The Elements of Evolution (9) The Cryogenian Cooler

The Cryogenian Cooler

The Great Oxidation Event triggered a loss of methane and its greenhouse effect, cooling the planet. Earth had its 1st major glaciation 2.2 BYA. Ice sheets extended to the tropics, nearly covering the entire planet.

The organisms that had arose in the relative warmth largely disappeared. More than a billion years later, glaciation would again set back the proliferation of life.

The term Cryogenian refers to the frigid climate of the time. During the Tonian (1000–720 MYA) and Cryogenian (720–625 MYA) periods, Earth shivered through 4 of the most severe ice ages in its history: the Kaigas (770–735 MYA), Sturtian (715–680 MYA), Marinoan (660–635 MYA), and Gaskiers (585–582 MYA).

Glaciers stretched and shrank in rhythmic pulses. The Sturtian and Marinoan were the most severe.

The period 800–600 MYA is termed Snowball Earth. Much of the world was frozen over. Ice extended to the equator.

Normally atmospheric carbon dioxide maintains an equilibrium with the oceans. The ice layer cut off that exchange. This allowed atmospheric CO2 to soar to 13% over a period of 4–30 million years; 325 times that of today (0.04%). The elevated level of this greenhouse gas, along with volcanic and microbially-released methane, created a global warming feedback loop.

Volcanoes can be a climatic doubled-edged sword. Volcanoes spouting soot into the atmosphere can rapidly cool, as happened to bring on the Sturtian. Conversely, volcanic release of greenhouse gases has a longer-term warming effect.

In periods of Earth’s history when it was very warm, volcanic cooling would not have been very important. In cooler conditions, Earth becomes uniquely vulnerable to volcanic perturbations of climate. Context matters. ~ American geologist Robin Wordsworth

The ocean’s acritarch populations were decimated during the Cryogenian period. The acritarchs of note were planktonic algae, dating to 3.2 BYA. They were first to evolve into simple eukaryotes.

The algal die off did not defeat a continued rise in oxygen level during the Cryogenian; setting the stage for the explosive growth that would follow when the ice retreated and the seas warmed again.

The cold ravaged a lot of potential for life. There were spots where the chill was not so severe. The end of narrow inlets, like the Red Sea, may have been relatively ice-free.

From 1.5 BYA to well over a half-billion years later, Earth was home to microbes and algae alone. Evolution seemed stalled in the slow lane, but there were innovations. By 809 MYA, organisms had armored themselves via biomineralization. These exoskeletons protected microbes from predation.

Ancient microbes luxuriously employed calcium phosphate to fashion their shells. Today, phosphate is limited, and microbes avoid wasting it. Modern microbes make their shells out of calcium carbonate.

Some cyanobacteria evolved the ability to fix atmospheric nitrogen ~800 MYA. This revolutionary event fertilized the oceans.

These oxygenating cyanobacteria were able to colonise the vast oceans and be fertilised by enough bioavailable nitrogen to then produce oxygen – and carbohydrate food – at levels high enough to facilitate the next ‘great leap forward’ towards complex life. This played a pivotal role in the evolution of life on Earth. ~ Columbian biologist Patricia Sánchez-Baracaldo

The last common ancestor of all animals, a sponge, arose nearly 800 MYA, during the Tonian. Metazoans developed further during the Cryogenian.

Phosphorus was the critical element which allowed the evolution of metazoa. Until 800 MYA, phosphorus availability was limited in the shallow marine environments which cradled nascent animal evolution. The phosphorus surge was likely instigated by volcanic and tectonic activity which released benthic phosphorus deposits from long-dead microbes.

Oxygen was scarce for the earliest animals, which could survive at 0.5–4.0% of present atmospheric levels. The oxygenation of the oceans ~635 MYA, though appreciated by those breathing, was not necessary to foster animal life.

By 580 MYA, atmospheric oxygen approached 10% of what it is now. Gills and circulatory systems evolved during this time. These evolutionary advances reflected ocean oxygenation and conditions in the deep.

While ocean surface waters had sufficient oxygen for life to evolve, the bottom waters of shelf seas were oxygenated by early animals. Sponges feed by filtering planktonic nutrients from the water. Besides putting oxygen into the water at depth, early filter feeders also removed the essential nutrient phosphorus. This in turn reduced ocean ecosystem productivity, which reduced oxygen demand. Adaptation to these conditions resulted in the 1st mobile marine predators.

Marine worms grew to nearly 1 meter, but were only 2.5 mm thick, thus providing maximum surface area to absorb oxygen and nutrients directly from ambient seawater. Numerous species turned carnivorous.

Conditions in the Cryogenian demanded that early life establish a developmental toolkit of robustness at the cellular level. The success of metazoans in the Ediacaran and Cambrian periods that followed – especially the facility for rapid speciation – owed to the new forms of developmental regulation created during the Cryogenian.

The conditions that led to the chilly Cryogenian, and its conclusion, came via continental configuration. At the onset of the Cryogenian, the Earth’s landmass consisted of Rodinia: a single supercontinent. Rodinia was barren, as it existed before the ozone layer formed, and so was too irradiated by ultraviolet sunlight for life to withstand.

The continent of Rodinia was mostly tropical. This fostered the big freeze of the Cryogenian, as land mass is more reflective than open ocean, and so absorbs less heat. Most of Earth’s solar absorption today is in the tropical oceans.

Earth came out of the freezer from the breakdown of the carbon cycle. This evolved from Rodinia breaking up, which took over 100 million years. A cooling mantle finally started the modern-style subduction that greases continental drift through tectonics.

There were dramatic sea level changes during this period. Sea levels initially rose in response to both mantle upwelling from tectonic activity and the mid-Cryogenian glaciations.

Widespread flooding created vast areas of shallow continental shelf under water. Sea levels receded as the world warmed and the continents drifted apart.

One impact of continental events was the release of phosphorus, which, coupled with abundant carbon dioxide, triggered a cyanobacteria population explosion. This in turn triggered rapid reoxygenation of the atmosphere, contributing to the rise of Ediacaran biota.

Another related dynamic was the slow release of calcium, allowing uptake by oceanic organisms to build exoskeletons, which set the stage for the Cambrian.