from 540 to 248 million years ago

Introduction: If you could see a satellite view of the Earth as it was 540 million years ago, you would not recognize it as home.   Most of the land was joined into one continent with a few small, stray continents nearby.  The big continent, Gondwanaland, was near the south pole, putting what is today Siberia and China near the equator!  If you were to land on this strange-looking Earth, you would be even more confounded by the alien landscape.  There were no trees, grass or plants carpeting the ground—only bare rock and loose, rocky soil.  There were no birds, insects or frogs then, and so the only sound would be ocean waves slapping the shores of rocky beaches and the wind blowing in your ears.  On a windless day far away from the shore, there was a silence like no human has ever known in the outdoors.  And there was nothing, absolutely nothing, to eat on land.  In the time before the Paleozoic (the Precambrian Era) life had spread through the ocean (taking about 3 billion years to do so), so in the ocean you might have found some things to eat:  algae, sponges, other soft-bodied invertebrates, coral, arthropods (such as trilobites), polychaete worms (6 families of which survive today!), mollusks, and some rather bizarre-looking echinoderms (starfish and urchins) — but nothing I would care to ingest.

The greatest change in Earth's appearance occurred during the Paleozoic Era, spanning only 292 million years.  By the end of the Paleozoic the barren, lifeless continents of Earth transformed into lush forests and swamps.  At last the air filled with the buzzing of insects and the sounds of amphibians and reptiles.  In the ocean life went from the primitive reefs and simple herbivorous invertebrates of Precambrian time to having swimming invertebrate predators and the first fish.

Motion of the Continents:  Over the course of the Paleozoic Era, Gondwanaland partially broke up and the continents moved around the ocean in a slow dance.  Gondwanaland was made up of Africa, South America, Australia, Antarctica, Asia, a little piece of land which later became Florida, and most of the east coast of the USA.  Gondwanaland traveled so far south during this time that the south pole found itself in what is today Morocco in north-west Africa, in southern Brazil, and then in South Africa by the end of the Paleozoic.  North America, Greenland and Scotland were joined as one and collided with the British Isles and a continent made up of Scandinavia and Europe.  By the end of the Paleozoic Era, all the continents had come back together to make a super-continent which we call Pangaea.  It was at this point that South America joined North America, while northern Africa rammed into the part of North America which would become the east coast of the US, causing the Appalachian Mountains to rise.  When this happened, New York was about 20 degrees south of the equator and Toronto and Quebec were on the equator!

Life:  In the Paleozoic Era, life diversified rapidly and fantastically on land and in the ocean. Life in the sea changed drastically during this time.  Trilobites, mollusks, urchins, and star fish appeared at the beginning of the Paleozoic as part of the "Cambrian Explosion." Later, jawless fish evolved, then fish with jaws, and even sharks filled the seas by the end of the Paleozoic. Species diversification of vertebrates and invertebrates alike went quite wild, and some fairly bizarre creatures emerged:  including an armored fish 30 feet long (Dunkleosteus) and many nightmarish-shaped invertebrates.

The appearance of plants on land set the stage for the eventual colonization of the continents by animals.  Not only would plants eventually be a source of food for these animals, but plants changed properties of the land itself.  Before plants, there were no roots to stabilize the soil against erosion.  And this also affected the flow of rivers; the more stable soil caused rivers to meander more and so rivers deposited sediment differently, creating diverse habitats for future animals to occupy.  Even the death of plants affected the land.  Dead plants mixed into the soil making humus, the dark organic richness of nutrient-laden soil.  Large trees that died in swamps eventually became large coal deposits.  In all these ways and more, plants prepared the land for the invasion of animals.  After the proliferation of land plants, it did not take long for animals in the sea to evolve for life ashore.  By the middle Paleozoic, scorpions and flightless insects were among the first creatures to live on land.  It took another 90 million years for vertebrates to appear on shore.  By the end of the Paleozoic, there were giant horsetail plants as tall as trees, early conifer forests, flying insects, amphibians (some of them pretty big and bizarre), reptiles, and therapsids (called mammal-like reptiles).  Yet the total absence of grass, flowers, and birds surely would strike modern humans as strange and eerie — Earth still displayed quite an alien scene.

Death:  Although life took a great leap during the Paleozoic Era, several mass extinctions of life took place.  With each extinction up to 60% or more of the species living at the time died; yet after each event, new species appeared very quickly and diversified into even more species.  Then, at the end of the Paleozoic, the greatest mass extinction since life evolved occurred (the "Permian Extinction").  On land, most of the therapsid species were wiped out, but it was much more serious in the ocean.  More than 90% of marine species suddenly disappeared. Although climactic change has been suggested as the cause, there is no direct evidence in the fossil record.  After this horrendous event, the world entered a new time of change, called the Mesozoic Era, also known as the Age of Dinosaurs.