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In this Journal Club, Anja Spang discusses the discovery of nanosized archaea, transforming our knowledge of archaeal phylogeny and shedding light on their roles in ecology and evolution.

Analysis of eukaryotic gene sequences using a relaxed molecular clock methodology indicate that eukaryotes emerged 3.0–2.25 billion years ago as a result of mitochondrial endosymbiosis with complex archaea that already possessed an elaborated cytoskeleton, membrane trafficking, endomembrane, phagocytotic machinery and a nucleus.

Analysis of the eukaryotic gene repertoires mediating central carbon metabolism identifies ancestral contributions from Alphaproteobacteria, Asgardarchaeota and other microbial taxa, followed by gene loss, transfer and subcellular retargeting, which have remodelled central carbon metabolism over time.

The symbiont Ca. Nanohaloarchaeum antarcticus is dependent on its host Halorubrum lacusprofundi for lipids due to a lack of certain biosynthetic genes. Here, the authors characterize the lipidome dynamics of this symbiotic relationship.

The timing of cellular evolution is poorly constrained. Here, the authors used improved molecular dating approaches to study the evolution of the ATP synthase in light of a dated tree of life thereby providing an absolute timescale for cellular evolution including eukaryotic origins.

Comparative genome and phylogenetic analyses of Asgard archaea reveal their metabolic potential and lead to the proposal of a revised model for eukaryogenesis, termed the reverse flow model.

A new large-scale genomics study reports a vastly expanded tree of life that is based on genomic data from over 3,000 species, including many uncultivated and poorly characterized prokaryotes.

The evolutionary relationships within Archaea remain unresolved. Here, the authors used genomic approaches to study the Undinarchaeota, a previously uncharacterized clade of DPANN, shed light on their position in an updated archaeal phylogeny and illuminate the history of archaeal genome evolution.

DPANN archaea are a group of microorganisms that require direct cell contact with other archaeal host species for growth. Here, Hamm et al. show that a DPANN archaeon engages in parasitic interactions with its host leading to host cell lysis, thus providing experimental evidence of a predatory-like lifestyle for an archaeon.

This study identifies a clade of archaea that is the immediate sister group of eukaryotes in phylogenetic analyses, and that also has a repertoire of proteins otherwise characteristic of eukaryotes—proteins that would have provided the first eukaryotes with a ‘starter kit’ for the genomic and cellular complexity characteristic of the eukaryotic cell.

Regulatory macrophages (Mreg) can directly suppress T effector cell responses. Here the authors show that human Mreg also elicit TIGIT⁺ regulatory T cells by integrating multiple differentiation signals, and that donor Mreg-induced recipient Tregs may promote kidney transplant acceptance in patients.

Integration of phylogenetics, comparative genomics and palaeobiological approaches suggests that the last universal common ancestor lived about 4.2 billion years ago and was a complex prokaryote-grade anaerobic acetogen that was part of an ecosystem.

Natural gas reservoirs in the oceanic subsurface sustain complex communities of anaerobic microbes. Here, Seitz et al. describe a previously unknown archaeal phylum, Helarchaeota, belonging to the Asgard superphylum and with the potential for oxidation of hydrothermally generated short-chain hydrocarbons.

Rumen methanogenic archaea are major sources of methane emissions and potential targets for methane mitigation strategies. Poulsen et al.now show that dietary rapeseed oil (RSO) supplementation can reduce the abundance of methanogenic Thermoplasmata archaea inhabiting the bovine rumen.

The Micrarchaeota lineage includes poorly characterized archaea with reduced genomes that likely depend on host interactions for survival. Here, the authors report a stable co-culture of a member of the Micrarchaeota and its host, and use multi-omic and physiological analyses to shed light on this symbiosis.

A study of the first genomes of the marine Hikarchaeia, the closest known relatives of Haloarchaea, is presented. Their inclusion in ancestral reconstructions unveils an intermediate stage in the evolutionary transition from ancestral anaerobic methanogens to modern day aerobic halophiles.

This work describes the Asgard superphylum, an assemblage of diverse archaea that comprises Odinarchaeota, Heimdallarchaeota, Lokiarchaeota and Thorarchaeota, offering insights into the earliest days of eukaryotic cells and their complex features.

The discovery and genomic characterization of a new group of extreme halophilic methanogens sheds light on the origin of methanogenesis and the evolution of the Haloarchaea.

For several cancer types, mouse models indicate that metastasis occurs early. Here, the authors show that human melanoma cells disseminate early and illustrate a genetic colonisation signature that evolves in parallel at different sites.

The Archaea was recognized as a third domain of life 40 years ago. In this Review, Emeet al. outline a brief history of the changing shape of the tree of life and examine how the recent discovery of diverse archaeal lineages has changed our understanding of the evolutionary relationships between the three domains of life and the origin of the eukaryotic cell.

In this Consensus Statement, the authors discuss the issue of naming uncultivated prokaryotic microorganisms, which currently do not have a formal nomenclature system due to a lack of type material or cultured representatives, and propose two recommendations including the recognition of DNA sequences as type material.