Two newly cultivated Heimdallarchaeia strains have branching membrane arms, but no nucleus or other internal compartments—a glimpse of what some of life’s closest ancient relatives may have been like.
The strains were grown and studied by a team led by Hiroyuki Imachi and Masaru K. Nobu of the Japan Agency for Marine-Earth Science and Technology, together with Satoshi Nakagawa of Kyoto University. Heimdallarchaeia are archaea, single-celled organisms, and are considered among the closest known archaeal relatives of eukaryotes. Eukaryotes include animals, plants and fungi, whose cells have complex interiors. The work, published in Nature, gives researchers a chance to examine these archaea directly rather than infer their traits from environmental DNA.
How eukaryotic cells first arose remains a major question in biology. One leading idea is that an archaeal cell formed a partnership with a bacterium, which later became the mitochondrion, the cell structure that helps release energy from food. In 2020, the JAMSTEC team cultivated another archaeal relative, strain MK-D1, from deep-sea sediment and used its features to propose the E3 model for eukaryotic origins. Later DNA studies pointed to Heimdallarchaeia as even closer relatives, but no one had grown them in the lab to check what their cells were actually like.
The two new strains had simple interiors, without membrane-bound structures such as a nucleus. Their outer surfaces, by contrast, sent out multiple long, branching membrane protrusions. The researchers found that these extensions helped the cells attach to other microorganisms and form partnerships. Both strains lived without oxygen, a way of life called anaerobic. Yet one strain had several systems that helped protect it from oxygen and could grow when small amounts were present, provided oxygen-using bacteria were also there.
These observations support a possible route from simple cells to the complex cells found in animals, plants and fungi. The researchers suggest that an early archaeal cell may have stayed simple inside while building relationships with bacteria through its branching surface. Such a partnership could have been an early step toward the internal relationship that eventually placed a bacterium inside the archaeal cell as a mitochondrion. The findings also raise the possibility that this first partnership began where oxygen was scarce, with the bacterial partner helping the archaeon cope with some oxygen.
The study does not recreate the moment eukaryotes began, and the team presents a possible explanation rather than a settled account. But having living Heimdallarchaeia to study gives researchers a practical way to test ideas about their cell structure and microbial partnerships. The next work may help clarify how a cell with a simple interior, and a surface built for close contact, could become part of the long story that led to complex life.