The Real Common Ancestor?

[seer]

Isn't the real Common Ancestor for all living things really a single, primitive cell? Can't all life on earth (in the evolutionary model) be traced back to one cell?

[Leonhard]

Isn't the real Common Ancestor for all living things really a single, primitive cell? Can't all life on earth (in the evolutionary model) be traced back to one cell?

I think so yes, genetic sequences from all animals were compared, and the hypothesis that we descend from one species was found to be two billion times more likely than that we all descended from any multiple of species. And since it had to be a precursor of archea (extremophiles), prokaryotes (basically all bacteria) and eucaryotes (us, animals, plants, yeast, algeas), it was a simple primitive cell like creature.

[robrecht]

Wow, isn't science wonderful?

[seer]

I think so yes, genetic sequences from all animals were compared, and the hypothesis that we descend from one species was found to be two billion times more likely than that we all descended from any multiple of species. And since it had to be a precursor of archea (extremophiles), prokaryotes (basically all bacteria) and eucaryotes (us, animals, plants, yeast, algeas), it was a simple primitive cell like creature.

So one cell happens to survive, divides, they happen to survive long enough, and so on. I'm not sure yet, but something doesn't look right with this picture. There were no other cells that started a different genetic line?

[JonF]

Current thinking has it that things were somewhat muddled back then. It's not at all clear that the CA was something we would recognize as a cell. There's also some reason to believe that there was a lot of horizontal transfer of genetic material in the early days. (note that I did not say genes). So the root of the tree is unlikely to be one cell.

[seer]

Current thinking has it that things were somewhat muddled back then. It's not at all clear that the CA was something we would recognize as a cell. There's also some reason to believe that there was a lot of horizontal transfer of genetic material in the early days. (note that I did not say genes). So the root of the tree is unlikely to be one cell.

So then wouldn't we eventually have different lines that were not genetically related?

[JonF]

Apparently not. Horizontal transfer. Whatever existed back then homogenized genetically.

[seer]

Apparently not. Horizontal transfer. Whatever existed back then homogenized genetically.

But why would they be genetically homogenized? It seems to me that once you had different lines you had the real possibility of different genetic outcomes.

[Carrikature]

But why would they be genetically homogenized? It seems to me that once you had different lines you had the real possibility of different genetic outcomes.

Horizontal transfer means genetic material was copied from one to the next and vice versa. You start with different lines which then homogenize as material is transferred back and forth. Originally, the possibility of different genetic outcomes may have been possible. Once that material was interchanged enough times, that ceases to be the case. After enough transfer, you no longer have distinct genetic lines.

[seer]

Horizontal transfer means genetic material was copied from one to the next and vice versa. You start with different lines which then homogenize as material is transferred back and forth. Originally, the possibility of different genetic outcomes may have been possible. Once that material was interchanged enough times, that ceases to be the case. After enough transfer, you no longer have distinct genetic lines.

Transferred back and forth and interchanged between what? Different cells?

[Ucchedavāda]

Transferred back and forth and interchanged between what? Different cells?

Populations of cells which were capable of exchanging genetic material.
You might be interested in this paper, which explains this in the context of the optimization of the genetic code:
Collective evolution and the genetic code

The authors argue (among other things) that access to a much greater gene-pool (through a common genetic code) allowed those organism who shared that genetic code to out-compete those populations who had access to a more restricted gene-pool.

[seer]

Populations of cells which were capable of exchanging genetic material.

So we know that these early cells were capable of exchanging genetic material? Do cells today exchange genetic material? And wouldn't this still lead us back to one primitive cell? I mean for cells to exchange genetic material you would think that they needed the same parent.

[Carrikature]

Transferred back and forth and interchanged between what? Different cells?

Not necessarily cells. Transfer of genetic material is possible without both entities being a cell. Viruses are a great example of this.

So we know that these early cells were capable of exchanging genetic material? Do cells today exchange genetic material? And wouldn't this still lead us back to one primitive cell? I mean for cells to exchange genetic material you would think that they needed the same parent.

Yes, they still exchange genetic material. Check out this wiki article on Horizontal Gene Transfer.

I think it could be said to lead us back to one population of primitive cells. I don't think there's any indication it came down to a single cell, but I could be wrong.

No, genetic material can be exchanged in a number of ways without needing the same parent. Viruses actually insert RNA or DNA into a host cell as part of their replication cycle. Sometimes that new section of code can be passed on to descendants, but it's relatively rare afaik.

[Ucchedavāda]

So we know that these early cells were capable of exchanging genetic material? Do cells today exchange genetic material?

It seems likely that past primitive cells could take up DNA from the environment, since modern cells are capable of doing this, both passively and actively. And as Carrikature points out, viruses offers another means of transferring genetic material between cells. Furthermore, if we assume that proto-life existed without cellular membranes at some point, then exchange of genetic material would have been unavoidable.

And wouldn't this still lead us back to one primitive cell? I mean for cells to exchange genetic material you would think that they needed the same parent.

No, they need a (reasonably) compatible genetic code. See the paper I linked above. Furthermore, if you do have exchange of genetic material in this manner, the notion of pointing to "one primitive cell" becomes rather meaningless, since any cell you can point to will be a mosaic.

[seer]

It seems likely that past primitive cells could take up DNA from the environment, since modern cells are capable of doing this, both passively and actively. And as Carrikature points out, viruses offers another means of transferring genetic material between cells. Furthermore, if we assume that proto-life existed without cellular membranes at some point, then exchange of genetic material would have been unavoidable.


No, they need a (reasonably) compatible genetic code. See the paper I linked above. Furthermore, if you do have exchange of genetic material in this manner, the notion of pointing to "one primitive cell" becomes rather meaningless, since any cell you can point to will be a mosaic.

Ok, that sounds plausible. Well, if it actually happened.