Science & Technology

The first signs of life on a forming world.

It’s time once again to travel back in time to the early age of Earth’s formative years – This time, to just after the violent Hadean Eon. When Earth finally emerged from the fire and turbulence of that first formative eon, something remarkable began to happen. The Archean Eon, which scientists believe ran from about four billion to 2.5 billion years ago, was a time of change, mystery, and – most importantly – the dawn of life. The Hadean Eon saw the formation of our planet, but it was during the Archean Eon that Earth came to life.

The Archean Eon: What Came After Chaos

The Archean world looked very different than it did during most of the Hadean – but it would still seem alien to us. There were no continents, not as we think of them, anyway. Rather, there were small land masses, rocky islands poking up from the vast, planet-wide ocean. Volcanoes raged, pumping gases into the still-young atmosphere. The air was thick with carbon dioxide, methane, and ammonia. The sun was dimmer, and Earth’s heat, left over from the violent birth, kept the planet hot. It was inhospitable to most life as we know it today.

Still, amid all that chaos, life emerged. Tiny, single-celled organisms – which, today, we call prokaryotes – found a way to survive in this harsh environment. These early forms of life were probably very similar to the bacteria and archaea of today, thriving under the seas near volcanic vents.

One thing that made the Archean Eon different from those that followed was the lack of free oxygen in the atmosphere. But all that was about to change, thanks to those tiny microbes.

Simple Life, Big Impact

Eventually, these ancient single-celled organisms picked up a handy trick for survival in a carbon dioxide-rich world: photosynthesis. Using carbon dioxide, water, and sunlight, these early cyanobacteria – often called “blue-green algae” – created their own food and generated free oxygen as a waste product, which they pumped back into the water. Communities of these microbes formed layered structures called stromatolites, some of which can still be found in Australia today – Earth’s oldest fossils.

That oxygen was initially absorbed by the oceans and rocks, but eventually it began to collect in the air and build up in the atmosphere, setting the state for Earth’s next big leap: The Great Oxidation Event. This caused a mass extinction in anaerobic life (that is, life that doesn’t rely on oxygen), cleared most of the methane from the atmosphere, and triggered an early ice age called the Huronian Glaciation. It was devastating to the little single-celled organisms that caused it – but it made the way for all future life on Earth.

Today, we look back at the Archean Eon and remember it for two things: the birth of life and the beginning of continents. In this early chapter of Earth’s existence, the life was simple and the “proto-continents” were small. Then came the Proterozoic Eon, and everything changed – but that’s a story for another time.

1. The second eon in Earth’s geologic history is the Archean Eon.
2. During this eon, most of the world was covered by a global ocean, and there was very little oxygen in the atmosphere.
3. Life began during this eon. The single-celled, photosynthetic organisms helped increase the oxygen in the atmosphere and made way for more complex forms of life.