  Modern biological tools have made it possible to unequivocally demonstrate
the deep relationship among species in terms of genes and basic molecular
mechanisms. In addition, results from genetic, physical and physiological
approaches applied to individual model cell systems have led to the mapping of
large networks of macromolecular interactions with similar general properties.
Although gene, mechanism and network structure similarities among species tend
to suggest the existence of important constraints applied to organisms, it is
surprising that the elucidation of the precise general mechanisms by which
natural selection could have operated appears to have taken a back seat in
most, if not all, studies concerning the evolutionary development of cell
macromolecular networks. Herein, a possible explanation is presented for how
cells could have evolved into the sophisticated and integrated organisms we
know today, using natural selection. Based on the new concept of gene toxicity,
essentially applied to unicellular organisms with their rapid multiplication
rate, it is proposed that natural selection likely exerts its effects in two
opposing and complementary directions to integrate a new gene function into the
host system; a simple and basic qualitative model for the buildup of cell
macromolecular networks is suggested. Finally, as a radical consequence, it is
proposed that complex life evolution has proceeded through a long alternation
of unicellular and multicellular states.
