Mysterious microbes living in wetlands in China have helped to narrow down when the ancestor of all animals and plants lived 鈥� and the oxygenation of Earth鈥檚 atmosphere may have played a key role in its emergence.
The new data brings us a step closer to understanding 鈥渁n event which happened billions of years ago鈥�, says at Aarhus University in Denmark.
This event was the origin of complex eukaryotic cells, of the type found inside our bodies and other complex organisms. The first living cells were small and had few internal structures. They are represented today by bacteria and their sister group the archaea. In contrast, eukaryotic cells are many times larger 鈥� though still microscopic 鈥� and have many internal structures. These include a nucleus, where the DNA is stored, and sausage-shaped mitochondria that supply energy.
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Since the 1960s, it has become clear that eukaryotes formed when one cell somehow came to live inside another. The energy-giving mitochondria were once free-living cells, which ended up living inside a larger host. Genetics suggests that the host cell was an archaean, while the original mitochondrion was a bacterium, says Avc谋.
A big break came in 2015, when researchers drilling in the Atlantic seabed discovered a new group of archaea, called the Asgard archaea after the home of the gods in Norse mythology. The Asgard archaea have many genes that were previously only found in eukaryotes, and they are the closest living relatives of eukaryotes. It seems the archaeal host that took in a bacterium to become the first eukaryote was an Asgard archaean, or something closely related.
But which Asgard archaean, and when, and how? There are many subgroups, all with Norse-inspired names like Lokiarchaeia, Thorarchaeia and Heimdallarchaeia. To find out which one gave rise to eukaryotes, a team led by at East China Normal University in Shanghai took samples from 14 coastal wetlands in China. The researchers sequenced all the DNA they could find, and identified 223 new Asgard archaea, including 16 new subgroups.
鈥淭his is a big number,鈥� says Avc谋, who was not involved in the research, and means we now have a much broader picture of the diversity within the group.
A 2023 study concluded that . However, Dong鈥檚 team found that eukaryotes as a group are older than Heimdallarchaeia, so they couldn鈥檛 have emerged from within that group. Instead, eukaryotes and Heimdallarchaeia are closely related 鈥渟ister鈥� groups.
Dong says previous studies inadvertently went wrong by including a group of Asgard archaea called the Njordarchaeales. These, it turns out, carry a lot of DNA from non-Asgard archaea, throwing off the analyses.
The new genomic data also allows Dong and his team to estimate when the different groups emerged. They say the last common ancestor of Asgard archaea lived 3.72 billion to 3.06 billion years ago. Then the Heimdallarchaeia group diverged 3.12 billion to 2.26 billion years ago, suggesting the eukaryotes emerged during the same period.
Dong says this fits with several previous studies. that the ancestors of eukaryotes split from other Asgard archaea around 2.7 billion years ago. However, they didn鈥檛 acquire mitochondria and become 鈥渢rue鈥� eukaryotes until much later, perhaps 1.84 billion years ago. Perhaps in line with that, , published in March, found that the oldest confirmed examples are 1.89 billion years old. Likewise, a 2023 analysis found that mitochondria split from other bacteria .
From about 2.5 billion years ago, oxygen appeared in the air, released by photosynthetic bacteria. Dong says this was key to the evolution of eukaryotes. Some bacteria could tolerate oxygen, so the Asgard archaea formed partnerships with them. 鈥淎s atmospheric oxygen levels continued to rise, the symbiotic relationship became increasingly integrated, ultimately resulting in the fusion of the partners,鈥� says Dong.
We can鈥檛 be too confident about any of these specific findings, because of our limited sampling of Asgard archaea, says Avc谋. 鈥淚 think we just see the tip of the iceberg now,鈥� he says, so additional genomes could change the picture again.
Nature
