While I was studying the period before the Ediacaran early this year, I learned of the Boring Billion, a vast 900-million-year stretch of time during which it seems (at least at first glance) that absolutely nothing happened, either in evolution or in geology.
And then -- once again -- the muses started that loud singing, and I had to write.
EDIT: Shame on me, I forgot to credit Tyellas for beta'ing this!
The Boring Billion
Content: paleontology, a few unavoidable big words
Trigger warnings, yadda yadda: references to mass extinctions (the victims are all one-celled life forms)
Word count: 789
@Copyright Maureen Lycaon, 2017. All rights reserved under the Bern Convention.
One billion years.
For one billion years, planet Earth has been locked in stasis. The great supercontinent of Rodinia sprawls across the tropics and does not move, its vast surface barren of life. The atmosphere is suffocating, nearly empty of oxygen; the methane-rich sky is a bizarre pink.
Still, weather happens. Clouds form; storms spawn into existence; rain falls onto bare rock and gravel. The runoff flows into fast-running rivers. It carries dissolved chemicals from the exposed rocks -- iron, manganese, phosphorus, and (importantly) sulfur. The rivers course into great deltas and finally into the oceans.
In places, the sulfur feeds marine blooms of sulfur-eating bacteria, which release vast quantities of toxic hydrogen sulfide. When they die off, their decay consumes what little oxygen there is. These bacteria outnumber all other forms of life.
The rest of the ocean has few nutrients. Even the sulfur-eaters remain rare. Still, life persists, scattered thinly across the planet: other bacteria, hardy archaea, and early eukaryotes (still one-celled). Deprived of resources, they, too, are caught in stasis. The oxygen-starved atmosphere will not support anything larger and more complex. Evolution stands still.
Once, life seemed destined for greater things.
Some nameless species of bacteria evolved a new kind of metabolism that created energy from sunlight. The process had a curious byproduct: oxygen.
At first, photosynthesis was slow, halting, and inefficient. Nevertheless, their descendents -- the cyanobacteria -- prospered. Some of them began to live in colonies, forming long filaments. Others formed great mats that over time formed mounds and pillars -- stromatolites.
As the cyanobacteria evolved, so too did their photosynthesis, becoming more efficient. The cyanobacteria began to out-compete all other bacteria. As a byproduct, they released more and more oxygen.
For a while, this innovation changed the world. Cyanobacteria waste filled the ocean and atmosphere with oxygen. For most other life forms, this meant extinction: oxygen destroyed them. A few species, however, could endure the higher oxygen levels, and now they had an opportunity to thrive as they never had before.
Then a eukaryote evolved that actually *breathed* oxygen. Fueled by this energy source, its descendents evolved the first multi-cellular life forms. Curious coin-like, star-like, and other-oddly shaped creatures appeared in the oceans.
Mysteriously, the oxygen vanished. Whatever the cause may have been, within 200 million years most of it was gone, and Earth's atmosphere had returned to its former state of methane and carbon dioxide.
The revolution died, stillborn.
The multi-cellular life forms died out, leaving no descendants -- just a few traces in certain rocks. The sulfur-eating bacteria assumed dominance. They have held it ever since.
Finally, something does change.
Below the Earth's surface, the heat of its formation lingers. Meanwhile, the Earth's asthenosphere -- the planet's thin upper layer -- has been inexorably cooling. More than a billion years ago, it congealed from churning, molten rock to a more solid substance, forming the earliest continents.
Now, as the asthenosphere continues to cool, it reaches another tipping point. It has become solid enough for plate tectonics.
Slowly, inexorably, Rodinia begins to come apart as its pieces drift away from each other.
This has happened before. Cracks would appear in the supercontinent along lines of stress, and fill with magma from the churning, hot mantle beneath. Each time, the geological processes dwindled and died, spent. The rifts fused again, disappearing.
This time, the process does not stop. The narrow rifts yawn ever wider and deeper; the ocean flows in to fill them.
Over millions of years, the widening rifts become small ocean basins in their own right. Erosion washes sediments from newly-exposed rock into the new basins.
In those shallow basins, something else begins to happen. Perhaps the extra nutrients now washing into the ocean have something to do with it. Perhaps the washed-in sediment is burying the tiny corpses of dead cyanobacteria faster than they can rot, thus saving oxygen. Perhaps the cause is something else entirely.
Whatever the cause, oxygen again begins to build up, diffusing through the water and the air, with one difference: whatever used to remove it can no longer do so.
The atmosphere, formerly pink with methane, begins to turn blue. There still isn't much oxygen -- less than two percent of the atmosphere -- but the percentage keeps rising.
Planet Earth has irrevocably changed.
The sulfur-eating bacteria decline. The eukaryotes, on the other hand, begin to thrive again, and to evolve.
A mere 40 million years later, a new multi-cellular creature, Otavia, lives in great colonies offshore. Though it does not yet have spicules to form a skeleton, it already resembles a sponge.
Ahead of life lies a difficult road, but the billion years of stagnation has ended.