Atmospheric Oxygen
Oxygen has been the volatile actor in life’s evolution. Before the origin of life, in the Hadean eon, atmospheric oxygen was negligible. Similarly, oxygen also exists in space, scattered about in minuscule quantities.
Anaerobic organisms arose that produced oxygen as exhaust, gaining usable carbon in a CO2 conversion catalyzed by sunshine: photosynthesis. Such phototropic life appeared within a billion years of Earth’s formation. Planetary oxidation had begun by 3.4 bya.
But not without counterforce. Methanogens thrived in the nickel-rich seas billions of years ago, belching methane into the air. The methane reacted with atmospheric oxygen, creating carbon dioxide and water.
Eventually, the methane party wound down. Oceanic nickel levels began to drop 2.7 bya. Nickel levels halved by 2.5 bya. The heyday of the methanogens had passed.
The mantle of the early Earth was so hot that dynamic tectonic plate flow with subduction did not begin until 3.0–2.7 bya. Oxidized material from Earth’s surface began being recycled into the mantle. Pressure at depth resulted in oxygen release into the atmosphere.
Relatively rapid oxidation, albeit fractional, resulted in Earth’s first extinction event. Much microbial life had evolved to survive in an oxygen-poor environment. The ascent of O2 spurred adaptation to greater oxygen tolerance. Over 2.5 bya, cyanobacteria evolved, and begat the slow oxygenation of the planet, by inhaling carbon dioxide and exhaling oxygen.
It took 500 million years for the atmosphere to begin oxidizing. The oxygen produced by photosynthesis readily reacts with ferrous iron and other elements to form precipitates, such as insoluble ferric oxide (e.g., rust).
1,500 different minerals were found on Earth prior to life arising, generated by dynamic mantle and crust processes during the first 2 billion years. Oxidation of Earth created 2,500 new minerals, many of those being oxidized and weathered products of predecessor minerals.
As the atmospheric oxygen level rose, every mineral that could be oxidized was. Once the weathering of iron-rich rocks abated, photosynthetic cyanobacteria belched so much oxygen so quickly as to overwhelm the planet’s ability to soak it all in.
Free oxygen poured into the air and oceans, radically altering geochemical dynamics, as well as the biochemical evolution of life. Rising atmospheric oxygen facilitated the evolution of eukaryotes: a significant step from purely prokaryotic life.
Life added to Earth’s mineral stock. Over 4,400 different mineral species have been cataloged. 400 have been added since eukaryotes arose.
Aerobic respiration may have presaged the oxygen surplus from photosynthesizers. 2.9 bya, the most ancient aerobic process produced pyridoxal (C8H9NO3), the active form of vitamin B6, and an oxygen-based enzyme, manganese catalase. The enzyme detoxifies hydrogen peroxide by breaking it down into oxygen and water. These early aerobic organisms may have got the oxygen needed for pyridoxal production by busting up hydrogen peroxide (H2O2), which might have come from glacial ice being bombarded by ultraviolet radiation, which generates generous amounts of H2O2.
UV levels were very high at the time, as the atmosphere had yet to form its later ultraviolet shield. The UV shield that eventually formed was an ozone layer in the upper atmosphere, a byproduct of atmospheric oxygen proliferation.
Despite unfiltered radiation from the Sun, early life prodigiously evolved. A few factors were in its favor.
1st, the Sun burned less brightly. 2nd, early life was in the oceans, which provided some protection from UV. 3rd, primordial bacteria developed protective mechanisms to limit DNA damage from UV radiation.
Without atmospheric O2, there is no O3 (ozone). Though the ozone layer accounts for only 0.00001% of the volume of atmospheric gas, its accumulation in the upper stratosphere, and its ability to absorb 99% of incoming ultraviolet rays, provides a blanket of protection for life on the surface.
Atmospheric oxygen stops water loss from the planet. Hydrogen released from water bumps into oxygen in the air before it wafts into space and is recaptured in rain droplets.
What started as a primitive organism waste product became organic fuel. The energy that can be derived from fermentation, or the early-evolved methane-sulfate reactions, are puny compared to the potency of aerobic respiration.
Nothing else could have powered multicellular life. All plants and animals depend upon oxygen for at least part of their life cycle. Only the early risers, microscopic life, managed to eke out an existence before the oxygen bloom.
The proof is prehistoric. 300 mya, atmospheric oxygen levels were 66% higher than today. The higher oxygen levels greatly affected the species that adapted to intake oxygen as an energy source. Paleozoic amoebas were 100 to 1,000 times larger than they are now.
During the atmospheric oxygen bloom, insects supersized. Dragonflies had wingspans of 70 centimeters. There were millipedes over a meter long.
Oxygen transport in vertebrates is through the bloodstream. But insects move air through their bodies by trachea: an internal network of channels. So, for insects, a higher oxygen level readily supports rapid evolution of a larger body.
While life adjusted to an oxygenated atmosphere, its toxicity at the cellular level remains a challenge. Both plant and animal tissues create anoxic conditions to generate stem cells.
Oxygen is a diffusible signal involved in the control of stem cell activity. ~ Italian botanist Francesco Licausi et al
