As far as we know, there is no place like home. Our planet, a rocky world that's 92 million miles from a pretty standard star in a spiral galaxy called the Milky Way, is so far the only world known to have the exact, perfect conditions for the development of life forms advanced enough to give it a name: Earth.
How did it get here? How did it develop into the ideal environment for life to form? Where did its water come from? Its atmosphere? The volatiles that would eventually form the proteins that became the building blocks of all life as we know it? Why didn't its rocky siblings, Venus and Mars follow the same path?
What do we know about Earth's ultimate fate?
The history of our planet is a fascinating one, and given how it remains unique among the planets of the universe—for now, at least—knowing how it came to be is an important part of searching for other planets like our own around other stars, and possibly even for alien life itself.
In the beginning
It may have all started with a molecular cloud of swirling gas and dust, known as a solar nebula. About 4.6 billion years ago, something disturbed this molecular cloud enough that it obtained angular momentum, most likely a shockwave from a nearby supernova.
This spinning motion gradually flattened the cloud into a disk, and material in the center of the cloud (or at least the point of greatest density) built up enough pressure from its own gravity that it was able to ignite nuclear fusion and become the sun.
The sun would continue to accumulate material, eventually accreting 99 percent of the molecular cloud into itself. The remaining material in its gravitation influence formed an accretion disk around it, stretching beyond the orbit of Neptune and into what is now the Kuiper Belt.
As the radiation and solar winds from the new star pushed lighter elements, such as hydrogen and helium, further out into the disk, it left behind heavier material that then started to gather together in ever-growing globs that would start to exert their own gravitational influence. Farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants.
As these growing bodies orbited the sun, they would start to clear their orbits by accreting material in their paths to form rings. In one of these rings, the early Earth was forming into a proper planet.
Hadean eon: 4.5 to 4 billion years ago
The early Earth was something of a hellscape, so much so that it inspired this era's name, the Hadean, after Hades.
The interior of the planet retained nearly all of the heat of its formation, which was a series of constant collisions with smaller bodies. It had cooled enough, however, that elements and minerals were able to start to stratify by density and had an outer layer of minerals that had cooled enough to crystalize and solidify, forming into a mostly solid crust around the planet.
This period is also characterized by hundreds of millions of years of bombardment from the material in its ring path, as well as material coming from outside of it that was shot inward by the massive, fast-growing gas giants like Jupiter.
These likely included a large number of icy comets, which may have provided the water ice that would eventually form Earth's oceans, for which there is evidence even as far back as the Hadean.
The earliest rocks we've been able to date come from the tail end of this era, about 4 billion years ago, and it is sometime during this era that the Moon may have formed after the early Earth collided with a smaller protoplanet that has been given the name Theia.
Moon rocks collected during the Apollo missions were dated back to roughly 4.5 billion years ago, meaning the Moon may have formed less than 100 million years after the solar system itself was formed.
The surface of the Earth at this time would have been partially molten, inhospitably hot, and with an initial atmosphere of hydrogen and helium, accreted from the original material of the molecular cloud during its formation from the sun's accretion disk.
This atmosphere would not have lasted long though, as the solar winds would have carried it away, as Earth had not yet developed a magnetic field.
It is possible that the earliest forms of life may have formed at the tail end of this era around thermal vents within the Earth's early crust, but the surface would have been way too inhospitable for life at this point.
Archean eon: 4 to 2.5 billion years ago
The Archean Era is defined by several major changes from the Hadean era.
First, the end of the Late Heavy Bombardment period. As the gas giants formed in the outer solar system, their gravitational influence would have sent asteroids and comets into the inner solar system with increasing regularity, so that the tail end of this period might have seen more impact events on Earth than earlier in its life.
Since the temperature further out into space is considerably lower than it is around the inner planets, many of the objects hitting Earth during this late period would have had a large volume of water ice, which could only have formed farther away from the sun in the outer solar system.
Though unquestionably violent, this era is likely where Earth acquired its oceans, which would be key to another of the major changes from the Hadean eon: the emergence of life.
The earliest forms of life on Earth, the prokaryotes, were single-celled organisms belonging to the Archaea domain and lacked a cellular nucleus. The earliest of these date to around 3.7 billion years ago, and would have blanketed the floors of the early oceans in microbial mats. There is also evidence of bacteria developing at this time as well.
By about 3.2 billion years ago, there is fossil evidence for microbial life on land, so the Earth's surface would have to have cooled enough by this time for these organisms to survive.
These early forms of life would likely have been RNA-derived, meaning that they both encoded their genetic material and replicated using RNA, rather than DNA. Because of this, it wasn't possible to encode stable genomes, as offspring could have radically different genetic sequences than its parent cell.
This would lead to a biosphere that was made up entirely of individual organisms but no speciation. Eventually, more complex DNA formed and began to allow for more stable genetic transfer between generations. The last universal common ancestor to all life on Earth can be traced back to this era, approximately 3.5 billion years ago.
This would have been a single-celled organism that replicated using DNA, but likely lacked defined organelles or a nucleus. It is also likely that rather than a single cell, a population of such cells developed through gene transfer and together became the original "species" from which all life descended.
The other significant change during the Archean period was the emergence of plate tectonics. The intense heat of the mantle beneath the Earth's crust would have produced convective currents beneath parts of the crust.
The mantle's temperature at this time was about three times as hot as it is today, which would have led to faster-moving convection currents in the mantle and the breakup of the early crust into many more, smaller plates than the fewer, larger plates that we have today.
The process was thought to have begun toward the tail end of the Archean, but recent evidence shows that it might have begun by about 3.2 billion years ago, roughly halfway through the eon.
Finally, the Archean is the latest that Earth developed its magnetic field, with solid evidence that it had formed 3.2 billion years ago, though there is some evidence that it may have formed a billion years earlier in the late Hadean.
Proterozoic eon: 2.5 billion to 542 million years ago
The key defining feature of the Proterozoic was the creation of an oxygen atmosphere.
Early forms of life during the Archean were anaerobic, meaning they did not require oxygen to produce food. During the proterozoic period though, single-cellular organisms started using photosynthesis for food production.
This process took carbon dioxide, water, and sunlight and converted them into oxygen and energy, in the form of sugars, similar to the way plants do today. Back during the Proterozoic, instead of planets, you would have had vast microbial mats similar to blue-green algae all around the Earth converting the carbon dioxide-rich atmosphere into food and producing oxygen as a byproduct.
Until this time, there was little, if any, oxygen in the atmosphere, which would have been full of greenhouse gases like carbon dioxide from two eons of volcanism. This meant that the conversion of the atmosphere from one rich in carbon dioxide to one rich in oxygen took a very long time, from about 2.4 to 2 billion years ago.
In the end, though, these first photo-synthesizing organisms got the job done, and by the end of the Proterozoic, there was more than enough oxygen in the atmosphere to support aerobic lifeforms. On the other hand, oxygen was toxic to these early forms of life, leading to the extinction of nearly all life on the planet.
Another consequence of the oxygenation of the atmosphere was the creation of an ozone layer in the atmosphere. As oxygen rose into the upper atmosphere, it absorbed UV radiation from the sun, which split O2 molecules into individual oxygen atoms.
Free oxygen atoms are highly reactive and so bond quickly to the surrounding O2 to form ozone (O3). UV radiation would then hit these O3 molecules, creating O2 and free oxygen atoms, starting the process all over again. This kept a significant amount of UV radiation from hitting the surface, allowing organisms to thrive in shallow water, and even on land.
Eukaryotes, single-celled organisms with defined nuclei, developed sometime between 2.7 and 1.6 billion years ago, and initially are thought to not have been capable of metabolizing oxygen. At some point after this, distinct mitochondria and chloroplasts were introduced to these cells, likely as a form of a symbiotic association of aerobic bacteria and cyanobacteria with the ancestors of eukaryotes. This allowed the eukaryotes to use the oxygen in the atmosphere, though when exactly this occurred is still debated.
The other major change in the Proterozoic was the development of larger tectonic plates. As the Earth's mantle cooled, the speed of the mantle's convection currents slowed. The many small plates started to consolidate into larger plates to form supercontinents like Columbia and Rodinia.
It is also during this eon that the Earth is believed to have experienced the first of its Snowball Earth glaciation periods, about 700 to 600 million years ago. According to the theory, during this period, massive glaciers extended from the poles all the way to the equator.
Evidence for glaciation as far as the equator comes mostly from the geological record, which shows compelling evidence of glacial deposits into sea sediment that would have been located along the equator at the time of its formation between 700 and 600 million years ago.
The extreme conditions of the Snowball Earth period would have challenged life on the planet to adapt or die, prompting the evolution of more complex organisms that were better able to endure the extreme conditions.
This adaptation towards more complexity set the stage for the most important event in Earth's history, at least as far as life is concerned.
Phanerozoic eon: 542 million years ago to present day
The Phanerozoic is the current geological eon and began with the so-called Cambrian Explosion. This is the period when nearly all animal phyla suddenly—in geological terms, at least—appeared in the fossil record.
Whether this is because animal life developed hard shells that were preserved in the fossil record or something prompted rapid speciation over a period of about 25 million years is still hotly debated. Regardless of its cause, it appears that nearly all complex life emerged during the Phanerozoic.
The period also saw the last of the supercontinents, Pangaea, split up around 250 million years ago during the Triassic period and begin to drift apart to form the continents as they exist today.
This eon also saw several transitions between greenhouse Earth and icehouse Earth conditions. Some of these transitions are characterized by mass extinction events, like the Late Ordovician mass extinction, wherein about 85 percent of marine life on Earth died off and oxygen levels plummeted.
Despite the fluctuations in its biosphere, Earth's geology was fairly stable during this period, all things considered. Although there was constant (in geological terms) swinging between tropical, hothouse climates and major ice ages, nothing as extreme as a full-on Snowball Earth took place during this time.
This doesn't mean that the planet wasn't still scarred by the occasional asteroid impact, the most famous of which brought an end to the Cretaceous period 66 million years ago. This impact event killed off 75 percent of all life on Earth, including all non-avian dinosaurs.
However, this also led to the rise of mammalian life on the planet, and eventually to the evolution of primates and Homo sapiens, which is where we find ourselves today.
What's in store for Earth in the future?
While volcanism, climate changes, continental drift, and extraplanetary factors will surely continue to influence how Earth develops, it is difficult to predict many of these events, but there are some things that we do know for sure.
In about 250 million years from now, all of the continents will have merged back into a supercontinent and eventually split apart again about 250 million years after that.
We also know that the sun's luminosity increases about 6-10 percent every billion years, so in 1 billion years, the sun will be significantly hotter than it is today. This will lead to a breakdown in carbon dioxide in the atmosphere, which will suffocate nearly all plant life on Earth.
Without plants to produce oxygen, the oxygen level in our atmosphere will plummet, leading to the mass extinction of all animal life on the planet, returning Earth to an Archaean-like state where the only life that remains is anaerobic microbes.
Those microbes will also be fighting for the last remaining pools of water left on the planet since the increased temperature will also boil away the Earth's oceans. Any remaining water will have to exist in small pockets at the poles or in underground aquifers.
At the latest, by around 1.85 billion years from now, all life on Earth will have died. At about the 2 billion year mark, the Earth's molten outer core might have cooled enough to weaken Earth's magnetic field. Without its protective magnetosphere, the intensifying solar winds will strip away most of Earth's atmosphere in short order.
At this point, Earth's average temperature will be about 300°F, so assuming there is any life on Earth in subterranean caves, it will be killed off and Earth will be fully sterilized of any life.
Beyond 3 billion years, Earth will resemble its early Hadean era, if not more extreme, and will remain like this until about 5.4 billion years from now, when the sun burns up hydrogen fuel and begins its transformation into a red giant.
There is a possibility that the forces involved in that transformation might push Earth's orbit further back toward Mars, but if not, the planet will end its life here as it is swallowed up by the expanding sun.
Assuming it does get pushed out to a "safer" orbit, it will be orbiting within a fraction of an AU from the surface of the red giant sun, which will still be hot enough to melt the mostly silicate crust of the Earth into a molten state.
This molten material will eventually bleed off into space, leaving behind Earth's iron core. After the sun sheds its helium outer layer about a billion and a half years after turning into a red giant, it will become a white dwarf and the remnant of Earth's orbit will decay until it eventually falls into the white dwarf many tens of billions of years later.
There are other possible ends for the Earth, including being gravitationally disrupted by a passing star and being shot out into interstellar space as a rogue planet, but this isn't as likely. The fate of our planet is almost certainly to fall into the sun, either when it is a red giant or when it is a white dwarf, though we will be long gone by then.
Even if humanity escapes the confines of the planet before the sun ends all life on Earth, human beings will have evolved into something else entirely or died out as countless other species have done before. Nothing lasts forever, after all, not even the Earth.