Stanford professor lauds ability to independently date key oxygenation transitions in Earth history

The origin of complex cells known as eukaryotes may have been misconstrued for decades, according to new research.

A team of scientists from Stanford and Exeter University have challenged the conventional wisdom that the emergence of eukaryotes was a response to the oxygenation of Earth's surface environment, according to a news release.

The team's review suggests recent breakthroughs in the Earth and life sciences "decouple" eukaryogenesis from rising oxygen levels and instead point to an anoxic, or oxygen-free, environment in the ocean as the birthplace of eukaryotes.

"We can now independently date eukaryogenesis and key oxygenation transitions in Earth history," said Stanford's Daniel Mills. "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," he added.

Mitochondria are the energy-producing "powerhouses" of eukaryote cells, and their emergence is now thought to be the defining step in eukaryogenesis. The new research addresses the possible origin of mitochondria's unique DNA, which is different from the cells in which they reside. 

The 2015 discovery of "Asgard" archaea offers a major clue, according to Mills. These single-celled organisms live in anoxic ocean sediments and can live symbiotically with bacteria. 

This relationship may have led to the "metabolic coupling" that created the first eukaryote cells. The DNA in modern Asgard archaea is more closely related to the DNA found in eukaryote nuclei today than it is to other archaea, providing additional evidence that the host that took in the bacterium was an archaeon. 

The new evidence supports the "hydrogen hypothesis," which suggests that mitochondria were acquired in anoxic conditions, as first put forward in 1998 by Bill Martin and Miklos Müller. 

The idea that oxygen led to eukaryogenesis has long been taken for granted but, in reality, mitochondrial aerobic respiration probably emerged later, only becoming globally widespread within the last billion years as atmospheric oxygen approached modern levels. 

The review aims to bridge the gap between biology and geology, as connections were waiting to be made following breakthroughs in both disciplines. The paper, entitled "Eukaryogenesis and Oxygen in Earth History," was published in the journal Nature Ecology & Evolution on April 27, 2022.