The creation of multicelled organisms

[CPL.Luke]

how does a single celled organism give "birth" to a multicelled one?

[Kyrisch]

Since when do they?

[CPL.Luke]

at one point (don't worry this isn't and I proved evolution wrong thread) in the distant past a single celled organism must have given "birth" to a multicelled organism

[Kyrisch]

Oh, you mean evolution-wise...

 

This happened the same way frogs are sometimes born with more than four legs. A single (or multiple) gene(s) mutate and cause some abnormal behaviour to occur. Five or more legs is an example of maladaptive evolution in frogs, but two or more cells could have come in handy back then, and whatever mutation that caused that to happen persisted.

[Bluenoise]

how does a single celled organism give "birth" to a multicelled one?

 

This is believed to have happened through processes that compartmentalised the single celled organism. It is possible for a single cell to be multinucleated so it shouldn't be too hard to imagine a membrane forming between them.

 

Oh' date=' you mean evolution-wise...

 

This happened the same way frogs are sometimes born with more than four legs. A single (or multiple) gene(s) mutate and cause some abnormal behaviour to occur. Five or more legs is an example of maladaptive evolution in frogs, but two or more cells could have come in handy back then, and whatever mutation that caused that to happen persisted.[/quote']

 

I think you're wrong there. AFAIK frogs with greater than 4 legs are the result of non-genetic developmental irregularities. Most are believe to be caused by parasites that split or interupt development of a body part or limb.

 

To create a from with more than 4 legs with genetics would most likely require the change of 100's of genes. Body plans are deeply rooted in development.

[LucidDreamer]

There is a sort of progression in complexity of multi-cellular form: single celled organism->colony of single-celled organisms->sponge->jelly fish->multi-cellular. The colony, such as certain phototrophic cells, is made of cells that are loosely connected. The sponge is made up of a collection of the same kind of cell. A jellyfish is slightly more complex but there is very little differentiation in its cells.

 

I'm not saying that the origin of complex multi-celled organisms on earth followed exactly that pattern but I believe it followed a similar pattern. It started out as a colony where the individual cells benefited by staying together.

 

So a single cell would start the colony, clone itself, and then the descendant cells would continue to asexually reproduce. The cells were located in different spots in the colony and so the colony benefited by some cells producing slightly different proteins or performing the slightest difference in function depending on where it was. The cell knew where it was in the colony because it received different amounts of sunlight or nutrients based on whether it was in the center or the end. Each cell was identical but each of the cells contained a variety of genes that could be turned off and on through regulation

 

The differentiation that resulted proved to be incredibly useful to face the pressures confronting it. Each cell had an identical genome, as complex multicultural organisms do today, but the colony benefited by having specialized cells. Perhaps the cells on the outer layer benefited by producing more proteins that helped protect itself from competitors or predators. Perhaps the top layer receive more sunlight and the bottom layers absorbed more nutrients from the sea, so the two layers benefited by sharing resources.

 

Since function is dependent on structure the colony/organism benefited by having some cells with different structures than the other cells. The colony/organism also benefited by having the cells begin to differentiate earlier and more completely so they could be highly specialized and efficient. Very specific control mechanisms and regulation evolved because it was very advantageous for an organism that had already become a primitive multi-cellular organism.

 

This pattern continued until you observe to complexity that you see today. You can think of a fertilized egg in a modern vertebrate as a sort of seed cell for a very complex colony of cells.

[Mokele]

Lucid pretty much has it, and the fossil record seems to support it: The first multicellular anything bigger than colonial algae were what's called the "Vendian fauna" (though it's debatable whether they were plants, animals, the precursor to both, or what). They've very simple things, look like branching ferns, but the 'leaves' are the whole organism. They disappear shortly after the Cambrian explosion, which is believed to have been caused by gene-duplication events allowing more complex developmental patterns to arise, leading to 'animals' proper.

 

Mokele

[Skye]

Sponges are differentiated: pinacocytes (epidermis), porocytes (form pores in some phyla), choanocytes (flagellated feeding cells) and amoebocytes (do everything else, differentiate into all the others, and into gametes).

[CPL.Luke]

hmm that brings me to anouther question why aren't there more plant-animals, it would seem that the added energy given from the ability to "metabolize" sunlight would still be advantageous if the creature were also actively hunting other creatures

[Kyrisch]

Oh' date=' you mean evolution-wise...

 

This happened the same way frogs are sometimes born with more than four legs. A single (or multiple) gene(s) mutate and cause some abnormal behaviour to occur. Five or more legs is an example of maladaptive evolution in frogs, but two or more cells could have come in handy back then, and whatever mutation that caused that to happen persisted.[/quote']

 

I think you're wrong there. AFAIK frogs with greater than 4 legs are the result of non-genetic developmental irregularities.

 

Scientists have recently discovered a set of genes that act as master switches for the development of the body. A specific one, called Cerberus (Based on the Greek legend of Cerberus the three-headed dog) has been isolated to be the master switch for the growth of the head, arms, legs, and fingers. For instance, after a head have been grown, Cerberus is activated after a complex chain reaction of chemicals and the "sprout point" on the neck is deactivated. If this gene is corrupted, Cerberus is never activated and another head begins to grow from "sprout point". Cerberus is also present in the bases of the extremities.

[Mokele]

Sponges are differentiated: pinacocytes (epidermis), porocytes (form pores in some phyla), choanocytes (flagellated feeding cells) and amoebocytes (do everything else, differentiate into all the others, and into gametes).

 

True, but the ability of sponge cells to de-differentiate, as well as the limited number of cell types, makes them a good model for early multicellular life.

 

hmm that brings me to anouther question why aren't there more plant-animals, it would seem that the added energy given from the ability to "metabolize" sunlight would still be advantageous if the creature were also actively hunting other creatures

 

Not actually. Like everything, photosynethesis costs energy in term of protiens and such. How much you get out of it depends on many things, like light intensity, surface area, etc. And actually, it doesn't really generate that much energy. In order to make a living at it, plants have basically modified just about every part of their anatomy into light-collecting leaves, maximizing surface area relative to the volume of body they need to maintain. For an animal not prepared to make that level of commitment, the energy gained would actually be less than the cost of protiens.

 

If you think about it, what a small animal eats in a day take a moderate-sized plant weeks to grow.

 

Mokele