Single-cell organisms, such as bacteria and protozoa, have been so successful in adapting to a variety of different environments that they comprise more than half of the total biomass on earth. Unlike animals, many of these unicellular organisms can synthesize all of the substances they need from a few simple nutrients, and some of them divide more than once every hour. What, then, was the selective advantage that led to the evolution of multicellular organisms?
A short answer is that by collaboration and by division of labor it becomes possible to exploit resources that no single cell could utilize so well. This principle, applying at first to simple associations of cells, has been taken to an extreme in the multicellular organisms we see today. Multicellularity enables a plant, for example, to become physically large; to have roots in the ground, where one set of cells can take up water and nutrients; and to have leaves in the air, where another set of cells can efficiently capture radiant energy from the sun. Specialized cells in the stem of the plant form channels for transporting water and nutrients between the roots and the leaves. Yet another set of specialized cells forms a layer of epidermis to prevent water loss and to provide a protected internal environment (see Panel 1-2, pp. 28-29). The plant as a whole does not compete directly with unicellular organisms for its ecological niche; it has found a radically different way to survive and propagate.
As different animals and plants appeared, they changed the environment in which further evolution occurred. Survival in a jungle calls for different talents than survival in the open sea. Innovations in movement, sensory detection, communication, social organization - all enabled eucaryotic organisms to compete, propagate, and survive in ever more complex ways.
The only problem with this is that it seems purely speculative. It assumes the truth of evolutionary theory and proceeds to show how something may have happened in accordance with said theory. That, however, does not constitute evidence that it did happen that way.
What I find so fascinating about this question is that it seems to posit that there is a mechanism for a) recognizing a competition exists for limited resources, and b) a process for gaining an advantage by collaboration and a division of labor.
ReplyDeleteAll this, packed into a single-celled organism. An organism with an awareness of a world outside itself, even an awareness of time beyond itself.
Because what would be the point of that organism reproducing. The organism doesn't gain by it. In fact, reproduction demands that it dies, so its can be broken down and fed upon, and to clear the way for subsequent generations.
The existence of reproduction demands the answer to the question: why? If the first cells spontaneously arose out of the right brand of cosmic soup, then why not let it go on that way? A variable suddenly arising in one creature that says, "Ah-ha! I see how the game is played. I'll reproduce myself and beat the other S-COs" implies either that the source of life lies in randomless or intention.
I hope someone brings up anything that drives the conversation further than my speculation.