Life on Earth is at least 3.5 billion years old, and possible chemical traces reach back to rocks about 3.7 billion years old. For most of that time, organisms were microscopic. Animals, forests and humans occupy only the latest pages of a much longer history.
Early microbes and the rise of oxygen
The oldest evidence is difficult to interpret because rocks have been altered, broken up and recycled. Chemical patterns in some very old rocks may record microbial activity. Stromatolites, layered structures built by microbial communities, provide a clearer record by about 3.5 billion years ago. Neither clue identifies every organism that made it.
By at least 2.4 billion years ago, cyanobacteria were releasing oxygen through photosynthesis. Oxygen accumulated gradually and changed ocean chemistry and the atmosphere. It was harmful to many organisms adapted to oxygen-poor settings, while other lineages evolved ways to use it for cellular energy. This transition is associated with the Proterozoic, a vast interval that included later changes in cell complexity.
Complex cells and multicellular life
Cells with nuclei appeared long after the earliest bacteria. The details of how eukaryotic cells arose are reconstructed from fossils, chemistry and comparisons among living organisms. Multicellularity evolved independently in more than one branch of life, so it was not a single ladder climbed by all organisms.
During the Ediacaran Period, from about 635 to 538.8 million years ago, the seas held large soft-bodied organisms and early animals. Some have no clear living equivalent. Their shapes and modes of life are debated because soft bodies fossilise only under unusual conditions.
The Cambrian record becomes richer
At the start of the Cambrian Period, about 538.8 million years ago, animal fossils become more diverse and more abundant. Arthropods, molluscs, sponges, echinoderms and early chordates appear in a growing range of forms and feeding modes. Exceptional sites preserve soft tissues that are missing from most deposits.
The phrase “Cambrian explosion” describes a rapid increase in diversity over millions of years, not the instant creation of all modern animal groups. Some lineages and ecological experiments began earlier. The Cambrian record is a major transition in what fossils preserve as well as in animal diversity.
Life spreads onto land
Microbial communities occupied land before forests. Plants and arthropods followed, though the timing and pathways differed across groups. In the Devonian Period, the first forests and early four-limbed vertebrates transformed terrestrial habitats. Later, Carboniferous wetlands supported extensive forests, amphibians and the first amniotes.
An amniotic egg protected an embryo from drying and allowed reproduction farther from open water. This innovation helped reptiles and their relatives diversify on land. It did not make every later animal independent of water, and it was one of several changes that shaped terrestrial ecosystems.
Extinctions and new ecosystems
Evolution has no predetermined direction. Mass extinctions removed many branches, while survivors sometimes diversified into ecological roles that had become vacant. The end-Permian crisis was the largest known mass extinction; its aftermath reshaped life before the Mesozoic Era, when dinosaurs, pterosaurs, marine reptiles, early mammals and birds diversified.
About 66 million years ago, the end-Cretaceous extinction eliminated non-avian dinosaurs and many other groups. Mammals and birds then expanded into a range of habitats. The Cenozoic Era is the time in which the human lineage evolved, but it was not an endpoint toward which earlier life had been moving.
Humans occupy only a small part of the timeline
The human and chimpanzee lineages separated only several million years ago, and Homo sapiens is younger still. Every living species represents a branch with its own history. The sequence from microbes to humans is a useful way to organise dates, but it should not be mistaken for a ladder of progress.
To follow the earliest animal branch in more detail, see the first animals on Earth. The disciplines that read animal and plant fossils together are described in palaeontology and palaeobotany.
Frequently asked questions
When did life first appear on Earth?
Possible chemical traces occur in rocks about 3.7 billion years old, while stromatolites provide evidence of microbial communities by roughly 3.5 billion years ago. The oldest clues remain open to interpretation.
What changed during the Great Oxidation?
Oxygen produced by photosynthetic microbes accumulated in the environment by about 2.4 billion years ago. It changed ocean and atmospheric chemistry and altered which forms of metabolism were possible.
Was the Cambrian explosion a sudden event?
No. Animal diversity increased rapidly on a geological timescale over millions of years, and some lineages existed before the Cambrian began.
Is human evolution the goal of life's history?
No. Evolution has no planned destination. Humans are one recent branch among many, and all living species have their own long histories.