The earliest known life appears, possibly developing from self-reproducing RNA molecules. Life is continually constrained by limited resources like energy, space, and available organic chemicals, resulting in natural selection. DNA molecules soon take over as the main replicators, developing inside enclosing membranes that provide a stable physical and chemical environment conducive to replication: proto-cells.

The atmosphere does not contain any free oxygen.

Photosynthesizing cyanobacteria evolve. The oxygen concentration in the atmosphere subsequently rises, acting as a poison for many other bacteria.

The moon's orbit is still very close to the Earth, causing tides a thousand feet high. The planet is continually wracked by hurricane-force winds.

Sponges, the earliest multicellular animals, develop from cell colonies. They have partially-differentiated tissues, but no muscles, nerves, internal organs, or capacity for locomotion.

Following sponges, jellyfish emerge. Jellyfish are some of the earliest creatures to have neurons, in the form of a simple net, with no central nervous system. They have the earliest known muscular tissue, digestive systems with mouths, and radially symmetry body plans.

Flatworms, the earliest animals to have a rudimentary brain and the simplest animals with bilateral symmetry, also emerge. There are still no organisms with a true circulatory or respiratory system.

The ozone layer forms, filtering some of the sun's intense radiation and allowing life's first major excursions onto land.

The first jawed fishes emerge. Their jaws evolve from the first of their gill arches. Their head and thorax are covered by articulated armored plates, while the rest of the body is scaled or naked.

The first simple plants and fungi move onto land.

Some lobe-finned fish develop legs and give rise to early four-limbed animals. These fish live in shallow, swampy freshwater habitats, using their fins as paddles to navigate shallow waters choked with plants and detritus—the likely origin of front limbs bending backward at the elbow and hind limbs bending forward at the knee. Eventually, early tetrapods resembling modern amphibians use their rudimentary legs to move out onto land for brief periods, perhaps to hunt insects. Lungs and swim bladders evolve.

Plants evolve seeds, structures that protect plant embryos and enable plants to spread quickly on land.

The supercontinent Pangaea forms as all of the earth's continents fuse into one.

Evolution of the amniotic egg gives rise to reptiles, who can reproduce on land. Insects resembling enormous dragonflies evolve flight.

Vast forests of clubmosses, horsetails, and tree ferns cover the land. When these decay, they eventually form coal and oil.

Archoasaurs, a branch of the reptile family, diversify into crocodilians, dinosaurs, and pterosaurs. From another reptile branch, the synapsids, evolve the first tiny precursors to mammals.

Conifers are the dominant land plants. Plant-eating animals begin growing larger to accommodate the long digestive tracts needed to digest nutrition-poor pine trees.

Dinosaurs survive a mass extinction and grow to huge sizes. Modern amphibians emerge, including frogs and salamanders.

The supercontinent Pangaea begins to break up into several land masses. The largest is Gondwana, made up of what are now Antarctica, Australia, South America, Africa, and India. Antarctica is still a land of forests. North America and Eurasia are still joined, forming the Northern supercontinent Laurasia.

The first true mammals emerge.

Last common ancestor of humans and mice. Birds with teeth roam the Northern Hemisphere.

The Cretaceous-Tertiary extinction event wipes out about half of all animal species , including all non-avian dinosaurs. A popular theory implicates a cooling of the climate following the impact of an enormous asteroid in the ocean off the Yucatan Peninsula.

Without the presence of giant diurnal dinosaurs, mammals can (and do) increase in diversity and size. A group of small, tree-dwelling, insect-eating mammals branches into what will eventually become primates, treeshrews, and bats.

Primates diverge into lemurs/lorises and tarsiers/monkeys/apes. The earliest elephant is the size of a large pig.

Grasses evolve from among the angiosperms.

The 'dawn dog' appears in North America; canids will eventually spread throughout the world.

India collides with Asia, causing the rise of Himalaya and the Tibetan plateau. Cut off from humidity, Central Asia becomes a desert.

The fox-sized 'dawn bear' hunts in the tree tops. This oldest branch of the bear family has one surviving member today, the giant panda.

The African plate collides with Asia.

Gigantic animals roam South America, including eighteen-foot-long giant sloths and flying birds with twenty-foot wingspans.

The climate begins to dry. Savannas and grasslands take over the forests.

Monkeys proliferate, and the apes go into decline. Human ancestors speciate from gorilla ancestors.

Volcanoes erupt and create the small area of land that joins North and South America. Mammals from North America cross the bridge and cause extinctions of South American mammals. Human ancestors speciate from chimpanzee ancestors. The largest known primates reach twelve feet tall in Asia.

Meat-eating Homo species coexist with Paranthropus, hominins who eat plants and termites. Homo habilis uses stone choppers in Tanzania. Broca's area, the speech region of the modern human brain, begins to emerge.

Homo erectus emerges in Africa and migrates to other continents, primarily South Asia. A large-scale extinction of marine life is triggered by some event, possibly a supernova. Many seabirds, which have dominated the shores and coastal waters for some twenty million years, become extinct; marine mammals diversify and take their place.

Armadillos the size of Volkswagen Beetles live in southern Peru.

Jared Diamond's "great leap forward." Humans paint and hunt mammoths in France. Most large mammal species disappear, directly or indirectly due to the expanding human population.

Neanderthals die out, leaving Homo sapiens and Homo floresiensis as the only living species of the genus Homo. In Eastern Europe, humans invent textiles and press weaving patterns into pieces of clay before firing them.

Humans in the Middle East develop agriculture. Plant domestication begins with cultivation of Neolithic founder crops. This process of food production, coupled later with the domestication of animals, accelerates the increase in human population, continuing to the present day.

Hunter-gatherers in Japan create the earliest known pottery. Humans reach Tierra del Fuego at the tip of South America, the last continental region to be inhabited by humans (aside from Antarctica.)

The human population approaches seven billion. The human-caused extinction event accelerates, with the observed rate of extinction rising dramatically in the last fifty years.

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