Script

Evolution of the Continents
INTRODUCTION
The continents on which we live are on the move, albeit at an average rate of only several centimeters each year. The continents move because they ride on top of gigantic plates that, in turn, float on a molten layer of Earth, called the mantle. Energy, released from radioactive decay in Earth's core, heats up the mantle and sets up convection currents that propel the plates around Earth's surface. The movement of the plates, and the continents that ride on them, is called continental drift.
At times in Earth's history, the continents have coalesced into giant landmasses, but at other times they have traveled away from each other. The positions of the continents affect Earth's climate, the sea levels, the distributions of organisms, as well as the birth and extinction of species. In addition to depicting continental drift, this animation also provides a summary of the state of life at each corresponding period in Earth's history.
ANIMATION SCRIPT
The continents lie on massive plates that are in constant motion on Earth's surface. Their movement, called continental drift, sometimes forces continents together and other times pulls them apart. The continents as we know them today are still in motion, so millions of years in the future their arrangement will look much different.
Let's go back 540 million years, to the beginning of the Cambrian period, when Earth's continents were mostly in the southern hemisphere and were coalescing into larger landmasses. The largest, Gondwana, included the future South America, Africa, India, Australia, and Antarctica.
Around this time, during a period of about 60 million years, a rapid diversification of life took place, known as the Cambrian explosion. Many of the major animal groups represented by species alive today first appeared during these evolutionary radiations.
Over the next 200 million years, the large landmasses continued to approach each other. At the end of the Ordovician period, massive glaciers formed over the southern continents, sea levels dropped about 50 meters, and ocean temperatures dropped. About 75 percent of all animal species became extinct, probably because of these major environmental changes.
During the Silurian period, marine life rebounded, and the first vascular plants and terrestrial arthropods (scorpions and millipedes) evolved. Fishes diversified as bony armor gave way to the less rigid scales of modern fishes, and the first jawed fishes appeared.
In the Devonian period, many animal groups radiated on land and in the sea. Fish diversified and some became formidable predators. Tall trees with fernlike leaves dominated newly appearing forests. The end of the Devonian is marked by a massive extinction of about 75 percent of all marine species.
In the Carboniferous period, about 350 million years ago, extensive swamp forests grew on the tropical continents. The fossilized remains from the forests formed the coal we now mine for energy. The diversity of terrestrial animals increased greatly. Amniotes, with their well protected eggs, evolved, as did giant amphibians and winged insects—the first animals to fly.
During the Permian period, the continents merged into a single supercontinent called Pangaea. On land, the amniotes split into two lineages: the reptiles, and a second lineage that would eventually lead to the mammals. Toward the end of the Permian, conditions for life deteriorated. Massive volcanic eruptions produced ash and gases that blocked sunlight and cooled the climate. The resulting death and decay of forests rapidly used up atmospheric oxygen. In addition, much of Pangaea was located close to the South Pole by the end of the Permian. All of these factors combined to produce the most extensive continental glaciers since the "snowball Earth" hundreds of millions of years earlier.
At the low O2 concentrations at the end of the Permian period, most animals would not be able to survive at elevations above 500 meters, so about half of the land area would have been uninhabitable. Scientists estimate that about 96 percent of all multicellular species became extinct. The few organisms that survived the Permian mass extinction found themselves in a relatively empty world.
In the Mesozoic era, atmospheric oxygen concentrations gradually rose. Pangaea remained largely intact through the Triassic period. On land, conifers became dominant plants, and frogs and reptiles began to diversity. The radiations of reptiles eventually gave rise to crocodilians, dinosaurs, and birds. The first mammals appeared during the Triassic. The end of the Triassic was marked by a mass extinction that eliminated about 65 percent of the species on Earth.
Pangaea began to break up and, by the late Jurassic period, Pangaea became fully divided into two large continents: Laurasia, which drifted northward, and Gondwana in the south. Most of the large terrestrial predators and herbivores of the period were dinosaurs. The earliest known fossils of flowering plants are from late in this period.
During the Cretaceous period, Laurasia and Gondwana broke apart. During the Cretaceous period, Earth was warm and humid. Flowering plants began the diversification that led to their current dominance of the land. By the end of the Cretaceous, many radiations of animal groups, on both land and sea, had occurred. A meteorite caused a mass extinction at the end of the Cretaceous. On land, almost all animals larger than about 25 kg in body weight, including the non-avian dinosaurs, became extinct.
Several significant continental collisions occurred during the Tertiary period. By about 35 mya, the Indian Plate ran fully into the Eurasian Plate, and the Himalayas began to be pushed up as a result. Africa subsequently collided with Eurasia, and South America with North America. In this period, flowering plants dominated on land, and a rapid radiation of mammals occurred. Although the early Tertiary was hot and humid, Earth's climate began to cool, and grasslands spread over much of Earth.
We are living in the Quaternary period, during which four major and about 20 minor "ice ages," have occurred. During ice ages, massive glaciers spread across the continents, and the ranges of animal and plant populations shifted toward the equator.
The last of these glaciers retreated from temperate latitudes less than 15,000 years ago. Organisms are still adjusting to this change. Many high-latitude ecological communities have occupied their current locations for no more than a few thousand years.
It was during the Quaternary period that divergence within one group of mammals, the primates, resulted in the evolution of the hominoid lineage, eventually leading to our modern human species—Homo sapiens. It may be hard to visualize, but humans are incredibly recent arrivals on this 4.5 billion-year-old planet. Even the Cambrian explosion, at about 500 million years ago, could be considered a recent series of events. Life has been evolving on Earth for about 3.8 billion years. The continents and Earth's biological communities have been constantly changing during this time and will continue to change into the future.
CONCLUSION
The changing positions of the continents have had dramatic effects on the Earth's climate and on its living organisms. For example, when the enormous landmass Gondwana formed over the South Pole 500 million years ago, the Earth entered a period of glaciation. Water became trapped in the frozen glaciers, which lowered the sea level dramatically. As the sea level dropped, continental shelves (submerged parts of the continents) became exposed, and the organisms that thrived there would have either died, adapted, or moved to other locations. The temperature of the oceans also dropped. During this time in Earth's history, 75% of marine species became extinct.
The positions of continents over time also explain some interesting features of the distribution of flora and fauna around the globe. For example, the island of Madagascar, currently near the southern tip of Africa, is home to animals that are remarkably similar to animals living in India. India and Madagascar are separated by 4000 km (~2500 miles) of ocean, much too great a distance for land animals to cross. However, at one time, India and Madagascar lay adjacent to each other. Until 90 million years ago, when India and Madagascar split, they could share species.