Evidence from living organisms indicates that all living things are descended from a single common ancestor. Some progress has been made in understanding the origin of life, but a great deal remains unknown.
The first fossil evidence of life dates from about 3.5 Gya, about 1 Gy after the formation of Earth. The earliest life forms of which we have evidence were prokaryotes.
Eukaryotes evolved about 1.8 Gya. Their mitochondria and chloroplasts evolved from endosymbiotic bacteria.
Although stem lineages of some modern phyla evolved long before the Cambrian period, the fossil record displays an explosive diversification of the animal phyla near the beginning of the Cambrian, about 541 Mya. The causes of this rapid diversification are debated, but may include a combination of genetic and ecological events. Jawless, limbless vertebrates evolved by the late Cambri-an.
Terrestrial plant and arthropod fossils are found first in the Silurian, and insects in the Devonian. Vertebrates (fishes) with jaws and limbs (fins) evolved in the Silurian, and tetrapods evolved in the late Devonian from lobe-finned fishes.
The most devastating mass extinction of all time occurred at the end of the Permian (about 252 Mya). It profoundly altered the tax-onomic composition of Earth’s biotas.
Seed plants and amniotes became diverse and ecologically dominant during the Mesozoic era (252–66 Mya). Early mammaliaforms evolved in the Triassic, and archosaurs, including especially the dinosaurs, dominated Jurassic and Cretaceous landscapes. Flying dinosaurs, the antecedents of birds, evolved in the Jurassic, and gave rise to some lineages of modern birds in the late Cretaceous. Flowering plants and plant-associated insects diversified greatly from the middle of the Cretaceous onward. A mass extinction (the K/Pg or K/T extinction) at the end of the Mesozoic included the extinction of the last nonavian dinosaurs.
The climate became drier during the Cenozoic era, favoring the development of grasslands and the evolution of herbaceous plants and grassland-adapted animals.
Most orders of placental mammals originated in the late Cretaceous, but underwent adaptive radiation in the early Paleogene. Many groups of mammals were once more diverse than they are now, and some are extinct. A few groups, such as rodents and artiodac-tyls, maintained high diversity.
A series of glacial and interglacial episodes occurred during the Pleistocene (the last 2.6 My), during which some extinctions oc-curred and the distributions of species were greatly altered.
Humans have caused species extinctions since the spread of agriculture or earlier. Human population growth and technology have had an accelerating impact on biological diversity, and have initiated another major extinction.