Timeline by Kimberly Sandoval Ramirez

4.6 billion years ago (bya)

sandovk27 — 2025-07-16 15:10:33 UTC

Around 4.6 billion years ago, Earth had just formed and did not have much of an atmosphere. The surface was very hot and covered in melted rock. Over time, volcanoes began to erupt and release gases such as carbon dioxide and methane into the air, which slowly started to form the planet's first atmosphere.

4.0 bya-2.5 bya

sandovk27 — 2025-07-16 15:11:26 UTC

Between 4 and 2.5 billon years ago, the sky was filled with thick haze, and there was no oxygen in the air yet. The first simple living things formed in the ocean and used sulfur and other elements to get energy. After some time, tiny blue-green bacteria started using sunlight to make its food, which allowed them to released oxygen into the air. This slowly began to add small amounts of oxygen and change the atmosphere.

2.6 bya- 400 million years ago (mya)

sandovk27 — 2025-07-16 15:12:13 UTC

During this time, oxygen levels in the atmosphere kept rising because of photosynthesis from tiny living organisms. As oxygen increased, it reacted with methane and other gases, changing the atmosphere a lot. Around 2 billion years ago, the sky became clearer and started to turn blue. The oxygen also combined with iron in the ground, creating large areas of red-colored rocks.

400 mya- 290 mya

sandovk27 — 2025-07-16 15:12:55 UTC

Between 400 and 290 million years ago, plants were growing all over the land and started changing the air by adding more oxygen. At one point, oxygen levels were higher than they are today. Huge forests and swamps covered low areas, and dead plants started building up without breaking down because there was not enough oxygen touching them. Over million of years, those buried plants turned into coal, which we still use today for energy. As this happened, carbon dioxide in the air dropped, which made Earth's temperature cooler, with some areas even freezing while others stayed warm.

290 mya- present

sandovk27 — 2025-07-16 15:25:41 UTC

Over the past 290 million years, Earth's climate has changed a lot, with some periods being much warmer than today. There were times when the polar regions had little to no ice, and trees and animals lived near the North Pole. In the Eocene period, the heat spread more evenly around the world, so even faraway places stayed warm most of the time.