Approximately 1 billion years ago, a wider variety of microscopic cells had appeared, including some that may have had nuclei. In deposits approximately 600 million years old, imprints of soft-bodied invertebrates are found. In sediments 570 million years old (the beginning of the Cambrian Period), the first skeletal invertebrates appeared--mollusks, brachiopods, and trilobites. The appearance of these invertebrates, in rocks deposited at the beginning of the Paleozoic Era, coincided with the first widespread signs of burrowing.

In rocks of the Ordovician Period (500-425 million years ago), researchers have found fossil animal burrows that are evidence of the earliest known land animals; these trace fossils indicate that terrestrial ecosystems may have evolved sooner that was once thought. Most of the modern classes of invertebrates as well as the Ostracoderms (the fishlike organisms), were represented by this time; marine faunas had become much more diverse.

In the Silurian Period (425-400 million years ago) landmasses were colonized by rapidly evolving higher plants, whose supporting structures and water-conducting vessels now made life on dry land possible.

In the Devonian Period (400-345 million years ago) the main groups of Fish--coelacanths (see Coelacanth), Lungfish, sharks (see Shark), bony fish, and the extinct arthrodires (see Arthrodire)--were differentiated. Forests and the first primitive insects (see Insect) appeared on land, as did Amphibians.

The Carboniferous Period (345-280 million years ago), known for its great coal deposits, witnessed the development of reptiles (see Reptile), the first animals having an amniote egg, which enables the embryo to develop on dry land.

The Mesozoic Era (225-65 million years ago), which includes the Triassic, Jurassic, and Cretaceous periods, is known particularly for the evolution of gigantic reptiles, both in the sea (Ichthyosaurus, Plesiosaur, and Mosasaur) and on land (Dinosaur).

Flying reptiles (Pterodactyl) and birds (see Bird), as well as mammals (see Mammal), appeared during the Jurassic Period (190-135 million years ago). On land, forests of conifers (see Conifer) and cycads (see Cycad) had largely replaced the lycopod- and seed-fern-dominated forests of the Paleozoic Era. In the sea tiny calcite-armored, photosynthetic unicells called coccolithophorids appeared, and massive calcium carbonate (chalk) deposition began in the deeper oceans (see Ooze, Deep-Sea).

The Cretaceous Period (135-65 million years ago) is the time of origin of two great groups of plants--the flowering plants (Angiosperm) on land and the Diatom in water. Flowering plants changed the face of the Earth in many ways and triggered a great wave of evolution among the insects. At the end of Cretaceous time, the extinction of dinosaurs resulted in the spectacular evolution of terrestrial mammals, and giant sharks and marine mammals replaced large reptiles in the sea.

The group of mammals known as the primates (see Primate)--now represented by lemurs, monkeys, apes, and humans--dates back to the beginning of the Cenozoic Era (65 million years ago to the present), but human-like creatures (see Prehistoric Humans) are known only from the last few million years, the Pliocene and Pleistocene epochs. Homo sapiens, modern humans, appeared in the Old World during the Pleistocene but did not reach the Americas until its latter part.

Fossils record the progressive evolutionary diversification of living things, the progressive colonization of habitats, and the development of increasingly complex organic communities. The development of new species and such larger groups of species as genera and families has gone on throughout time, but so also has the loss of species by EXTINCTION. The rate of extinction at some times in Earth history greatly exceeded the rate of speciation, and the faunas and floras of the world became reduced. Notable biotic crises occurred in Cambrian time, near the end of the Devonian time, at the close (PERMIAN PERIOD) of the Paleozoic Era, and at the end of the Cretaceous Period. Various theories have been advanced to explain the cause of these extinctions.

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