Simpler Ancestors of Complex Cells

     The origin of the eukaryotic cell is shrouded in mystery. Animals, plants, fungi, and a vast array of single-celled creatures are made of such cells. Eukaryotic cells contain a membrane-enclosed nucleus and membrane-enclosed organelles. They also contain a system of active, folded membranes; and the cells’ shapes and movements result from an internal skeleton. As I described in my June 2nd article, some of the organelles in eukaryotic cells, the respiratory and photosynthetic ones, are descendants of primitive bacteria. How did all this internal machinery evolve?
      We can assume that such a complex system came from a simpler system, because this is true of all evolution. And for the same reason, we can assume that the complexity arose a little at a time, rather than all at once. The simpler system was the result of gene activity, so the complex system must have resulted from gene mutations and/or additions. Based on such assumptions, biologists can hypothesize about the origins of eukaryotic cells. A wonderful hypothesis comes from Christian de Duve, winner of the 1974 Nobel Prize in Physiology and Medicine, and still active today.
     DeDuve starts with the problem of how an ancestor of modern eukaryotic cells could have taken in the bacteria that became respiratory organelles (the mitochondria). Such an ancestor had to have extended its outer membrane to surround a bacterium in a process called “endocytosis,” because this is the way cells take in other cells as prey. Bacteria and archaea never perform endocytosis, so some primitive cell must have evolved this ability and started a new line that eventually led to eukaryotes. But bacteria and archaea, presumably the descendants of more primitive cells, all have cell walls outside their outer membranes. For the primitive ancestor of eukaryotes, cell walls would have prevented extending an outer membrane to surround prey. So the first mutation in an ancestral cell might have been the loss of the cell wall.
      Meanwhile, until the loss of the cell wall, the ancestral cell would have practiced exterior digestion. That is, it lived within its food source, produced digestive enzymes on the exterior of its outer membrane, and digested its food down to molecules small enough to diffuse inward through channels in its outer membrane.
      To be continued…

DeDuve, Christian. May 007. The origin of eukaryotes: a reappraisal. Nature Reviews/Genetics. 8:395-403