Father Earth

Introduction

This document gives a brief description of how our planet was formed and in turn, how it formed humanity. The text is a kind of crash course into "Earth sciences", centered about geology, ecology and biology, but also touches on astronomy, chemistry, anthropology, economy and others.
It is called "Father Earth" because Earth as it turns out, is not much like a mother gently tending to her children. Instead, it is acting more like a stern father who whips them into adulthood. Read on to see if you agree with that viewpoint or not.

The birth of a planet

And there was light

A long, long time ago, more than 4.5 billion years in the past, at the fringes of a galaxy called the Milky Way, a sparse cloud of gases and dust and ice started to condense. Most of it consisted of simple hydrogen atoms and a little helium, but there were other elements too, some even clustered together in molecules or even small clumps of matter. All the tiny particles exerted a little bit of gravitational force on each other. Not much; in fact very little, but enough to slowly pull the loose stuff together. The denser the jumble got, the stronger the gravitational pull became. As the cloud had inherited some spin from the galaxy it formed in, it gradually flattened into a disc, with a bulge in the center where the spin was (and still is) the weakest. As in that same center gravity is the strongest, the most matter condensed there. The middle gathered so much matter that it became dense rather than sparse, compressing the matter strongly and building up tremendous pressure and heat. So much, that a process called nuclear fusion started up. In basic nuclear fusion hydrogen atoms fuse together into helium. The resulting helium atoms have a tiny bit less mass than the hydrogen atoms that formed them. The missing mass is transformed into energy via Einstein's famous formula E=mc² and that energy is pushed outwards as heat and radiation. This created an outward pressure that compensated the inward pull of the gravity. The result was a stable, radiating object that we call a star. This particular one has since received a name: we call it the "Sun".
The Sun is the engine for life on the planet. It continually showers us with light and heat, driving the climate, plant photosynthesis and recently some of our industry directly too. Without the Sun, Earth would be a cold and dark ball of rock and ice, much like asteroids and comets further out in the solar system.

A planet is born

Meanwhile, in the outer regions of the spinning gas disc, things were also happening. Here too, matter was condensing into chunks by a process called "accretion", i.e. pulling the stuff together by gravity. Because they were not in the center, these concentrations of matter gathered less and remained smaller. These bodies never accumulated enough mass to start nuclear fusion. Instead they became planets, asteroids and comets. Because of their size, they wielded enough gravity to scoop up loose matter in their path, "vacuuming" the interplanetary space. The remaining gas and dust and ice was taken on by the "solar wind", which is not wind but radiation from the sun. This blasted out most of the debris to the outer fringes of the solar system, the Kuiper belt and the Oort cloud. The inner planets got the most of this "wind", stripping them of most of the light elements like hydrogen and helium and leaving them the heavier ones, so they became solid "rock" planets. Further away from the sun the solar wind is weaker, so there planets managed to retain these elements and grow into giant "gas giants" and "ice giants".
But planets like we see them today were not yet around in the young solar system. First there were "proto-planets", smaller and more numerous and with wild and chaotic orbits. They frequently crashed into each other, breaking each other up but more often, because of the effects of gravity, combining into larger entities, until they finally became the planets and moons we know now. One of the inner planets, Earth, was probably formed 4.54 billion years ago, estimated in just 10 - 20 million years.
Earth's position is close to optimal for life. Too close to the Sun temperatures soar too high and a planet gets too hot, like Venus, where the surface temperature is around 750 degrees Kelvin. Too far away it is too cold, like on the frozen moons of Jupiter and beyond. Between the extremes there is band that is called the "habitable zone", where liquid water, an essential prerequisite for life as we know it, can exist for prolonged periods. Our Earth just happens to be inside that zone.
Not only is the position right, but also the size. Smaller planets like Mars do not have enough gravity, inner heat and magnetism to retain oceans and a thick atmosphere, sooner or later turning into desert worlds. Larger planets accumulate too much water, becoming ice worlds like Jupiter's moon Europa, or even gas giants like Jupiter itself.

The Moon

Earth's axial tilt, image from Wikicommons

A recent but solid theory states that our planet, only some tens of millions of years after its creation, was hit by another protoplanet called Theia. The impact knocked off a large part of the planet mass and liquefied the rest. Some of the matter that was shattered spun off into space, some fell back onto Earth and some formed another planetary body which has accompanied us ever since: the Moon. The Moon is an anomaly in the solar system. Most moons are many times smaller than their host planet, but ours is quite large, big enough to be a planet on its own if had not crashed into Earth. When it had just formed, it was much closer to the Earth than now and appeared as a massive disc over the horizon. Gravitational friction has caused it to gradually move away to the roughly 400,000 kilometers that it is now distant from us. Over time it has become tidally locked with Earth, which means that it rotates just as fast along its own axis as it does around the Earth, so the same side of it is always turned our way. This tidal lock does not mean that tidal effects themselves are gone too. The Moon exerts a strong tidal force on the Earth, which can be seen in the Earth's seas. It even stretches out the Earth's lithosphere a bit when it passes overhead.
More important than tidal forces are two other effects of the Moon. The first is that when Theia slammed into Earth, it knocked the planet over, tilting its rotation axis 23 degrees towards the plane in which it travels around the Sun. As a result, the northern hemisphere gets more sunlight in the summer and less in the winter and on the southern hemisphere it is the other way around. This causes the seasons, which force life on Earth to continually adapt to different weather and may have been crucial in driving evolution. The second effect is that the presence of a relatively heavy moon orbiting close not only caused that 23 degrees axial tilt, but also keeps it stable. It prevents the plane from "wobbling". Compare our situation to for example the planet Uranus, which has an axial tilt of 98 degrees, so it is "lying flat", roughly putting one half of that planet's surface in the eternal dark and the other in continual light. If Uranus would be as close to the Sun as Earth, one side would be frozen and the other fried. Without our Moon our axial tilt would probably oscillate between 20 and 25 degrees, but possibly much more, totally screwing up our seasons.

All things die

An artist's impression of Venusian weather

Like living beings, planets, stars and even the universe have a beginning and an end. The fate of the universe falls outside the scope of this document, but our own star is relevant because the life of the Earth is so tightly bound to it. The Sun is what astronomers call a "yellow dwarf", which means that it is just a little smaller and lighter than the average star. Which is fortunate for us, as neither a large nor a small star is a good host for life bearing planets.
A star that is very large is also very heavy. All that weight causes a very strong gravitational pull, which in turn prompts very intense nuclear fusion. So intense, that large stars "burn up" all their hydrogen in a relatively short time, sometimes no more than a few million years. When hydrogen runs out, they start fusing helium into carbon and oxygen and these into heavier elements, up to iron. After that nuclear fusion costs more energy than it yields, so such a star cannot keep up its outward pressure. It implodes and then, under the enormous strain that that creates, explodes in what is called a supernova, blasting everything in the neighborhood. If our Sun would have been a heavy star, it would have blown up before life on Earth had gotten underway.
The sun could also have been a red dwarf, smaller, lighter, longer living and more common in the universe. That would not have been a good host either. In order to receive enough light and warmth from the star, Earth would have needed to be much closer to it. That would increase the tidal forces between planet and star and just like with Earth and Moon our planet would soon have become tidal locked with the sun, ending up in the theoretical Uranus-close-to-the-sun scenario described above.
Yet our sun is just the right size, a yellow dwarf with a life expectancy of about 10 billion years, which means we are not even halfway down the road. In about 5.4 billion years the Sun will start running out of hydrogen, though because it is not so heavy as giant stars, it will not go supernova. Instead it will swell up to a "red giant", then alternately shrink and expand while heavier elements like helium are burned up. When all the fuel is depleted, it will shrink to "white dwarf" star that shines for many trillion years more, but only very very weakly. By that time Earth may have been swallowed up during the red giant phase, or still orbit the Sun. Either way, it will first have been fried, then chilled and ended up uninhabitable.
The Sun as life bringer has even stricter limits than those set by its 10 billion years lifespan. In astronomical terms the Sun is currently on the "main sequence", a long period of stability between its birth and death. Stability does not mean that it does not change. The slow depletion of hydrogen is steadily changing the balance of the Sun's inner physics. When it was created, the sun had only 70% of its current brightness. This is still puzzling scientists, as a fainter sun in the early years should have led to a colder Earth, but there is strong evidence that that was not the case. The most popular explanation is that our planet had more greenhouse gases in the atmosphere in the past, which trapped heat. The Sun's brightness is still slowly increasing. In about 1.1 billion years from now the brightness will be at 110% of current level and over 4.9 billion years at 140%. Here is where the trouble comes. The 10% increase in solar brightness in 1.1 billion years time is enough to make our planet uninhabitable. Even if we could strip the Earth of all the heat-trapping greenhouse gases and turn off the heat from the inside of the planet, it would not be enough to counter the then deadly rays of the Sun. So by then either we should have left, or will be cooked.

Hadean

The first geological eon of Earth's history is called the Hadean. It lasted about 600 - 700 million years from the planet's formation. The name "Hadean" derives from Hades, the Greek underworld. While the mythological Greek counterpart is rather dark and dull, the Hadean eon was one hellish turmoil. Earth was a ball of liquid stone, metals and other elements, flowing and bubbling all the time. At first there was no distcinction between lithosphere (rock), hydrosphere (oceans) and atmosphere (air).
Gradually things settled down and the Earth got its current structure. In the center a solid inner and a liquid outer core of mostly iron (Fe) formed. Because the planet is spinning around its axis, this causes an electromagnetic field. If the Earth would not rotate or have negligible iron, our modern compasses would not work. More importantly, the electromagnetic field shields us from the most of the solar wind. Without it, that would strip away most of our atmopshere, something that has happened to Mars.
The planetary core is surrounded by a thick mantle of molten rock and topped by a thin crust of solid rock. That solid crust, which nowadays covers the entire planet, gradually formed during the Hadean, starting off as small patches, drifting on a boiling sea of lava. Over time it came to cover the entire surface of the planet. The crust is now between 5 and 50 kilometers thick, while the mantle is nearly 2,900 kilometers thick.
Meanwhile the solar system was still full of all kinds of small objects swinging around. Many of them hammered into the planet as meteorites, the largest of them obliterating parts of the surface that had just formed. The proto-planet Theia, mentioned earlier, was so large that it destroyed the entire shape of the planet and flung its formation a couple of million years backwards. This does not mean that their attacks were baneful to our young planet. For example, it is estimated that meteorites brought in a sizeable part of the Earth's current water supply, after much water had been lost during the early turmoil. Because initially the crust was not complete, oceans could not form and the atmosphere was a haze of gases, water vapor and even vaporized rock.

Plate tectonics

Earth's tectonic plates; Winkel Tripel projection map

At the end of the Hadean the Earth had somewhat quieted down. There was a thin but complete crust, steam had settled into oceans of liquid water and more volatile elements formed an atmosphere. The planet was still more turbulent than nowadays, with much higher levels of volcanic activity and meteor showers. Since then, it has steadily become calmer, but never completely dormant. This supports a unique geology which has proven vital in the development of life. Despite more than 4.5 billion years of gradually leaking warmth to the cold emptiness of space, the temperature of the Earth's core is estimated to be still around 6,000 degrees Kelvin. About 20% of all the heat energy is a residue from the original planetary accretion, when kinetic energy from impacts with other solar bodies was converted into warmth. Continuous gravitational friction with the Moon also contributes a small part. But the most important source of energy is the slow but steady radioactive decay of minerals inside the planet, especially from Potassium, Uranium and Thorium.
This energy keeps the interior of the planet hot, the rocks molten and therefore flowing. Over hundreds of millions of years plumes of hot magma rise up from below, spill against the crust, lose part of their heat and then sink back down to pick up new energy. These convection movements drag the crust horizontally, i.e. in line with the surface of the Earth, with them and break it into several "plates". In some places, especially mid-ocean ridges, these are drawn apart. In the fissures in between magma wells up and condenses into new crust. In other places they are dragged to smash into each other. At these collisions one plate moves over the other, pushing it back into the mantle where it melts back into magma. This process is called "subduction". At the surface of subduction zones the crust crumples, is pushed upwards and forms mountain ranges. This continuous moving and reshaping of tectonic plates causes disasters like earthquakes and volcanic eruptions, but is vital to life on Earth, as we shall see.
The situation on Earth is quite different from other planets and planetoids in the solar system. For example on Mars there is one single plate with one volcano (Olympus Mons) where the planet periodically blows off heat. Because Mars is much smaller than Earth it has cooled off much faster and now has a much lower internal temperature. Accordingly, Mars mantle convection is not strong enough anymore to drive any plate tectonics. On our own Moon, which is relatively large but even smaller than Mars, there is not even a single volcano.

Archean

The next eon in Earth's history is called the Archean. During this time, the first continents formed, though they were much smaller than today. With planets having swept the solar system mostly clean of debris, the heavy meteorite rains subsided substantially. Once the basics were available and the surface was not too violent anymore, life appeared very quickly. Early life used water, minerals and possibly geothermic energy in simple biochemical processes. The first life forms, archea and bacteria, were prokaryotes, i.e. consisting of a single cell. They were nothing like the oxygen-breathing animals we see around us today. That would have been impossible, because there was no free oxygen (O₂) in the Earth's atmosphere at the time, which was a mix of mainly nitrogen (N₂), carbon dioxide (CO₂) and water vapor (H₂O). Instead, the archea and bacteria were "chemotrophs" that munched on simple chemicals like hydrogen (H₂), hydrogen sulfide (H₂S), ammonia (NH₃) and ferrous iron (FeO). The microbes altered the chemical balance of the Earth's atmosphere, lowering the level of carbon dioxide but increasing the level of methane (CH₄). The system had an inherent stability. Whenever one species ate too much food they produced chemicals that others could eat in turn. Eating basic chemicals called for simple, slow lives, something which early prokaryotes were well suited for.
This could have gone on for a long time, but near the end of the Archean, about 2.7 billion years ago, a fresh phylum of bacteria called "cyanobacteria" invented a new way of living. They tapped into a thus far unused source of energy: sunlight. Using a process called "photosynthesis", they started to combine water and carbon dioxide into sugars. This yielded more energy to frolic around with.
A byproduct, no more than waste from the viewpoint of the cyanobacteria, was oxygen, an aggressive chemical element. It bound quickly with iron and other chemically reducing elements, creating mostly ferrous (FeO) and ferric (Fe₂O₃) iron, or in everyday language: rust. Over several hundreds millions of years these "banded iron formations" steadily built up, creating sources of iron that are being mined in our time.
Another side-effect of photosynthesis was the preservation of water, which is vital for life on Earth. Among the molecules that the free oxygen from the cyanobacteria bound to, was hydrogen. Molecular hydrogen is very light and easily escapes into space. Had nothing happened, Earth would gradually have lost its hydrogen and with it its source of water. That would have meant less weathering, thus more carbon dioxide in the atmosphere and possibly ending up like Venus: dry, hot and barren. But instead microbes bound it to oxygen in water, to carbon in methane, to sulfide in hydrogen sulfide and other more complex molecules. These molecules are a lot heavier and more easily trapped on Earth.

Proterozoic

The Proterozoic started about 2.5 billion years ago and lasted until 542 million years ago. At the start of this eon, unbound surface iron started to run out. With no more iron left to bind it, the level of free oxygen in the atmosphere rose up for the first time in Earth's history. About 2.4 - 2.2 billion years ago the level was 2%, ten times less than today. The oxygen reacted with methane, one of the major greenhouse gases that trap heat in the Earth's atmosphere and removed it from the air. As a result, an "oxygen crisis" occurred, in which the planet cooled. So much, that it plunged right into the first ice age, the Huronian. This period of freezing lasted a whopping 300 million years, the longest ice age the Earth has ever known.
This was a heavy blow for life on Earth, pushing it back into the margins. It is thought that some microbes survived in places where conditions were still favorable, with warmth, liquid water and minerals: "black smoker" thermal vents under the ocean, ice-free regions in the tropics and patches of water underneath the glaciers.
Eventually it was plate tectonics that came to the rescue. Volcanoes continued to spill out lava into the atmosphere, but also gases like hydrogen sulfide and carbon dioxide. This may have been accelerated near the end of the Huronian by the breakup of the supercontinent Kenorland (see below). The greenhouse gases trapped heat like they did before and because there was hardly any life left to stop them, the planet warmed up again. Of course then life also picked up and started to produce oxygen anew, but this time things stayed more or less in balance. One balancing factor was that, though oxygen was poison to the oldest life forms, some new ones evolved that actually started to consume it. By now Earth had two whole new branches of life that lived in shallow waters: the photosynthesizing cyanobacteria that thrived on sunlight and carbon dioxide, and others that ate the oxygen. Meanwhile the oldest anaerobic life forms were not gone. They "went underground", in anoxic areas in the deep ocean and underground, where they still live today.
All the oxygen-related changes paved the way for further stages in the evolution of life. Not all oxygen molecules in the air ended up as two-atom ones. Some had three; these are called ozone (O₃). Ozone high up in the atmosphere started (and still acts today) as a shield against the most destructive ultraviolet radiation from the sun. The shielding allowed for more complex life forms. These were the "eukaryotes", or multi-cellular organisms. The oldest of these were simply colonies of prokaryotes living close together, but gradually different parts of the colony started taking on different functions, diversifying the whole into true organisms. The first eukaryotes may have appeared already in the Archean, but it was not until after the Huronian ice age that they started to proliferate.
At the end of the Proterozoic things were changing once more. Somewhere around 1.2 billion years ago life, which was once again thriving, achieved a new milestone: sex. Sexual reproduction allows mixing genes, greatly accelerating mutations and evolution. This was one of the factors that allowed some microbes to adapt to dry conditions and to colonize the land, which until then had consisted of barren rock.

Wilson cycle

Mantle plumes, image by Helge Gonnermann

As explained earlier, the continuous slow movements of hot plumes of magma in the Earth's mantle drives the plate tectonics. Over time, the movements tend to drive the various smaller continents together into a single landmass, a "super-continent". But these then start acting as a kind of lid over the hot magma plumes in the mantle. Sooner or later the plumes melt down parts of the crust and then break through, tearing the supercontinent apart again. The whole process is repetitive, so that the continents oscillate between the joined and fragmented states.
This geological dance is very slow; one full cycle is estimated to take 300 - 500 million years. The youngest supercontinent was Pangea, which lasted roughly from 300 to 200 million years ago. The number of its predecessors is still a matter of debate among geologists, as it is not sure when the movements became cyclic. The recurring reformation of the edges of continents has destroyed virtually all evidence of the shapes and sizes of early continents. According to one theory that there have been several: Pannotia, 600 - 540 million years ago; Rodinia, 1250 - 750 million years ago; Columbia 1.8 - 1.5 billion years ago; Kenorland 2.7 - 2.1 billion years ago; Ur 3 billion years ago and first of all Vaalbara 3.6 - 2.8 billion years ago. Another theory states that there has been but a single supercontinent before Pangea, from 2.7 billion to 600 million years ago. Whether there were two supercontinents or more, it is certain that the size of the landmass on Earth has been growing slowly through time. Vaalbara and Ur are estimated at about half the size of modern Australia, though being the only continents on the planet in their time.
The arrangement of the continents has profound effects on Earth's climate. When a supercontinent is present, most oceanic crust is relatively old, cool and low, causing low sea levels. But when the continents are dispersed, the ocean floor tends to be young, less cool and higher, causing high sea levels. The difference in planet wide sea level can be as large as several hundred meters. On an Earth with a single supercontinent the climate is generally cooler and dryer, while when the continents are dispersed heat flows more easily, leading to a warmer and wetter world.
The recycling of continents is not just something of the past. The current plates are still moving. The Atlantic ocean is still widening; the Pacific shrinking; Africa, India and Australia are pushing north against Eurasia; eastern Africa and the western USA are shearing apart. The breakup will run its course and afterwards the continents will gobble together again, forming a new supercontinent in about 250 million years.

Snapshots of the layout of continents in the past and the "near" future: http://www.scotese.com/earth.htm

More cycles

The composition of our atmosphere is variable. Carbon dioxide is removed from the atmosphere by a process called carbonation, a form of weathering. As water rains down from the clouds it absorbs the carbon dioxide into carbonic acid, which binds with minerals on the surface. Over time, the carbon is washed down by rivers and streams to seas and lakes, where it settles into sediments and eventually becomes part of the Earth's crust. This is not an one-way process, as the plate tectonics described earlier overtime cause volcanoes to spill the gas back into the atmosphere. The rate of weathering is increased when the temperatures rise, because this causes more water to evaporate from the oceans and consequently also more water to rain down. But carbon dioxide is a powerful greenhouse gas that allows more heat from the Sun to pour in than to radiate back out, effectively keeping the planet warm. So higher temperatures mean increased weathering, less carbon dioxide and therefore lower temperatures. This is an example of what scientists call a "negative feedback cycle", which dampens extremes and has a natural tendency to converge on a stable level. Whenever temperatures run high or low on Earth, the weathering phenomenon is a long term guarantee to get the planet back to normal. Life, since it appeared in the Archean, has provided us with other feedback cycles.
So there exist several processes which stabilize the atmosphere, the oceans and the temperature on Earth. But they are all slow long-term processes, spanning millions or even billions of years. In the short term, variations can occur.

The Cryogenian ice age

When the supercontinent Rodinia started to break up, about 850 million years ago, the level of free oxygen rose again, to around 10%. As before, this prompted a severe ice age. This second ice age, the Cryogenian, like the Huronian, lasted hundreds of millions of years, to 635 million years ago. Some scientists think that during that time the entire planet may have been covered in ice, turning it into "Snowball Earth". Others estimate that a small zone around the equator remained ice-free, hence "Slushball Earth".
Like before, it was volcanoes that broke the spell. They slowly but steadily pumped more and more carbon dioxide into the atmosphere. On a warm Earth, weathering acts a brake on volcanic carbon dioxide release. But on an Earth covered in snow, there is but very little rain and likewise far less weathering. This allows the atmospheric carbon dioxide to build up. Eventually it reached a very high level: 13%, which is 350 times more than today, for Snowball Earth; or somewhat less for Slushball. This warmed the planet so much that the ice started to melt again.
Surface ice has a high albedo: is white and reflects most of the sunlight that falls onto it. But where it is gone, land and sea are exposed, which have a low albedo, reflecting much less light and absorbing more. So melting ice warms up the planet even more. This is a "positive feedback cycle", the counterpart of the negative feedback cycle mentioned earlier. As a result, ice ages can end abruptly, melting worldwide sheets of ice of kilometers thick maybe in as little as 1000 years.

Earth scientists on the Snowball Earth theory: http://www.snowballearth.org/

Paleozoic and Mesozoic

The Cambrian explosion

The Proterozoic eon ended with the Ediacaran period. Though geologically and formally, the Ediacaran period belongs to that older eon, biologically it is more like the Cambrian period that followed it. The Ediacaran lasted from 635 to 542 million years ago, the Cambrian from 541 to 485 million years ago.
In the middle of these two geological periods there was a 70 - 80 million year explosion of life that biologists call the "Cambrian explosion". During this time, the rate of evolution accelerated by an order of magnitude. Complex, multicellular lifeforms evolved, which spread through the seas and also onto the land. First, in the late Ediacaran, the Ediacaran biota sprung into life. The first arthropods appeared, but there were also stranger lifeforms, many of which are hard to classify as either plant or animal. The Ediacarans were suddenly wiped out at the end of the period, for reasons that are still obscure. But they were immediately replaced by others, who became the predecessors of all modern lifeforms. The most common in the Cambrian were the trilobites, remains of whom can be found all over the Earth. These animals evolved shells and eyes, while early fish were the first vertebrates.
This biological spurt seems to have been caused partly by geological events and partly by life itself. The melting of all the ice of the previous ice age flushed a lot of phosphorus into the ocean, which combined with the plenty of carbon dioxide provide the perfected breeding ground for cyanobacteria. These started a massive photosynthesis boom that replaced most of the carbon dioxide with oxygen again. The fluctuations between oxygen-rich and -poor and between heat and cold seem to have accelerated biological evolution. More and more advanced multicellular lifeforms appeared and also took from the seas to the land. The higher level of oxygen in the air played a double part in this. First, early lifeforms lacked a complex circulatory system, which meant that they had trouble supplying the center of their body with enough fresh air. A larger concentration of oxygen in the air was a boon that allowed larger organisms to evolve without making an evolutionary leap. Second, higher levels of oxygen also meant higher levels of ozone high up in the atmosphere. This strengthened the natural "radiation shield", reducing the harmful ultraviolet radiation to which multicellular lifeforms are especially susceptible.

The horn of plenty

A swamp from the Carboniferous, image by Mary Parrish

During the geological periods known as the Ordovician, Silurian, Devonian, Carboniferous and Permian, all part of the Cambrian, life expanded everywhere. About 470 million years ago the first plants ventured onto land, turning the barren rocks green. It took them 60 million years more to develop stems and roots. 30 million years later the first trees evolved and 10 million years after that seed-producing plants. Soon the land was covered everywhere with forest.
This created a legacy which 2½ centuries ago was essential to fuel the Industrial Revolution: coal. Coal consists of the remains of dead plants, compressed by the weight of later layers of Earth on top of it. Much of today's coal was formed during the Carboniferous, which is named after it.
Meanwhile animals evolved too. The first insects appeared 400 million years ago, not long after plants had conquered the land. Conditions for them were very favorable. The exuberant plant growth had increased the oxygen level to about 35%, compared to 21% in our time. The oxygen plenty allowed insects to grow very larg; some ancient dragonflies had wingspans of 2/3 meters. 40 million years later the first vertebrates followed them: the amphibians. Like insects, arthropods and amphibians also grew to giant sizes. Monster creepers up to 10 meters long crawled and slithered the Earth. But it was not to last.

More extinctions

During the Phanerozoic eon, life evolved steadily but was frequently set back by climate changes and smaller and larger disasters. Some of these were devastating, wiping out the majority of animal and plant species, the worst up to 75% - 85%. The three largest were the Ordovician-Silurian extinction event between 450 and 440 million years ago; the Permian-Triassic extinction event at the end of the Paleozoic, 252 million years ago; the Triassic-Jurassic extinction event 201 million years ago. The first two corresponded with new ice ages: the Andean-Saharan and the Karoo respectively. The causes of all these disasters are a bit unclear; probably they were triggered by a combination of factors.
One possible cause is the impact of a meteorite. This is hard to verify, as the resulting damaged Earth crust has long since been recycled in plate tectonics. Meteorite strikes are described in more detail below in the section on the Chicxulub meteorite, which is recent enough to verify.
Another candidate is volcanism. Mantle plumes bursting through continents in massive flood basalt eruptions increased the level of carbon dioxide, causing global warming, or spilled out dust clouds, leading to temporary cooling.
A third possible cause is the sudden heavy release of methane, previously locked up in ocean beds or permafrost, by warming or geological movements. Methane is a powerful greenhouse gas that could have caused global warming. It can also have made the deep oceans anoxic, killing off deep marine life. This in turn may have boosted sulfate-reducing bacteria, caused a massive release of hydrogen sulfide, weakened the ozone layer and thus causing an increase of deadly radiation.
Finally there are purely geological effects. Variations in the rates of volcanism on one side and weathering on the other can have affected the levels of carbon dioxide and oxygen in the atmosphere. The movement of continents may have removed shallow seas, where most of the marine life thrives.
None of all the causes listed above explains the sudden and sometimes nearly total extinctions that can be seen in the fossil records. Hopefully in the near future scientists will reach an understanding of how all the processes interact and why they can sometimes lead to such sudden extinctions, because we know they were real.

Pangea

Gradually, over the course of most of the Phanerozoic eon, all loose continents once more clustered into the single big landmass called Pangea, surrounded by a vast ocean called Panthalassa. The process was complete when the last independent continents clumped together around 300 million years ago. Conditions for life on land changed dramatically. Much of the land got so far removed from the coast that rainclouds seldom reached it, turning it into desert. Amphibians, which need water to reproduce in, were reduced to the fringes. In response, 310 million years ago the first reptiles appeared. These laid eggs with hard shells that could withstand drought and this enabled them to rule the land. Pangea was not only dryer, but also colder than the smaller continents that had formed it. So the plants adapted too. About 10 million years after the reptiles the first conifers evolved, which have several adaptations to cope with cold and snow.
The Permian-Triassic extinction wiped out many species, but the evolutionary advantages that had been developed were not lost. Small, adaptable species survived and started to evolve again, diversifying to fill all the niches of life once more. After about 20 million years some had evolved to giant size. Of course they where reptiles, more specifically archosaurs: the dinosaurs. They ruled the Earth for 165 million years in biodiversity and numbers (if the hordes of insects and smaller creatures are omitted), especially after the Triassic-Jurassic extinction event 201 million years ago. The dinosaurs and other reptiles were not the only vertebrate animals on land. Amphibians were still around and new classes evolved and occupied small niches left by the reptiles: mammals around 190 million years ago and birds around 150 million years ago. In the plant world, there was a revolution 140 million years ago when angiosperms evolved. They rapidly conquered the Earth, pushing other plants into the margins.

The Tethys ocean

When Pangea started to break up around 200 million years ago, the first rift was between the north, called Laurasia and the south, called Gondwana. In between these two flowed a new ocean, the Tethys. For much of its existence and for much of its area it was rather shallow, just like other early oceans that were starting to form between the fragments of Pangea. As sea levels were high, large parts of continents like proto-Europe were flooded, also creating shallow seas. The Tethys itself was centered around the equator where the climate was warm and sunny, but other waters were warm too. During a brief period in the mid-Cretaceous temperatures worldwide were the highest of the entire Phanerozoic, on average 10° Kelvin above modern level, with temperate forests covering the poles. All these factors combined created a perfect habitat for marine life. Much of this consisted of shellfish that lived and died in great numbers. When a shellfish dies, its shell, full of calcium carbonate (CaCO₃) alias chalk, usually sinks to the ocean floor en eventually ends up in the sediments. So many shells are in the layers deposited in the Cretaceous that the period was named after them. Like with coal, the remains of Cretaceous life benefit us still. Not only chalk was deposited in massive amounts, but also organic material. Since the Cretaceous the weight of later sediments have pressed this together to create petroleum and natural gas, which are heavily being exploited in our time. It is no coincidence that the richest fields of oil and gas are found right where once the shallow seas of the Cretaceous lay.

Grass

Towards the end of the Cretaceous there was another revolution in plant life, one that may have looked rather insignificant at the time, but that ultimately enabled human civilization. It was the evolution of grasses. From the appearance of the first trees 380 million years ago, forests had dominated all the fertile land, covering the continents almost entirely under a blanket of green, shrouding the undergrowth in darkness. Grasses changed that by adopting a different strategy. While trees grow slow and tall, grasses stay small but grow quickly. Trees were occasionally destroyed by fires started by lightning or volcanoes. When that happened, grasses could repopulate the ash wastes much faster than trees could and often established dominance over them. In some areas, the fact that dry grass is highly flammable caused a frequent recurrence of fires. This killed young trees and grass alike, but of course the grass again recovered much faster. They could also grow in dry and cold areas too harsh for trees.
The result was a more varied landscape, with forests here and grass plains there. Some animal species adapted to the new terrain. They evolved into specialized grass-grazers, which did not require great leaps of evolution. Nonetheless during this period the seeds of human civilization were sown because the remote ancestors of our dearest plant and animal companions were created: grains and horses.

The Chicxulub meteorite

An artist's impression of the Chicxulub impact

At the crossing between the Cretaceous and Paleogene periods, 66 million years ago, life on Earth was reminded that the interplanetary space outside the atmosphere is not entirely empty. A meteorite crashed into Earth at what is now the Yucatan peninsula in Mexico. It released 100 teratons of TNT of energy, 2 million times more than the most powerful human-made nuclear bomb and good enough for a crater nearly 200 kilometers wide. The impact caused earthquakes of magnitude greater than 11, forest fires and megatsunami's that travelled all the way around the planet. But worse, it threw up massive amounts of dust, which dimmed the sunlight, causing acid rain and a global winter that lasted many years. This caused the fifth large mass extinction in Earth's history, known as the Cretaceous-Paleogene extinction event.
It is often stated that this was the meteorite impact that killed the dinosaurs and paved the way for the mammals. But there are other theories that say that the dinosaurs were already in decline and the meteorite was just one in a series of events. Whatever the exact causes, the dinosaurs were finished off. As the largest animals, often at the top of the food chain, they were the most vulnerable to ecological disasters. The event killed not just the dinosaurs, but 75% of all species on the planet. Like at other extinctions, the smallest and most adaptable ones survived, including many mammals and birds, so the statement above is partly right.
Fortunately, impacts as large as Chicxulub are rare, about once every 100 million years. Measurements have shown a direct inverse correlation between the impact of meteorite strikes and the frequency at which they occur. In other words: if an impact is X times heavier, it will occur X times less often, or if Y times lighter, then Y times more often. Indeed, evidence of the more frequent lesser impacts abounds, up and into human history, for example the Tunguska explosion in 1908, estimated between 3 - 30 megatons TNT and as recently as 2013, the 460 kiloton TNT asteroid crashing near Chelyabinsk.

The Cenozoic

Mountains

Meteorites shake up the planet occasionally; plate tectonics continually. The latter are vital for the landscape that we have come to know and depend upon. Without the constant friction of the plates, wind and especially water would gradually erode the higher parts of the Earth's surface away and eventually create a planet wide flat plain, topped by a layer of water several kilometers deep. Earth would be an ocean world without any dry land. Thanks to the planet's inner energy, that is not the case. Instead we have a varied landscape.
But the movements of the plates do more. They bring up minerals from deep within the mantle, for example metals that are vital to our industry; "rare earth minerals", used in electronics; phosphorus for living organisms; precious metals, making up money; etcetera. Some minerals like diamonds are created by plate tectonics themselves, in "cratons", at the bottom of continental plates where they meet the mantle.
But mountains also play a key role in weather, drinking water supply and agriculture. In some areas like Tibet north of the Himalayas and the western Andes, mountains block rainclouds and create dry deserts. On the other sides, India and the Amazon rainforest, water pours down in large quantities, creating fertile areas. It is not just water that allows agriculture, but also the minerals that are washed from the mountains and deposited in fertile silt.
Mountain ranges have varying ages. The Rocky Mountains, Andes, Atlas and all mountain ranges across southern Eurasia are the result of recent subduction processes. The Appalachian and Ural Mountains are examples of older mountains that where once as high as the Himalayas today, but have since eroded. Eventually they will completely flatten, but by then plate tectonics will have created new ones elsewhere.

Humans arrive

Monkeys have been around since the start of the Cenozoic. Hominidae, alias great apes, evolved 15 million years ago; apes walking on two legs like Australopithecus 4 million years ago; our own biological genus Homo 2.5 million years ago and our own species Homo sapiens just 200,000 years ago. Humans originated in Africa, at a time that that continent was (and still is) centered around the equator, so relatively warm. That is one of the reasons we could develop a unique trait in the animal kingdom: (mostly) hairless skins and the ability to sweat to lose heat.
We stayed in Africa for several 10,000 years before migrating to Asia and beyond and colonized the planet in bounds and leaps: South Asia maybe 100,000 years ago; Australia and Europe 50,000 - 40,000 years ago; North Asia 25,000 years ago; America 15,000 - 12,000 years ago; New Zealand only 1,500 years ago.
Of course we learned to produce clothing first before venturing into the colder areas. Clothing is a typical human solution to problems we stumble upon in our environment. Humans make tools; our hands and brains are for a large geared to inventing and making tools. The fact that we walk upright and have such large brains may be a result from our strategy of tool-making. They allowed us to make the (partial) transition from scavengers to hunters. The energy-rich meat in turn allowed our gut to become shorter and helped the development of our brain.
Throughout history, we have used resources from the Earth to make our tools. We made knives out of flint stone, spears and campfires out of wood, clothing out of animal skins and fur.

The ice ages

The first two ice ages mentioned in this document, the Huronian and the Cryogenian, were triggered by rises of the level of oxygen in the atmosphere. The other two, the Andean-Saharan and the Karoo, are more vague, but probably caused by changes in the layout of continents. More recently a fifth ice age has dawned, usually simply called the "current ice age". This is the ice age that spans the whole Quarternary geological period, from 2.58 million years ago to the present day. Most people speak of an ice age when great glaciers cover large parts of the planet, but scientists call these "glacial periods" alias "glaciations". They consider the entire Quarternary, both glaciations and warmer "interglacials" in between, a single ice age, as even the interglacials are relatively cold when compared with earlier periods of Earth's climate.
Like the third and fourth ice age, the main cause of the fifth also seems to be the changes in the layout of the continents, specifically the breakup of Pangea. Shortly after Pangea tore apart, an ocean called the Tethys appeared, that spanned the entire planet, roughly around the equator. This allowed warm water to circulate easily and heat up colder areas. The effect did not only distribute warmth more evenly, but also warmed the planet as a whole. But recent events have killed the Tethys. 40 - 50 million years ago India joined with Asia and closed the east, later Africa and Arabia the center and about 3 million years ago the two Americas joined up. Some continents benefitted from these changes. Europe for instance got a steady supply of southerly warmth from the newly created Atlantic Gulfstream. But the North pole got surrounded by land on most sides, cutting it off from the major part of ocean flows and it cooled. On the southern hemisphere a new planet-wide ocean current formed, around Antarctica, which had become isolated. Due to the difference in latitude, this had the opposite effect from the Tethys: it cooled, rather than warmed, making the South pole the coldest place on today's Earth.
All these changes have brought worldwide average temperature down and caused the Quarternary ice age. There have been many glaciations in the current ice age. The last one, the Würm, started 110,000 years ago and ended 11,700 years ago, though the Fennoscandian ice sheet lingered on until 8,500 years ago. We now live in the Holocene geological epoch, that started at the end of the last glaciation, which means we are currently in an interglacial. It will not last forever, as the greater ice age still goes on. It is estimated that if levels of greenhouse gases stay low, we may be in for a new glaciation in 10,000 - 15,000 years time. If greenhouse cover is increased, for instance by human industrial activity, we may be able to fend it off up to 50,000 years, though this way we may well suffer the reverse, severe global warming, in the meantime.

The Neolithic

The Holocene climate

During the Holocene the Earth's climate has been quite stable, which probably has been a key factor in the development of modern civilization. So much, that our ancestors thought and many modern people still do, that climate is an unchangeable thing. But it turns out that climate, even with the Holocene, is not static. Climatologists divide the Holocene into five "chronozones": the Preboreal (10,000 - 9,000 years ago); the Boreal (9,000 - 8,000 years ago); the Atlantic (7,500 - 5,000 years ago); the Subboreal (5,000 - 2,500 years ago) and the Subatlantic (2,500 years ago - present). There were marked differences among these periods. For example the Atlantic was a period that is sometimes called the "Holocene maximum". During this period the climate was warmer and wetter than today, so much that the Sahara was a savanna rather than a desert.
And even within the chronozones there have been fluctuations, sometimes lasting only a few centuries or even decades. These have all had impact on humanity. It was a period of drought, combined with over-farming, that brought down the Maya empire around 1000 CE. Vikings could probably not have colonized Greenland in the Medieval Warm Period if the climate had not been warmer than today. As recently as from 1550 CE to 1860 CE, the Little Ice Age tempered population growth and can still be seen in paintings that depict many white winter scenes. The most recent change is the steep warming caused by the exhaust of greenhosue gases in the industrial world. Climate drives weather and weather affects humanity strongly. It can yield good or bad harvests, can dampen or enhance the spread of epidemics, make or break military campaigns and cause extreme phenomena like hurricanes.

Agriculture

When the ice sheets retreated, Earth got warmer and wetter. Humans had been experimenting with agriculture before, possibly tens of thousands of years ago. Because of the cold and dry climate these attempts had never really yielded much. But in the Holocene conditions were right and agriculture, together with animal husbandry, took off. It did so first in the Middle East, in Mesopotamia to be exact. That was the place because conditions were ideal: a mild climate, varied terrain and a position right in the middle of three large continents: Asia, Africa and Europe, where all kinds of species could mix, mingle and evolve. This is why the area is rich in plant and animal species, including some of the most nutritious and/or domesticatable: wheat, sheep, goats and cattle. Agriculture quickly spread to fertile neighboring flood plains: the Nile in Egypt and the Indus in the east. After some millennia of refinement it spread further west and east. Spread to other latitudes was slower because the crops were slow to adapt to different climates. In areas where there existed strong barriers, this severely hampered the evolution of domesticated species and with it, the development of civilization. Prime examples are the Saharan desert and rainforests of Africa, which blocked off South Africa, and the narrow land bridge between the two American continents. While some areas, especially South America with its maize and potatoes, had a lot to offer, the narrow selection of crops and draft animals made the Americans lag behind in the cultural arms-race. Only when Europeans started to sail over the oceans, did the center and the fringes start to exchange influences.

The Anthropocene

During the ice ages, humanity was limited to a life of hunting and gathering. We may have been top predator, were intelligent and had developed a culture, but ecologically we made no more impact than other animals. That started to change when the ice sheets retreated back to the poles and we came up with agriculture. Within a few thousand years the first cities sprung up. We built roads, dug canals and raised monumental buildings like the Egyptian Pyramids and the Great Wall of China, which can even been seen from space. Still, all that building had little impact on the planet.
What did have an impact was the ways in which we gathered and gather food. Despite alternative explanations, it seems that is was human hunters who dealt the death blow to mega fauna in America, Australia and New Zealand. Native American Indians frequently burned large areas of forest to create space for grazing animals on which they fed. Farmers burned forests too and replaced them with farmland. As a result, forests have yielded in all but the more inaccessible areas. We have even claimed land from the sea, for instance in the Dutch polders.
Still humanity's impact on the surface of the Earth was limited. Things really started to change in the 18th century CE, when the Industrial revolution started. Some scientists call it the beginning of the "anthropocene", the age of humans, though others extend it back to the Atlantic or even the advent of agriculture. Since the Industrial revolution we have started to change the ecosystem by damming rivers, cutting down forests on a massive scale, increasing the amount of greenhouse gases and exploiting natural resources wherever we could find them. Our meddling with nature is often ill-balanced. Changes like pollution with chemicals, silting of land due to over-farming, erosion as a result of deforestation have turned entire landscapes into deserts and brought down whole civilizations. Biodiversity is decreasing so much that some biologists speak of a sixth extinction event. Humanity is hitting Earth so hard that the ecosystem that we depend upon may break down under our pressure and bring us down with it. Earth itself will not seriously be harmed; there is almost nothing that a few tens of millions years of untampered natural processes cannot undo. But it is doubtful if humanity has so much patience.