Lab-grown Asgard archaea closest to eukaryotes are simple inside

Two Heimdallarchaeia strains, isolated after three years of lab work from a Japanese gas well, have no nucleus or lasting internal membranes, yet grow arms that can hold more volume than the cell body itself.

Peer reviewed · Laboratory cultures and genome analysis · Two strains

The archaea that sit closest to our own branch of life are simple on the inside. A team led by Hiroyuki Imachi and Masaru K. Nobu of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) grew two strains of Heimdallarchaeia, the Asgard group that genome studies place nearest to eukaryotes, and reports in Nature that neither has a nucleus or any lasting internal compartment. That cuts against proposals that eukaryotes’ archaeal ancestor was already complex inside. The catch: these are two living microbes, not the ancient ancestor, and where eukaryotes branch on the tree is still argued.

Evidence at a glance

Is this a big deal?

A solid step. Earlier Asgard isolates came from branches further from eukaryotes, so claims about the group nearest to us rested mostly on DNA. Two live strains let researchers check them. What the ancestor looked like is still argued, now with better evidence on one side.

What scientists found

Eukaryotes are organisms whose cells keep their DNA in a nucleus: animals, plants, fungi and many microbes. Most biologists think the first one arose when an archaeal cell teamed up with a bacterium that became the mitochondrion. Asgard archaea, now formally the phylum Promethearchaeota, carry many genes once thought unique to eukaryotes, and HC1 and SC1 rank in the top quarter of their class for them.

Even so, neither strain showed a nucleus or persistent inner membranes; a few cells held faint vesicles that came and went. The complexity was on the outside. Most HC1 cells sprouted branching protrusions about 370 nm wide, which on average held about 1.4 times the volume of the cell body (itself 1.3 µm across).

Picture a one-room cabin with long walkways out to the neighbours, rather than a house divided into rooms. The analogy has a limit: nobody knows yet whether material moves along the arms.

How they found it

The microbes came from a biofilm in gas well KTG3 on Japan’s Boso Peninsula, where geyser-like bursts of methane-rich brine expose it to air now and then. The team fed cultures powdered milk and other protein-rich extracts and used antibiotics to weed out bacteria. Three years after the first 2019 sample, HC1 grew with just one methane-making partner. It doubles every 7 to 12 days; SC1 takes 14 to 20.

Blocking the partners’ methane production stopped growth, so both archaea rely on partners that mop up the hydrogen they release. Both needed peptides, short chains of amino acids. Dividing cells never had protrusions, and extra food made cells and arms bigger without speeding division, unlike most microbes.

Oxygen was the sharpest test. HC1 carries genes for cytochrome c oxidase, an enzyme oxygen-breathing cells use. Yet trace oxygen cut HC1 numbers roughly a hundredfold. It grew in low-oxygen medium only when oxygen-using bacteria were added, and yields still did not rise. The authors read the enzyme as protection, not respiration, and a classifier they built put every analysed Heimdallarchaeia genome among the anaerobes.

Why it matters to you

This won’t change anything in your daily life. It is about where your cells came from: if the leading theory is right, from a merger of an archaeon and a bacterium. For the parts these archaea lack, see how plant and animal cells differ.

What the evidence does not show

It does not show what the ancestor looked like. Only three Heimdallarchaeia strains have been studied under the microscope, and lineages gain and lose features over billions of years. The authors’ scenario needs “a steep transition in cell structure” later on, which leaves the biggest step unexplained. Neither archaeon grows without its partners, and the “closest relatives” label depends on which family tree you trust.

How this fits what we already knew

In 2020 the same JAMSTEC group grew the first Asgard archaeon, MK-D1, after 12 years of work; it too was simple inside, with protrusions. Eme and colleagues (2023) placed eukaryotes as sister to Hodarchaeales, the order HC1 and SC1 belong to; Zhang and colleagues (2025) placed them before all sampled Heimdallarchaeia. Appler and colleagues (2026) proposed from 404 genomes that the ancestor may have breathed oxygen, the view this paper pushes back on. For the oxygen backdrop, see oxygen before the Great Oxidation Event.

Independent perspective

SciQuest found no on-record comment from a researcher outside the author team as of 29 October 2026, 08:17 UTC, and has not yet interviewed one. Independent work cuts both ways. A November 2025 preprint by MacLeod and colleagues (MRC Laboratory of Molecular Biology, Cambridge) describes another Heimdallarchaeia, ‘Ca. Yibarchaeum umbracryptum’, with many internal vesicles early in growth. It is not yet peer reviewed, and the new paper treats those vesicles as transient, not lasting organelles.

How other headlines framed it

• Nature’s Research Briefing presented the strains as clues to how complex cells arose.
• Jiji Press led with the Chiba well and said the protrusions let the cells move. The paper reports no movement in these strains: their arms sway by Brownian motion, the random jostling of molecules, and grip partner cells.
• JAMSTEC’s release stressed external over internal complexity and called these the first Heimdallarchaeia grown in culture.

What happens next

HC1 and SC1 grow faster and larger than MK-D1, the only Asgard strain publicly available so far, which makes them workable lab models. The paper leaves open what the protrusions are for and whether the transient vesicles relate to the eukaryote-like genes.

In one sentence

The first isolated members of the archaeal group nearest to eukaryotes are oxygen-avoiding, partner-dependent cells with plain interiors and elaborate arms, which hints that inner complexity came after the partnership began.

Read next

• Plant vs animal cells: the organelles these archaea lack.
• Bleached corals recover by touching healthy clones: symbiosis at work today.
• Are humans aliens? What the evidence says about life’s origins.

Sources and disclosure

1. Imachi H, Nobu MK, Ishii S, et al. (26 authors). “Eukaryotes’ closest relatives are internally simple syntrophic peptidotrophic archaea.” Nature, published online 28 October 2026. doi:10.1038/s41586-026-11170-7.
2. JAMSTEC and partner institutions, “Capturing our closest archaeal relatives offers clues to the origin of eukaryotes“, 29 October 2026.
3. Imachi H et al. “Isolation of an archaeon at the prokaryote–eukaryote interface.” Nature 577, 519–525 (2020). doi:10.1038/s41586-019-1916-6.
4. Eme L et al. “Inference and reconstruction of the heimdallarchaeial ancestry of eukaryotes.” Nature 618, 992–999 (2023). doi:10.1038/s41586-023-06186-2.
5. Zhang J et al. “Deep origin of eukaryotes outside Heimdallarchaeia within Asgardarchaeota.” Nature 642, 990–998 (2025). doi:10.1038/s41586-025-08955-7.
6. Appler KE et al. “Oxygen metabolism in descendants of the archaeal-eukaryotic ancestor.” Nature 652, 405–415 (2026). doi:10.1038/s41586-026-10128-z.
7. MacLeod FI et al. “An Asgard archaeon with internal membrane compartments.” bioRxiv preprint, November 2025. doi:10.1101/2025.11.06.686947.
8. Coverage read for framing: Nature Research Briefing (summary only; full text paywalled), Jiji Press.

Disclosure: This article was drafted with AI assistance and checked against the sources above, including the full open-access paper. The authors declare no competing interests. Calculation by SciQuest: protrusion volume of 1.7 µm³ at 142% of the main cell body, as the paper reports, is about 1.4 times the body’s volume. The graphic is a SciQuest original built from values in the cited papers. Spotted an error? See our corrections policy.