Metazoans appeared 800 million to one billion years ago by derivation
from preexisting unicellular organisms which most probably were flagel
lates. Several transitional steps are proposed in the paper to explain
the evolution from unicellular to pluricellular animals. Specifically
, it is argued that the earliest metazoans arose not by aggregation of
solitary cells, but by scission of plurinucleated cells.