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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 and building phylogenetic trees.

Fossils are preserved remains or imprints of organisms, such as bones or footprints. Together, they form a record of life’s history and provide strong evidence for evolution.

A phylogenetic tree is a diagram that shows evolutionary relationships among organisms. Its branching pattern shows how different groups share common ancestors.

For example, one might assume that whales and fish are closely related because both are aquatic animals with fins and tails used for swimming. However, anatomical comparisons show that modern whales share structural similarities with humans and other mammals, such as similar forelimbs.

Also, phylogenetic analyses suggest that whales share a more recent common ancestor with humans than with fish.

Phylogenetic analyses and fossil evidence show that whales evolved from terrestrial mammals—four-limbed tetrapods that moved from land back into the water. The similar body plans of whales and fish evolved independently through a process called convergent evolution.