All complex life may be descended from one group of single-celled organisms, whose modern descendants live in mud. These microbes have unusual abilities that would have enabled them to form more intricate cells – and ultimately animals and plants.
The microorganisms are called Asgard archaea, after the mythological home of the Norse gods. For the first time, scientists have isolated one and grown it in the laboratory. żěè¶ĚĘÓƵ first reported the achievement in August 2019, and the results have now been published.
The first Asgard archaea were described in 2015, after their DNA was found in sediments on the Atlantic seabed. Biologists immediately recognised that they could help explain one of the most important steps in evolutionary history: the origin of eukaryotes.
Advertisement
The oldest living things are bacteria and archaea. They are all single-celled, with simple internal structures. In contrast, eukaryotes have larger, more intricate cells. All multicellular organisms, from mosses to humans, are eukaryotes. The question is how eukaryotes evolved.
Our single-celled ancestors
The Asgard microbes are crucial because, while they are archaea, they carry many genes that were only previously found in eukaryotes. This implies that they are our closest non-eukaryotic relatives – and that, billions of years ago, eukaryotes evolved from an Asgard archaeon.
It turns out that Asgard archaea are common. All are named for Norse gods, for example the Heimdallarchaeota group, for a guardian with the ability to see the future. Most recently, a November study identified Gerdarchaeota, named for the Norse goddess of fertile soils, in coastal sediments (bioRxiv, DOI: ).
Furthermore, the link between Asgard archaea and eukaryotes looks increasingly solid. In December, Tom Williams at the University of Bristol, UK, and his colleagues analysed more than 3000 gene families in archaea and eukaryotes. They confirmed that Asgard archaea are the closest known relatives of eukaryotes, and that Heimdallarchaeota is the closest of all (Nature Ecology & Evolution, DOI: ).
But it is still unclear how Asgard archaea gave rise to eukaryotes.
Eukaryotes’ most crucial feature is arguably mitochondria: sausage-shaped structures within the cells that provide energy. Mitochondria were once bacteria, which were somehow engulfed by a eukaryotic cell.
The newly cultured Asgard archaeon, Prometheoarchaeum, hints at how this happened. When Hiroyuki Imachi at the Japan Agency for Marine-Earth Science and Technology and his colleagues grew it, they found it had long tentacle-like protrusions. It looked like a squashed spider.
They suggest that Asgard archaea cooperated with bacteria, which first lived swaddled in these protrusions, before ultimately moving inside the Asgard cells.
Robert Robinson at Okayama University in Japan has evidence that fits this story. He and his team recently found that Asgard archaea have complex mechanisms for assembling and breaking down a protein called actin (bioRxiv, DOI: ).
This had been predicted on the basis of the microbes’ DNA, but Robinson’s team showed how the system works. It even functions when transferred into human cells, which suggests that this molecular machinery has changed very little, even over billions of years – a finding Robinson calls “astonishing”.
Actin forms struts within cells, so it is crucial for their ability to change shape by deforming their outer membranes – which is necessary for swallowing other cells. Robinson says the Asgard archaea probably use actin to grow their protrusions.
Cells swallowing cells
This is neat, but it can’t explain everything, says PurificaciĂłn LĂłpez-GarcĂa at the French National Centre for Scientific Research (CNRS).
LĂłpez-GarcĂa says the outer membranes of eukaryotes resemble those of bacteria, not those of archaea – which makes no sense if eukaryotes are primarily descended from Asgard archaea. Many eukaryotic genes also come from bacteria. She suspects that more than two microorganisms contributed to eukaryotes.
In line with this, a Japanese group recently described a bacterium that can engulf other microorganisms (Nature Communications, DOI: ). This ability was thought to be unique to eukaryotes.
“We are going to discover that these types of mechanisms are more widespread than we thought before,” says LĂłpez-GarcĂa. In that case, the Asgard archaea may be just one player in a larger story of microbes swallowing each other during the origin of eukaryotes.
Nature
