Eukaryotic Cells Emerged in Anoxic Environment, New Study Suggests

Apr 27, 2022 by News Staff

Since the 1960s, many scientists have argued that the emergence of eukaryotes — cells containing a clearly defined nucleus — happened in response to the oxygenation of Earth’s surface environment. But new research led by Stanford University and University of Exeter scientists suggests eukaryotes in fact emerged in an anoxic environment in the ocean.

Asgard archaeon. Scale bar - 500 nm. Image credit: Imachi et al., doi: 10.1038/s41586-019-1916-6.

Asgard archaeon. Scale bar – 500 nm. Image credit: Imachi et al., doi: 10.1038/s41586-019-1916-6.

“We can now independently date eukaryogenesis and key oxygenation transitions in Earth history,” said lead author Dr. Daniel Mills, a researcher in the Department of Geological Sciences at Stanford University, the Department of Biology at Portland State University, and the Penn State Extraterrestrial Intelligence Center.

“Based on fossil and biological records, the timing of eukaryogenesis does not correlate with these oxygen transitions in the atmosphere (2.22 billion years ago) or the deep ocean (0.5 billion years ago).”

“Instead, mitochondria-bearing eukaryotes are consistently dated to between these two oxygenation events, during an interval of deep-sea anoxia and variable surface-water oxygenation.”

The emergence of mitochondria — the energy-producing powerhouses of eukaryote cells — is now thought to be the defining step in eukaryogenesis.

Mitochondria have different DNA to the cells in which they live, and the new study addresses the possible origin of this symbiotic relationship, famously championed by the biologist Lynn Margulis (then Lynn Sagan) in 1967.

“The 2015 discovery of Asgard archaea offers a major clue,” Dr. Mills said.

“Mitochondria-bearing eukaryotes likely resulted from a merger between archaea and bacteria, and the DNA in modern Asgard archaea is more closely related to the DNA found in eukaryote nuclei today than it is to other archaea.”

“This is additional evidence that the host that took in the bacterium was an archaeon.”

Asgard archaea live in anoxic ocean sediments, and they can live symbiotically with bacteria — possibly the same situation that led to the metabolic coupling that created the first eukaryote cells.

“The new evidence supports the hydrogen hypothesis — that mitochondria were acquired in anoxic conditions — first put forward in 1998 by Bill Martin and Miklos Müller,” said senior author Professor Tim Lenton, director of the Global Systems Institute at the University of Exeter.

“The idea that oxygen led to eukaryogenesis has been taken for granted.”

“In fact, mitochondrial aerobic respiration probably emerged later, having only become globally widespread within the last billion years as atmospheric oxygen approached modern levels.”

“Our review was intended to bridge a gap between biology and geology, as connections were waiting to be made following breakthroughs in both disciplines.”

The team’s paper was published in the journal Nature Ecology & Evolution.

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D.B. Mills et al. Eukaryogenesis and oxygen in Earth history. Nat Ecol Evol, published online April 21, 2022; doi: 10.1038/s41559-022-01733-y

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