Maybe This Is How the Stage Was Set for Eukaryotes

     In my last article (7/8/09), I described a little of Christian de Duve’s ideas of how early eukaryotic cells might have gotten started. De Duve follows the clue of mitochondria, the respiratory organelles descended from ingested bacteria. We know that today, eukaryotic cells extend their outer membranes like tentacles to surround prey. Then the eukaryotic cells pull the prey into their cytoplasm in membrane bubbles called vacuoles, where they digest them. This “endocytosis” or “phagocytosis” is the only way known that early eukaryotic cells could have taken in the respiratory bacteria that became permanent symbiotic residents. Such bacteria likely were taken in as food, but turned out to resist digestion and to become useful organelles.
      However, we have to back up, for before an ancestral eukaryotic cell could take in a respiratory bacterium, it had to change from being bacterial itself. De Duve guesses the ancestral cell had to lose its cell wall by genetic mutation. This would have allowed the much more flexible outer membrane to change shape enough to perform phagocytosis and take in prey or other food particles. But such motion also would have required a rudimentary cytoskeleton to push membrane tentacles out and pull the food bubble in: more mutations or extra genes could have provided such cytoskeletal beginnings.
      Evolution by natural selection requires that these genetic changes provide extra fitness, so that the new cell type would survive better and reproduce in greater numbers than others. De Duve points out advantages for such a transformed cell. Instead of having to live right in its food source, so as to be constantly digesting and taking in needed molecules (See my last article, 7/8/09.), the new, flexible cell could swim around, take in food when it found some, and continue swimming and searching for food while digesting at the same time!
      Such an advantage could have led to more and more of the outer cell membrane being inside the cell. This extended inner membrane could have folded to fit inside a cell, and been supported by a more and more extensive cytoskeleton. New uses could have evolved for the membrane/skeletal system, always conferring a selective advantage. This story is beginning to remind us of a lot of the inner machinery of modern eukaryotic cells, like the E.R. and Golgi apparatus. Eventually the innermost part of this membrane system could have surrounded the genes of the ancestral cell, forming a nucleus.
      So after my intriguing encounter with Norman Pace at the AAAS Annual Meeting in February, here we are in July, contemplating some inspired thought about a possible way for the eukaryotic cell nucleus to have evolved into existence.