By Mathew Goldstein
Alphaproteobacterium is a class of aerobic bacteria within the phylum Proteobacteria that have diverse metabolic capabilities, including nitrogen fixation and photosynthesis. The Great Oxidation Event that changed earth’s atmosphere and triggered the development of multi-cellular life, thanks to photosynthetic bacteria, occurred more than 2.4 billion years ago. The common ancestor of animals, plants, fungi, seaweeds, and various related unicellular organisms was a eukaryote cell that originated when an archaea cell non-destructively engulfed an Alphaproteobacterium. The archaea that most resembles eukaryotes, because they have the same genes, are the Asgard archaea. The merged pair of prokaryotes formed an ongoing symbiotic relationship, with the oxygen respiring bacteria becoming an oxygen respiring energy-producing organelle, known as the mitochondria, of the archaea. The mitochondria rely on genes found only in Alphaproteobacterium. This was one of the steps needed to transition the cell from an archaea to a eukaryote. This perspective is rooted in a taxonomic classification scheme with three unicellular domains, Archaea, Bacteria, and Eukarya, where the latter domain emerges from this merger of Asgard archaea and Alphaproteobacterium cells.
However, it has been unclear how anaerobic Asgard archaea that live only in places without oxygen and that cannot metabolize oxygen would meetup with, and merge with, an aerobic bacterium that lives only in places with oxygen. A new analysis of a single celled organism named Heimdallarchaeia demonstrates it is an Asgard archaea that has several proteins closely resembling those used by eukaryotes for oxygen-based, energy-efficient metabolism (without the mitochondria, membranes, and nucleus found in eukaryotes). Heimdallarchaeia are found in shallow coastal sediments that alternate between oxic and anoxic states, among other places. Heimdallarchaeia is currently the only known mixed anaerobic and aerobic lifestyle Asgard archaea and therefore is now the leading candidate for being the closest extant organism to the common ancestor of all eukaryotes.
The conclusions asserted above are best fit with the available evidence conclusions where the available evidence is, and plausibly always will be to some extent, incomplete. We have no videos of all of the relevant historical events that occurred step by step several billion years ago. We do not yet know if the first eukaryotic common ancestor (FECA) acquired mitochondria before acquiring membranes and a nucleus, or acquired membranes and a nucleus before mitochondria. These unavoidable limits in our ability to fully know everything that happened more than a billion years ago are too weak of an excuse for rejecting altogether the conclusions we arrive at this way about the past given how tightly and consistently the many different pieces of evidence we have all converge on this same general conclusion regarding our origins. There is equal opportunity for evidence to favor any one of various mutually exclusive conclusions, so when the relevant evidence all points us towards the same conclusion then there cannot be a better reason for favoring any other conclusion. For those people who find the larger implications of conclusions like this regarding our origins depressing, I recommend this wonderful 2024 essay by philosopher Maarten Boudry What if False Beliefs Make You Happy.