane.

Furthermore, every known lifeform uses the same genetic code. For example, RNA strands are always decoded in triplets, and the code itself is uniform (so CGG always codes for arginine, while UAA is a stop codon).

At this point, we don't really have much evidence beyond Occam's razor. But which do you think is more likely -- that several puddles independently decided to use the same set of molecules, and even to give them the same meaning, or that the decision was taken once and then became widespread?
Click to expand...

It's also possible that many viable populations occured, and that one supplanted the others.
 
Also, haven't there been several near extinction events? Not talking dinosaurs here, but way before that. PreCambrian - before the evolution of more complex life forms. The "common ancestor" may not have prevailed due to being best at utilizing resources, but through sheer luck at having survived one of the extinction events. No way to know whether all the life forms before those events were genetically related or not.
 
manny said:
That's exactly the piece of science I'm curious about in this matter. Has it been demonstrated (theoretically, of course -- I'm aware that even the "normal" way hasn't been replicated artificially) that there could be self-replicating molecules that use different sequences or that code up in quads, for example? My admittedly non-trained application of Occam would have it that it's more likely that there is a way for life to form which is chemically far easier than other ways and that that particular way developed independently in several locations and times than it is that there was only one master life appearance or that a single appearance outcompeted all the others.
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I'm not aware of anything that gets coded in quad, but I do know other self-replicating molecules have been made using different proteins and bases.

What I was referring to though, is that there are several thousand potential ways to code the replication sequence all life on Earth has, in DNA with our 20 amino acids, but only one of those is expressed, suggesting a single ancestor.

The current theory is that there was a pre-RNA stage, sometimes called "PNA World", which became the "RNA World", possibly (but not yet certainly) due to superior efficiency, and then DNA evolved and we have the above.

John's comment also applies. It is known that life could not have evolved in our current environment. What did evolve was what was probably best-suited to the environment at the time, and once Earth became sufficiently oxigenated, it was no longer possible for another rival to show up.
 
Well, I can't say as I think either abiogensis or evolutionary theory covers this.

Consider:
We have several pathways for some of the more basic reactions in cells.
We have the halophile, acidophile, and thermophile archeobacteria, that are "mostly different" in pretty much all regards.
There is always the opportunity (see "mitochondria") for one form to invade, colonize, or otherwise parasitize/symboyote with another organism, such as the way that mitochondria are thought to potentially be originally an invasion of one cell by another.

So, I don't think we could tell if it was all one puddle or not.

One thing that I think we have to consider is that the liklihood of a self-reproducing system is probably low enough that as soon as one exists, it's going to spread like gangbusters. I mean, the spread of a bacterium is slow by automotive standards, but by geological standards it's lightning-fast.

Oh, and no, I surely don't mean the original self-replicating whatever-it-was was anything as complex as a bacteria. Lipid spheres, or something like that, perhaps.
 
Could the conditions in one puddle vs another have been so similar as to create the same signiture in our DNA? Would the conditions/contents of the puddle matter? Well, obviously the contents would, but could slightly different contents result in a drastically different arrangement?
 
eri said:
Could the conditions in one puddle vs another have been so similar as to create the same signiture in our DNA? Would the conditions/contents of the puddle matter? Well, obviously the contents would, but could slightly different contents result in a drastically different arrangement?
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It really seems unlikely that separate puddles would create the exact same sequence - many other sequences should be just as likely.
 
Obviously, this will never be known but I would think it would depend on the chances of life occurring and how fast the first life spread.

If life will spontaneously occur once every 100 million year and the first organism descendants will engulf the the earth in a million years, it would be likely that there is one ancestor. If you switch the odds, then it is likely that two or more ancestors combined to form a viable organism.

CBL
 
Belz... said:
It's also possible that many viable populations occured, and that one supplanted the others.
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From what I've read in this thread so far, everyone seems to be assuming that no two populations ("puddles") would be compatable. Why wouldn't they? Conditions along the equator, say, would be similar all around the planet, and chemistry is chemistry. If you duplicate the conditions of the experiment, you duplicate the result. A nutrient-laden lake in coastal Peru (using modern reference points for illustration) probably looks pretty much like a nutrient-laden lake along the Ivory Coast. Assuming everything but location to be the same, why couldn't two different but genetically compatible life forms arise in each place? If their decendents then eventually met...
 
I'm not a biology expert, but I find a lot of meaning in the knowledge that, according to evidence, the eye has evolved multiple times.

I simply take that to mean that evolution is a resilient process, and probably isn't so delicately balanced on certain "lucky" happenings like a certain individual puddle creating life.

Life probably evolved in multiple "puddles," whenever the necessary ingredients showed up. They may have originally had different DNA, but perhaps a certain type was best for reproduction...which lead to all life today having that type.

Over time, it seems that evolution might just naturally lead towards certain patterns that are most effective, with chance not really playing a part. If the eye can continually show up, then maybe homo sapiens and other common life forms will continue to show up also over time, even if the current population becomes extinct.

So, in short...I'd guess that life did show up in multiple puddles, starting with diversity, and simply evolved towards the most efficient form, which is why so many life forms today use similar methods and have similar DNA...

Of course, if there is an actual explanation, the biology people will know.
 
Xeriar said:
It really seems unlikely that separate puddles would create the exact same sequence - many other sequences should be just as likely.
Click to expand...

My own thinking is that DNA wasn't an issue at the point that life began, but came about afterwards. There are other, less-stable self-replicating molecules, and ones that do not take large, chance-designed enzymes, to decode and recode, as well.

The fact that not all species and organisms decode DNA quite the same would argue that there is some diversity, but I think what argues more is the kind of diversity shown by things like archeobacteria, some kinds of fungi, and a variety of niche organisms.
 
jj said:
My own thinking is that DNA wasn't an issue at the point that life began, but came about afterwards. There are other, less-stable self-replicating molecules, and ones that do not take large, chance-designed enzymes, to decode and recode, as well.
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I agree. The first was probably RNA, which can self-replicate more-or-less by itself.
 
IIRC, there's a fossil bed in Canada that had all sorts of strange creatures in it. They think the creatures co-existed with the trilobites, but were far superior in design, at least as far was we can tell. They existed in a lake, actually, that wasn't connected to the regular oceans.

The lake dried up.

Such is evolution.

I saw this on a PBS show, quite some time ago(10 yrs?).

So if someone could fill in the blanks, I would gladly accept the correction.

I toccurs to me that three is an optimal number for the coding. Two isn't enough and four is wasteful. There is reason to believe that the nucleus in Eurkaryotes and the mitochondria both were originally symbiotes with a different cell. Three ponds and several millions of years of evolution later...
 
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