The Primitive Phagocyte
THE PICTURE IS CLEAR. When Giardia's lineage branched from what was to become the main eukaryotic trunk, probably more than two billion years ago, almost all the key features of eukaryotic cells, with the exception of endosymbiont- derived organelles, had already emerged. The crucial prokaryote-eukaryote transition occurred some time during the 1.0 to 1.5 billion years following the primeval forking that led to the eukaryotic branch. During that time, a simple prokaryote developed into a primitive phagocyte, a large nucleated cell capable of capturing food and digesting it intracellularly. What pathway did this momentous transformation follow? And, especially, why was this road actually taken in reality?
Extant organisms offer a number of valuable clues to the first question, but we have only educated guesses to help us answer the second. Remember the rule: foresight excluded. There was no goal, no eukaryotic ideal beckoning from the distant future, inviting evolving cells to overcome hurdles and vanquish difficulties. Every step of this extraordinary voyage was taken in its own present context, the consequence of some chance mutation that happened to confer an immediate benefit favoring the survival and proliferation of the affected cell there and then. What hidden selective forces cut open this trail, step by step, over an immensely long period of time, to produce what was probably the most epoch-making innovation in the history of life? This question will be with us as we try to retrace the main steps of the voyage.
From what we have seen of Giardia, there are really two major developments to be accounted for within the context of an enlarging cell: cytomembranes and cytoskeletal elements, with a fenced-off nucleus arising through a special combination of the two. We have no clues to the origin of the cytoskeleton, which may be a true innovation. But we know the origin of eukaryotic cytomembranes. According to all available evidence, they come from the ancestral prokaryotic cell membrane.