There’s no place like home. Of all the wondrous worlds we’ve studied in our Solar System and beyond, none are quite like Earth. Our planet is covered in liquid water and harbors an oxygen-rich atmosphere that has allowed a multitude of plants and animals to evolve — including us.
It wasn’t always that way. If you could travel back in time to early Earth, your lungs would choke on an atmosphere devoid of oxygen. The gas that humans need to survive didn’t appear in significant quantities until roughly halfway through our planet’s current lifespan.
So how did Earth’s atmosphere get its oxygen? The simple answer is that early microorganisms produced it using a process you may have learned about in elementary school: photosynthesis. Photosynthesis is the process by which plants and other organisms use sunlight, water, and carbon dioxide to produce energy. As a byproduct, oxygen is released, which has over time given Earth an oxygen-rich atmosphere suitable for complex lifeforms.
Exactly how the Cosmos set the stage for this to happen is not known with complete certainty, but we know the basics. Let’s start at the beginning.
The care and feeding of early Earth
Our Sun formed about 4.57 billion years ago from a cloud of collapsing dust and gas, in an unremarkable corner of the Milky Way galaxy. The material left over the Sun’s formation coalesced into everything else in our Solar System, including Earth, which was born around 4.55 billion years ago.
Earth was still practically a baby when a Mars-sized world named Theia is believed to have plowed into the planet about 4.51 billion years ago. Most of Earth’s mantle was melted by the impact, and molten remnants from the collision formed the Moon. Vaporized rock and silicate clouds circled Earth, making our world shine like a small, fiery Jupiter.
Our planet was pelted by more space rocks throughout its early history. These impacts may have spiked between 4.0 and 3.8 billion years ago, when the orbits of the giant planets shifted, flinging objects throughout the inner Solar System. This event is known as the Late Heavy Bombardment.
We don’t know exactly when life arose on Earth. It’s possible life came into existence and was wiped out multiple times by giant impacts before taking hold for good. Our earliest direct evidence of life dates back to about 3.7 billion years ago. Life as we know it needs water, so Earth had some when life arose. But where did that water come from?
One theory is that Earth was born with the elemental precursors of water locked in its rocks. A second possibility is that water-rich asteroids bombarded Earth, bringing water here. It’s also possible that some combination of both theories happened.
The building blocks of Earth were probably not icy, since the Sun is hot enough in the inner Solar System to sublimate ice — turning it directly from a solid to a gas. However, it is possible that the chemical precursors to water could have been present inside the rocks that formed Earth.
The European Space Agency’s Rosetta mission showed that comets like 67P/Churyumov-Gerasimenko are not a good match for Earth’s water. Nevertheless, water-rich asteroids are still a possibility. Evidence in support of asteroids recently came from Japan’s Hayabusa2 mission, which returned samples of asteroid Ryugu to Earth. Another recent study theorizes that Theia, the giant world that impacted Earth and formed our Moon, could have been the source of Earth’s water.
From cyanobacteria to homo sapiens
Early organisms lived within an atmosphere bereft of oxygen and full of carbon dioxide, similar to present-day Venus and Mars. How these microbes survived in such harsh conditions is not entirely clear, but scientists have found present-day microbes that live without oxygen in the salty depths of the Mediterranean Sea, and in an arsenic-laden river in Chile’s Atacama Desert.
Then, something extraordinary happened: A microscopic organism named cyanobacteria emerged and began using sunlight, carbon dioxide, and water to produce food in a process called photosynthesis. The byproduct of photosynthesis is oxygen. The estimated time at which this happened varies from 3.0 billion to 2.8 billion to 2.7 billion to 2.5 billion years ago.
At first, the oxygen produced by cyanobacteria was sequestered in minerals and seawater. But between 2.4 and 2.5 billion years ago, cyanobacteria were producing enough oxygen to be stored in Earth’s atmosphere. This time period, when oxygen levels in the atmosphere began to appreciably rise, is known as the Great Oxidation Event.
Meanwhile, microbes began innovating. Some began living inside other microbes, and then grouping together, forming the precursors of more complex life. Simple, oxygen-based life began to emerge, such as sponges, which arrived on the scene as early as 760 million years ago.
Around 350 million years ago, Earth’s oxygen levels hit 20%, which is roughly the percentage they are at today. Oxygen concentrations continued to rise to 35%, before a cooling climate and the large-scale death of many plants sent concentrations plummeting to 12%. Levels continued to fluctuate before stabilizing at today’s 21%.
Oxygen set the stage for some legendary animals, including us. Non-bird dinosaurs evolved and roamed the Earth from 245 million to 66 million years ago — an extraordinary run that ended when a 10-kilometer-wide (6.2-mile-wide) object struck the planet. Our homo sapiens ancestors didn’t appear until 300,000 years ago, making the age of humans a relative blip on the cosmic timescale.
A guidebook to other worlds
Knowing Earth’s backstory is helpful when studying other worlds. While the detection of an oxygen-rich exoplanet atmosphere could be cause for celebration, an atmosphere without oxygen does not necessarily mean a planet lacks life. Such a hypothetical exoplanet might simply be gearing up for its own Great Oxygenation Event, or could host life that doesn’t produce or depend on oxygen at all.
Saturn’s moon Titan has an atmosphere that may be similar to early Earth. Methane produced by early Earth organisms could have formed an orange haze around our planet similar to Titan’s, shielding early life from the Sun’s ultraviolet rays. NASA’s Dragonfly mission is scheduled to launch in 2027 on a mission to reveal the secrets of this mysterious moon.
Today, photosynthetic organisms in Earth’s oceans produce roughly half of the planet’s oxygen. One particular species, Prochlorococcus, churns out 20% of the oxygen in our biosphere. Prochlorococcus is a type of cyanobacteria, the same type of organism that originally pumped oxygen into our atmosphere. The processes that shaped our world billions of years ago are not all that different from those today.