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.