Guest post by John Tillman

In 1962 paper, Roger Stanier and C. B. van Niel established the division of cellular organization into prokaryotes and eukaryotes, defining prokaryotes as those organisms, such as bacteria, which lack a cell nucleus.

Ernst Mayr (1904-2005) called the evolution of eukaryotes–cells with nuclei–“perhaps the most important and dramatic event in the history of life”. Eukaryotic cells also contain various organelles, such as mitochondria, their powerhouses, and in plants and algae, photosynthetic chloroplasts, which use sunlight to make sugar from water and carbon dioxide. This momentous milestone probably occurred between 1.8 and 2.2 billion years ago, but possibly longer.

In 1966, Lynn Margulis (1938-2011), the first Mrs. Carl Sagan, proposed that eukaryotic cells resulted from endosymbiosis, via engulfment of the ancestor of mitochondria by another prokaryote. Experimental evidence for this hypothesis came in 1978, when Robert Schwartz and Margaret Dayhoff demonstrated the descent of mitochondria from bacteria and of chloroplasts from cyanobacteria. During the 1980s, the DNA of mitochondria and chloroplasts was found to differ from their host’s nuclear genetic material, which validated endosymbiosis as a real evolutionary process.

Meanwhile, in 1977, Carl Woese (1928-2012) and George Fox defined Archaea as a third domain of life, with fellow prokaryotic Bacteria and with the much more complex Eukaryota.

In 1999, M. W. Gray, et. al., found strong phylogenetic evidence that an alphaproteobacterium was the ancestor of the mitochondrion. This left open the question of what kind of prokaryote engulfed the first proto-mitochondrion. Similarities in the membranes of archaea and eukaryotes suggested that the endosymbiosis wasn’t between two bacteria, but was a union of two separate domains. If so, then phylogenetically, there were arguably only two domains, not three, since the nuclear DNA of us eukaryotes descends from archaea.

In 2017, various researchers identified the Asgard superphylum as the closest archaean relatives of eukaryotes, but this group was known only from its DNA, recovered from seafloor sediments. Without knowing what Asgard archaeans even look like, let alone their behavior, scientists couldn’t shed light on how endosymbiotic events might have happened.

But know long, hard work by real scientists has helped to unravel this mystery. Thanks to the remarkable persistence and painstaking practice of Japanese microbiologists, science now knows enough about one member of the Asgard superphylum to form an educated hypothesis regarding ancient endosymbiosis and the origin on unicellular eukaryotes and their multicellular descendants, ie animals, fungi and plants.

Please read this Science article for the fascinating details:

https://www.sciencemag.org/news/2019/08/tentacled-microbe-could-be-missing-link-between-simple-cells-and-complex-life

This discovery offers another instance of the importance to life in general of all kinds of symbiosis. We eukaryotes swim in a sea of microbes, most of which aren’t parasites or pathogens. Many make our lives possible. Even the simplest animals–sponges–often form symbiotic relationships with oxygen-producing cyanobacteria. Similarly, lichen are mutualistic partnerships between fungi and algae or cyanobacteria. “Coral symbiosis is a three-player game”:

https://www.nature.com/articles/d41586-019-00949-6

While H. sapiens relies less on gut prokaryotes than do termites, ruminants and other animals needing to break down cellulose, our bodies still contain about 1.4 microbes for each human cell.

Gut prokaryotes: not just for methane any more!