How difficult is the transition to multi-cellular life?

[Avalancheon]

Our galaxy is over 13 billion years old, and contains 400 billion stars. This strongly implys that many advanced civilisations ought to have come into existence by now. And yet, we find no real evidence of intelligent life beyond our own. Therefore, there must be some barrier that prevents life from evolving to the point where it can spread among the galaxy.

Lately, I have been wondering what kind of evolutionary step could be so incredibly difficult as to leave the galaxy barren and lifeless for this amount of time. Even with billions of potential biospheres that could spawn intelligent life, it has ended up with nothing. In my opinion, therefore, the great filter must make an appearance at a very early stage. It stops the progress of life that is young and primitive.

What I am undecided on is whether or not the transition from single cell to multi cellular life could be a candidate. I have found two different sources making opposite arguments for this. Richard Grosberg claims that the transition isn't that hard, as it has independently happened many times in earths history. However, Nick Lane argues that the transition was a freakish anomaly, and that it has only happened once in earths history.


I would like to get a discussion going on this subject. Of the two authors, who do you think made the better case? Just how likely was the eukaryotic transition? Could it have realistically happened in other biospheres?

[iancampbell]

Our galaxy is over 13 billion years old, and contains 400 billion stars. This strongly implys that many advanced civilisations ought to have come into existence by now. And yet, we find no real evidence of intelligent life beyond our own. Therefore, there must be some barrier that prevents life from evolving to the point where it can spread among the galaxy.

Lately, I have been wondering what kind of evolutionary step could be so incredibly difficult as to leave the galaxy barren and lifeless for this amount of time. Even with billions of potential biospheres that could spawn intelligent life, it has ended up with nothing. In my opinion, therefore, the great filter must make an appearance at a very early stage. It stops the progress of life that is young and primitive.

What I am undecided on is whether or not the transition from single cell to multi cellular life could be a candidate. I have found two different sources making opposite arguments for this. Richard Grosberg claims that the transition isn't that hard, as it has independently happened many times in earths history. However, Nick Lane argues that the transition was a freakish anomaly, and that it has only happened once in earths history.


I would like to get a discussion going on this subject. Of the two authors, who do you think made the better case? Just how likely was the eukaryotic transition? Could it have realistically happened in other biospheres?

You seem to be talking about two different things here, and possibly confusing the two.

There are almost certainly more single-celled eukaryotes (in terms of species) than multicellular ones. In terms of biomass, there is an even larger difference. And, BTW, the transition has happened at least twice, in succession, to any eukaryote that has chloroplasts as well as mitochondria; this argues against it being difficult.

The transition to multicellular life appears to be easy, because many distinct evolutionary lines have made it. The simplest version appears to be that used by various algae, which merely stick together in a line. The next simplest is probably Volvox.

I would also like to make another point. On Earth, as far as I know, there are no multicellular prokaryotes with any form of specialised cell types or mobility; bacterial biofilm mats probably don't count. However, this does not mean that the idea is impossible; the probable reason it hasn't happened on Earth is that eukaryotes are better at it and out-competed them long ago.

[JohnnyYesterday]

Our galaxy is over 13 billion years old, and contains 400 billion stars. This strongly implys that many advanced civilisations ought to have come into existence by now. And yet, we find no real evidence of intelligent life beyond our own. Therefore, there must be some barrier that prevents life from evolving to the point where it can spread among the galaxy.

We haven't been looking that long. 

Perhaps xeno-civs hundreds of millions of years old don't conform to our naive speculations that they should have a megastructure-fetish and blast gamma ray death beams out their tailpipes.

[Avalancheon]

You seem to be talking about two different things here, and possibly confusing the two.

There are almost certainly more single-celled eukaryotes (in terms of species) than multicellular ones. In terms of biomass, there is an even larger difference. And, BTW, the transition has happened at least twice, in succession, to any eukaryote that has chloroplasts as well as mitochondria; this argues against it being difficult.

The transition to multicellular life appears to be easy, because many distinct evolutionary lines have made it. The simplest version appears to be that used by various algae, which merely stick together in a line. The next simplest is probably Volvox.

I would also like to make another point. On Earth, as far as I know, there are no multicellular prokaryotes with any form of specialised cell types or mobility; bacterial biofilm mats probably don't count. However, this does not mean that the idea is impossible; the probable reason it hasn't happened on Earth is that eukaryotes are better at it and out-competed them long ago.

You're right, I was conflating 'multi-cellular' with 'eukaryote.' That was an embarassing flub on my part.

Just an FYI, but there is a theory (not widely accepted) that chloroplasts actually had a single origin. In other words, they used to be homologous to all eukaryotes, until they started diversifying into separate phylums. Those that didn't evolve to use photosynthesis eventually lost their chloroplasts, because they were functionally useless and vestigal.


Nick Lane contends that the evolution from prokaryote to eukaryote was very difficult: "The emergence of complex life, then, seems to hinge on a single fluke event - the acquisition of one simple cell by another. Such associations may be common among complex cells, but they are extremely rare in simple ones. And the outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendants could flourish."

"This does not bode well for the prospects of finding intelligent aliens. It means there is no inevitable evolutionary trajectory from simple to complex life. Never-ending natural selection, operating on infinite populations of bacteria over billions of years, may never give rise to complexity. Bacteria simply do not have the right architecture. They are not energetically limited as they are - the problem only becomes visible when we look at what it would take for their volume and genome size to expand. Only then can we see that bacteria occupy a deep canyon in an energy landscape, from which they are unable to escape."

[stevebowers]

As I see it there were three events that led to eukaryotic life as we know it; the evolution of the cell nucleus, followed by the inclusion of endosymbiotic mitochondria, followed by the inclusion of endosymbiotic chloroplasts. There is some doubt about the order in which these events occurred, but here is Wikipedia's version.


It should be noted that this need not be the way that life has developed on other worlds; in fact I would go as far as saying that eukaryotes as such can only be found on Earth, and any instances of a similar arrangement which evolves elsewhere would need to be classified separately. Maybe the nucleus is not necessary as a way of separating out the nuclear DNA, or maybe there are several different nuclei, or maybe there are different endosymbiotes, or none.

Multicellular life has evolved several times on Earth, and maybe most forms of multicellular life in the universe are colonial organisms rather than Earth-like cellular organisms. We have at least one non-cellular species of macroscopic organism in OA (the Soft Ones) and these might be representative of a life-type that is at least moderately common.

[Avalancheon]

As I see it there were three events that led to eukaryotic life as we know it; the evolution of the cell nucleus, followed by the inclusion of endosymbiotic mitochondria, followed by the inclusion of endosymbiotic chloroplasts.  There is some doubt about the order in which these events occurred, but here is Wikipedia's version.

It should be noted that this need not be the way that life has developed on other worlds; in fact I would go as far as saying that eukaryotes as such can only be found on Earth, and any instances of a similar arrangement which evolves elsewhere would need to be classified separately. Maybe the nucleus is not necessary as a way of separating out the nuclear DNA, or maybe there are several different nuclei, or maybe there are different endosymbiotes, or none.

Multicellular life has evolved several times on Earth, and maybe most forms of multicellular life in the universe are colonial organisms rather than Earth-like cellular organisms. We have at least one non-cellular species of macroscopic organism in OA (the Soft Ones) and these might be representative of a life-type that is at least moderately common.

Hi Steve, thanks for the answer. My question about eukaryotes was asked in the context of the great filter. I.E., the idea that there is some evolutionary barrier that stops 99.9999% of biospheres from producing intelligent lifeforms (that can spread through space).

In your opinion, how likely is it that an alien biosphere composed solely of prokaryotes could evolve into eukaryotes? In his book, Nick Lane contends that the symbiotic relationship between prokaryote and mitochondria was an exceedingly unlikely event.

[iancampbell]

As I see it there were three events that led to eukaryotic life as we know it; the evolution of the cell nucleus, followed by the inclusion of endosymbiotic mitochondria, followed by the inclusion of endosymbiotic chloroplasts.  There is some doubt about the order in which these events occurred, but here is Wikipedia's version.

It should be noted that this need not be the way that life has developed on other worlds; in fact I would go as far as saying that eukaryotes as such can only be found on Earth, and any instances of a similar arrangement which evolves elsewhere would need to be classified separately. Maybe the nucleus is not necessary as a way of separating out the nuclear DNA, or maybe there are several different nuclei, or maybe there are different endosymbiotes, or none.

Multicellular life has evolved several times on Earth, and maybe most forms of multicellular life in the universe are colonial organisms rather than Earth-like cellular organisms. We have at least one non-cellular species of macroscopic organism in OA (the Soft Ones) and these might be representative of a life-type that is at least moderately common.

Hi Steve, thanks for the answer. My question about eukaryotes was asked in the context of the great filter. I.E., the idea that there is some evolutionary barrier that stops 99.9999% of biospheres from producing intelligent lifeforms (that can spread through space).

In your opinion, how likely is it that an alien biosphere composed solely of prokaryotes could evolve into eukaryotes? In his book, Nick Lane contends that the symbiotic relationship between prokaryote and mitochondria was an exceedingly unlikely event.

Another question, IMHO, is whether multicellular prokaryotes are possible.

[stevebowers]

Well, if we use the term 'prokaryote' to mean 'everything that is not a eukaryote' then I hope that this would be possible, since I've suggested that eukaryotes are unique to Earth. But instead there will be other, analogous forms, eukaryotoids perhaps, that can readily form multicellular clonal organisms like ourselves. Maybe the 'endosymbiotes' are external, making them exosymbiotes; or maybe they extend between cells like hyphae or even like springs. (a spiral-type microbe might handily join two or more cells together).

The important aspect of a clonal multicellar organism on Earth is that all the cells are identical genetically, but are modified locally to perform different functions. These cells are the result of sexual reproduction. Could there be an alternative process on other worlds? Maybe the cells are all haploid, like germ cells, and only come together to mate, producing a fruiting body of some sort. Or maybe the cells of an organism reproduce asexually, and only the endosymbiotes reproduce sexually.

[tmazanec1]

IMVHO, the abiogenesis of a protocell capable of reproducing and evolving is the Great Filter.

[Drashner1]

Re the Great Filter - why assume that there is only one?

It's been a while since I last checked, but IIRC there are easily a dozen or more ideas for why we don't see evidence of intelligent life all over the universe. The big issue with nearly all of them seems to be that it would have to work that way for all civilizations everywhere. But what if instead it's a case of all the various reasons all being in play at once?

Advanced life (beyond bacteria or simpler stuff) might be comparatively rare. Out of those, intelligent life might be comparatively rare. Out of those some number destroy themselves for one reason or another. Out of those that don't, some turn to pursuits that don't result in space travel. Out of those that do become spacefaring only some go to the stars. Out of those, only some decide to go in for activities that are visible across interstellar distances. Out of those, only some happen to be close enough to us in space and time to be engaged in anything we would recognize as a civilization or could detect. Out of those, only some have any interest in contacting non-spacefaring civs or in spreading far enough to get anywhere near us. And so on and so forth.

The upshot of all this is there could be millions of civilizations in the galaxy - but only a very tiny percentage occupy a range of types and activities and cultures that we could potentially detect or contact. And who knows how far away the nearest one could be from us?

Just some thoughts,

Todd