THE HISTORY OF THE SOLAR SYSTEM
First it is necessary to understand the new story of the solar system. The planets, comets, and other bodies orbiting the sun were born, some 4.5 billion years ago, from a spinning disk of dust and gas called the solar nebula. It was long assumed that things formed more or less where they orbit now – nine planets orbiting in well-determined orbits like clockwork, forever in the past, and forever in the future.
But when dust flakes from a comet were captured, a new theory was needed to explain the metals found in the dust. That comet had theoretically resided outside of Neptune’s orbit for its entire life, but exotic minerals, like tungsten and titanium nitride, that could only have been formed near the newborn sun in temperatures of more than 3000 F were in the dust. Some violent process must have thrown them into the outer solar system. The solar system had literally turned itself inside out at one time.
Since Newton, it has been recognized that the planets must interact with one another. Their gravitational tuggings are far weaker than those of the sun, but over time, they affect the paths of neighbors. As a result, there is no such thing as a circular orbit.
Pluto provided the first evidence of interplanetary interaction. The oddball of the solar system, it dips far above and below the pancake-like plane in which the eight planets travel; it swoops on an elongated orbit that takes it from 30-50 times the Earth’s distance from the sun. But the most curious thing about Pluto is its bond with Neptune. It’s called resonance: For every three times that Neptune orbits the sun, Pluto orbits twice, and in such a way that the bodies never approach each other.
A theory has been proven to explain how that synchrony could have evolved. When the solar system was young and full of asteroids and comets, Neptune was closer to the sun. If one of those bodies approached Neptune, the planet’s powerful gravity would either fling the comet or meteor closer to the sun or out of the solar system entirely, in a cosmic version of crack the whip. Because action begets reaction, Neptune’s orbit would shift a tiny bit too. The effect of trillions of such interactions compelled Neptune to migrate away from the sun. That led it to “capture” Pluto, which was already farther out and sweep it into gravitational lockstep.
Further proof was provided by the Kuiper belt, a dark region extending far beyond Neptune. Telescopes have unveiled bunches of Plutinos – icy dwarf worlds that have the same two-to-three resonance with Neptune. That could only have happened if Neptune had advanced toward the Kuiper belt like a gravitational snowplow, piling up dwarf planets into new orbits.
The planets had not condensed gently from the solar nebula; instead they had grown to full size by absorbing planetesimals – rocky asteroids, icy comets, and larger objects – that smashed into them at high speed. One theory suggests that the moon coalesced from the spray of molten rock that was blasted into orbit when a body the size of Mars collided with Earth. All this happened in the first 100 million years of formation of the solar system.
Another enigma was revealed in the Kuiper Belt. Besides Plutinos, it was littered with bodies on wildly different orbits. Some were grouped in a flat disk, some in a puffy doughnut-shaped cloud; others in orbits even more crazily eccentric and elongated. The smooth outward migration of Neptune should have not strewn debris so widely. Hundred of planets around other stars have now been detected. Some are in tightly bunched orbits, much closer together than the planets in our solar system. Some are Jupiter- or Neptune-size worlds that race on insanely hot orbits close to their suns. Others loop deep into space on weird trajectories – on average the orbits of extrasolar planets are more eccentric than those in our solar system. Some planets even float freely in interstellar space. That is not what would be expected from planets born in a spinning disk around a star and staying quietly in their birthplace. That process should produce well-spaced near-circular orbits.
It is predicted that the solar systems history was anything but smooth – that it had somehow endured a “global gravitational instability.” It was proposed that the solar system’s four giant planets –Jupiter, Saturn, Uranus, and Neptune – had started out closely packed together, in nearly circular orbits, next to the sun. 4.4 billion years ago, Jupiter was closest, then Saturn, and Uranus may have been outside Neptune. Early on they were embedded in the disk-shaped solar nebula, which was still full of icy and rocky debris. As the planets absorbed those planetesimals or flung them away after close encounters, they cleared gaps in the disk. Their orbits slowly shifted as they cleared away this debris of comets and asteroids. A dense comet belt lingered beyond Uranus.
Because the planets were also tugging on one another, the whole system was fragile – almost infinitely chaotic. The most powerful gravitational link was between Jupiter and Saturn, the two biggest planets and a yank on them could jolt the whole system. When the solar system was about 500-700 million years old, 3.8 billion years ago, as the planets interacted with planetesimals, their own orbits shifted. Jupiter moved slightly inward; Saturn moved slightly outward, as did Uranus and Neptune. Everything happened slowly – until at a certain point, Saturn was completing exactly one orbit for every two of Jupiter’s. That was the tipping point. That one-to-two resonance wasn’t stable like the one between Neptune and Pluto; it was a brief vigorous yank. As Jupiter and Saturn approached each other repeatedly at the same point in their orbits, those near circular orbits were stretched into the ellipses we see today. That soon ended the precise resonance, and Jupiter’s gravity pumped Saturn closer to Uranus and Neptune. This drove them into the comet belt, flinging comets all over – including at Earth. Those two planets hurtled violently outward and may have even swapped places.
The planets grew to full size by absorbing rival planet embryos in a series of titanic collisions – one of which probably gave Earth its moon. A Mars-size protoplanet struck Earth, vaporizing itself and part of Earth’s rocky mantle. Rocky debris blasted into orbit coalesced into a moon – or maybe two – in less than a century. Most of the incoming protoplanet’s iron sank into Earth’s core, so the moon is less dense than Earth. Lunar gravity raised a tidal bulge on Earth; its spinning in turn accelerates the moon, causing it to spiral slowly outward. A sister moon, about a third as wide, orbited at a distance. Within tens of millions of years, the moon reeled in its sister. Splatting onto the moon’s far side, it creates a highlands there – a striking contrast to the low plains, called maria, on the side we see.
As Uranus and Neptune plowed through zones of the solar system that were still full of icy planetesimals, they triggered a devastating cascade. Ice balls were catapulted in all directions. The giant planets captured a few as oddly orbiting moons. Many objects were scattered into the Kuiper Belt. An untold number – perhaps a trillion – were banished even farther to the Oort cloud, a vast cocoon of comets reaching halfway to the next star. A lot of comets were also hurled into the inner solar system, where they crashed into planets or fell apart in the heat of the sun.
The Kuiper belt is a region of the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System’s formation. Although some asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed “ices”), such as methane, ammonia and water. The classical belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, are also believed to have originated in the region.
Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets’ true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.
Pluto is the largest known member of the Kuiper belt, and one of the two largest known TNOs, together with scattered disc object Eris. Originally considered a planet, Pluto’s status as part of the Kuiper belt caused it to be reclassified as a “dwarf planet” in 2006. It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as “plutinos”, that share the same 2:3 resonance with Neptune.
Meanwhile the giant-planet migrations also disrupted the rocky asteroid belt between Jupiter and Mars. Scattering asteroids joined comets from farther out to create the Late Heavy Bombardment. Our moon suffered badly then and earlier in its history: Its entire crust was deeply fractured. Earth would have caught even more flack, but shifting tectonic plates have erased the craters. Any early life could only have survived deep underground. The Late Heavy Bombardment lasted less than 100 million years.
Once Uranus and Neptune swept up most of the comets from their new orbits (and also switch places), the Late Heavy Bombardment ends. The four giant planets settle into their current, slightly elliptical orbits.
When an asteroid slams into Earth, tiny droplets of molten rock are lofted high into the atmosphere, and they later rain out as solid, glassy beads called spherules Deposits of spherules from the six-mile-wide asteroid that hit the Yucatan some 65 million years ago, wiping out the dinosaurs, have been discovered all over the world. At least 12 comparable spherule beds have been found dating from 1.8-3.7 billion years ago. A now vanished inner rim of the asteroid belt, which shed asteroids for two billion years after Jupiter disturbed it. As many as 70 may have struck Earth, each comparable to the one that extinguished the dinosaurs. Vesta, at 300 miles across, is the third largest asteroid in the belt between Mars and Jupiter. It endured eons of impacts, and it is believed that chips off Vesta make up 6% of the meteorites that fall on Earth.
Solar system evolution is dynamic and violent. Ours is probably on the mild side compared to others. That mildness was needed in order to have a habitable planet.
New telescopes will expose far more objects in the Kuiper belt and maybe even in the Oort cloud. It could be littered with lots of planets the size of Earth or Mars.
It is likely that the four giant planets have finished wandering and will be on the same orbits five billion years form now, when the aging sun is expected to balloon outward and engulf the inner planets: Mercury, Venus, Earth and Mars. There is a 1% chance the solar system will go dramatically unstable in the next 5 billion years. The problem is a weird long-distance connection between Jupiter and Mercury. When Jupiter’s closest approach to the sun lines up with Mercury’s noticeably squashed orbit in just the right way, Jupiter exerts a slight but steady tug. Over billions of years this gives Mercury a 1-in-a-100 chance of crossing the orbit of Venus. There is a further 1-in-a-500 chance that if Mercury goes nuts, it will also perturb the orbit of Venus or Mars enough for one of them to hit Earth – or miss it by several thousand miles, which would be almost as bad. The entire Earth would get stretched and melted like taffy.
WHY EARTH HAS LIFE
Earth is one special planet. It has liquid water, plate tectonics, and an atmosphere that shelters it from the worst of the sun’s rays. But many scientists agree our planet’s most special feature might just be us.
It’s the only planet we know of that has life. Though other bodies in our solar system, such as Saturn’s moon Titan, seem like they could have once been hospitable to some form of life, and scientists still have hope of eventually digging up microbes beneath the surface of Mars, Earth is still the only world known to support life.
The closest star system to our own made headlines with the announcement that it hosts a planet about the mass of Earth — a tantalizing discovery so close, astronomically speaking, to us. While the newfound planet may be Earth-sized, researchers say it is almost certainly barren. Astronomers detected the alien world around the sunlike star Alpha Centauri B, which is a member of a three-star system only 4.3 light-years away from our solar system. This planet, known as Alpha Centauri Bb,is about as massive as Earth, but its hot surface may be covered with molten rock — its orbit takes it about 25 times closer to its star than Earth is from the sun.
None of this is a revelation, but understanding what’s special about Earth is crucial for finding other planets out there and predicting what they might be like.
The fact that Earth hosts not just life, but intelligent life, makes it doubly unique. From our anthropocentric viewpoint, we naturally separate ourselves from the planet that we live on, but if one adopts the point of view of an external observer, it is the ‘planet’ (taken as a whole) that has done these remarkable things.
Earth has not been chosen. Our existence is the result of countless random occurrences that combined to make what we are today. Though Earth has the necessary ingredients for life, it’s unclear whether the development of life here might have been a one-time fluke, or if it’s something that happens pretty much everywhere the conditions are right.
So what makes a world such as ours able to host life? Why is Earth so special?
There are a few key ingredients that scientists often agree are needed for life to exist — but much debate remains as to what limits there actually might be on life. Even Earth hosts some strange creatures that live in extreme environments.
First, you’d need some kind of liquid, any place where molecules can go react. In such a soup, the ingredients for life as we know it, such as DNA and proteins, can swim around and interact with each other to carry out the reactions needed for life to happen. The most common contender brought up for this solvent is the one life uses on Earth: water. Water is an excellent solvent, capable of dissolving many substances.
Water contains oxygen. It exists in all three states.
Water, as almost all other substances, contracts when cooled, but in contrast to virtually all other materials (there are very few exceptions, such as rubber and antimony), it contracts when cooled only until it reaches 4° Centigrade, then it amazingly, expands until it freezes. If water continued to contract when cooled, it would become heavier and thus sink to the bottom of the ocean. One result of this is that the ocean bottom would be extremely cold—and many fish would die. In time, more and more of the ocean would become ice as more froze on the surface, sank, and accumulated at the bottom.
Thus, for much of the Earth, the ice that forms in seas, oceans, and lakes stays near the surface where the sun and the warm water below melts it in the summer. Water that is warmer than 4°, being heavier, sinks to the bottom and warms the depth of the oceans. This process of surface water warming and sinking to the bottom, plus the Coriolis effect produces ocean currents. These currents, among other things, insure that most of the ocean stays in a liquid form.
No one knows why Earth has the exact amount of water it does, which is relatively small considering that water molecules outnumber silicate molecules in the galaxy. The Earth is remarkable for its precisely-tuned amount of water, not too much to cover the mountains, and not so little that it’s a dry desert, as are Mars and Venus, our ‘sister’ planets. Having the right amount of water produces relatively stable weather.
Water is unique in that it absorbs large amounts of heat without much alteration in its temperature. Its absorption speed is extremely rapid—about ten times as fast as steel. During the day, the seas rapidly soak up a great deal of heat, thus the Earth stays fairly cool. At night, the oceans release the vast amounts of heat that they soaked up during the day, which combined with atmospheric effects, keeps the surface from getting too cold at night. If it were not for the tremendous amount of water on the Earth, there would be far greater day and night temperature variations. Many parts of the surface would be hot enough to boil water in the day and the same part would be cold enough to freeze water at night. Water is an excellent temperature stabilizer. The large oceans on Earth are a vital part of our survival.
Of course, alien life may not play by the rules we’re used to on Earth.
Astrobiologists increasingly suggest looking beyond conventional habitable zones. For instance, while liquid water might not currently persist on the surface of Mars or Venus, there may have been a time when it did. Life might have evolved on their surfaces in that time, and then either fled to safer locales on those planets, such as underground, or adapted to the environment when it became harsh, much as so-called extremophile organisms have on Earth, or both.
Astronomers looking for extraterrestrial life most often focus on planets in the so-called habitable zones of their stars — orbits that are neither too hot nor too cold for liquid water to persist on the surfaces of those worlds. Earth happened to hit the Goldilocks mark, forming within the sun’s habitable zone. Mars and Venus lie outside it; if Earth’s orbit had been just a bit further inside or outside of where it is, life may likely never have arisen and the planet would be a cold desert like Mars or a cloudy furnace like Venus. Earth’s water is also special in that it has remained liquid for so long.
The extremely fine line between an environment where life can and cannot exist is illustrated by the fact that it is estimated that a one-degree temperature change in the average worldwide temperature would, in time, seriously affect life on the Earth, and a two-degree temperature change could be disastrous to life. The tolerances are extremely small, and if there are any other planets in the universe, it is unlikely that any of them could have life, due to the extremely rigid conditions necessary for life to exist. The right temperature to grow plants is necessary for photosynthesis, producing oxygen and using carbon dioxide.
It is important that the temperature does not go from one extreme to the other. Mercury can be anything form -200 C to +375 C when water would be only gas and the planet would be completely dry. Venus has a surface temperature of +480 C. Mars, although it can reach +25 C, it can be as cold as -140 C.
Our day length is important for photosynthesis, taking just 24 hours and each side receives light regularly. Venus takes 243 days to spin once on its axis
If evolution works to evolve life to fit the existing environments, why has it not equally conquered all of the environments here and elsewhere? Earth is far better suited for life than any other planet, yet most of the environments even here, are either too hot or too cold, too far underground or too far above ground to support much life. In the several thousands of miles of changing environments from the center of the Earth to the edge of its atmosphere, there are only a few feet of habitable environment, and therefore almost all creatures are forced to live there. Although only the Earth is inhabited in our solar system, even on the Earth only a thin slice is ideally suited for life.
This thin section, though, is teeming with life. It is estimated that an acre of typical farm soil, six inches deep, has several tons of living bacteria, almost a ton of fungi, two hundred pounds of one-cell protozoan animals, about one hundred pounds of yeast and the same amount of algae.
Some type of life is found in every niche on the Earth. From the top of the atmosphere to the bottom of the oceans, from the coldest part of the poles to the warmest part of the equator, life persists here.
If the Earth traveled much faster in its 292-million-mile-long orbit around the sun, centrifugal force would pull it away from the sun, and if too far, all life would cease to exist. If it traveled slightly slower, the Earth would move closer to the sun, and if it moved too close, all life would likewise perish. The Earth’s 365 day, 5-hour, 48-minute and 45.51-second-round-trip is accurate to a thousandth of a second! If the yearly average temperature on Earth rose or fell only a few degrees, most life on it would soon roast or freeze. This change would upset the water-ice and other balances, with disastrous results. If it rotated on its axis slower, all life would die in time, either by freezing at night because of lack of heat from the sun, or by burning during the day from too much sun.
3. Atmosphere – the miracle of air
Earth has a breathable atmosphere that contains the right amount of oxygen and carbon dioxide. Carbon dioxide makes up most of the atmosphere of Venus and Mars making them unable to support life. As the atmosphere is kept on the planet by gravity, Mars and Venus are too small to keep atmosphere – Venus has no atmosphere and Mars’ is very thin.
The atmosphere Is thick enough, and with the help of ozone, to prevent ionizing radiation from penetrating.
If our atmosphere were thinner, many of the millions of meteors which now are burned up would reach the Earth’s surface, causing death, destruction and fires everywhere.
On the land, the opposite of what happens in water, occurs. Air, after it is warmed, rises—and the air close to the surface of the Earth is heated via light energy from the sun. The air near the surface then rises upward. The result is that the air near the Earth’s surface maintains a temperature in which life can exist. If air acted the same way that water did, the temperature on the Earth’s surface would be unbearable—and life could not survive for very long. The temperature a few hundred feet above the surface, on the other hand, would be quite cold and, likewise, life could also not exist there. The only habitable region would be a thin slice of air, but even here life could not exist for long. Plants and trees, necessary to support the life in the atmosphere, could not survive as they would be in the cold zone. Thus birds would have no resting place, or food, water or oxygen. But air rises when heated and thus life can exist on the Earth.
The movement of warm air from the surface rising upward creates air currents (wind) which are an important part of the Earth’s ecological system. They carry away carbon dioxide from areas which overproduce, such as cities, and move oxygen to areas in need of it, as large urban population centers.
The mixture of gases usually found in the atmosphere, without man’s pollution, is perfect for life. If it were much different (more oxygen, less carbon dioxide, etc., or the atmospheric pressure was much lighter or heavier), life would cease to exist on Earth.
4. The Sun and energy
Life needs energy. Without energy, virtually nothing would happen.
The most obvious source of energy is a planet or moon’s host star, as is the case on Earth, where sunlight drives photosynthesis in plants. The nutrients created by photosynthesis in turn are what the bulk of life on Earth directly or indirectly relies on for fuel.
It is important how little variation there is in our sun’s radiation compared with more volatile stars. As stars go, our sun is relatively weak and cool. Compared to every other star that is visible, the earth is the weakest.
Of all the energy the sun gives off, only one billionth of its daily output is picked up by the Earth. The sun does provide the Earth with more than 130 trillion horse power each day, about fifty thousand horse power for each current resident. Even though there are likely several hundred billion galaxies in the universe, there is only one atom for every 88 gallons of space, which means most of the universe (the vast majority, actually) is empty space!
If the Earth was not tilted 23° on its axis, but was at a 90° angle in reference to the sun, we would not have four seasons. Without seasons, life would soon not be able to exist here—the poles would lie in eternal twilight, and water vapor from the oceans would be carried by the wind towards both the north and south, and would freeze when close enough to the poles. In time, huge continents of snow and ice would pile up in the polar regions, leaving most of the Earth a dry desert. Eventually the oceans would disappear and rainfall would cease. The accumulated weight of ice at the poles would cause the equator to bulge and, as a result, the rotation of the Earth would drastically change
Still, countless organisms on Earth subsist on other sources of energy as well, such as the chemicals from deep water vents. There may be no shortage of energy sources for life to live off.
Scientists have argued that habitable worlds need stars that can live at least several billion years, long enough for life to evolve, as was the case on Earth.
Some stars only live a few million years before dying. Still, life might originate very fast, so age is not that important. For instance, the Earth is about 4.6 billion years old. The oldest known organism first appeared on Earth about 3.5 billion years ago, meaning that life might conceivably evolve in 1.1 billion years or less. However, more complex forms of life did take longer to evolve — the first multicellular animals did not appear on Earth until about 600 million years ago. Because our sun is so long-lived, comparatively, higher orders of life, including humans, had time to evolve.
5. Plate tectonics and Recycling
Researchers have suggested that plate tectonics is vital for a world to host life — that is, a planet whose shell is broken up into plates that constantly move around.
Plate tectonics as essential in recycling the molecules life needs. For instance, carbon dioxide helps trap heat from the sun to keep Earth warm. This gas normally gets bound up in rocks over time, meaning the planet would eventually freeze. Plate tectonics helps ensure this rock gets dragged downward, where it melts, and this molten rock eventually releases this carbon dioxide gas back into the atmosphere through volcanoes. The fact that Earth has plate tectonics allows for the carbon-silicate cycle to operate over geological timescales. With the carbon-silicate cycle, the levels of carbon in the atmosphere get regulated to keep the surface temperature around that of liquid water.
The slip-sliding movements of Earth’s crust that are thought to have created the planet’s towering mountain ranges and plummeting ocean depths.
Plate tectonics is useful but probably not imperative. Volcanism might very well provide enough fresh supplies of whatever life might need.
Plate tectonics and water are inextricably linked. Not only does plate tectonics enable liquid water to exist by way of regulating the temperature, but many scientists have argued water enables plate tectonics to happen.
Without water the planet would be geologically dead. Water is what lubricates plate tectonics, which is what leads to the extreme difference between continents and seafloors, the large amount of earthquakes and volcanoes, fresh mountain-building. Venus has no water, no plate tectonics, no deep sea floor, no steep mountains, no continents, probably few earthquakes or volcanoes. A much less geologically interesting place!
6. Size of Earth
Another “just-right” aspect of Earth is its size: If it was much smaller, it wouldn’t be able to hold on to our precious atmosphere, but much larger and it might be a gas giant too hot for life. If the earth were larger than 5 times its present size, the weight of the atmosphere would crush us.
7. Presence of Jupiter
The presence of our big brother planet, Jupiter, farther out in the solar system blocking Earth from much of the incoming debris, has also helped Earth become a safe haven for life. Jupiter acts like a giant broom, sweeping the solar system of debris – rocks as small as cars and as huge as moons that could snuff out life in one fatal blow. This protective effect was particularly helpful in the solar system’s early years, when Earth still got pummeled but, scientists say, not nearly as bad as would have been the case without Jupiter.
8. A friendly moon
Life on Earth may also owe a debt to our nearest celestial neighbor, the moon.
Earth’s moon stabilizes our planet’s rotation, preventing drastic movements of the poles that could cause massive changes in climate that some scientists think could have doomed any chance for budding life to form or evolve. The moon is just the right size and distance. Our moon is the result of a collision many millions of years ago between our planet and a planetoid the size of Mars The earth was totally destroyed and that’s how we ended up with the moon and a big iron core at the center of the earth.
The moon also helpfully pulls the ocean’s tides, which scientists suggest might have been the perfect place for early life to begin evolving to survive on land.
9. A magnetic field. To have an iron core big enough to produce a magnetic field strong enough to protect us from the sun’s radiation and solar storms of charged particles. Violent bursts of radiation could have scoured life from Earth in its early, fragile stages.
10. Rare Earth
All of these features make Earth special among known planets near and far.
You hear all the time how Earth-like Mars is, but if you were taken to Mars you wouldn’t feel happy there at all. It’s not Earth-like. And Titan, when the [Huygens] probe landed, there was all this stuff in the media about how Earth-like it is. Earth-like? It is completely different. It has all this methane on the surface. Venus has about the same mass [as Earth], almost the same distance from the sun. But it’s a totally different place – no oceans, no plate tectonics and it’s not a place you would want to be.
So far, we haven’t seen any planet outside the solar system come very close to Earth either. Of the nearly 300 new worlds glimpsed elsewhere in the galaxy, most are “hot Jupiters” – large planets that orbit close to their stars, on which life and liquid water are unlikely to exist.
It is doubtful that in our galaxy typical stars have planets just like Earth around them. There are lots of planets in the galaxy that are somewhat similar to Earth, but the idea that this is a typical planet is nonsensical.
The extremely fine line between an environment where life can and cannot exist is illustrated by the fact that it is estimated that a one-degree temperature change in the average worldwide temperature would, in time, seriously affect life on the Earth, and a two-degree temperature change could be disastrous to life. The tolerances are extremely small, and if there are any other planets in the universe, it is unlikely that any of them could have life, due to the extremely rigid conditions necessary for life to exist.
As our planet-hunting technology improves, many planet hunters expect to find Earth’s twin. The search has led scientists to debate whether Earth is really as special as we think it is. There are two points of view.
The chances of a planet being just the right size, the proper distance away from the right star, etc., are extremely minute, even if many stars have planets circling them, as some speculate. The mathematical odds that all of these and other essential conditions happened by chance are astronomical—something like billions to one!
The optimists believe otherwise. In the past 10 years, everything has been pointing in the direction of the solar system, which we thought was unique, is not unique at all. Many scientists think it’s likely that some form of life exists on some of those countless other planets out there. It is almost certain that life is actually quite common. There may be literally billions of them in the galaxy.
EVOLUTIONARY HISTORY OF LIFE
The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on the planet first originated until the present day. Earth formed about 4.5 Ga (billion years ago) and life appeared on its surface within 1 billion years. The similarities between all present-day organisms indicate the presence of a common ancestor from which all known species have diverged through the process of evolution.
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them. The evolution of oxygenic photosynthesis, around 3.5 Ga, eventually led to the oxygenation of the atmosphere, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Bilateria, animals with a front and a back, appeared by 555 million years ago.
The earliest land plants date back to around 450 Ma (million years ago), although evidence suggests that algal scum formed on the land as early as 1.2 Ga. Land plants were so successful that they are thought to have contributed to the late Devonian extinction event. Invertebrate animals appear during the Ediacaran period, while vertebrates originated about 525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252.2 Ma. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates, displacing therapsids in the mid-Triassic; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the dinosaurs, mammals increased rapidly in size and diversity. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous (130 to 90 Ma) probably helped by coevolution with pollinating insects. Flowering plants and marine phytoplankton are still the dominant producers of organic matter. Social insects appeared around the same time as flowering plants. Although they occupy only small parts of the insect “family tree”, they now form over half the total mass of insects. Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over 6 Ma. Although early members of this lineage had chimpanzee-sized brains, there are signs of a steady increase in brain size after about 3 Ma.
TIMELINE OF THE EVOLUTIONARY HISTORY OF EARTH
Axis scale: millions of years ago.
Dates prior to 1000 million years ago are speculative.
This timeline of evolution of life represents current scientific theory outlining the major events in the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, and individual organisms and molecules such as DNA and proteins. The similarities between all present day organisms indicate the presence of a common ancestor from which all known species, living and extinct, have diverged through the process of evolution.
The dates given in this article are estimates based on scientific evidence.
In its 4.6 billion years circling the sun, the Earth has harbored an increasing diversity of life forms:
for the last 3.6 billion years, simple cells (prokaryotes);
for the last 3.4 billion years, cyanobacteria performing photosynthesis;
for the last 2 billion years, complex cells (eukaryotes);
for the last 1 billion years, multicellular life;
for the last 600 million years, simple animals;
for the last 550 million years, bilaterians, animals with a front and a back;
for the last 500 million years, fish and proto-amphibians;
for the last 475 million years, land plants;
for the last 400 million years, insects and seeds;
for the last 360 million years, amphibians;
for the last 300 million years, reptiles;
for the last 200 million years, mammals;
for the last 150 million years, birds;
for the last 130 million years, flowers;
for the last 60 million years, the primates,
for the last 20 million years, the family Hominidae (great apes);
for the last 2.5 million years, the genus Homo (human predecessors);
for the last 200,000 years, anatomically modern humans.
Periodic extinctions have temporarily reduced diversity, eliminating:
2.4 billion years ago, many obligate anaerobes, in the oxygen catastrophe;
252 million years ago, the trilobites, in the Permian–Triassic extinction event;
66 million years ago, the pterosaurs and nonavian dinosaurs, in the Cretaceous–Paleogene extinction event.
Dates are approximate.
In this timeline, Ma (for megaannum) means “million years ago”, ka (for kiloannum) means “thousand years ago”, and ya means “years ago”.
Hadean Eon 4000 Ma and earlier.
4600 Ma The planet Earth forms from the accretion disc revolving around the young Sun; complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.
4500 Ma According to the giant impact hypothesis the moon is formed when the planet Earth and the planet Theia collide, sending a very large number of moonlets into orbit around the young Earth which eventually coalesce to form the Moon. The gravitational pull of the new Moon stabilises the Earth’s fluctuating axis of rotation and sets up the conditions in which life formed.
Archean Eon 4000 Ma – 2500 Ma
4000 Ma Formation of Greenstone belt of the Acasta gneisses of the Great Slave Region, in Canada, the oldest rock belt in the world.
4100–3800 Ma Late Heavy Bombardment: extended barrage of impact events upon the inner planets by meteoroids. Thermal flux from widespread hydrothermal activity during the LHB may have been conducive to life’s emergence and early diversification.
3900–2500 Ma Cells resembling prokaryotes appear. These first organisms are chemoautotrophs: they use carbon dioxide as a carbon source and oxidize inorganic materials to extract energy. Later, prokaryotes evolve glycolysis, a set of chemical reactions that free the energy of organic molecules such as glucose and store it in the chemical bonds of ATP. Glycolysis (and ATP) continue to be used in almost all organisms, unchanged, to this day.
3800 Ma Formation of Greenstone belt of the Isua complex of the western Greenland Region, whose rocks show an isotope frequency suggestive of the presence of life.
3500 Ma Lifetime of the last universal ancestor; the split between bacteria and archaea occurs.
Bacteria develop primitive forms of photosynthesis which at first do not produce oxygen. These organisms generate ATP by exploiting a proton gradient, a mechanism still used in virtually all organisms.
3000 Ma Photosynthesizing cyanobacteria evolve; they use water as a reducing agent, thereby producing oxygen as waste product. The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere slowly rises, acting as a poison for many bacteria. The Moon is still very close to Earth and causes tides 1,000 feet (305 m) high. The Earth is continually wracked by hurricane-force winds. These extreme mixing influences are thought to stimulate evolutionary processes. (See Oxygen catastrophe). Life on land likely developed at this time.
Proterozoic Eon 2500 Ma – 542 Ma
2500 Ma Great Oxidation Event led by Cyanobacteria’s oxygenic photosynthesis. Commencement of plate tectonics with old marine crust dense enough to subduct.
2000 Ma Diversification and expansion of acritarchs.
By 1850 Ma Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages. The appearance of red beds show that an oxidising atmosphere had been produced. Incentives now favoured the spread of eukaryotic life.
1400 Ma Great increase in stromatolite diversity.
By 1200 Ma Meiosis and sexual reproduction are present in single-celled eukaryotes, and possibly in the common ancestor of all eukaryotes. Sex may even have arisen earlier in the RNA world. Sexual reproduction first appears in the fossil records; it may have increased the rate of evolution.
1200 Ma Simple multicellular organisms evolve, mostly consisting of cell colonies of limited complexity. First multicellular red algae evolve
1100 Ma Earliest dinoflagellates
1000 Ma First vaucherian algae (ex: Palaeovaucheria)
750 Ma First protozoa (ex: Melanocyrillium)
850–630 Ma A global glaciation may have occurred. Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution.
600 Ma The accumulation of atmospheric oxygen allows the formation of an ozone layer. Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land.
580–542 Ma The Ediacaran biota represent the first large, complex multicellular organisms — although their affinities remain a subject of debate.
580–500 Ma Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion.
560 Ma Earliest fungi
550 Ma First fossil evidence for ctenophora (comb jellies), porifera (sponges), and anthozoa (corals & anemones)
Phanerozoic Eon 542 Ma – present
The Phanerozoic Eon, literally the “period of well-displayed life”, marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, which are divided by major mass extinctions.
Paleozoic Era 542 Ma – 251.0 Ma
535 Ma Major diversification of living things in the oceans: chordates, arthropods (e.g. trilobites, crustaceans), echinoderms, mollusks, brachiopods, foraminifers and radiolarians, etc.
530 Ma The first known footprints on land date to 530 Ma, indicating that early animal explorations may have predated the development of terrestrial plants.
525 Ma Earliest graptolites.
510 Ma First cephalopods (Nautiloids) and chitons.
505 Ma Fossilization of the Burgess Shale.
485 Ma First vertebrates with true bones (jawless fishes).
450 Ma First complete conodonts and echinoids appear.
440 Ma First agnathan fishes: Heterostraci, Galeaspida, and Pituriaspida.
434 Ma The first primitive plants move onto land, having evolved from green algae living along the edges of lakes. They are accompanied by fungi, which may have aided the colonization of land through symbiosis.
420 Ma Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions.
410 Ma First signs of teeth in fish. Earliest nautiid nautiloids, lycophytes, and trimerophytes.
395 Ma First lichens, stoneworts. Earliest harvestman, mites, hexapods (springtails) and ammonoids. The first known tetrapod tracks on land.
363 Ma By the start of the Carboniferous Period, the Earth begins to be recognisable. Insects roamed the land and would soon take to the skies; sharks swam the oceans as top predators, and vegetation covered the land, with seed-bearing plants and forests soon to flourish.
Four-limbed tetrapods gradually gain adaptations which will help them occupy a terrestrial life-habit.
360 Ma First crabs and ferns. Land flora dominated by seed ferns.
350 Ma First large sharks, ratfishes, and hagfish.
340 Ma Diversification of amphibians.
330 Ma First amniote vertebrates (Paleothyris).
320 Ma Synapsids separate from sauropsids (reptiles) in late Carboniferous.
305 Ma Earliest diapsid reptiles (e.g. Petrolacosaurus).
280 Ma Earliest beetles, seed plants and conifers diversify while lepidodendrids and sphenopsids decrease. Terrestrial temnospondyl amphibians and pelycosaurs (e.g. Dimetrodon) diversify in species.
275 Ma Therapsids separate from synapsids.
251.4 Ma The Permian–Triassic extinction event eliminates over 90-95% of marine species. Terrestrial organisms were not as seriously affected as the marine biota. This “clearing of the slate” may have led to an ensuing diversification, but life on land took 30M years to completely recover.
Mesozoic Era From 251.4 Ma
The Mesozoic Marine Revolution begins: increasingly well adapted and diverse predators pressurize sessile marine groups; the “balance of power” in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
245 Ma Earliest ichthyosaurs.
240 Ma Increase in diversity of gomphodont cynodonts and rhynchosaurs.
225 Ma Earliest dinosaurs (prosauropods), first cardiid bivalves, diversity in cycads, bennettitaleans, and conifers. First teleost fishes. First mammals (Adelobasileus).
220 Ma Gymnosperm forests dominate the land; herbivores grow to huge sizes to accommodate the large guts necessary to digest the nutrient-poor plants., first flies and turtles (Odontochelys). First Coelophysoid dinosaurs
200 Ma The first accepted evidence for viruses that infect eukaryotic cells (at least, the group Geminiviridae) exists. Viruses are still poorly understood and may have arisen before “life” itself, or may be a more recent phenomenon.
Major extinctions in terrestrial vertebrates and large amphibians. Earliest examples of Ankylosaurian dinosaurs
195 Ma First pterosaurs with specialized feeding (Dorygnathus). First sauropod dinosaurs. Diversification in small, ornithischian dinosaurs: heterodontosaurids, fabrosaurids, and scelidosaurids.
190 Ma Pliosaurs appear in the fossil record. First lepidopteran insects (Archaeolepis), hermit crabs, modern starfish, irregular echinoids, corbulid bivalves, and tubulipore bryozoans. Extensive development of sponge reefs.
176 Ma First members of the Stegosauria group of dinosaurs
170 Ma Earliest salamanders, newts, cryptoclidid & elasmosaurid plesiosaurs, and cladotherian mammals. Sauropod dinosaurs diversify.
165 Ma First rays and glycymeridid bivalves.
161 Ma Ceratopsian dinosaurs appear in the fossil record (Yinlong)
155 Ma First blood-sucking insects (ceratopogonids), rudist bivalves, and cheilostome bryozoans. Archaeopteryx, a possible ancestor to the birds, appears in the fossil record, along with triconodontid and symmetrodont mammals. Diversity in stegosaurian and theropod dinosaurs.
130 Ma The rise of the Angiosperms: These flowering plants boast structures that attract insects and other animals to spread pollen. This innovation causes a major burst of animal evolution through co-evolution. First freshwater pelomedusid turtles.
120 Ma Oldest fossils of heterokonts, including both marine diatoms and silicoflagellates.
115 Ma First monotreme mammals.
110 Ma First hesperornithes, toothed diving birds. Earliest limopsid, verticordiid, and thyasirid bivalves.
106 Ma Spinosaurus, the largest theropod dinosaur, appears in the fossil record.
100 Ma Earliest bees.
90 Ma Extinction of ichthyosaurs. Earliest snakes and nuculanid bivalves. Large diversification in angiosperms: magnoliids, rosids, hamamelidids, monocots, and ginger. Earliest examples of ticks.
80 Ma First ants.
70 Ma Multituberculate mammals increase in diversity. First yoldiid bivalves.
68 Ma Tyrannosaurus, the largest terrestrial predator of North America appears in the fossil record. First species of Triceratops.
Cenozoic Era 66 Ma – present
66 Ma The Cretaceous–Paleogene extinction event eradicates about half of all animal species, including mosasaurs, pterosaurs, plesiosaurs, ammonites, belemnites, rudist and inoceramid bivalves, most planktic foraminifers, and all of the dinosaurs excluding their descendants, the birds.
From 66 Ma Rapid dominance of conifers and ginkgos in high latitudes, along with mammals becoming the dominant species. First psammobiid bivalves. Rapid diversification in ants.
63 Ma Evolution of the creodonts, an important group of carnivorous mammals.
60 Ma Diversification of large, flightless birds. Earliest true primates, along with the first semelid bivalves, edentates, carnivorous and lipotyphlan mammals, and owls. The ancestors of the carnivorous mammals (miacids) were alive.
56 Ma Gastornis, a large, flightless bird appears in the fossil record, becoming an apex predator at the time.
55 Ma Modern bird groups diversify (first song birds, parrots, loons, swifts, woodpeckers), first whale (Himalayacetus), earliest rodents, lagomorphs, armadillos, appearance of sirenians, proboscideans, perissodactyl and artiodactyl mammals in the fossil record. Angiosperms diversify. The ancestor (according to theory) of the species in Carcharodon, the early mako shark Isurus hastalis, is alive.
52 Ma First bats appear (Onychonycteris).
50 Ma Peak diversity of dinoflagellates and nanofossils, increase in diversity of anomalodesmatan and heteroconch bivalves, brontotheres, tapirs, rhinoceroses, and camels appear in the fossil record, diversification of primates.
40 Ma Modern-type butterflies and moths appear. Extinction of Gastornis. Basilosaurus, one of the first of the giant whales, appeared in the fossil record.
37 Ma First Nimravid carnivores (“False Saber-toothed Cats”) — these species are unrelated to modern-type felines
35 Ma Grasses evolve from among the angiosperms; grasslands begin to expand. Slight increase in diversity of cold-tolerant ostracods and foraminifers, along with major extinctions of gastropods, reptiles, and amphibians. Many modern mammal groups begin to appear: first glyptodonts, ground sloths, dogs, peccaries, and the first eagles and hawks. Diversity in toothed and baleen whales.
33 Ma Evolution of the thylacinid marsupials (Badjcinus).
30 Ma First balanids and eucalypts, extinction of embrithopod and brontothere mammals, earliest pigs and cats.
28 Ma Paraceratherium appears in the fossil record, the largest terrestrial mammal that ever lived.
25 Ma First deer.
20 Ma First giraffes, hyenas, bears and giant anteaters, increase in bird diversity.
15 Ma Mammut appears in the fossil record, first bovids and kangaroos, diversity in Australian megafauna.
10 Ma Grasslands and savannas are established, diversity in insects, especially ants and termites, horses increase in body size and develop high-crowned teeth, major diversification in grassland mammals and snakes.
6.5 Ma First hominin (Sahelanthropus).
6 Ma Australopithecines diversify (Orrorin, Ardipithecus)
5 Ma First tree sloths and hippopotami, diversification of grazing herbivores like zebras and elephants, large carnivorous mammals like lions and dogs, burrowing rodents, kangaroos, birds, and small carnivores, vultures increase in size, decrease in the number of perissodactyl mammals. Extinction of Nimravid carnivores
4.8 Ma Mammoths appear in the fossil record.
4 Ma Evolution of Australopithecus, Stupendemys appears in the fossil record as the largest freshwater turtle, first modern elephants, giraffes, zebras, lions, rhinos and gazelles appear in the fossil record.
3 Ma The Great American Interchange, where various land and freshwater faunas migrated between North and South America. Armadillos, opossums, hummingbirds, and vampire bats traveled to North America while horses, tapirs, saber-toothed cats, and deer entered South America. The first short-faced bears (Arctodus) appear.
2.7 Ma Evolution of Paranthropus
2.5 Ma The earliest species of Smilodon evolve
2 Ma First members of the genus Homo appear in the fossil record. Diversification of conifers in high latitudes. The eventual ancestor of cattle, Bos primigenius evolves in India
1.7 Ma Extinction of australopithecines.
1.2 Ma Evolution of Homo antecessor. The last members of Paranthropus die out.
600 ka Evolution of Homo heidelbergensis
350 ka Evolution of Neanderthals
300 ka Gigantopithecus, a giant relative of the orangutan dies out from Asia
200 ka Anatomically modern humans appear in Africa. Around 50,000 years before present they start colonising the other continents, replacing the Neanderthals in Europe and other hominins in Asia.
40 ka The last of the giant monitor lizards (Megalania) die out
30 ka Extinction of Neanderthals, first domestic dogs.
15 ka The last Woolly rhinoceros (Coelodonta) are believed to have gone extinct
11 ka The giant short-faced bears (Arctodus) vanish from North America, with the last Giant Ground Sloths dying out. All Equidae become extinct in North America
10 ka The Holocene Epoch starts 10,000 years ago after the Late Glacial Maximum. The last mainland species of Woolly mammoth (Mammuthus primigenius) die out, as does the last Smilodon species
6000 ya Small populations of American Mastodon die off in places like Utah and Michigan
4500 ya The last members of a dwarf race of Woolly Mammoths vanish from Wrangel Island near Alaska
613 ya (1400) The moa and its predator, Haast’s Eagle, die out in New Zealand
386 ya (1627) The last recorded wild Aurochs die out
325 ya (1688) The dodo goes extinct
245 ya (1768) The Steller’s sea cow goes extinct
130 ya (1883) The quagga, a subspecies of zebra, goes extinct
99 ya (1914) Martha, last known Passenger Pigeon, dies
77 ya (1936) The Thylacine goes extinct in a Tasmanian zoo, the last member of the family Thylacinidae
61 ya (1952) The Caribbean monk seal goes extinct
5 ya (2008) The Baiji, the Yangtze river dolphin, becomes functionally extinct