The Story of Earth by Robert Hazen
The story of Earth is a story of constant change and disruption. Earth has had its surface reshaped and remade more than any other body in the solar system. Thanks to the rock record, we have a decent general idea of what Earth looked like throughout various phases of its history.
Formation and Stratification (4.5 to 4 billion years ago)
Earth formed through the gravitational coalescing of outer material from the nebula that formed the sun. Like the other three inner plants, Earth is made up primarily of middleweight elements like iron, oxygen, silicon, magnesium, calcium, and aluminum.
The early solar system featured frequent collisions and mergers. Very soon after Earth’s initial formation, a Mars-sized planetoid (usually referred to as Theia) struck Earth in an oblique collision. Most of Theia’s mass, including its heavy metal core, ended up getting absorbed into Earth, helping to make Earth both the largest and the densest of the four inner plants. Some of Theia’s surface-level rocks, about 1/80th of Earth’s new mass, ended up getting expelled into space as debris, eventually coalescing to form the Moon. Compared to Earth, the Moon has a very small inner metal core, a very thick outer silicate layer, and an almost complete absence of lightweight volatile chemicals at its surface. The Moon was initially 1/16th of its current distance from Earth but has been gradually moving away. As the Moon moved further away, conservation of angular momentum has slowed down Earth’s rotation from a five-hour day to its present 24-hour day.
The Theia collision left Earth’s entire surface liquid and molten. The Moon’s strong tidal forces impeded Earth’s cooling by causing huge tidal waves of lava that circled Earth’s surface every day. However, eventually Earth’s outer crust cooled and became solid. Earth’s first solid surface was made of black basalt and was covered with volcanoes.
Earth stratified into more clearly-defined layers, with heavy compounds sinking towards the center and lighter compounds rising to the surface. Volcanoes expelled lightweight volatiles above the crust, most notably water, carbon dioxide, and nitrogen. Water soon covered Earth’s surface while nitrogen became the dominant gas of Earth’s atmosphere. Earth became almost entirely covered in a global ocean, with black volcanic islands being the only dry land. The sun was less luminous than it is now but Earth compensated with a stronger greenhouse effect.
Earth’s internal convection heated the mantle from below and began driving plate tectonics. In some places, basalt partially melted and then cooled as granite, which is lighter and therefore floats on top of basalt. Mounds of granite rose above the sea surface and formed Earth’s first land continents. The first continents were grey in color and completely barren. Earth’s continents have been constantly moving and have merged into a giant supercontinent at least five separate times.
The Rise of Microbes (4 to 2.5 billion years ago)
Common jolts of energy such as volcanic heat, lightning, and UV light caused water, carbon dioxide, and hydrocarbons to combine into complex organic molecules. These organic molecules often ended up sticking to the surfaces of common undersea rocks where other molecules could join them. The early Earth was a giant decentralized laboratory of organic chemistry, with collections of molecules regularly assembling, modifying, copying, and destroying each other. It is unclear how the first living cell formed, but at some point a clump of molecules became able to consume other molecules, harness chemical energy to power itself, encode the instructions needed to create its proteins, and split into copies of itself. Earth’s first life formed under the sea, most likely near an energy-rich volcanic vent.
There is undisputed evidence of microbial life on Earth by 3.5 billion years ago and disputed evidence that goes back even earlier. Earth was an unusually interesting and dynamic world even without any life, but the rise of life made it geologically and chemically active in new and unique ways. Earth has many properties that made it possible for microbes to survive long-term, including liquid water, abundant sources of energy, a strong magnetosphere, and a stable roughly-circular orbit around a stable star.
The Great Oxidation Event and its Aftermath (2.5 to 0.6 billion years ago)
Early microbes powered themselves with a wide variety of anaerobic chemical reactions. Around 2.5 billion years ago, some microbes evolved a new photosynthetic reaction which used sunlight to build sugars and expelled oxygen gas as a byproduct. Earth’s atmosphere now contained oxygen for the first time, though initially it was only a fraction of a percent. Even at small concentrations, oxygen’s extreme reactivity had profound effects on Earth. Oxygen-intolerant life could now only live in oxygen-free areas such as the deep oceans or underground. Surface-level rocks got oxidized and Earth’s still-barren continents turned from a granite grey to a Mars-like red. The diversity of Earth’s rocks and minerals skyrocketed; Earth now has about 4500 known types of minerals, about two-thirds of which are not known to exist anywhere else.
After the initial shock of the Great Oxidation Event, Earth entered a period of remarkable stability. The period from 1.9 to 0.9 billion years ago is often called the “boring billion.” There were no major innovations by life, no significant changes in the surface or atmosphere, and no major changes in climate. Microbes lived mainly in the sunlit shallow coastal seas next to the lifeless red continents, blotching the sea in many colors.
Earth’s stasis was shattered 0.9 billion years ago by the breakup of the giant equatorial desert supercontinent Rodinia. Rodinia’s breakup created much more shallow sea habitat for photosynthetic algae while also triggering more continental rain and weathering. So much carbon was sequestered that Earth’s greenhouse effect collapsed, leading to runaway global cooling. In the resulting “snowball Earth,” all the continents down to the equator got glaciated and most of the seas also froze over. Volcanoes eventually replenished the atmosphere with greenhouse gases but there was no strong negative feedback loop to keep them in check. Earth swung in the opposite direction and developed a very hot climate.
The snowball-hothouse cycle repeated at least three times over the next 300 million years, each cycle having a smaller amplitude than the one before. Earth’s overall biomass increased during the hothouse periods and became better able to balance Earth’s carbon cycle. Microbes expanded beyond the seas and evolved the ability to live on low-lying swampy lands. A significant rise in photosynthetic algae increased the atmosphere’s oxygen content from less than one percent to about 15%.
The Rise of Complex Life (0.6 billion years ago to present)
600 million years ago is the first time a modern human would be able to stand on Earth’s surface without suffocating from low oxygen or getting burned by UV light because of a lack of a substantial ozone layer. In the most recent 600 million years of Earth’s history, Earth’s climate has been more dynamic than in the boring billion but less so than in the snowball-hothouse periods. Sea levels have swung hundreds of meters both above and below current levels. The atmosphere’s oxygen content varied between 15% and 33% (it is currently 21%).
The high oxygen content of the atmosphere enabled the emergence of increasingly complex and energy-intensive life. A major biodiversity explosion occurred around 541 million years ago when life evolved the ability to make shells and other hard body parts out of minerals; many of today’s major phyla originated during this period. Life finally started to colonize the continents and changed them to their present-day green color. The first land plants were mosses; they were followed by spindly plants, leafy plants, trees, and finally flowering plants. The first vertebrate animals were fish and the first vertebrate land animals were fish that could walk on their fins and breathe atmospheric air.
The rich fossil record of this period shows five major mass extinctions, of which the Permian extinction of 251 Mya was the most severe. After the Permian, amphibians and reptiles competed for dominance, with reptiles eventually winning. Reptiles dominated Earth’s megafauna until a mass extinction 65 million years ago, after which mammals stepped up to fill many of the old niches left behind.
Earth’s Future
Earth will remain habitable for the foreseeable future. Its surface will be repeatedly reshaped through continental drift, volcanic activity, and impact events, just as it always has. There will inevitably be more mass extinctions, but biodiversity should recover after each one.
The biggest long-term threat to life on Earth is the sun’s slowly-increasing luminosity. Between one and two billion years from now, the hot sun will have evaporated enough of Earth’s oceans for the water vapor to cause a runaway greenhouse effect. UV light will dissociate atmospheric water and the hydrogen atoms will be lost to space in the solar wind. Two billion years from now, Earth will have lost its water and will become a desert planet, just as Mars did several billion years ago. Complex life will go extinct but microbes could still potentially survive for a long time in specific places.
Five billion years from now, the sun will have exhausted its hydrogen fuel and will swell into a much larger and cooler red giant. Earth’s fate in this scenario is unclear. It may end up engulfed by the expanding sun and be vaporized instantly. Alternatively, it may end up outside the red giant sun, orbiting it as a scorched cinder.