On 03/08/2011 05:49 PM, Warp wrote:
>    I think that hypotheses about how different organisms could have been
> evolved are fascinating. It's also fascinating to try to come up with
> plausible hypotheses.

The difficult part is figuring out what /really/ happened. After all, 
most of this stuff happened a *really* long time ago. Usually there's 
not much evidence to go on.

>    One particular thing that has fascinated my imagination is how
> multicellular organisms could have formed from unicellular ones. After
> all, the jump is a rather huge one, nothing like a minuscule variation
> in surface coloring or something. And if current estimates are even close
> to correct, the jump *was* rather drastic.

OK, you seem to be talking about three separate concepts:

* The creation of the eukaryote cell.

* The creation of multi-cellular organisms.

* The Cambrian explosion.



Regarding the first: Best indications are that the the eukaryote cell is 
several cells mashed together. Whether this happened by digestion gone 
wrong, symbiosis or parasitism is unclear. But over time this state of 
affairs became the norm.

To a first approximation, bacteria and archaea show a vast range of 
cellular chemistries, while eukaryotes have just one (or at least, a 
very small number). For example, algae, yeast, daffodils, crocodiles, 
ants and fungi all contain genes for the protein "actin", and these 
genes are all 85% identical. Yes, you and I have some of the same genes 
as a cabbage or a hummingbird. Bacteria, on the other hand, all 
seemingly differ wildly in their chemical abilities. (E.g., no animal 
can digest cellulose, but bacteria readily do this.)

The obvious explanation for this is that somewhere back in deep time, a 
cell swallowed one particular bacterium species, and ended up having one 
particular body chemistry as a result. Or perhaps it was separate but 
similar events for each of the major eukaryote groups - the plants, 
animals and fungi, plus all the rest that nobody has ever heard of. Who 
can say whether these groupings came before or after the eukaryote cell?

http://tolweb.org/Eukaryotes/3



Regarding the second, note that not all multicellular lifeforms are 
eukaryotes. (!) To my mind, you can treat cells a bit like animals.

Some animals live alone. (Especially predators.) They will usually 
attack any other members of their species that they see.

Some animals live in herds. Each member of the herd is essentially "the 
same", and they are all "separate" animals, but they habitually live and 
travel together. In some cases, it's just for protection against 
predators. In other cases, it's vital to their survival. (E.g., hunting 
behaviour that only works with multiple individuals.)

And then there are the "social insects" - ants, termites, bees, wasps, 
etc. Each animal is a separate individual, and they all live together. 
But they all /work together/ as well. They're not just standing next to 
each other, they're actually working towards a shared goal. In the case 
of bees, all the animals are more or less the same. But in the case of 
some ant species, the workers, warriors, scouts, etc., are so different 
from each other you almost wouldn't believe they're the same species.

Now compare that to a culture of cells. Think about how a muscle cell is 
different from a nerve cell - so different you might think they were 
from different animals. But they aren't, and they all work together in a 
cohesive system.

It's not hard to imagine how a solitary animal could find advantage 
living in herds, and then find advantage in large-scale group 
cooperation, and then find that certain individuals happen to be better 
at different roles, right up until you end up with complex ant 
societies. I suspect a similar thing happened with cells.

Note that you find living cells in various degrees of single-celled or 
multi-celled life. For example,

http://en.wikipedia.org/wiki/Portuguese_Man_o%27_War

It's not one animal, but a colony of them, all fused together. These 
animals cannot survive alone, but related species can. (It's also /not/ 
a jellyfish, by the way.)

So just as animals have varying levels of cooperation, it seems that 
cells are similar.



Regarding the Cambrian explosion... It seems current theory suggests it 
wasn't as big a bang as previously thought. Lots of species are now 
known to have existed before the Cambrian, although currently the 
evidence still says that there was /something/ of an increase in the 
rate of speciation.

http://en.wikipedia.org/wiki/Cambrian_explosion

Like most things involving evolution, I guess we'll probably never know.

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