|A bacterial tree of life, created by comparing gene sequences of modern bacteria species.|
Mitochondria first emerged approximately 2 billion years ago, in what was a turning point for the advancement of complex life. That emergence has long been something of a mystery, one which we may now know the answer to.
We know that mitochondria are almost certainly descended from separate organisms because they possess a different set of genetic code from that contained in the nuclei of eukaryotic cells, which derive their names from the Latin words for "good kernel" because they all contain at least one DNA-enveloping cell nucleus.
Biologists have long believed that mitochondria first emerged when ancient host cells absorbed simple bacterial cells; these eventually evolved into the mitochondria found in virtually every modern eukaryotic cell. Thus, from the beginning, it was thought there was a symbiotic - mutually beneficial - relationship between the hosts and their resident mitochondria.
However, according to University of Virginia biologists Zhang Wang and Martin Wu, mitochondria are in fact descended from parasitic bacteria which originally preyed upon the cellular energy of early plant and animal ancestors, before evolving into the beneficial energy-producing organelles they are today.
Drs. Wang and Wu used state-of-the-art DNA sequencing to match mitochondrial DNA with that of its most closely-related bacterial cousins - 18 strains of bacterial parasites.
This indicates that mitochondria were almost certainly a type of bacterial parasite which stole ATP energy from their hosts. Only later did they begin switching the direction of ATP transport outward, thus providing the energy which would enable complex, multicellular life to emerge.
Source: "New U.Va. Study Upends Current Theories of How Mitochondria Began" press release, University of Virginia newsroom, October 16, 2014