A basic source of the confusion is that in the course of evolution whole
suites of genes have apparently been transferred sideways among the
major branches. Among animals, genes are passed vertically from parent
to child but single-celled creatures tend to engulf each other and
occasionally amalgamate into a corporate genetic entity. It has long
been argued that mitochondria, the tiny organelles that handle the
energy metabolism of eukaryotic cells, were once free-living bacteria
that were enslaved by an early eukaryote. Mitochondria still possess
their own, bacteria-like DNA but many of their genes have emigrated into
the eukaryotic cell's own DNA in the nucleus.
