[Note: this is a rough draft, and may contain factual errors. Any comments, corrections, and suggestions are welcome!]
According to biologists, life arose on earth sometime between 4.2 and 3.5 billion years ago. Not much is known about why or how this occurred. But it is worth considering how it might have happened, because life -- from fungus to humans -- is surely the most wondrous and intriguing thing in the universe.
Life requires very special conditions to arise. Planets with no atmosphere, such as Mercury, cannot have oceans -- because fluid boils in a vacuum. And without any oceans, you can't have life. Why? Because chemicals in solid form (such as rocks) do not interact readily enough with one another. On the other hand, chemicals in gaseous form (such as air) can interact, but they are too diffuse to form any structures.
So you've probably got to have liquid of some kind for life. Worse, certain liquids will not work. For example, magma (as found on the surface of Venus) or liquid hydrogen (as found inside Jupiter) are too dense. In fact, the only common substances of the right sort appear to be water (H2O), liquid methane (CH4), and ammonia (NH3). Only Earth and a few of the moons of Jupiter and Saturn have such oceans.
Some scientists believe that there may be life on some of these moons (especially Titan or Europa), but it does not appear likely. The conditions on these moons are somewhat extreme, featuring intensely hot and cold temperatures, and Earth has a much larger habitable area than these moons. This is important because, as we will see, evolution works better when there is room for a teeming diversity of species and subspecies.
The Earth was fortunate to have just the right conditions. If it were much closer to the sun, the oceans would evaporate, leading to greenhouse warming from water vapor, and a thick, dense atmosphere like Venus. Venus is so hot that lead melts on the surface. On the other hand, if it were much farther away from the sun, the oceans would freeze over, reflecting the sunlight and leading to even colder conditions. If the earth were too small, it couldn't have an atmosphere. If it were too large, it's atmosphere would be too thick.
Our star, the sun, is also just right. If it had been 2.5 times larger (like the star Vega), it would have burned itself out after a billion years, which is too short a time for complex life to evolve. And if it had been half the size, it would be unable to support such a large solar system, and any planets close enough to have life would become "tidally-locked" -- that is, one side would permanently face the sun, and be too hot for life, while the other side would face away, and be too cold (like Mercury in our solar system).
Also, our solar system, as I explained in the last section, was formed from elements created in giant stars and ejected in at least one supernova explosion. That means that if our solar system had been formed a few billions years earlier, or in a different part of the galaxy, it wouldn't have had the right elements for life. For example, phosphorous is essential to DNA, but is never formed in stars less than 10 times heavier than our sun. The same goes for iron, an essential metal in the rise of human civilization.
Nevertheless, it bears to keep in mind that our universe is huge. The part that we can see contains 80 billion galaxies, each with about 2 trillion stars. That's a total of about 160 billion trillion stars. Though the conditions for life are quite special, there may well be billions of solar systems similar to ours out there. So even if life is unlikely to arise spontaneously, it may have already had plenty of chances.
My point here is not to go through these calculations exactly. Scientists have already been debating the probability of extraterrestrial life for decades. (Just google "Drake equation.") My point is simply to convey, in a qualitative sense, the specialness and fragility of life.
The other important question to ask is how the first living thing arose from nonliving matter. About 4 billion years ago, you finally had stable conditions on Earth. What used to be a ball of magma had cooled and formed a solid outer crust. The heavy bombardment from meteors had ended, and you had shallow pools and underwater volcanic vents where new organic chemicals could form and co-mingle.
If the first life indeed arose from chemicals reacting in a shallow pool (the so-called "primordial soup") then it must have been very simple. The simplest life we know of today is the virus. But viruses are unable to replicate on their own -- they are so simple that they cannot produce their own proteins and rely on a host cell for replication. The simplest life that has its own metabolism is cellular life, but even the simplest known cell is much too complex to have arisen spontaneously in the primordial soup. This means that the first life on Earth was completely different from anything we know of today. We do not even have any fossil remains of it, since the oldest fossils we have are of cellular life.
The question of what the first life was like, therefore, is a speculative one. It's a question for philosophers. What I'm going to do next is a tell a story that seems compatible with the evidence. I won't go into the chemical details (which we don't know anyway), but will try to stick to the essence of what might have happened. Most scientists would probably agree with the broad outline I will give here, even though there is still a great deal of disagreement about the details.
Life requires two things, energy and raw material. You need energy to break chemical bonds and make new ones. And you need the right material -- organic molecules -- because these substances can act like building blocks to create more and more complex structures. The most important atom for life is carbon, because it can form four strong (covalent) bonds. Other important elements include nitrogen (3 bonds), oxygen (2 bonds), and hydrogen (1 bond). Life on earth has also evolved to use many other elements, especially phosphorous, sulfur, and a number of salts (ions). Again, without a diversity of such building blocks, you can't build very complex structures. Fortunately, our planet was blessed with an abundance of such raw materials, as well as plenty of water, and energy from the sun and volcanic vents.
So, about 4 billion years ago, everything that life needs was in place. All that was missing was life itself. But here we have a puzzle, because all known self-sufficient life forms are actually quite complex. Even the simplest bacteria have a number of specialized organs (called organelles), including DNA, ribosomes, a membrane, cytoplasm, and a number of specialized proteins. The element that is missing from our primordial soup is organization.
Fortunately, the early Earth probably did not just have one primordial soup, but many. Scattered all along the coastlines of the world, were shallow pools were chemicals could be mixed and combined by the action of sun and lightning (which was supposed to be very common back then). The earth was also very volcanic, so you probably also had a number of underwater volcanic vents which released organic chemicals and energy in the form of heat.
Now each of these primordial soups probably had its own character, since different elements will be more common in different places, and different complex molecules would have formed by chance. The thing to remember about organic molecules is that there are practically an infinite number of kinds. Just as you can build endless different things out of Legos, organic elements can be combined to form not only proteins but countless other complex molecules.
What makes complex organic molecules interesting is their ability to catalyze chemical reactions. "To catalyze" just means to make faster or more likely. For example, you have catalysts in your saliva that help dissolve sugar placed in your mouth. If you put an M&M in a glass of water, it will sit there for days. But if you put it in your mouth, it will dissolve in a matter of seconds. Catalysts can also put things together. In cheesemaking, it is the action of certain catalysts (a type of protein) that causes the cheese to solidify.
Catalysis is essential to life. It is chemical reactions that make living things do what they do, and it is proteins (also called enzymes) that catalyze these reactions. Of everything that goes on in your body, you might say 90% of it is catalysis. Most of this catalysis is carried out by proteins, and all proteins are coded either in your DNA, or in the DNA of the animal or plant you ate to obtain it.
Very roughly:
DNA --> Proteins --> Catalysis --> Most of what happens in your body
The puzzling thing about the origin of life is that it probably did not involve DNA. DNA is actually a very complex mechanism for storing information. It requires all kinds of complicated proteins just to read it and interpret it. Most biologists agree that DNA was an adaptation that occurred later in the history of life.
So where did the first proteins come from, if not from DNA? A number of experiments have indicated that proteins can arise spontaneously in a primordial-soup-like mixture. One idea that's been proposed is that these early proteins catalyzed the formation of more proteins, and then these proteins catalyzed still more proteins, until somewhere along the line you got a collection of proteins that catalyzed one another. Once you get an "auto-catalytic set" like this, you might say you have the first life form, since you've got a system capable of producing more systems like itself.
I think this idea is on the right track. But it is important to keep in mind that not all organic molecules are proteins. Other examples include RNA, DNA, fats, cellulose, and sugars. Remember, organic elements can be put together in countless combinations, including categories not listed here. Scientists have tended to debate whether it was DNA, RNA, or protein that came first, but there are plenty of other possibilities -- in fact more than we could possibly conceive of. But this is actually good, because it means that there are probably countless ways for something living to arise. If there really were many primordial soups, and these were allowed to evolve for millions of years, then perhaps it wasn't all that unlikely that some self-perpetuating system would arise somewhere and spread through the rest of the world.
Of course, there is no guarantee that a certain primordial soup will lead to anything like life, even after hundreds of millions of years. Assuming that the catalysis of new molecules eventually settles down into some kind of equilibrium, this equilibrium would most likely be something boring, such as a collection of molecules that catalyze themselves, but fail to produce anything new. Fortunately, if there were many primordial soups, then the boring ones would eventually be overtaken by the more adaptive ones. This sort of evolutionary creativity is important to keep in mind, because it will reappear again and again in the history of life, as well as the history of human civilization. So let's take a closer look at how it would work.
Imagine the early earth, with its shallow pools and volcanic vents scattered everywhere. Many of these shallow pools are home to "boring" collections of molecules. These molecules simply catalyze more of the same molecules, or maybe simpler and simpler molecules until you've got nothing really interesting. But fortunately, you also have a number of "chaotic" primordial soups. These consist of molecules that are good at catalyzing new, different molecules of unpredictable types. Though these systems are unstable, they may lead to something interesting. Of course, they may also eventually become boring, or simply remain chaotic and unpredictable.
These chaotic soups, if creative enough, may spread some of their volatile molecules to nearby soups, changing them from boring to chaotic as well. As long as there is at least one chaotic soup of molecules somewhere, the whole system can keep changing in interesting ways. And even if one day all of the soups become boring, perhaps some extreme weather will hit, such as lightning or hurricanes, that can stir the soups up and make them chaotic again.
In all this flux, over millions of years, it doesn't seem implausible that eventually you'd have a primordial soup that is somewhere between chaotic and boring. Such a soup would remain more or less stable, but at the same time it would be able to spread its molecules to nearby pools and "wake them up" as well. Moreover, it would also catalyze the creation of new molecules -- slowly enough to prevent the system from changing into something completely different, but quickly enough that it could give rise to "children," that is, new primordial soups somewhat different from itself. Once you've reached this point, you've got evolution. These soups have the capability of reproduction, mutation, and natural selection, as fitter, more vigorous soups replace the less adaptive ones.
Here I've tried to describe life in its simplest form. It involves diversity, creativity, and competition. All of these elements are important. If there was no competition among early collections of molecules, there would be no way to guarantee that the more "alive" ones would spread. If there had been no diversity, then there would be nothing to choose among. And if there had been no change or creativity, you would have no diversity to begin with.
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