Timeline
If we examine the accomplishments of man in his most advanced endeavors, in theory and in practice, we find that the cell has done all this long before him, with greater resourcefulness and much greater efficiency. ~ Belgian cytologist Albert Claude
Life emerged 4.1 billion years ago (BYA), in the relative respite from continual bombardment by bolides. Jupiter acted as something of a shield, and so engendered Earth’s prospects for life.
Even before the planetary bombardment let up, early Earth was not as hellishly hot as long presumed. Instead, the planet was surprising cool, with liquid water on its surface, sloshed around by boulders from space.
Earth is on the cold side of the habitable planetary zone in the solar system. Further, solar energy during the Hadean was 25% less than what it is now. Early Earth should have been a big ball of ice. It was not because of greenhouse gases in the atmosphere.
A gas has a warming effect by having an electric dipole moment (EDM) that allows interaction with electromagnetic radiation. H2O is a greenhouse gas because of its permanent EDM. CO2 also has an EDM, but has to distort asymmetrically to create it, as it is not a linear molecule, unlike H2O.
Atmospheric nitrogen and oxygen have no electric dipole moment, nor can these symmetric, diatomic molecules bend or stretch to create one.
The compositional dynamics of Earth’s early atmosphere remain uncertain, but hydrogen certainly played a critical role in making the planet warm enough to support life.
Atmospheric hydrogen (H2) has warming potential by absorbing radiation that excites it to a higher rotational state. Owing to volcanic outgassing, the presence of 10% H2 in Earth’s early atmosphere might have increased surface temperatures 10–15 ºC. The abundance of atmospheric N2 would have abetted this dynamic.
Once methanogens evolved, they reduced atmospheric H2 levels. In doing so they produced methane, a potent greenhouse gas, as well as converting carbon dioxide in water vapor: CO2 + 4 H2 → CH4 + 2 H2O. Outgassing of both waste products kept Earth warmer than it would have been otherwise.
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In the earliest eon of life, microbes made a living chewing rocks: unlocking energy from sulfur, nitrogen, iron, and hydrogen. This created sediment that acted as a geological viscous lubricant, which was instrumental in generating tectonic plate subduction. By this, microbes facilitated the rise of continents.
The first ecosystems were actually quite complex. ~ English biogeologist Simon Darroch
Microbial life was not confined to the oceans. By 3.7 BYA, archaea and bacteria were well-ensconced in soils around the world.
Cyanobacteria arose by deriving energy from fermentation, which does not require oxygen. By 3.6 BYA, cyanobacteria had acquired the quantum trick of photosynthesis, having got the original genic formula from their cousins, archaea.
Exactly how photosynthesis evolved is unknown, but it was an incremental adaptation. Methanospirillum hungatei was an ancient methogenic archaeon that existed before photosynthesis developed. It had the essential genetics for the metabolic pathway used during photosynthesis. M. hungatei simply lacked the trick of activating the pathway with light.
The evolution of chlorophyll literally changed the world. Animal life on Earth was made possible by the by-product of photosynthetic bacteria: oxygen.
During the first half of Earth’s history, the majority of life forms were probably capable of photosynthesis. ~ Columbian life scientist Tanai Cardona
Carbon is the basic building block of biomatter, and an element fundamental to atmospheric temperature regulation. Photosynthesis greatly accelerated Earth’s carbon cycle; even more so when plants colonized land.
Though the energy of the Sun was 20% weaker than now 3 BYA, Earth was not frozen because of greenhouse gases which retained heat in the atmosphere. Atmospheric carbon (CO2, CH4) dissolves in water to form acid rain. With substantial continents having formed, acid weathering of the early crust reduced atmospheric carbon and increased oxygen release from the rocky surface. In the process came continental drift and global cooling. Earth had its first ice age 2.5 BYA.
2.45 BYA came the so-called Great Oxidation Event: an upsurge of atmospheric oxygen. For a few hundred million years, oxidized material on Earth’s surface had been recycled into the mantle. Pressure at depth resulted in oxygen release into the atmosphere. Meanwhile, oxygen generators on the surface of the seas infused the atmosphere with their waste. The ocean depths remained anoxic for quite some time.
The oxidation that began 2.45 BYA was not lasting, nor so significant to be labeled a ‘great event’. Atmospheric oxygen remained low for over 1.5 billion afterwards. Continuing tectonic plate movements brought buried carbon sediment to the surface, which reacted with the atmospheric oxygen to keep O2 at ~10% of present-day levels.
Oxygen was at first poisonous to life. Organisms adapted to appreciate the accessible energy that free oxygen offered.
The capacity to detoxify reactive oxygen species must have evolved prior to oxygenic photosynthesis and aerobic metabolism, or the first aerobic organisms would have poisoned themselves. This poses a puzzle as to what drove evolution of protection from O2 toxicity before there were significant amounts of O2 in the environment. Carotenoids, which protect against H2O2, are found in ancient anaerobic bacteria and do not require O2 for their production yet protect against its toxic effects. ~ English Earth scientist Timothy Lenton
Oxidation altered every form of life. Even prodigal oxygen producers had to adapt to their own success.
Bacteria learned to succor oxygen as a fuel source. Along with archaea, bacteria had the planet to themselves for nearly 2 billion years: plenty of time to diversify and settle into every possible niche that can support life.
During that time, microbes perfected metabolism near the optimality afforded by chemistry, with the slight trade-off of being able to adjust to alternative nutritional conditions. This efficacy goes a long way in explaining the diversity and staying power of microbes.
This time in Earth’s history was a bit of a catch-22 situation. It wasn’t possible to evolve complex life forms because there was not enough oxygen in the atmosphere, and there wasn’t enough oxygen because complex plants hadn’t evolved. It was only when land plants came about did we see a more significant rise in atmospheric oxygen. ~ Timothy Lenton
Oxygen is one of the key variables in Earth’s biotic system. Once photosynthesis arose, oxygen’s transformative power upon Earth’s surface was destined. Progressive oxidation paved the way for eukaryotes to evolve: first single-celled, then multicellular. Oxygen was the fuel for larger and more metabolically active life.
Oxygen levels were very dynamic, going up and down until they passed a threshold and pushed the planet to a different state. ~ American biogeochemist Noah Planavsky
