Of all the topics covered so quickly in this series, the history of earth from 542 mya onward could and does fill books and constitute a chain of college classes. The Paleozoic Era spans from 542 to 252 mya, and its early portion, the Cambrian Period from 542 to 485 mya, is known for an amazing proliferation of life into many different forms, both terrestrial and marine, known as the Cambrian Explosion. Large animals with exoskeletons like mollusks, and vertebrates like fish appeared. By the end of the Cambrian, most modern phyla–one of the highest level of classifications of living species–were represented.
With the tremendous expansion of species and populations came extinction events, when large changes in climate wiped out large portions of existing species. Earth had reached the point where life was widespread and abundant enough that it would be dramatically affected by major, planet-wide changes. These will be the subject of future posts.
Accumulation of oxygen in the atmosphere led to the formation of the ozone layer in the upper atmosphere, reducing UV radiation on the earth’s surface and allowing larger animals and plants to develop. The earliest evidence of land plants–not living near the water—appears around 480 mya, and the first animals, arthropods (think bugs, but lobsters and crabs are arthropods too) around 458 mya. With several intervening extinction events, life forms on earth alternately bloomed and waned, in forms ever closer to the types of animals, like lizards, amphibians, fish, and later birds and mammals, extant today.
Around 355 mya the most recent supercontinent, Pangea, assembled, spanning roughly from pole to pole across the equator, surrounded by one huge ocean, Panthalassa. Pangea broke up in two phases, with proto-North America rifting away between 175 and 150 mya, proto-Eurasia rotated clockwise toward proto-Africa, closing the Tethys Sea and creating the deposits which have become the Arabian oil reserve. Between 150 and 140 mya several other continents, proto-South America, Australia, India and Australia. separated.
The increase in volcanic rifting activity led to a global increase in atmospheric CO2 concentration, creating a warm climate in the mid-Mesozoic Era (252-66 mya). At the same time, the active mid-ocean spreading ridges expanded due to their increased temperature (a function of their increased activity), raising sea level globally by ~300m. In this world the dinosaurs proliferated until the Chixculub Meteor strike 65.5 mya which ended the Mesozoic era and the time of the dinosaurs and made room for mammals.
During the Cenozoic (“new life”) era, from 65.5 mya onward, India has collided with Asia, Australia has separated from Antarctica, the Arctic Ocean has opened and North and South America have been connected by the land bridge of Central America. Homo sapiens evolved around 300,000 years ago, meaning our species has existed for a little less than .07% of the age of the earth.
Tomorrow: extinction events.
This post was previously published on Dailykos.com and is republished on Medium.
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