Earth forms from space dust, rocks, and gases. It is not a nice place to live. In fact, nothing lives there. © Microsoft Clipart

First prokaryotes (single-celled organisms with no nucleus) arise. They are the ancestors of modern day bacteria and cyanobacteria.

First single-celled organisms with nuclei (eukaryotes) arise. They are the ancestors of modern day protists.

The first soft-bodied invertebrates evolve. They will evolve into sponges, jellyfish, mollusks, and worms. They are the first animals.

The first hard-bodied invertebrates evolve. They will become the arthropods (crustaceans, insects, and arachnids).

Fish first appear on Earth. They are very similar in appearance to lampreys of today They are the first vertebrates.

First vascular land plants. They were fern-like organisms.

The first amphibious creatures crawl onto land. They are ancestors of frogs and salamanders. They breathe through lungs as well as through the skin. © G.R. Morton, 1997

The first true reptiles appear. They have scales, lungs, and lay eggs with hard, leathery shells. © Microsoft Encarta

The first dinosaurs and the first mammals appear. They are warm-blooded. They will lead way to bigger dinosaurs, bigger mammals, and birds.

Evolution of the first birds. They are originally dinosaurs with feathers. They also develop hollow bones to make them lighter.

The first placental mammals evolve. They nourish their unborn offspring with placentas and give birth to live young. They soon become the dominant mammalian form on Earth.

The first human beings appear. They are almost like us, but not quite, being a little smaller and a little less upright, and a lot hairier. Homo sapiens (us) will not show for another 1.7 million years.

ClipArt courtesy of Microsoft ClipArt Amphibian image courtesy of Morton, G.R. retrieved August 27, 2002 from All other images © Microsoft Corporation. Microsoft Encarta Encyclopedia 2002.