The First Complex Life Forms May Have Come From Two Merging Microbes

New evidence strengthens the hypothesis that the first complex life forms, called eukaryotes, arose from the merger of two simpler life forms, researchers report.

The same team previously identified living relatives of ancestors of eukaryotes, while this latest study shows how those ancestors might have shared the work of metabolism with bacteria they acted as hosts for.

"This study is exciting because it provides important clues about how interactions in nature led to the origin of complex life," says Brett Baker, assistant professor at the University of Texas Marine Science Institute. The research appears in the journal Nature Microbiology.

Scientists have proposed that around 2 billion years ago, when all life on the planet was single-celled organisms, something new happened between a couple of them: namely, a microbe belonging to the Archaea group gobbled up a bacterium, and the bacterium survived.

Because the new living arrangement benefited both the landlord and the tenant, the two previously free-living microbes spawned a single, hybrid offspring — a new form of life whose descendants evolved into plants, fungi, animals, and even humans. All these life forms, called eukaryotes, have cells with complex internal structures.

In a previous study from 2017, which appears in the journal Nature, the researchers found that a newly identified group of microbes called Asgard archaea had many genes that were thought to be exclusive to eukaryotes.

At the time, the researchers interpreted that to mean that the archaea that merged symbiotically with bacteria 2 billion years ago to form the first eukaryotes were likely members of this Asgard superphylum. This latest study strengthens that interpretation by showing more about how that symbiosis worked.

The researchers helped compare the genomes of the Asgard archaea to those of other organisms in order to piece together their metabolic pathways. They found that these archaea use complex carbohydrates as growth substrates — something that requires the intermediary involvement of a bacterial partner.

"This study advances our understanding of how the archaeal host survived in nature and it appears they rely on a symbiotic partner," Baker adds.

Ultimately, the researchers would like to experimentally verify the new hypothesis, which is purely based on genomic analyses. This would, however, require growing Asgard archaea in the laboratory, something that researchers haven't yet achieved. The microbes thrive in extreme environments.

Support for the work came from the European Research Council, the Swedish Foundation for Strategic Research, the Swedish Research Council, the NWO-I Foundation of the Netherlands Organization for Scientific Research, the European Commission, the Wenner-Gren Foundations in Stockholm, the European Molecular Biology Organization, the Natural Sciences and Engineering Research Council of Canada, the Australian Research Council, and the National Science Foundation.