I.1.3. The Origin of Life

[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.

I.1.2: Errata

In the previous post, I mistakenly stated that elements heavier than helium must be formed within a supernova.   A friend of mine was kind enough to point this out:

Are abundant supernovae actually required to produce a sufficient abundance of carbon, nitrogen, and oxygen? My impression (possibly mistaken) was that somewhat smaller stars readily produce those elements. Of course, supernovae are required to produce a host of other biologically important elements, so I'm not disputing the underlying point. Just a niggling concern about accuracy, if I were editing for publication.

My response:

You are entirely right about nucleosynthesis. I was mistaken -- stars about the size of our sun are capable of producing everything up to carbon and oxygen. Elements from nitrogen to iron are formed inside massive stars (even before they go supernova). Supernovas are only needed to (1) produce elements heavier than iron and (2) distribute the elements the star has already produced. I'll have to fix that post ... I appreciate the heads up.

I.1.2 The Atomic Elements

Whether one believes in God or not, the universe we live in is undoubtedly special. You need a rich, complex environment for living things to arise. Our universe is special because it has the right kind of qualities for intelligent beings like ourselves to evolve.
 
One of the most extraordinary things about our universe is the number of different atomic elements it has, each with its own special properties, such as helium, sodium, oxygen, silver, lead, hydrogen, and gold. (An element cannot be broken down into simpler substances. Compounds, on the other hand, are made of combinations of atomic elements. A water molecule, for example, has two hydrogen atoms, and one oxygen atom.)
 
Without such diverse elements, you could never get complex life forms. As I mentioned in the last section, when the universe began you only had the simplest kinds of elements -- hydrogen and helium. But hydrogen can only bond with one other atom, and helium doesn't bond at all. So for a while there was nothing in interstellar space but helium atoms and hydrogen molecules (made of two hydogren atoms).
 
Eventually this boring state of affairs changed. Most of the atomic elements in your body were forged in the core of a massive star (about 10 times bigger than the sun) and distributed by a supernova, the enormous explosion that occurs when a large star dies. (These explosions are so big that they shine as bright as an entire galaxy.) Our sun is, in fact, a second or third generation star. It was formed when gravity pulled together the remnants of previous stars that went supernova.
 
If it weren't for these ancestors of our sun, there would be no carbon, oxygen, or nitrogen -- the three main kinds of atoms that, along with hydrogen, make up your body and most living things. In fact, if the Big Bang had been only slightly more powerful, the massive stars needed to quickly synthesis the elements and then explode as supernovae would have been too rare, and our solar system would have been composed mostly of hydrogen and helium. And these atoms are too small and light to form solid objects on their own.
 
Here's the periodic table of elements. Hydrogen and helium are marked as "H" and "He." Basically nothing else on this chart would exist if the Big Bang had been only slightly more powerful. (And remember, if it had been slightly less powerful, you wouldn't have any atoms at all -- only black holes.)

 
But what's so special about carbon, nitrogen, and oxygen? Why were these particular atoms needed to make up living things? The answer to this question is actually very interesting, but I don't have space to go into the details here. (If you're interested, find an organic chemistry textbook and read the first few chapters.)
 
The basic idea is this. Certain kinds of atoms are good at making strong bonds (so-called "covalent" bonds) with other atoms. Hydrogen can only make one such bond, oxygen can make two, nitrogen can make three, and carbon can make four. If there were only hydrogen, you could have pairs of atoms bonded (H2), but nothing else. If there were only hydrogen and oxygen, things would still not be very interesting. You could theoretically have chains of oxygen atoms strung together, but in practice more than two oxygen atoms in a chain will not form a stable compound. Nitrogen is similar in this respect. Only carbon has the ability to form stable molecular chains (and perhaps silicon, a much heavier element). In this way, carbon can form indefinitely large structures, such as the proteins that make up animals, and the cellulose that makes up plants. DNA, which is just a long molecule that can store information, is another important structure made primarily of carbon. In fact, there's an entire field of study devoted exclusively to carbon-based compounds -- organic chemistry.


 
But carbon is only part of the story. The structure of our planet, for example, is largely dependent on an abundance of iron -- it forms the liquid core and provides a strong gravitational pull and a magnetic "shield" that protects us from the ions emitted by the sun. Oxygen is important because it is a key component of water, and of the atmosphere. Sodium, potassium, and other ions serve essential functions as electrolytes in most animals. Ultimately, it is the richness and diversity of atomic elements -- their varied structure and properties -- that has proven most important in the rise of life and human civilization. One can easily imagine a universe with a different set of elements that could still give rise to life and intelligence -- but only if those elements were diverse enough to combine into interesting, complex structures.
 
The theme that I am touching on here is creativity. For a world to be creative, it must show properties such as richness, diversity, and stability. This is the point of view I'm going to use in the coming chapters to examine the evolution of life, and the rise of civilization. Again and again we will see that our world has just the right properties to allow living things to evolve in divergent, complex, and interesting ways. This is important to understand because it will also eventually form the backdrop of my account of the rise of human values, ethics, and spirituality.
 
 
Appendix: The Physics of Chemistry
 
In this appendix I explain a few more details concerning the properties of atoms. For a more thorough treatment, you'll really need a physics textbook.
 
Physicists have observed hundreds of different kinds of particles, but only three of them ever combine to form atoms: electrons, protons, and neutrons.
 
Electrons are tiny -- about a trillionth the size of a pea. Electrons have what physicists call "negative charge," which means that they repel other negative charges, and attract positive charges. You can observe this effect when you rub a balloon on your head. The balloon collects electrons from your head and becomes negatively charged. Your hair, which is now positively charged, is attracted to the balloon and stands on end.
 
But why does your hair become positively charged when it loses electrons? Because now it has extra protons, which are positively charged. Most atoms contain an equal number of protons and electrons. In this state, the atom has a "neutral charge," which means that it does not attract or repel other atoms. Atoms that lose electrons have extra protons and become positively charged. But because positive charges attract negative charges, most atoms do not stay this way for long, and eventually attract extra electrons and become neutral again.
 
You've just learned the most fundamental rule in chemistry. Atoms like to have an equal number of protons and electrons, because opposite charges attract, and like charges repel. Much of chemistry is ultimately explained by this rule. Covalent bonds occur because certain atoms have an uneven distrubtion of electrons around the nucleus. Parts of an atom that have more electrons have a somewhat negative charge, and can attract parts of other atoms that have a somewhat positive charge. (Keep in mind that this way of thinking about it is somewhat inexact; it would require a mathematical treatment using quantum mechanics to provide a full explanation.)
 
But why is chemistry important? Because everything that happens in your body is chemical. All living things are really walking chemistry labs. Chemical reactions give you energy, send signals through your body, and even read the code contained in your DNA.