Evolutionary history of life
Part of the Biology series on
Evolution
Introduction
Mechanisms and processes
Adaptation
Genetic drift
Gene flow
Mutation
Natural selection
Speciation
Research and history
Evidence
Evolutionary history of life
History
Modern synthesis
Social effect
Theory and fact
Objections / Controversy
Evolutionary biology fields
Cladistics
Ecological genetics
Evolutionary development
Human evolution
Molecular evolution
Phylogenetics
Population genetics
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The evolutionary history of life on Earth traces the pro-
cesses by which living and fossil organisms evolved. It
stretches back over 3,000 million years ago, possibly as
far as 3,800 million years ago, and there is evidence that
evolution continues, even in humans. All present-day
organisms use the same large set of complex chemical
reactions, which indicates that all modern organisms
share a common ancestor. However even the simplest
modern organisms are too complex to have emerged
directly from non-living materials. Some scientists have
proposed that
life on Earth was "seeded" from
elsewhere, but most research concentrates on how sim-
pler forms of life could have arisen independently on
Earth.
For about 2,000 million years multi-layered microbial
mats were the dominant life. The evolution of oxygenic
photosynthesis led to the oxygenation of the atmo-
sphere, beginning about 2,400 million years ago. While
eukaryotic cells may have been present earlier, their
evolution accelerated when they began to use oxygen in
their metabolism. The earliest evidence of complex euk-
aryotes with organelles, dates from 1,850 million years
ago. Later, around 1,700 million years ago, eukaryotes
evolved into multicellular organisms. The earliest
known fossil animals are cnidarians from about 580 mil-
lion years ago, although the earliest animals must have
appeared before then. The earliest modern-looking bi-
laterian animals appear in the Early Cambrian, along
with several "weird wonders". Vertebrates remained an
obscure group until the first fish w