Is there a unique ancestor of all life?

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Grinkle
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Summary:

Is there known to be a single ancestor for all life that exists today?

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I don't know enough biology to ask this question in a well framed manner.

I don't know at what point a primitive self-replicating thing is considered a living individual - I expect that is a matter where reasonable people can disagree.

I am sure that is not the only ill-defined concept in my question.

Is it known that there was a single individual living entity that is a common ancestor for all living inidividuals on earth today?

Or is it possible that life started more than once independently on earth and there are living individuals today on earth that do not share a common ancestor?
 
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phyzguy
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No one knows for certain, but I think it is widely believed that all living things on Earth are descended from a common ancestor. Charles Darwin said, "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."

In support of this, there are aspects of the genetic code which are common to all living things, and it seems highly unlikely that the same code would evolve twice independently.
 
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Summary:: Is there known to be a single ancestor for all life that exists today?
I presume you mean Earth life, because it is pretty implausible to suggest abiogenesis being a one-time event on the scale of the universe.

Is it known that there was a single individual living entity that is a common ancestor for all living inidividuals on earth today?
Either abiogenesis (an incredibly rare event that might occur less than once per galaxy) occurred multiple times, or yes, there was one such event which gave rise to all life as we know it.

Or is it possible that life started more than once independently on earth and there are living individuals today on earth that do not share a common ancestor?
This requires one further implausible premise: That life started a second time and that somehow both unrelated lines live to this day. No, one would have quickly wiped out the other I'd think.
 
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phinds
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... abiogenesis (an incredibly rare event that might occur less than once per galaxy)
Considering that there are a few billions of solar systems in the galaxy and likely two or three times that many planets, that would seem to be an incredibly low chance. We don't know of course, but I'm just sayin' ...
 
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BillTre
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I would first consider what is meant by "single ancestor".
Based upon the aforementioned logic (@phyzguy, @Halc), would expect something equivalent to an inter breeding species of individual pre-organism-things, to be what preceded and lead to the first "living individual".
Of course, it depends on how you want to define when it (the pre-organism) is living.
I am using organism and pre-organism to label things of either side of the live/not yet alive divide. But I don't really think those terms are all that well defined.

My expectation of how "living organisms" arose via abiogenesis involves many individual of molecular collections, housed in a bi-lipid membrane, that can generate useful metabolic energy, and reproduce and reconstruct themselves using stored genetic information.

The details of how this evolved and which steps occurred in what order, are not obvious.
However, it is widely thought that however the eventually living things were generated, they probably came out of a population of pre-living things which exchange genetic information and molecular components from time to time. This would be similar to lateral gene transfer. (Lateral: between units of the same generation as opposed to Vertical: inherited by offspring unit from parent unit.)
The leading theories of abiogenesis (alkaline hydrothermal vent site or hydrothermal ponds on early volcanoes) are based on large numbers of similar units (but with individual differences in their molecular compositions) are generated by geo-chemical forces.
The generated units compete for resources and probably exchange materials (molecular and genetic) as they interact.
This whole mass might gradually cross the line to the "living", not all at once but as a population over the span of several generations.

Such a transition could be seen as a single ancestral group, but not a single individual.
At the individual level it might be a flurry units achieving "life" in a relatively short time period, while at the population level it could be seen as a single transition of the population (defined by some degree of exchange), extended over a number of generations.
The dominant view of speciation would probably be the more similar to the population view, than a single individual founding organism, although cases (like this clonal origin) are possible.
Therefore I would consider this kind of origin (from the population point of view) as a single event.

Should abiogenesis processes in another location approach the living state independently, they might be able to achieve an independent origin of life on earth.
However, if the second group was too far behind in time, the previously formed and now rapidly evolving organisms would have had time to make great advances in their biological functions (like metaboism and genetics), which would put any newer (and therefore simpler) living forms at great selective disadvantages to the older ones (which have become through evolution, better tuned to their particular environments).

More recently originated versions of life might have been able to use leftover parts (molecules) of dead living entities to add (through these material acquisitions) to their informational/functional repertoire (importing pre-evolved genetics).
 
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