How Probable is Life?

A curious phenomenon you may have encountered before is called the birthday paradox. If there's just
one person in a room, then the probability is one that that person, being the only person in the
room, doesn't share a birthday with anyone else in the room. On the other hand, if there are 367
people in the room, then since there are only 366 possible birthdays, two must share a birthday.

So now the question is this: If we keep adding people to the room, starting with one, then two, and
so on, at what point is there a 50-50 chance that there are two people in the room that have the
same birthday? The answer is surprisingly small: 22. (At 21, the odds are nearly 50-50, at 22, the
odds are slightly better.)

But at a party with 22 people, when, as happens half the time, two people discover that they've got
the same birthday, they'll probably be amazed, perhaps exclaiming "What are the chances of that?"

Life is discussed in the same terms. Life is a wonderful thing, which may lead us to believe that
it's a rare occurrence. But is it?

To begin to examine the question, we need to understand the Principle of Selection, as we've
discussed before. If something is more probable, it probably happens more. Everything we know
about chemistry boils down to physics, and everything we know about physics boils down to methods
for calculating probabilities of things.

So what is most probable? On our planet Earth, we have a constant stream of energy arriving from
the Sun. When any two chemicals collide, there's an electomagnetic interaction that occurs, and
the outcome of that depends on things like what their precise orientation was when they collided,
how energetically they collided, what the possible result states are, and what their energies are.
Some reactions absorb energy, others release energy. The flow of energy from the Sun tends to
favor reactions that absorb energy. And here's the thing - all reactions are in competition with
all other reactions. If a given reaction absorbs some energy, that energy is no longer available
to power some other, competing reaction. In the long run, when we're all dead, the most probable
state tends to be the one of lowest free energy, due to the constant pressure situation we find
ourselves in due to the atmosphere weighing down on us. But in the shorter term, during which
life is happening, it's the _fast_ reactions that win. If a reaction is fast, it soaks up the
free energy, starving out competing reactions.

What reactions are fast? The ones that are catalyzed. What catalyzes a reaction, is some geometric
arrangement that lowers what's called the activation energy. The probability of a reaction dies
incredibly fast as the activation energy rises.

The trouble, of course, is that the catalyst has to be in the right place at the right time. But
there's one foolproof way for a chemical reaction to guarantee that the catalyst is always present
- the reaction can be self-catalyzing. A self-catalyzing reaction, if it exists at all, will
outrun anything. And that's damn close to a definition of life. RNA catalyzes the construction of
matching RNA. RNA, in turn, catalyzes the construction of matching proteins, which in turn are the
catalysts that run all the processes of life. This process of soaking up energy we call "growth",
and we keep growing until we get as big as we feasibly can. At just the point we slow down
growing, we resume that growth by reproducing, delegating to offspring the work of continuing to
soak up energy.

A relatively small piece of RNA is self catalyzing, in the presence of amino acids. However long
it might take for such a piece to synthesize in the first place, once it did, it would grow
rapidly, and take over, as indeed it has, particularly in the diffusive medium provided by oceans.

Where do the amino acids come from? There's an interesting possibility there, too. The answer may
turn out to be comets. With their irregular orbits, they sweep the far reaches of the solar system
and being ice balls, as they near the sun they shed themselves in the path of Earth's orbit. Also
being ice balls, they tend to quench the reactions they collide with. So if a molecule is in a
high energy state, before it can decompose it can be hit by an iceball, which freezes it in.

In the outer reaches of the solar system, where these comets sweep, it appears that there is
a high temperature gas, of just the sorts of things amino acids are made from. And at high
temperatures, all reactions are possible, all occur, if only for a moment. If they're frozen in,
then that moment can last a long time. Long enough for the piggyback ride to the sun, and the dive
into the ocean. Given the vast ranges these comets sweep, this mechanism for quenching and
ferrying interesting chemical formations, and depositing them onto planets, makes that bottom
rung beginning step of life far more probable than it might, at first blush, seem to be.