Thread: Dumb Questions VI

Did multicellular organisms evolve from a mutualistic symbiotic relationship between 2 unicellular organisms or did unicelllar organisms develop the ability to produce their own beneficial specialized cells? What does the evidence, if any, tell us?

Multicellularity (aside from the fleeting kind seen in Myxobacteria, slime moulds, and so on) generally requires a redox gradient sufficient to extract a good deal of energy from energy sources such as glucose. Put another way, multicellularity relies on aerobic respiration. Aerobic respiration ends with oxygen -a fabulous electron acceptor, and only a subset of microorganisms have managed to construct their metabolic enzymes to use O₂ in this fashion.

One such oxygen-utilising microorganism, and only one (to my knowledge), has been stably incorporated endosymbiotically into another cell. This endosymbiotic event was required for muticellularity to arise, and no other endosymbiotic event appears capable of extracting sufficient energy from glucose to allow multicellularity.

This is a roundabout way of saying that rise of multicellularity relied on a specific endosymbiotic event ... which is not precisely what you asked. Answering your question is a bit tricky, because the answer is in a sense 'yes' to both options you provided.

The endosymbiotic event at hand here, occured between two cells. That event resulted in a unicellular organism, in that the organism (although a cell-in-a-cell), was still a single cell. Such unicellular cell-in-a-cell type organisms can (and do) live quite happily in a unicellular state. The evidence here, is that there are many cellular organelles that have the earmarks of having once been free-living cells. Yet, all multicellular organisms have mitochondria - which indicates that despite the plethora of endosymbiotic events over evoutionary time, only one event - the phagocytosis of a cell that became the mitochondria - ultimately gave rise to multicellular lifestyles.

At some point one or a few of them mutated features that allowed multicellular associations between daughter cells, and bona fide multicellular organisms developed from there.

I see, so the more oxygen the better. Would the advent of mitochondrial organelles appearing within the cell occur only when conditions on the Earth allow it. I mean, does there have to be sufficient oxygen levels present. Would that be a reason for multicellular life to first appear in the oceans? Would you consider the mitochondrial presence an adaptation to the environment or an evolutionary partnership designed to improve survival chances?

Were the first multicellular organisms plant or animal or both?

Originally Posted by zinjanthropos

I see, so the more oxygen the better.

Although never forget that oxygen is a 'poison'. The use of oxygen only became fashionable in evolution once organisms appeared that produced oxygen on a wide scale.

Originally Posted by zinjanthropos

Would the advent of mitochondrial organelles appearing within the cell occur only when conditions on the Earth allow it. I mean, does there have to be sufficient oxygen levels present.

Yes, there was no need to incorporate mitochondria during the early phases of life since there was no free oxygen.

Originally Posted by zinjanthropos

Would that be a reason for multicellular life to first appear in the oceans? Would you consider the mitochondrial presence an adaptation to the environment or an evolutionary partnership designed to improve survival chances?

A symbiosis that increased the energy production of the proto-eukaryote cell.

Similarly the chloroplast is an ancient prokaryote incorporated into larger celled organisms.

The problem is that bacteria and archea are much better at chemical reactions than the eurkaryote cell was or still is. They have a much larger repetoir of chemical reactions. The eukaryote cell is still extremely limited in what kind of (bio)chemical reactions it can do.

Without the symbiosis the eukaryote cell would be even more limited.

Originally Posted by zinjanthropos

Were the first multicellular organisms plant or animal or both?

Probably plant. The plants and animal ancestors split off before there was a multicellular organism.

Something like the volvox could be a possibility.