A mitochondrion

Mitochondria are often described as the “powerhouse” of the cell. These organelles are responsible for production of the majority of the energy (in the form of ATP) used by eukaryotic cells and are thought to have arisen over 2 billion years ago when a proto-eukaryote “ate”, but did not digest, a prokaryotic cell. Over time, the host cell and its endosymbiont became more and more dependent on one another. If this symbiosis ceased to exist, that is, if all mitochondria suddenly died, then the world would once again belong to the prokaryotes.

Because of the dependence eukaryotic organisms have on their mitochondria and the interdependence of genes and gene products between the nuclear and mitochondrial genomes, loss of mitochondria would result in the death of all organisms that have them. Not only would all eukaryotes go extinct, but so would some species of bacteria and viruses that depend on eukaryotic hosts. Such numerous losses would constitute a mass extinction on the scale never before seen on earth that would effectively serve as a “reset” button for life on this planet. Despite widespread death of macroscopic organisms, certainly, some (if not many) bacteria (and archaea) would persist.

This new world re-dominated by single-celled life would be vastly different from our current one. Aside from the eukaryotic corpses littering the landscape, the oxygen in earth’s atmosphere would surely decrease since plants would no be adding it to the atmosphere, the oceans would contain massive “dead zones” lacking oxygen because of all the decaying eukaryotes, and the global temperature would likely rise in the absence of plants sequestering carbon dioxide. This last point could certainly be debated since humans will no longer exist, and thus no longer be putting out such large quantities of greenhouse gases.

In the absence of eukaryotes, there would be abundant ecological opportunity and open niche space for life to re-explore. Whether replaying the tape of life (albeit from a biased starting point) would yield similar or completely different outcomes is anyone’s guess. It might not be surprising if another endosymbiotic event occurred among all the thriving bacteria and archaea, that, in the absence of eukaryotic competitors, eventually led to “new”karyotic, multicellular life re-evolving.

In any case, we should be thankful that our personal powerhouses keep chugging along.

An added note to this post since it was first conceived: Eukaryotes have been found that no longer have mitochondria. However, they live inside the guts of animals that due utilize mitochondria, so the scenario described above would not be very different.

June 14, 2016