Evolution and oxygen interplay (Introduction)

by David Turell @, Tuesday, June 06, 2017, 15:23 (76 days ago)

First life was anoxic. Cyanobacteria had to evolve with a form of photosynthesis to create atmospheric oxygen:


"Scientists know that atmospheric oxygen irreversibly accumulated on Earth around 2.3 billion years ago, at a time known as the Great Oxidation Event, or GOE. Prior to that time all life was microbial, and most, if not all, environments were anoxic (that is, contained no oxygen). Oxygen was first produced some time before the GOE through the evolution of a group of photosynthetic bacteria known as cyanobacteria. Releasing oxygen as a by-product of splitting water in order to acquire electrons to be energized by light, this process led to dramatic changes in both the biological and geochemical processes on a planetary scale. Eventually, the continued accumulation of oxygen led to an oxidized surface, atmosphere, and ocean that persist to this day.

"Besides shedding light on a fundamental change in Earth's climate, it is hoped that understanding the GOE will help scientists gain insight into the rise of eukaryotes—cellular organisms like us humans, in which genetic material is DNA in the form of chromosomes contained within a distinct nucleus. Eukaryotes require oxygen to produce sterols, an important part of their cell membranes. Furthermore, eukaryotes also contain mitochondria, organelles descended from ancient bacteria that use oxygen to generate energy using aerobic respiration.

"There are currently two schools of thought regarding how oxygen levels rose: The first proposes a small initial rise at the time of the GOE, with levels low but stable until increasing again around 600 million years ago, approaching modern levels. The second posits a more oscillatory rise with a greater increase immediately following the GOE, and then a subsequent crash, with levels only increasing again 600 million years ago.


"Molecular oxygen (O2) readily changes to an extremely reactive "free radical" form with an unpaired electron called superoxide, a chemical highly damaging to many biological systems. Many organisms are protected against superoxides by superoxide dismutase enzymes that convert superoxide to hydrogen peroxide, the first step in detoxifying this compound. It is present in most extant bacteria (i.e. ones that are alive today) but is assumed to have originally appeared in response to the increasingly oxygen rich environment of the GOE."

Comment: Oxygen required built-in defenses because it is so useful in producing energy and yet so dangerous. The rest of the article describes techniques to understand the genomic changes. The other important point is that in origin of life, energy supply is not with oxygen and that source is probably in evidence in the extremophile forms on Earth now, using alternative. Their ancestors were the original archaea.

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