It was previously believed that eukaryotes evolved in the traditional way—prokaryotes became more complex through mechanisms of evolution, such as natural selection, until they were different to their ancestors. Yet there is no evidence of an evolutionary intermediate between prokaryotes and eukaryotes in the fossil record. Plants and fungi, for instance, two types of eukaryotes, did not develop from different types of prokaryotes. Instead, eukaryotes are monophyletic—a population of eukaryotes arose once, and all plants, animals, fungi, algae, and protists evolved from this original eukaryotic population. What is most fascinating about the origins of eukaryotes is that it can be seen as a singular event. In other words, either a eukaryotic population occurred only once in 4 billion years, or it occurred any number of times, yet only one population survived long enough to populate the planet. In either case, the birth of the eukaryotes is a complex and possibly extremely rare event. So what caused it?
The hydrogen hypothesis suggests what the nature of the event might have been. During the two billion years in which the Earth was populated entirely by prokaryotes, an endosymbiotic relationship arose between an archaeon and a bacterium. For this to have occurred, several conditions must be true of the host, in this case the archaeon: (1) it was anaerobic, (2) it possessed a hydrogen-based metabolism, and (3) it was strictly autotrophic, capable of providing itself with nutrients using inorganic substances. Similarly, the symbiont, the bacterium in this case, must have been able to provide the host with the hydrogen it needed. This results in a relationship in which the bacterium, the symbiont, lives within the cell membrane of its host, the archaeon, supplying its host with the hydrogen needed for metabolic processes since it is a byproduct of anaerobic respiration. Methanogens are archaea that satisfy the conditions above, strongly suggesting that the original eukaryotic cells had methanogens or a similar archaeon as the host. Finally, when the archaeon host is removed from geological hydrogen (for whatever reason), it becomes dependent on the hydrogen provided by the symbiont. This lethal selective force led to the survival of only those cells that had this endosymbiotic relationship, in the population in which the event occurred. Over time, this relationship permanently changed the population of surviving cells, as the bacteria developed into organelles, such as nuclei and mitochondria, and the archaeon host became a cell adapted to utilizing these internal structures. The eukaryotes, genetic chimeras with genes of their predecessors, then had the capability to evolve into the countless species that have existed on the planet.
Lane, Nick. The Vital Question: Energy, Evolution, and the Origins of Complex Life. New York: W.W. Norton, 2015. Print.
Martin, William, and Miklós Müller. "The hydrogen hypothesis for the first eukaryote." Nature 392.6671 (1998): 37-41.
- Ricardo Roche