Complex archaea that bridge the gap between prokaryotes and eukaryotes

The origin of the eukaryotic cell remains one of the most contentious puzzles in modern biology. Recent studies have provided support for the emergence of the eukaryotic host cell from within the archaeal domain of life, but the identity and nature of the putative archaeal ancestor remain a subject of debate. Here we describe the discovery of ‘Lokiarchaeota’, a novel candidate archaeal phylum, which forms a monophyletic group with eukaryotes in phylogenomic analyses, and whose genomes encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities. 

Our results provide strong support for hypotheses in which the eukaryotic host evolved from a bona fide archaeon, and demonstrate that many components that underpin eukaryote-specific features were already present in that ancestor. This provided the host with a rich genomic ‘starter-kit’ to support the increase in the cellular and genomic complexity that is characteristic of eukaryotes.


a, Schematic overview of the metagenomics approach. BI, Bayesian inference; ML, maximum likelihood. b, Bayesian phylogeny of concatenated alignments comprising 36 conserved phylogenetic marker proteins using sophisticated models of protein evolution (Methods), showing eukaryotes branching within Lokiarchaeota. Numbers above and below branches refer to Bayesian posterior probability and maximum-likelihood bootstrap support values, respectively. Posterior probability values above 0.7 and bootstrap support values above 70 are shown. Scale indicates the number of substitutions per site. c, Phylogenetic breakdown of the Lokiarchaeum proteome, in comparison with proteomes of Korarchaeota, Aigarchaeota (Caldiarchaeum) and Miscellaneous Crenarchaeota Group (MCG) archaea. Category ‘Other’ contains proteins assigned to the root of cellular organisms, to viruses and to unclassified proteins.

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