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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