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.

  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.

  After all, it took something like 3 billion years for this to happen,
and when it did happen, seemingly the evolutionary advantage was so
enormous, causing such a tremendous natural selection pressure, that it
produced the so-called cambrian explosion, ie. the extremely rapid
proliferation of a wide variety of multicellular organisms. (Well,
"extremely rapid" when talking about geological time. It was still a
period of time of something like 10 million years, which is quite a lot,
especially since simple organisms reproduce very frequently.)

  It took something like 1 or 2 billion years for the simplest of
self-replicating molecules to evolve into prokaryote and eukaryote cells.
If you think about it, that's actually quite a lot of evolution, given
how complex such cells are. It's not something that could happen overnight.
(The 1-2 billion years is effectively even longer when you consider that
single-celled organism reproduce very fast, often even several times a *day*.
That's like a thousand times more generations in a given timespan than more
complex multicellular organisms.)

  At this point unicellular organisms were probably so evolved that they
could perhaps have been able to form multicellular ones. However, it still
took another billion or so years more before it finally happened. (There
was still ongoing evolution of these unicellular organisms, of course,
and they constantly became even more and more adapted to the changing
conditions of the environment, but probably multicellularity could have
happened earlier under the right circumstances.) That's how big of a jump
it probably was.

  There were probably innumerable "false starts" during the entire history
of unicellular life, where perhaps some extremely primitive multicellular
organisms were formed, but they didn't survive (either because they did not
have an survival advantage, or just because of bad luck).

  However, during the cambrian something happened that changed this. One
or more organisms successfully made the transition in such a way that it
provided them significant survival advantage and were able to transmit
these traits to their offspring. Then the proliferation of multicellular
organisms exploded.

  But how did this happen? How can a unicellular organism become a
multicellular one?

  There are several hypotheses, but one which I like is that the first
multicellular organism was actually a unicellular organism which was born
with a genetic defect (caused by a mutation or natural genetic variation).

  This defect caused that when the unicellular organism reproduced (probably
via mitosis, or possibly via binary fission or budding), the offspring cell
failed to detach itself from the original cell, and they effectively became
conjoined inside the same membrane. Both of these cells could then further
reproduce, both offspring also failing to detach, and so on. If the genetic
mutation was just right, then this undetachable multiplication would have
stopped at some point (perhaps at a random point) so that this creature
would grow indefinitely, and instead further reproduction would have produced
a cell that did successfully detach from the mother organism. However, since
this separated cell also had the same genetic "defect", it also then grew
into a multicellular blob, and so on.

  At first this was just an amorphous blob of cells, each one of them
identical. However, over millions and millions of generations, with
subsequent mutations and genetic variation, the way in which the cells
multiplied could produce survival advantage over just a "blind" cellular
copying. In other words, if some of the cells that were produced this way
had certain properties, certain "roles", the resulting overall organism
could have had a better chance at survival. For instance, if the cells
produced like this were not placed randomly in the blob, but always ended
up at approximately the same places, and the properties of these cells
would be right, it would give the organism a clear advantage. For example
if cells that ended up on the surface of the organism were more resistant
to the environment, possibly at the cost of not being as efficient in
other tasks (such as nutrition), while cells that ended up inside the organism
would be less resistant to the environment but more proficient in other tasks
(such as processing and distributing nutrients to their surrounding cells),
the overall efficiency and survival rate of this organism would be
significantly increased.

  It's plausible that over millions of generations this could well produce
simple multicellular organisms (such as very primitive worms, slugs, medusas,
and so on). And from there only the sky was the limit (literally).

  I am wondering, however, if sexual reproduction happened before or after
this. (Unicellular organisms can reproduce sexually, but I'm wondering which
happened first, multicellularity or sexual reproduction.)

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
>   However, during the cambrian something happened that changed this.

  Btw, I have to correct this small mistake. There's clear evidence that
multicellular organisms existed prior to the cambrian (although it's not
completely clear when exactly they appeared). What makes the cambrian so
special is the sudden proliferation of lifeforms.

-- 
                                                          - Warp

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On Wed, 03 Aug 2011 13:22:05 -0400, Warp <war### [at] tagpovrayorg> wrote:
>   Btw, I have to correct this small mistake. There's clear evidence that
> multicellular organisms existed prior to the cambrian (although it's not
> completely clear when exactly they appeared). What makes the cambrian so
> special is the sudden proliferation of lifeforms.

Actually, what makes the Cambrian special is the sudden appearance of  
diverse forms in the fossil record.  Whether the fossils reflect the  
actual pace of diversification is not certain.

http://www.talkorigins.org/indexcc/CC/CC300.html - One way to learn neat  
things about evolution is to peruse scientific responses to YEC nonsense.


-- 
<Insert witty .sig here>

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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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Warp <war### [at] tagpovrayorg> 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.
>
....
>                                                           - Warp

I haven't really reached any firm conclusions with regard to the Endosymbiotic
Hypothesis (basically, the idea that symbiotic cooperation and merging of
simpler life forms played a major role in the development of more complex ones),
but I think it has enough going for it to merit serious consideration. A pretty
good introduction can be found here:

http://endosymbiotichypothesis.wordpress.com/history-the-formation-of-the-endosymbiotic-hypothesis/

I know this is a REALLY broad generalization... and that applying it to a
specific complex case is really nothing more than pointing to an approach that
MIGHT POSSIBLY yield some interesting results, BUT... mathematically speaking,
environmentally restricted interaction between otherwise closed systems has
tremendous potential for generating increasingly intricate structures and
feeding the "emerging complexity" phenomenon.  So far, the more I learn about
the endosymbiotic appraoch, the more I tend to think "Hey, this might just
work!"

Best Regards,
Mike C.

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On 04/08/2011 01:26 PM, Mike the Elder wrote:

> I haven't really reached any firm conclusions with regard to the Endosymbiotic
> Hypothesis (basically, the idea that symbiotic cooperation and merging of
> simpler life forms played a major role in the development of more complex ones),
> but I think it has enough going for it to merit serious consideration.

I've read a couple of references which treat this as an established 
scientific /fact/. (Which, I realise, doesn't necessarily mean a lot.) 
I'm no expert in this field, but it appears that most consensus agrees 
with this theory.

At least, at the moment. It is in the nature of almost /all/ scientific 
theories that one day we'll decide it's total nonsense and the truth is 
something completely different...

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