By Mathew Goldstein
Alphaproteobacterium are a class of bacteria within the phylum Proteobacteria with diverse metabolic capabilities, including nitrogen fixation and photosynthesis. The common ancestor of animals, plants, fungi, seaweeds and various unicellular organisms was probably an archaea that non-destructively engulfed an alphaproteobacterium. The Achaea that most resembles eukaryotes are the Asgard archaea. The merged pair of single cell organisms formed an ongoing symbiotic relationship with the bacteria becoming an energy-producing organelle, known as the mitochondria, of the archaea. This merger then resulted in the first eukaryotes which are the type of cell now found in fungi, plants, animals, etc.
However, it has been unclear how anaerobic Asgard archaea that live only in places without oxygen would meetup with, and merge with, a bacterium that live 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. Heimdallarchaeia is therefore now the leading candidate for being the closest extant organism to the common ancestor of all eukaryotes (or even possibly being that common ancestor).
Of course, 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 historical events that occurred step by step billions of years ago. These unavoidable limits in our ability to fully witness what happened 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. 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.
