Doubting Didymus

Is the common ancestor of all multicellular organisms a multicellular organism itself? That is, has multicellularity evolved only once, or had multiple origins?

I am particularly interested in the animal/plant common ancestor, with respect to the endosymbiosis of bacterial organelles. It seems to me as though animals and plants must have been separate unicellular lineages, at least at the chloroplast aquisition event.

If this is the case, then are there any plant or animal lineages that do not have a multicellular common ancestor with other members of its kingdom? It would be interesting to look at the homology.

lpetrich

Multicellularity happened more than once.

It's difficult to find a really good overview of eukaryotic phylogeny, but The Tree of Life has a <a href="http://tolweb.org/tree?group=Eukaryotes&contgroup=Life" target="_blank">reasonable, if cautious, one</a>. Also possibly interesting is <a href="http://www.denison.edu/~kuhlman/WOL/eukaryote.html" target="_blank">Peter Kuhlman's page on this subject</a>.

The animal kingdom had originated from choanoflagellates (collar flagellates), which are one-celled.

The animal-choanoflagellate group is closest to the fungi, which include one-celled and multicelled forms; the combined group is sometimes called "opisthokonts".

Plants are descended from green algae, which include one-celled and multicelled forms.

It is usually inferred that the ancestors of each of these two groups were one-celled.

So multicellularity was invented several times, though how many times is still unclear.

Doubting Didymus

So even the animal/fungi common ancestor was unicellular?

I am interested in reconciling multiple multicellularity events with my previous thread on the cambrian explosion and fundamental body plans. Can anyone tell me when, in relation to the cambrian explosion, plants and fungi make their first multicellular appearances?

Tom Sawyer

Multicellular life was started from one-celled organisms. These organisms split in two to reproduce. Somewhere along the lines, one of them split in two, but something happened and the split didn't work out correctly - sort of like the difference between conjoined twins and regular twins.

This may have happened many times and most of the time the organism died out as a result. At least once, though, this mutant offspring was able to successfully survive and when it split to reproduce, another two-celled organism was created. They, for whatever reason, tended to be more successful than one-celled organisms, or filled a niche that was previously open in the environment. The same sort of thing happened again and again over the course of a couple billion years and eventually, there were many species of mutli-cellular organisms around.

Evidence of this can be seen, IMHO, by the fact that every cell in the body contains a complete copy of the organism's DNA. Each cell only needs a small part of the genetic code to be able to work (ie to turn on gene #42 during the 3rd week of development in order to start building the spinal cord). Coming from a background of one-celled organisms, though, each cell was pased a complete copy of the genetic code and, for whatever reason, they never stopped doing this.

Whether all multi-celled organisms come from the same ancestor that split that one time, or if it was an event that happened numerous times and was successful in more than one, I don't know.

lpetrich

First, here's <a href="http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html" target="_blank">UCMP's page</a> on eukaryote phylogeny -- the branching may have to be revised, but the various groups appear to be well-defined.


To my mind, peteyh's comments are vague hand-waving. The pathway

one-celled (separate)
-&gt;
colonial (lookalike)
-&gt;
multicelled (differentiated)

is indeed a plausible one, but peteyh does not add anything about how often that had happened. So I'll take a stab at that subject.

Opisthokonts:
- Animals (most likely only once)
- Fungi

Green algae and plants

Red algae

Stramenopiles / Chromists / Heterokonts (diatoms, golden algae, brown algae, oomycetes, etc.)

Slime molds (may not be a well-defined group)


Multicellularity may have originated more than once in several of these groups


As to the fossil record, land plants start appearing in the Silurian and they proliferate in the Devonian; multicellular fungi also start appearing in the Devonian.

(Fixed UCMP URL)

There is some evidence of multicellular algae in the late Precambrian, but it is very ambiguous.

[ October 16, 2002: Message edited by: lpetrich ]</p>

Doubting Didymus

Lovely.

Thanks for the clarification.

lpetrich

I recommend the UCMP site as a reference to the fossil record -- it has some good thumbnail accounts of the fossil records of various groups.

Doubting Didymus

The site, she don't go.

lpetrich

That UCMP URL is now fixed.