WITH THE EMERGENCE of the primitive phagocyte, the major part of the prokaryote-eukaryote transition was accomplished. Considering the many innovations needed for this development, the uptake and adoption of endosymbionts may be seen as an almost banal event. Yet it was of paramount importance for future evolution. With rare exceptions, today's eukaryotes all belong to the post-endosymbiont era. There may be a good reason for this, possibly connected with oxygen.
The possession of flagella by Giardia tells us that the primitive phagocyte was a motile, fully emancipated cell that had long left the shelter of the bacterial colonies within which it supposedly was born. Perhaps it tended to browse around the rich and easily accessible food supply offered by its erstwhile abode, but it could also have taken advantage of its freedom to move out into any stream, lake, sea, or ocean where bacteria were present. Quite possibly, it spread in different directions and diversified into a variety of species adapted to different environments. Products of this early diversification that have survived to this day include the diplomonads, the microsporidia, and, perhaps, other members, still awaiting discovery, of the large, incompletely inventoried group of protists.
It is likely that our distant eukaryotic ancestor had improved its chances of heterotrophic survival by acquiring some of the properties that help phagocytes today. It probably possessed chemotactic surface receptors sensitive to certain types of molecules and connected to the flagellar apparatus in a manner that made the cell move toward a potential food supply and away from noxious substances. Most likely, it also had endocytic receptors to help it catch and engulf its prey. Perhaps, like our own white blood cells, it complemented the digestive enzymes of its lyso-