About two and half billion years ago, life on Earth was still in its infancy. Complex organisms such as plants and animals had not yet appeared, but the planet was teeming with microscopic bacteria that thrived in the temperate and nutrient-rich environment. Greenhouse methane lingered in the atmosphere and trapped the sun’s warmth, creating a very accommodating climate for the stew of microbes that made their home on primitive Earth.
Some time in prehistory, however, a billion years of bacterial evolutionary progress was suddenly stunted by a catastrophic global event. Geologists see no evidence of a great meteor impact or a volcanic eruption corresponding to the event, but they have uncovered the unmistakable geologic scars of rapid worldwide climate change.
Average temperatures, which were previously comparable to our present climate, plummeted to minus 50 degrees Celsius and brought the planet into its first major ice age. This environmental shift triggered a massive die-off that very nearly extinguished all life on Earth. Paleoclimatologists have good reason to believe that event was unwittingly caused by some of the planet’s most humble residence: bacteria.
The period in history is known as the Paleoproterozoic era, a time when the ocean was tinted green with iron, and the atmosphere was dominated by gases other than oxygen. The sea was host to a plethora of microorganisms, including a few members of a newly evolved variety: a blue-green algae known as cyanobacteria. These adapted bacteria were the first to use water and sunlight for photosynthesis, producing oxygen as a by-product of their metabolism.
The cyanobacteria were a struggling minority at first, but scientists believe that these new microbes began to dominate with the help of melt-water from a few glaciers scattered over the young continents.
These glaciers spent centuries scraping across the earth collecting minerals, ultimately depositing their rich nutrient payloads into the oceans. The cyanobacteria flourished in the presence of the increased minerals and the rapidly growing population was soon venting increasingly large amounts of corrosive oxygen into the environment.
At first the damage was limited to the ocean’s ecosystems. The oxygen began to chemically react with the abundant oceanic iron, eventually scrubbing the seas clean of the element through oxidation. The oxidized iron settled to the ocean floor and the oceans’ green tint began to fade.
This series of developments as nothing short of an ecological disaster – oxygen was poisonous to most of primitive Earth’s inhabitants, and many bacteria relied on the iron as an essential nutrient.
With very little competition for resources, cyanobacteria continued to proliferate and pollute. The free oxygen they produced began to seep from the sea into the air, gradually breaking down the methane that kept the Earth’s atmosphere warm and accommodating.
It took at least a 100,000 years, a short duration in geological terms, but the Earth was eventually stripped of her methane and with it her ability to store the heat from the sun. Temperatures fell well below freezing worldwide, and a thick layer of ice began to encase the oxygen-saturated planet.
Not even cyanobacteria were immune to the effects of this major ice age. The traits that had once given them such an evolutionary advantage had now created an environment that was completely inhospitable. As the centuries marched on, the surface became increasingly cold and frozen, with the ice at the equator eventually reaching up to one mile in thickness. Earth was an ice planet.
Thermal vents on the ocean floor provided pockets where some resilient bacteria managed to survive, and certain organisms that lived underground were insulated from much of the destruction, but these reservoirs of life were scarce. Almost every living thing on Earth died as a result of this massive event known as the oxygen catastrophe.
The survivors of the oxygen catastrophe eventually adapted to consume the abundant oxygen and produce carbon dioxide. This greenhouse gas very gradually made its way into the atmosphere, increasing in concentration and nudging temperatures back into the hospitable range over millions of years.
Had temperatures been slightly colder during the first major ice age – if Earth had been in a slightly more distant orbit – the planet would have remained an icy wasteland because the carbon dioxide would have frozen solid before it could promote the greenhouse effect.
Before the big thaw began, temperatures had reached as low as minus 50 degrees Celsius, and carbon dioxide freezes into dry ice at minus 78 degrees. Indeed, it seems that life on Earth was spared by a tiny margin.
Today all life on the planet can trace is lineage back to those few microorganisms that survived the great dying of 2,500,000,000 BC, and now cyanobacteria are among the most common bacteria on Earth.
In the billions of years since the first ice age, the environment has been dramatically altered on numerous occasions. Causes have included greenhouse gases that trap heat; shifting tectonic plates that reroute ocean currents; our sun’s varying radiation levels; and volcanic activity that alters the atmosphere.
But at least once in Earth’s long history, its own occupants seem to have unwittingly brought all life to the brink of extinction. The sun was warmer now than it was then, so such a “snowball Earth” is a bit less likely to occur . . . but the cautionary tale cataloged in ancient rocks warns us that the environment is certainly not impervious to the actions of those living in it.