Earth's History

What Was Earth’s Ancient Atmosphere Like?

Birth of a Planet
Image Credit: NASA

In Earth’s early history, the atmosphere was much different, and Earth did not have liquid water. Scientists theorize that a lot of debris or meteorites from space bombarded Earth, which caused the outer layer of Earth to melt. After the bombardment stopped, Earth began to cool. As the molten surface became solid, gases were released into the atmosphere. These gases consisted mostly carbon dioxide (CO2), with some nitrogen (N2) and water vapor (H2O), and other trace gases (methane, ammonia, sulfur dioxide, hydrochloric acid, and argon).

Image Credit: Microsoft Clip Art

As Earth continued to cool, the water vapor condensed to form clouds, and great rains began. The oceans formed, and the amount of water vapor and carbon dioxide in the atmosphere, in turn, decreased, leaving Earth with a nitrogen-rich atmosphere. Eventually, approximately 2.7 billion years ago, ancient organisms evolved to use carbon dioxide, water, and sunlight to make their food energy and release oxygen (known as photosynthesis). The oxygen content of the atmosphere slowly increased, and other forms of life were able to evolve.

A Brief Look at Earth’s History

The table below shows a brief look at Earth’s history over the past 4.6 billion years. The biological, geological, and climatic events appear in reverse chronological order with the oldest event at the bottom of the table and the most recent event at the top of the table. Within each geologic era, the oldest event is also listed at the bottom. Notice that the amount of time in each era, period, and epoch varies and that for most of Earth’s history, life was limited to simple marine organisms.