Thanks to a team of scientists led by Thijs Ettema, Asgard is now also the name of a large clan of microbes. Its members, which are named after Norse gods like Odin, Thor, Loki, and Heimdall, are found all over the world. Many of them are rare and no one has actually seen them under a microscope. But thanks to their DNA, we know they exist. And we know that they are singularly important to us, because they may well be the group from which we evolved.

If Ettema is right, then around two billion years ago, an Asgardian microbe (or an incredibly close relative) took part in a unique event that gave rise to the eukaryotes. That’s the group which includes humans, our fellow animals, plants, fungi, and every living thing made from large, complex cells—all the living things we’re most familiar with, and all the ones we can actually see. Our origins lie either in Asgard, or next door to it.

Eukaryotic cells are generally much bigger than either bacteria or archaea. They also have larger genomes. They have internal compartments that act like our organs, each with its own special job. They have an internal skeleton that acts as a transport network for molecules. There’s this huge gulf of complexity that separates them from the other two domains. It’s a gulf that has only ever been crossed once in life’s history. Bacteria and archaea are capable of amazing feats of evolution, but in over 3.7 billion years of existence, none of them have ever evolved into anything approaching a eukaryote-like cell—except that one time. Why?

One possible answer, which I’ve written about before, says that eukaryotes were created through an incredibly unlikely merger between members of the other two domains. Somehow, a bacterium found its way inside an archaeon and, rather than being digested or destroyed, became a permanent part of its host. In doing so, it provided the archaeon with an extra source of energy, which allowed it to get bigger, accumulate more genes, and evolve down new paths that were previously inaccessible to it. That fusion cell gave rise to the eukaryotes, and the bacterium eventually turned into the mitochondria—little bean-shaped structures that still power eukaryotic cells to this day.

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posted by f. sheikh