Cells into Organs: could it evolve?

There are several kinds of single celled organism that will congregate together into a colony under some circumstances. You can see a similar thing with schools of fish. Sometimes there is an advantage being in a group. There are also cases in which single cells will, under stress, combine to form a colony and then differentiate to fit different roles within the colony.A dramatic example is Dictyostelium discoideum. This is a "slime mold", and lives as a single celled organism, feeding on bacteria in forest litter. But when the food runs out, they form colonies, of about 10 to 50 thousand individuals. These colonies are effectively a multi-celluar organism... a "fruiting body", a bit like a slug. As part of this process, the individual cells differentiate into one of two types. This slug is better able to move up to the top of the litter in which it lives, and disperse spores which will then be new individual cells.There is a Dictyostelium-Homepage, which includes videos.

Stop right there. Why this extra constraint? What you should be looking for is beneficial; not "needed for survival".Needed for survival is not a useful or sensible extra constraint. Environments are very complex. They don't have a sharp boundary between do this and you survive, do that and die.Something is "beneficial" if it makes survival (or propagation) more common. You will still get cases in which an organism adopting a colony behaviour will die, and other cases in which individuals that go it alone survive. But in some lineages and some environments, single cells have adapted for a behaviour in which they form colonies and differentiate roles.Some organisms do this in reaction to stress, but in times of plenty they live as single independent cells. This conveys some benefits to the organisms, which means the trait propagates. The fact that they can also live as individuals gives a clear evolutionary pathway. And there is also scope for development in which organisms become committed to the colony lifestyle. Sometimes this works out, sometimes it doesn't.The ones for whom it worked out are the ones who propagate more relatives to themselves.End result, we have living creatures today showing a range of living strategies, with no sharp boundaries. The ones living today are the ones for whom circumstances and strategies worked out for their ancestors. I'm not crashfrog; but your analogy fails. Nearly all nations do have resources, in differing degrees. They are not absolutely dependent on each other, but going it alone means they can only support smaller populations, or accept a lower standard of living. So they adopt strategies of co-operation that have a payoff.They also adopt strategies of conflict; and this has a side effect of driving to the wall those competitors that opt for less effective strategies.I don't doubt that you can't see how co-operative behaviours and differentiation can arise from single cells. But it seems to me that you aren't thinking this through, or looking at the examples available to show how co-operation can be "chosen" by living cells or by nations, who are also able to live independently.Cheers -- Sylas

Our last two posts passed each other in the pipeline. I was writing number 13 while you were writing number 12. Actually, my previous Message 13 already answers your objections in Message 12, but let me spell it out. As far as I can see, they are pretty much required to do so for survival. We are not only talking about one species here; there are a range of species with this kind of behaviour, and I would expect them to differ in the degree of commitment they have to the colonial phase of their life cycle. Others may well be less committed.This behaviour persists because the individuals don't propagate all that well as individuals. They are individuals for some of their life cycle, but some slime moulds (I think) are now pretty much committed to going through the colonial phase for the next generation.As for how it evolved; this is pretty obvious. We don't have a record of all the exact ancestors, but we have a clear case in which individual cells sometimes come together for mutual benefit. There are plenty of other examples, in which individuals come together in much more loosely defined amalgamations, so there is no major gap or hurdle to overcome.Species evolve by accumulation of change. There is no question of "figuring out" anything. Living creatures just do their thing. Some survive, and some don't. This is due mostly the vagaries of fortune; a predator happens by, food runs out, a tree falls in the forest and sets up a suitable environment. But there is also a contribution of how well adapted living things are to take advantage of opportunity or to surmount obstacles.Two things drive evolution. First, living creatures are not exact replicas of their parents. Second, there are many more organisms born than manage to survive to the next generation.The sets up a kind of competition, in which change that works out persists, and change that does not is removed from the gene pool.This is not "working things out". This is just doing what comes naturally, with what comes naturally changing over time. The existence of many degrees of colonial organization plainly establishes that there are credible evolutionary pathways for cumulative change taking you from single independent cells to obligate multi-cellularity.Cheers -- SylasThis message has been edited by Sylas, 02-16-2005 17:44 AM

I don't think there is any "instinct" here at all. The word instinct, as I use it, refers to an ingrained behaviour mediated by the nervous system.The creatures we are looking at here don't have any nervous system. The level of organization above the level of the cell is minimal. The cells are reactive to stimuli. If this is "instinct", then any reaction is instinct, and the word becomes so inclusive that it no longer serves as a way of distinguishing different behaviours.But whether you call it instinct or not, the cellular chemistry of the cells responds to external stimuli in a manner characteristic of that species of cell. Evolution can change such instincts, just as they can change any other heritable aspect.The complexity of these creatures is not actually "irreducible" in the sense defined by Behe, because you can in fact remove or modify bits and pieces and have a workable system. It is only irreducible if you take a gross view of removing whole slabs of functionality, which is a complete non-sequitur, because evolution does not work by adding functionality in large slabs. Evolution tends to tweak or tinker with a system.The claim that this behaviour cannot evolve over time is false on the face of it. We see living creatures evolving over time just fine. We see slight graduations of behaviour in many related organisms, spanning the gammut of possibilities from single cells to obligate multi-cellularity, with no particular gap or hurdle that could pose the slightest problem for evolutionary change. The many degrees of behaviour in currently living cells shows right now, for those willing to actually look at biology in the real world, that there is ample opportunity for such behaviours to evolve.There are even experiments in which the colonial habit is shown to vary with evolutionary change. I don't have a reference right handy, but I'll see if I can find one.The claim that colonial behaviour and differentiation can't evolve is mere assertion, completely lacking in any sensible argument or evidence, and conflicting with every indication we have about how living things, and these critters in particular, work in the real world.You're still talking about "learning". This is totally irrelevant. They don't learn. They merely change, from generation to generation, and the changes that work out persist.You've been given the explanation. The way it happens is by evolution. The tendency to congregate under stress is inherent in the cells, and it can vary as the cellular chemistry and its mediating genetic basis varies. If the tendency to congregate tends to result in more of the organisms propagating to subsequent generations, than that tendency gets amplified, simply by selection. Traits are passed on because they are founded in a cells chemistry and genome, which is passed on. Traits of an organisms that are not passed on to the next generation cannot be used to identify a species; only an individual.The colonial habit of slime moulds is passed on. That's an observation. The replication of cells is complicated, but quite well understood by now. So what is your problem? I don't get it.Cheers -- SylasPS. LDSdude; check out Message 1. It can give some clues about quoting and formatting you may find handy.This message has been edited by Sylas, 02-17-2005 21:20 AM

Ouch!What a fascinating insight into the nature of substantive debate, and how real progress is made when all sides take seriously the input of others and attempt to make progress in discussion. And also the harsh uncompromising brick wall of the external shared reality we attempt to describe, and for which some descriptions must fail.Yet sometimes, as a vanquished sportsman sadly retires from the field at the close of play, the fans in the stadium rise to their feet and give spontaneous applause to a battle fought well and fairly, and for the contribution of the loser to the whole edifying event, and in recognition and hope that we shall see them again on the field another day.The raising and resolution of a problem helps give real insight into the examples, much beyond what would have been obtained in a bare recitation of most accurate perspective. The chagrin will be fleeting; you've made a real contribution to this matter. Thanks.Cheers -- Sylas