Did multicellularity evolve more than once?

[John Salerno]

I recently finished reading Richard Dawkins' book "The Ancestor's Tale" and when I reached the end I noticed that the family tree lists (multicellular) plants and fungi as preceding some single-celled organisms such as choanoflagellates and DRIPs.

 

I had always assumed (perhaps naively) that single-celled organisms came first, then multi-celled organisms evolved from them at one point, and that was that. (I understand that single-celled organisms still exist and did not die out when multicellularity appeared on the scene).

 

But unless I'm understanding the book incorrectly, it seems like certain multicellular organisms (e.g. plants and fungi) branched off from unicellular organisms early, and unicellular organisms persisted, and then branched again into other multicellular organisms, such as what eventually became animals. Is that correct?

 

 

[JN.]

Some fungi organisms are also single-celled, such as yeasts.

We do think, at least today, that the first ogranisms to appear were unicellular and simple ones, called procaryota. For example, bateria are procaryota, and they can be distinguished for they have not organels inside separated membranes.

 

Then the eucaryotic cells appear. The eucaryotic cells are bigger thant the procaryotic cells, and have different organels inside them, such as mitochondria. Our cells are eucaryotic, for example.

 

We do think that some organels, such as mitochondria and chloroplasts (and many other plasmids) were first independent procaryotic organisms, which suffered phagocytosis of a bigger procaryote organism. They were not digested, which means they stood inside the biggest cell, and they were kept there, because they could help the bigger organism getting some nutrients or converting them to energy. For the small organisms, being inside a bigger one was advantageous, because they were protected from other predators.

 

That's how we think now that eucaryotic cells evolved. By the way, we think that first appeared mitochondria, and only then chloroplasts, because although all eucaryotic organisms possess mitochondria, only plants have chloroplasts.

 

Then eucaryote cells gathered in colonies, and some cells became specialized in some functions, such as reproduction or locomotion. Although they were living in colonies, they could leave it, as they were independent organisms. However, they evolved to multicellular organisms. In multicellular organisms, the cells that compose them are not individual organisms, because they are all dependent of the other cells.

 

That's how I think evolution to multicellular life occured, and I think it is the most accepted theory. And, of course, single celled organisms persisted because they were also well adapted and evolution did not kill them. Not always multicellularity and a big size are a symbol of success.

[Mr Skeptic]

Multicellularity evolved multiple times independently. This is not entirely from scratch, since even unicellular life has adaptations to multicellular living both with self and other species as in colonies or biofilms. Prokaryotes such as myxobacteria are also multicellular.

[John Salerno]

Multicellularity evolved multiple times independently.

 

Hmm, how do you like that! All this time I just assumed it was a progression from single-celled to multi-celled. Talk about a paradigm shift.

[CharonY]

Multicellularity arose several times (even in prokaryotes, to some extent) throughout history, if that was the question.

 

Edit: cross-posted.

Edited August 10, 2010 by CharonY

[kitkat]

Multicellularity arose several times (even in prokaryotes, to some extent) throughout history, if that was the question.

 

Edit: cross-posted.

 

I have been previously taught that prokaryotes were strictly single celled organisms but now you are saying multicellularity arose several times including some prokaryotes. Is the nucleus now the only one that separates the eukaryotes from the prokaryotes? Do any single celled prokaryotes posses chloroplasts or mitochondria? What would make eukaryotes large enough to be able to ingest mitochondria in the first place? or is the digestion of mitochondria is why they are larger?

[CharonY]

Pro- and eukaryotes have very different cellular structures. In fact certain organelles as mitochondria were bacteria once. As such it is of course almost impossible that bacteria possess such organelles. While there are in fact bacteria that proliferate intracellular in other bacteria, they pretty much destroy the cell with their presence. Note that in prokaryotes only transient multicellular states are known (e.g. fruiting body formation) whereas in eukaryotes obviously stable multicellularity has evolved.

AFAIK it is still disputed how the first eukaryotic cell may have looked like, but IIRC it was assumed that it the first eukaryotic cell may have been a mix between archaea and bacteria. Also note that the first eukaryotic cells were very likely without mitchondria or plastids (mostly due to the comparisom of the timing when the nucleocytoplasm arose, relative to the ancestors of plastids and mitochondria).

Edited August 31, 2010 by CharonY

[kitkat]

Pro- and eukaryotes have very different cellular structures. In fact certain organelles as mitochondria were bacteria once. As such it is of course almost impossible that bacteria possess such organelles. While there are in fact bacteria that proliferate intracellular in other bacteria, they pretty much destroy the cell with their presence. Note that in prokaryotes only transient multicellular states are known (e.g. fruiting body formation) whereas in eukaryotes obviously stable multicellularity has evolved.

AFAIK it is still disputed how the first eukaryotic cell may have looked like, but IIRC it was assumed that it the first eukaryotic cell may have been a mix between archaea and bacteria. Also note that the first eukaryotic cells were very likely without mitchondria or plastids (mostly due to the comparisom of the timing when the nucleocytoplasm arose, relative to the ancestors of plastids and mitochondria).

 

How could the nucleocytoplasm arise when archaea and bacteria do not possess one? The mitochondria do not have a nucleus. Plastids what are they?

[CharonY]

The nucleocytoplasm contains elements that are most likely derived from archaea. There is a newer paper out there suggesting that the nucleus proper arose after the incorporation of mitchondria. Most likely the first eukaryotes did not possess a nuclear envelope, which evolved later from the endomembrane system. Note that the precise timing of the respective organelles are still under discussion, though.

[Maximus Semprus Veridius]

If i am correct in saying that if there is a niche fillable then an organism may well evolve into it. So I think that even though certain plants had evolved before certain Single-celled organisms then the niche for the Single celled organisms was available (and lets be honest how many plants infringe on single celled organism's niche). I may well have got the whole point of your thread wrong but it seems quite obvious that just because there are more advanced organisms (PLEASE IS THERE NO ABBREVIATION FOR THIS WORD???!!) that doesn't mean that less "advanced" organisms go extinct. (of course there are certain circumstances when exactly this happens!!!). Please correct me if im wrong. I would like to know if my logic (or lack of it) serves me well, for I do not acctually KNOW ther answer.