For a discussion of human evolution, the articles genetics, human and heredity.
Specific aspects of evolution are discussed in the articles coloration and mimicry.
Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution.
Biological evolution is a process of descent with modification.
Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time.
What is impressive is not just the numbers but also the incredible heterogeneity in size, shape, and way of life—from lowly bacteria, measuring less than a thousandth of a millimetre in diameter, to stately sequoias, rising 100 metres (300 feet) above the ground and weighing several thousand tons; from bacteria living in hot springs at temperatures near the boiling point of water to fungi and algae thriving on the ice masses of Antarctica and in saline pools at −23 °C (−9 °F); and from giant tube worms discovered living near hydrothermal vents on the dark ocean floor to spiders and larkspur plants existing on the slopes of Mount Everest more than 6,000 metres (19,700 feet) above sea level.
The virtually infinite variations on life are the fruit of the evolutionary process.
Beginning in the 1960s, a related scientific discipline, molecular biology, enormously advanced knowledge of biological evolution and made it possible to investigate detailed problems that had seemed completely out of reach only a short time previously—for example, how similar the genes of humans and chimpanzees might be (they differ in about 1–2 percent of the units that make up the genes).
This article discusses evolution as it applies generally to living things.
The earliest fossils resemble microorganisms such as bacteria and cyanobacteria (blue-green algae); the oldest of these fossils appear in rocks 3.5 billion years old ( Precambrian time).
The oldest known animal fossils, about 700 million years old, come from the so-called Ediacara fauna, small wormlike creatures with soft bodies.
Paleontologists have recovered and studied the fossil remains of many thousands of organisms that lived in the past.
This fossil record shows that many kinds of extinct organisms were very different in form from any now living.