Query: The last time the planet was even four degrees warmer, Peter Brannen points out in The Ends of the World, his new history of the planet’s major extinction events, the oceans were hundreds of feet higher.

Relevant passages:
Earth <=SEP=> Earth ( from Eorðe -LSB- Γαῖα , Gaia -RSB- Terra ) , otherwise known as the World or the Globe , is the third planet from the Sun and the only object in the Universe known to harbor life . It is the densest planet in the Solar System and the largest of the four terrestrial planets .   According to radiometric dating and other sources of evidence , Earth formed about 4.54 billion years ago . Earth 's gravity interacts with other objects in space , especially the Sun and the Moon , Earth 's only natural satellite . During one orbit around the Sun , Earth rotates about its axis over 365 times ; thus , an Earth year is about 365.26 days long . Earth 's axis of rotation is tilted , producing seasonal variations on the planet 's surface . The gravitational interaction between the Earth and Moon causes ocean tides , stabilizes the Earth 's orientation on its axis , and gradually slows its rotation .   Earth 's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years . About 71 % of Earth 's surface is covered with water , mostly by its oceans . The remaining 29 % is land consisting of continents and islands that together have many lakes , rivers and other sources of water that contribute to the hydrosphere . The majority of Earth 's polar regions are covered in ice , including the Antarctic ice sheet and the sea ice of the Arctic ice pack . Earth 's interior remains active with a solid iron inner core , a liquid outer core that generates the Earth 's magnetic field , and a convecting mantle that drives plate tectonics .   Within the first billion years of Earth 's history , life appeared in the oceans and began to affect the Earth 's atmosphere and surface , leading to the proliferation of aerobic and anaerobic organisms . Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago . Since then , the combination of Earth 's distance from the Sun , physical properties , and geological history have allowed life to evolve and thrive . In the history of the Earth , biodiversity has gone through long periods of expansion , occasionally punctuated by mass extinction events . Over 99 % of all species that ever lived on Earth are extinct . Estimates of the number of species on Earth today vary widely ; most species have not been described . Over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival . Humans have developed diverse societies and cultures ; politically , the world has about 200 sovereign states .

Ordovician <=SEP=> The Ordovician ( -LSB- pronɔrdəˈvɪʃən -RSB- ) is a geologic period and system , the second of six periods of the Paleozoic Era . The Ordovician spans 41.2 million years from the end of the Cambrian Period million years ago ( Mya ) to the start of the Silurian Period Mya . The Ordovician , named after the Celtic tribe of the Ordovices , was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods , respectively . Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods , and placed them in a period of their own . It received international sanction in 1960 , when it was adopted as an official period of the Paleozoic Era by the International Geological Congress .   Life continued to flourish during the Ordovician as it did in the earlier Cambrian period , although the end of the period was marked by the Ordovician -- Silurian extinction event . Invertebrates , namely molluscs and arthropods , dominated the oceans . The Great Ordovician Biodiversification Event considerably increased the diversity of life . Fish , the world 's first true vertebrates , continued to evolve , and those with jaws may have first appeared late in the period . Life had yet to diversify on land . About 100 times as many meteorites struck the Earth during the Ordovician compared with today .

Devonian <=SEP=> The Devonian is a geologic period and system of the Paleozoic , spanning 60 million years from the end of the Silurian , million years ago ( Mya ) , to the beginning of the Carboniferous , Mya . It is named after Devon , England , where rocks from this period were first studied .  The first significant adaptive radiation of life on dry land occurred during the Devonian . Free-sporing vascular plants began to spread across dry land , forming extensive forests which covered the continents . By the middle of the Devonian , several groups of plants had evolved leaves and true roots , and by the end of the period the first seed-bearing plants appeared . Various terrestrial arthropods also became well-established . Fish reached substantial diversity during this time , leading the Devonian to often be dubbed the `` Age of Fish '' . The first ray-finned and lobe-finned bony fish appeared , while the placodermi began dominating almost every known aquatic environment .   The ancestors of all four-limbed vertebrates ( tetrapods ) began adapting to walking on land , as their strong pectoral and pelvic fins gradually evolved into legs . In the oceans , primitive sharks became more numerous than in the Silurian and Late Ordovician . The first ammonites , species of molluscs , appeared . Trilobites , the mollusk-like brachiopods and the great coral reefs , were still common . The Late Devonian extinction which started about 375 million years ago severely affected marine life , killing off all placodermi , and all trilobites , save for a few species of the order Proetida .   The palaeogeography was dominated by the supercontinent of Gondwana to the south , the continent of Siberia to the north , and the early formation of the small continent of Euramerica in between .