For 4 billion years, Earth’s continental plates have restlessly migrated, forming giant continents that eventually split apart — three of which have been crucial to the origin of life as we know it.
Kenorland, one of Earth’s earliest super-continents, formed 2.7 billion years ago and was responsible for one of the planet’s greatest climate disasters — one that essentially allowed life as we know it to exist: the Great Oxidation Event (GOE).
Up until Kenorland’s formation, the young planet’s oceans and atmosphere were rich in methane, creating a favorable environment for anaerobic bacteria. Oxygen-loving bacteria were relegated to life on the ocean floor, where, deep in the abyss, they accumulated in microscopic rock-layer cracks, generating a biosphere. When the new landmass emerged from the collision of the continental plates, parts of the seafloor were thrust toward the surface, forming shallow lakes and inland oceans.
With a boost from the faint light of a weak sun, the exposed aerobic bacteria began to pump oxygen into the atmosphere, eventually displacing enough of its then-dominant gases — carbon dioxide, methane and nitrogen — that the anaerobic bacteria were unable to survive on the surface. When Kenorland disintegrated after some 300 million years, more shallow oceans and new coastlines formed, allowing even more oxygen-promoting bacteria to emerge.
The GOE left a clear trail in the geologic record: pyrite. Because it can only form in the absence of oxygen, geologists are able to use the dates of ancient pyrite layers to determine when Earth’s atmosphere began to oxygenate. It took a very long time, hundreds of millions of years, but once oxygen became established in the atmosphere, the stage was set for the emergence of the more complex life-forms that exist today.
Extent: North America, Greenland, the Baltics, Scandinavia, Western Australia, parts of Africa
Solar radiation: 20 percent of present levels
Atmospheric oxygen content: 0.1 percent
Temperature: Similar to today
A scientist’s description of the super-continent Rodinia would probably go something like this: a vast, desertlike landmass where barren plains stretching endlessly toward the horizon are drenched in a never-ending tropical rain that slowly eats away at the rocky landscape.
It might sound like an alien world, but Rodinia was the catalyst for a number of events that caused life as we know it to develop. It was the first supercontinent in which all of Earth’s present landmasses were united, and, more importantly, it was located along the equator.
Continents reflect more sunlight than oceans do, so Rodinia caused Earth to absorb less heat from the sun than it does in the present era, in which oceans cover large swaths of the equator. And as the relentless tropical rain eroded the rocks, minerals were exposed that reacted with the carbon dioxide in the air, helping to reduce the level of this greenhouse gas in the atmosphere.
This cooling process eventually resulted in the globe being covered by ice, creating a “Snowball Earth” that persisted for millions of years. When volcanic activity began spewing new greenhouse gases into the atmosphere, the planet warmed again; as the ice melted, glaciers moved huge amounts of mineral-containing sediments into the oceans, and life began to proliferate.
Formed: 7.1 billion to 750 million years ago
Extent: Almost all present landmasses
Solar radiation: Weaker than present levels, but deadly due to the lack of an ozone layer
Oxygen content: 4 percent of the atmosphere
Temperature: Similar to today, with ice ages
Life: Emergent marine life
Some 345 million years ago, three large continents began to assemble into one: Pangaea. This supercontinent stretched from pole to pole and was dotted with many shallow lakes and crisscrossed by rivers. Mosses and ferns grew in abundance as life thrived in the muddy swamps.
Biologists think that it was in those pools where fish began to evolve lungs in response to the water level dipping too low. Using their strong fins, they threw themselves onto the shore, developing into amphibians that could live both in water and on land.
The long “snowball” period and subsequent thaw had filled the atmosphere with such a high oxygen content that insects grew huge. But although Pangaea provided a breeding ground for life, the continent was warm and dry — not until it began to break apart did evolution gain momentum. Emerging coastlines formed seas and oceans, creating new, life-friendly environments that hosted the development of millions of new species, spurring incredible biodiversity across the globe. The youngest of the super-continents, Pangaea is still disintegrating.
Formed: 300 to 200 million years ago
Extent: The present seven continents
Solar radiation: Similar to present levels
Oxygen content: 30 percent of the atmosphere
Temperature: Similar to today, with ice ages
Life: Rich marine life, land snails and the first amphibians, followed by mammals and dinosaurs