Complex Cells' Origin Has Been Started Without Oxygen

Complex Cells' Origin Has Been Started Without Oxygen ...

According to new research, complex cells were originally formed without oxygen.

Many experts have argued that the emergence of eukaryotes (cells with a clear nucleus) was made due to the oxygenation of the Earth''s surface environment.

Recent advances in the Earth and life sciences, according to a team led by Stanford and Exeter, have weighed heavily on this notion.

These findings "decouple" the emergence of eukaryotes (hereinafter known as eukaryogenesis) from rising oxygen levels, and suggest that eukaryotes in fact grew in an anoxic (no-oxygen) environment in the ocean.

"We can now independently date eukaryogenesis and key oxygenation transitions in Earth history," says Dr Daniel Mills of Stanford University.

"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 of this, mitochondria-bearing eukaryotes are always dated to between these two oxygenation events, during an interval of deep-sea anoxia and variable surface-water oxygenation."

The appearance of mitochondria, the energy-producing "powerhouses" of eukaryote cells, is now believed to be the driving force behind eukaryogenesis.

The mitochondria are linked to different molecules than the cells in which they live, and the new study investigates the possibility of the symbiotic relationship, famously championed by Lynn Margulis.

"The discovery of "Asgard" archaea (single-celled organisms) in 2015 has provided a significant clue," Dr Mills said.

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

"This is another evidence that the host who took the bacteria was an archaeon."

Asgard archaea live in anoxic ocean sediments, and they can symbiotically with bacteria, possibly the same situation that erupted in the "metabolic coupling" that spawned the first eukaryote cells.

Professor Tim Lenton, the Director of Exeter''s Global Systems Institute, said the new evidence supports the "hydrogen hypothesis" (that mitochondria were acquired in anoxic conditions) which was first discussed in 1998 by Bill Martin and Miklos Muller.

"The notion that oxygen led to eukaryogenesis has been taken for granted," he said.

"In fact, mitochondrial aerobic respiration is likely to have begun later, though it has only become globally widespread in the last billion years as atmospheric oxygen reaches new highs."

As "connections were waiting to be made" following breakthroughs in both disciplines, Dr Mills said the study is intended to "bridge a gap" between biology and geology.

The paper, published in the journal Nature Ecology & Evolution, is entitled: "Eukaryogenesis and oxygen in Earth history."

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