Animals have been part of Earth’s story for a very long time—but life began earlier still. The oldest evidence of life points to microscopic organisms billions of years ago. Animals are multicellular organisms in the wider tree of life, and scientists combine fossils, genes and anatomy to investigate when animal lineages emerged.

Early animals and ancient seas

DNA-based estimates suggest that the earliest animals may have evolved around 800 million years ago, though such estimates are models and the exact timing remains uncertain. Sponges are among the simplest living animal groups, and evidence from fossils and molecules helps researchers study their early history.

By about 580 million years ago, the Ediacaran seas held a variety of soft-bodied organisms. Some were frond-shaped or quilt-like and unlike most animals alive today. The Cambrian Period began about 539 million years ago, according to the International Commission on Stratigraphy, and brought a striking diversification of animal forms in the fossil record. Many major animal groups were established by the end of that period.

Evolution is a branching tree

Animals did not evolve in a straight line from “simple” to “advanced.” Populations change over generations, branches split, and many lineages go extinct. The result is a branching tree with living species at its tips and shared ancestors deeper in the past. Every species alive today has had the same amount of time to evolve since its own ancestors diverged.

Across millions of years, some animal lineages adapted to life on land, while others remained in water or returned to it. Vertebrates diversified into fishes, amphibians, reptiles, birds and mammals. Mammals themselves include many branches, from bats and whales to primates.

When did humans appear?

Homo sapiens evolved in Africa about 300,000 years ago, according to the Smithsonian’s Human Origins Program. Our species is one recent branch among many human relatives. Humans and non-bird dinosaurs did not live together: non-bird dinosaurs disappeared about 66 million years before Homo sapiens emerged. Birds, however, are living dinosaurs.

From microscopic life to people, evolution is a deep, interconnected history. It has no predetermined destination; it records how living populations respond to changing environments over time.

Sources & further reading

Correction, 18 October 2026: the approximate start of the Cambrian has been updated from 541 to 539 million years ago to follow the current ICS chart.