You’ve probably heard the phrase, “you can lead a horse to water, but you can’t make him drink.” When considering the possibility of extraterrestrial civilizations, one could alter that phrase ever so slightly: “you can generate life pretty quickly, but you can’t make it become intelligent quite so easily.” That’s not intended to be a knock on humanity (every now and then I like to take a timeout from doing that). It’s just an acknowledgement of the one data point we have on how long it takes life to become intelligent.
Mr. Drake, your equation please:
N = R* × fp × ne × fl × fi × fc × L
In this equation,
- N is the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy
- R* is the rate at which new stars are created in our galaxy
- fp is the fraction of stars that have planets
- ne is the average number of planets per star that might support life
- fl is the fraction of life-supporting planets that actually develop life
- fi is the fraction of developed life that becomes intelligent
- fc is the the fraction of intelligent life that sends signals into space
- L is how long a signal-sending civilization survives and sends those signals
So far, we have estimated R* = 2 per year, fp = 0.99, ne = 0.35, and fl = 0.75. Now we set our sights on fi, the fraction of developed life that becomes intelligent. In the last post, we set fl pretty high – maybe even conservatively so – because in the grand scheme of things, life on Earth developed quite quickly after the conditions were right. It took far longer to arrive at what we might call intelligent life. To understand the dramatic difference in timelines there, I’m going to steal (not for the first or last time) from Carl Sagan’s “Cosmos”, where he mapped the history of the universe to a single calendar year. Only this time, I’m going to do that only with the history of the Earth, which we will call the Earth calendar here. Or at least that’s what I’m going to call it. You can call it Fred if you like. Actually, in honor of the late Fred Willard, that is precisely what I am going to do.
If you spread the 4.54-billion-year age of Earth across a 12-month Fred-year, each month on the Fred calendar equates to about 380 million years, or alternatively each day equates to a little over 12 million years. There is a considerable amount of debate as to exactly when the oceans formed and when life originated. But one reasonable way to express it would be that if Earth was born on Fred-January 1, the oceans began to form sometime in mid-Fred-January, and they were fully formed by late Fred-February. The earliest undisputed evidence of life on Earth dates to just after mid-Fred-March, but strong evidence likely pushes that back into mid Fred-February, which means life was forming around the same time the oceans were forming. Now, again, this is a bit of speculative math – but the bottom line is, it didn’t take life long to emerge once the conditions were right.
It’s also important to note how well-organized life was even at that moment, because the earliest evidence we can find suggests these were microbes – similar to today’s bacteria – meaning life had developed beyond mere DNA and fully organized itself into cellular structures. After that, progress was painfully slow. Cells evolved their ability to “eat”, which eventually also led to the tendency to “eat” each other, and perhaps as a defense to this troubling development, they would eventually organize into loose colonies. The next revolutionary step was the evolution of sex – whose humble beginnings date to late Fred-September. At that point, DNA was able to build off the diversity of genes from two organisms instead of one, which dramatically increased the potential combinations and new features. But even after *that*, life remained largely cellular or for nearly another two Fred-months. It wasn’t until about 541 million years ago – or mid Fred-November – that life began to proliferate into the vast array we see today.. So life may have taken only a matter of Fred-days to arise, but it took another nine Fred-months to advance in any meaningful way after that.
The event that changed it all around 541 million years ago is called the Cambrian explosion. It is called Cambrian because it happened at the beginning of the Cambrian Period, which lasted until around 485 million years ago. It is called an explosion because it is when nearly all major categories of animal life that we know of today originated. There are lots of hypotheses about why this explosion occurred. It could have been a combination of any number of factors, ranging from oxygen and ozone levels to accelerating “arms races” between predators and prey to the evolution of eyes. Whatever happened, it took only a few Fred-days, after nine months of life frankly not being all that creative.
As life became more complex, it also became more dependent on specific conditions, and therefore more susceptible to major shifts in climate. Occasionally, these shifts have been quite dramatic, leading to five major (and a number of other minor) mass extinctions on the planet. They occurred 444 million years ago (Fred-November 25), 375 million years ago (Fred-December 1), 251 million years ago (Fred-December 11), 200 million years ago (Fred-December 15), and 66 million years ago (Fred-December 26). The middle one of these was the most extreme – 96% of all species went extinct. Evidence suggests this was caused by a combination of a huge volcanic eruption, a subsequent massive release of methane into the atmosphere, temperature rises in response, and acidification of the oceans. Known as the Permian-Triassic extinction event, it essentially erased the majority of the progress that had been made since the Cambrian explosion. The most recent extinction event – the Cretaceous-Paleogene event – was the one that wiped out the dinosaurs, clearing the path for mammals to rise up and eventually evolve into us. An asteroid impact probably triggered this extinction.
All while these waves of change were sweeping the planet, the brain was steadily developing as well. Its origins trace all the way back to early cellular organisms that developed the ability to transmit electrical and chemical signals – an early ancestor of the neurons that transmit signals throughout our central nervous systems and between various sections of our brains. Just as other organs developed with more specific and advanced capabilities, so did the brain. I could spend another several blog posts talking about how that all came together, but for now it’s sufficient to recognize that the brain is the seat of our intelligence, which begs another question. Since we are ultimately going to all this literary trouble to develop an estimate for fi in the Drake equation, at what threshold do we decide a species has become intelligent enough to trigger this factor?
The next factor in the equation will determine how likely a species is to start communicating signals into space, so we don’t want to restrict our definition of intelligence so far that we are double-booking (or in that case we should just be calculating one factor). But we also don’t want to be too loose with the definition of intelligence, because we are interested in the kind of intelligence that eventually leads to a civilization that can communicate with those on other worlds. There are indications the dinosaurs were fairly intelligent, but they had a couple hundred million years to develop, and as far as we can tell, they never turned into a civilization. Even the modern-day dolphin, despite possibly being the second most intelligent animal on Earth, has seemed content to remain in the ocean and not develop any kind of technology – although one must wonder how much further they would evolve if we were not around. But for the purposes of the Drake equation, let’s just state that we define intelligence to be the development of Homo sapiens – our species of human. Having done that, we can go back to the Fred calendar to see where that happened.
The beginnings of the family of species that have evolved into modern apes and humans occurred in a timeframe centered around 12 million years ago – the beginning moments of Fred-December 31. This split into two branches – one became the great apes (including gorillas and chimps), and the other led to us. The human branch began to crystallize about 4 million years ago, or 4pm on Fred-December 31. The Homo genus arrived 2 million years ago, or 8pm. The earliest fossils of our species – Homo sapiens – are from around 200,000 years ago, or a little after 11:30pm. So – after life arose sometime in Fred-February, it took the entire remainder of the Fred-year to become intelligent, and it had to endure several mass extinctions along the way. What does all of this say about fi?
There are a lot of reasonable arguments out there. The length of time it took for intelligence to take hold, combined with the fragility demonstrated by mass extinctions, combined with the fact that of all the billions of species that have existed on Earth, only one has become intelligent, suggest that fi is pretty low. On the other hand, even though it took a long time on Earth, it arose with several billion years still remaining in the planet’s lifetime. Even though there have been several mass extinctions, none of them have wiped out all the life on Earth – and even the worst one saw a relatively quick recovery in the evolution of new species. And – perhaps most importantly, we only need one civilization to become intelligent here. It doesn’t really matter how many other species on the same planet didn’t. In fact, you could make a perfectly reasonable argument that fi = 1, since we are here.
At this point, I’m going to fall back on the same kind of reasoning I used for fl, the fraction of habitable worlds that develop life. A value of 0 is absurd and also flatly contradicted by our existence. A value of 1 is likely too optimistic, as some worlds are bound to experience greater cataclysmic events than ours has to date. Even though it takes a long time, life seems to have been hell-bent on Earth toward ever-increasing complexity, which includes the development of brains and intelligence. Given enough time, I would expect the forces of natural selection (survival of the fittest) to encourage the development of intelligence on any world with life – so it’s just a matter of how many worlds are given that time. Even with massive climate shifts and the occasional rude intrusion by asteroids, Earth has come through the other side with us. So I’m going to set fi = 0.75 – again, a conservatively optimistic number, but high enough to leave us with hope that other beings are out there somewhere. Our updated Drake equation is:
N = 2 × 0.99 × 0.35 × 0.75 × 0.75 × fc × L
In other words, a little over once every three years, a star forms somewhere in our galaxy that will someday preside over an intelligent species, capable of figuring out what stars and galaxies are, and beginning to wonder if they are alone in the universe.
In the next post, we will dive into fc, the fraction of intelligent civilizations that transmit signals of their existence into space. For now, a moment of reverence for the human brain and its largely untapped potential.