Life › Complex cells & first multicellular life › Eukaryotes: complex cells with a nucleus
Asgard archaea: the host cell
host lineage before c. 1.8 billion years ago; found 2015· Organism or group· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
A group of archaea, first found as DNA in Arctic seafloor mud in 2015, that are the closest known relatives of complex cells.
See it on the Life map →What happened
In 2015 Thijs Ettema's team sequenced DNA from sediment near a deep-sea vent field called Loki's Castle, between Greenland and Norway, and found 'Lokiarchaeota'. Relatives named Thor, Odin and Heimdall followed: the Asgard group. They carry genes once thought unique to eukaryotes, such as those for internal scaffolding and membrane handling. In 2020 a Japanese team, after 12 years of work, grew one in the lab: a tiny, slow-growing cell with long tentacle-like arms.
Why it matters
They suggest eukaryotes grew out of archaea, so the tree of life has two main branches (bacteria and archaea), not three.
The evidence (fossils, genes, rocks)
Genomes assembled from environmental DNA (metagenomics), the cultured strain Prometheoarchaeum (Imachi and colleagues, Nature 2020), and gene-tree analyses such as Eme and colleagues, Nature 2023.
Still debated
Which Asgard lineage is the true sister of eukaryotes, and how complex the host already was before taking in its bacterial partner.
Connections (3)
▸Asgard archaea: the host cell provided the host cell for Eukaryotes: complex cells with a nucleusStrong evidence
The connection
Gene trees place eukaryotes inside or next to the Asgard archaea, so an Asgard-type cell was probably the host that took in the bacterium that became the mitochondrion. Asgard cells already carried some eukaryote-style genes for membranes and internal scaffolding, raw material for the nucleus and other compartments.
When and how
Before about 1.8 billion years ago, probably in oxygen-poor seafloor sediments; recognized through Thijs Ettema's 2015 DNA study and later analyses.
How sure are we?
Shared 'eukaryotic signature' genes, gene trees (Eme and colleagues, 2023), and a lab-grown Asgard cell whose shape fits a possible precursor.
Debates and open questions
Some scientists question whether eukaryotes branch inside Asgard or beside it, and which Asgard group was the actual ancestor.
▸Asgard archaea: the host cell are a branch of Bacteria & archaea: the microbial worldStrong evidence
The connection
Asgard archaea belong to the archaea, one of the two great domains of microbes alongside bacteria. They went unnoticed until DNA was sequenced straight from environmental samples, showing how much of the microbial world was hidden and suggesting eukaryotes are an offshoot of archaea.
When and how
Seafloor sediments, hot springs and microbial mats worldwide; first described from sediment near Loki's Castle, Arctic Ocean, in 2015.
How sure are we?
Their genomes carry the core archaeal genes, and gene trees consistently group them with other archaea, including the strain grown in the lab in 2020.
Debates and open questions
How many Asgard groups exist, and whether eukaryotes should formally be counted as a branch within the archaea.
▸Asgard archaea: the host cell reshaped the root of Common descent & the tree of lifeStrong evidence
The connection
Carl Woese's tree had three great domains: bacteria, archaea and eukaryotes. Asgard archaea carry many genes once thought unique to eukaryotes, and on gene trees eukaryotes branch from within them. That suggests complex cells arose when an Asgard-like archaeon merged with a bacterium, leaving two primary domains, with eukaryotes as a later offshoot.
When and how
Lokiarchaeota DNA from Arctic seafloor mud near Loki's Castle, published by Thijs Ettema's team in 2015; first grown in a lab by a Japanese team, reported 2019–2020.
How sure are we?
Gene trees built from dozens of shared proteins place eukaryotes inside or beside Asgard archaea; the lab-grown cell makes eukaryote-like scaffolding proteins.
Debates and open questions
Some analyses still favor three domains, results depend on methods and gene choice, and which Asgard group is closest to eukaryotes (perhaps Heimdall-related) is debated.