Filmed live without oxygen, Asgard archaea crawl on protrusions that actin inhibitors stop

Asgard archaea are our closest known microbial relatives, and they sit at the centre of current models for how complex cells arose ^[4]. Until these films, the best pictures of them were electron micrographs. Those showed a round cell body ringed by delicate projections up to 20 times its length, and nothing about what the projections did ^[9]. Most of what is known about the group comes from those images and from DNA sequencing ^[9].

Filming them first required cultures. The first two specimens were grown only in 2020 and 2023, one at JAMSTEC in Japan and one in Christa Schleper's lab in Vienna ^[8]. The cells also had to stay alive under the lens, so the team filmed them in an oxygen-free environment ^[2]. The cells are tiny, about a thousandth the volume of a human cell ^[6]. If the two have similar shapes, that is roughly a tenth of the width ^[12].

In both strains, a Lokiarchaeon and a Heimdallarchaeon, the cells changed shape drastically every minute, attached to surfaces with their thin appendages and crawled across them ^[2][3]. That behavior had not been described in microbes before. The closest comparison the researchers offer is complex cells, including human immune cells ^[3].

The drug experiment is the control. Actin inhibitors suppressed the dynamic behavior. The team takes that as a sign of a central role for an actin-based cytoskeleton, the same class of machinery that shapes and moves human cells ^[11]. It is a pharmacological result, obtained on cultured cells under a microscope ^[2]. It ties the crawling to actin. How the protrusions generate force or grip a surface is a separate question.

The films do not show when crawling evolved. Current models put the first eukaryotes at about 2 billion years ago, formed when a bacterium fused with an ancestor of today's Asgard archaea ^[7]. The Vienna team argues that complex cell motility may be much older than previously thought and may have mattered in the ancient symbiosis that later produced mitochondria ^[13]. That inference rests on two living lineages. If Asgard archaea and eukaryotes both crawl on actin, the simplest explanation is a common ancestor that crawled. Separate origins stay possible until more lineages are filmed and their underlying proteins compared.

According to the University of Vienna, oxygen-free live-cell microscopy makes it possible for the first time to test models of the origin of complex life against experiment ^[14]. I think the method is the part of this work that will last. Sequence data and fixed images ^[9] describe what a cell encodes and how it looks. A film shows what the cell does, and the inhibitor test shows that the behavior can be switched off on purpose ^[11]. The press account does not report how many cells were tracked or how fast they moved.