"No one effing knows!"

There's an anecdote regarding a famous newspaper publisher who commissioned a 500 word article on whether there's life on Mars. The scientists assigned, cabled back "Nobody knows" repeated 250 times.

Lately there have been dueling articles here and there claiming aliens have existed, aliens exist, aliens will exist, and there are no aliens.

The truth is no one has much of a clue.

There's the Drake Equation, which does a good job estimating probabilities, except when Drake came up with it, only one of the quantities was roughly known. The rest were guesswork.

Now we know a few more quantities. we know how many stars are in the galaxy, in a rough approximation. We know, also in an approximate way, how many of those stars have planets (almost all of them!). We can guesstimate how many planets are suitable for life, if by the simple expedient of extrapolating extrasolar planet data to average the planets known which would receive about as much energy from their star as we do from ours.

That's all we know.

We don't know the fraction of suitable planets where life has arisen. We don't know what the fraction of life-bearing planets develop intelligent life. We don't know what fraction of intelligent species develop a technological civilization. And we certainly don't know how long such civilizations last.

Before the flood of extrasolar planets hit a few years back, all we could do was extrapolate from the Solar System. We now know it not to be too typical. As far as life goes, we have still only the one data point. Outside of Earth-related studies, this data point is useless. But it's all we have, so let's take a look.

The Earth was formed around 4.5 billion years ago. Life arose perhaps 3.8-4 billion years ago. It's hard to say when multicellular life arose, but it's certain it took its own sweet time. Let's be generous and say 2 billion years ago. Then we had to wait again for chordates (animals with a spinal column and skeleton) to come along, plus for life to move off the sea into land (opportunities for technological development at sea are limited). The point being multicellular chordate life on land is around 400-500 millions of years old. That's along time to us, but it's small potatoes to the history of life as a whole.

Now, while life does expand quickly to fill every niche (nature abhors a vacuum), all it need do is survive and reproduce. Or as Bill Bryson put it in A Short History of Nearly Everything: "Life wants to be. It just doesn't want to be very much." What this means is no particular organism requires intelligence in order to survive and reproduce. Furthermore, intelligence has a rather high cost. The brain uses up a lot of resources. No plant has ever developed a brain, or much of a centralized sensory system. Several animals lack brains entirely (the sea cucumber consumes its brain once it find a perch to live on).

But intelligence has a huge advantage. Humans can, through the tools we make because of our intelligence, live in almost every ecological niche in the world. The investment in the brain makes up for what we lack elsewhere. We're not particularly strong, fast or agile. We lack much in the way of claws or teeth that can hurt and kill prey, or hurt and kill predators. It's amazing what just a hand axe and a spear can do for us.

On that vein, consider how long humanity, including our immediate ancestors and relations, existed without civilization. One estimate is of a total lifespan for homo Sapiens of about 30,000 years, with perhaps 7-8 thousand with a civilization. Seen in the lifespan of the Earth, that's little less than nothing. But the point is we might have stalled intellectually at some point, and simply been the smartest animal and top predator for thousands of years, without ever settling down and building such works as the Pyramids of Giza, the Pantheon, or the Boeing 747.

Then, too a technological civilization (the two terms always go together) need not advance very far. Egypt was one of the wealthiest nations on Earth for thousands of years without ever developing iron-based tools (they did, eventually, but by then so did everyone else). If you compare the world from, say, 700 CE to that of 1700 BCE, there are many changes, but a person from one period would understand the technology of the other without trouble. Something we cannot say of two other randomly chosen periods like, say, 1600 CE and 1860 CE. The point is we might have remained at an early iron age level for millennia yet.

and we might also have advanced faster. There's no point in time when science might have revolutionized our understanding of the world, but there are a few eras here and there where it might have. It was more a matter of social, economical and political conditions, as well as religious and moral ideologies, among other factors, which kept science from being discovered. And not in all areas. The Babylonians and Sumerians and Chaldeans made astonishingly precise astronomical calculations, without the benefit of even a rational exponential notation number system.

See much to extrapolate from?

Not me.

Intelligence might have developed earlier, though not much earlier (a brain requires first a central nervous system), or never. Civilization might have arisen earlier, or never. Science and advanced technology might have flourished in Ancient Greece, or might be absent to this day.

No one f%&@ing knows!

Still no one knows. But there's an interesting number: one in 20 billion trillion, or 10^-22

What is it?

It's how low the probability has to be for a technological civilization to develop, in order that humanity be the only one such life form that has ever existed in the universe.

Wow!

That's a very small number. We can write it down easily: 0.0000000000000000000001. But try and picture it.

Keep in mind two things:

1) If the odds for a technological civilization to develop are higher than this, then it's likely there have been, or there are, aliens somewhere in the universe.

2) Even if the odds are much higher, and thus aliens exists or have existed, the phrase "technological civilization" is rather vague. Our remote hunter-gatherer ancestors employing stone, bone and wood tools qualify, though they'd be no more capable of communicating across interstellar distances than geese.

Then, too, land life, though plentiful, is not as abundant as sea life. A sea-based life form might develop technology, which would be very helpful to their continued survival and reproduction, yet never go past the stone age. You can shape stones in water as easily as in air, but try to smelt metals under water. Unless they found some means of conquering land (and this is not out of the question), they'd be very successful primitives forever.