Five Steps in the Evolution of Multicellularity in Animals.

[Darwin123]

Somebody suggested some time ago that I rewrite the five steps of
multicellular evolution down, starting a separate thread. This way,
we could eliminate all the baggage from the previous discussion. That
seems like a good idea to me. So here goes.


The picture that I have for the evolution of multicellularity in animals
approximates the picture for “colonial hypothesis” given in the following
link. However, I consider the pictures incomplete because they don’t
show a few stages of growth.

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

There is one row of pictures, 1-5, presented in the link above. However, I
think there is one column of pictures corresponding to each illustration. Thus, it really should have been presented as a matrix of pictures. Every column
corresponds to a stage of growth and every row corresponds to a step of
evolution.

As I understand it, every organism in the sequence has at least one
unicellularhaploid stage and one diploid stage. The pictures under colonial
hypothesis are only showing a diploid stage of the organism. So what is being
shown is the steps in the diploid evolution.

The five steps of diploid evolution, according to the link, are:
1. Unicellular flagellated protist.
2. Multiple flagellates make an aggregate where the aggregate is monoclonal.
3. The unspecified flagellate cells form a hollow sphere which is monoclonal.
4. Specialized reproductive cells form on the colony which is monoclonal.
5. Cells begin to fold in to make tissues which are monoclonal.

What the link doesn’t show is the steps in haploid evolution. Every
animal going all the way to human beings has a stage of growth corresponding to
unicellular haploid organisms. An unfertilized chickens egg is a unicellular
haploid organism. A sperm is a unicellular diploid organism.

Here are my candidates for steps 1 and 2.

Step 1 would be an organism like a dinoflagellate. The dinoflagellate has
many dipoid stages of growth. It turns from flagellate to amoeba to spheroid.
However, it does have at least one unicellular haploid stage.

Here is a link on dinoflagellates.
http://en.wikipedia.org/wiki/Dinoflagellate

I now introduce my candidate to stage 2. The aggregate cell ancestor of
all animals is somewhat like choaoflagellate. Some choanoflagellate species
form an aggregate made of unspecialized cells.

Here is a link on choanoflagellates.

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

Note the photograph of Sphaeroeca looks a lot like the aggregate
of stage 2 in the Wiki article on the Evolution of Multicellularity.

Each cell in the choanoflagellate colony (diploid) sometimes
divides by mitosis and sometimes divides by meiosis. Since the cells
in the colony are unspecialized, the ratio between the two processes
is constant throughout the colony. However, haploid cells do not form
colonies. Hence, the haploid stage is unicellular.


I think this general model is what most evolutionary biologists
believe today. However, any biologist out there can tell me where I
messed up. The outline of the theory is presented for facilitate
discussion.

Is somebody still interested in this?