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Around 4 billion years ago, Earth’s early atmosphere contained water vapor and gases released by volcanic eruptions. As the planet cooled, water vapor condensed into rain, making a significant contribution to the formation of early oceans.

Aquatic, single-celled organisms were the first living beings to inhabit Earth nearly 3.5 billion years ago. These early cells were prokaryotes - simple cells without a nucleus. Later, some prokaryotes, such as cyanobacteria, began using carbon dioxide to produce sugars through photosynthesis.

Oxygen, produced as a byproduct of photosynthesis, slowly accumulated in the atmosphere. Around 2.4 billion years ago, this increase led to the Great Oxidation Event, which raised atmospheric oxygen levels and changed Earth’s environment.

Evidence suggests that by around 2 billion years ago, some single-celled photosynthetic organisms may have begun colonizing moist terrestrial environments. Later, between 400 and 500 million years ago, multicellular organisms, particularly plants and fungi, established themselves on land, followed by animals.

Scientists reconstructed the evolutionary history of life on Earth—how life has changed over time—by studying fossils a