How Likely is Intelligent Life?
From a scientific perspective, just how unlikely was it that a civilization might develop on a planet such as the Earth? Here, I look at a couple of seemingly unlikely episodes in our planet’s history that I believe help to justify my low figure for the chances of intelligent life developing elsewhere in the galaxy (see the previous page, about the Alien Life. Arguably, without these unlikely events, no civilization could have developed. Then I will look at the Fermi Paradox.
Guess what? If the Earth didn’t have a Moon, and a big one at that, civilization may have been impossible. Indeed, life may have remained hardy and primitive because without the Moon, the Earth’s environment would have been very unstable, probably making it impossible for sophisticated life forms to develop.
The Moon acts to stabilise the axis of the Earth’s rotation. Without such a large satellite, the Earth’s axis, it is thought, would wobble randomly and relatively rapidly (perhaps over the course of thousands of years). Sometimes, it might be as it is now, pointing roughly at right-angles to the Sun and providing relatively genial weather and gentle seasons. But, a mere few centuries years later, it could be leaning over and pointing directly at the Sun, for example: one side of the world would be blisteringly hot, never experiencing darkness, while the other side would freeze in a thousand-year night. There would be tremendous storms driven by the extreme temperature differences. Without the Moon to stabilise its rotation, the Earth would simply tumble through space rather than spinning in a sedate and predictable way as it does now.
Life as we know it, sophisticated life that is, would have a hard time developing under such conditions. I can’t say that it would be impossible: the experiment hasn’t been run here, after all. But life as we know it requires a stable environment. In the modern world, as we are only too well aware, climate change leads inexorably to mass extinction. And that is only the relatively minor, as yet barely measurable global warming that we humans have perhaps triggered. The other big cause of mass extinction is loss of habitat, of course. But with climate change as drastic as it would be on a Moon-free Earth, loss of habitat would be automatic pretty much all the time.
Recent studies, for example, The Great Warming: Climate Change and the Rise and Fall of Civilizations and this article reported at the Zazna.com, suggest (controversially) that the rise and fall of some historical civilizations can at least in part be linked to climate change. Minor climate change, by these standards. When the Earth’s axis shifted by less than one degree several thousand years ago, the Sahara grasslands became the Sahara desert and the humans living there had to migrate. Some settled by the Nile. The Egyptian civilization began there, instead, perhaps, of in the once fertile Sahara.
The Earth’s axis shifted, by the way, in part because the Moon is gradually drifting away from the Earth, and its stabilising effect is therefore getting weaker. Eventually, this planet will be uninhabitable once more. When the Moon was formed about 4.6 billion years ago (according to the Orpheus Collision theory), it would have been only about 14,000 miles away – just far enough to avoid falling into the Earth, in fact. Currently it is about 230,000 miles away and drifting off at the rate of about 3cm per year.
So how was the Moon formed, and is it a likely event? Current theory suggests that there was another planet in the early solar system, probably orbiting between Earth and Mars. This planet has been named ‘Orpheus’ (sometimes ‘Theia’ or ‘Thea’) and it was about the same size as Mars: about half the diameter of the Earth. It struck the Earth a glancing blow in the plane of the Earth’s orbit and of course caused a mass of debris to be ejected. Probably, it tipped the Earth’s axis to approximately its current value as well. The Earth and Orpheus coalesced, and much of the vapourised rock that was ejected collected together to form the Moon. If there was any life on Earth at the time, it will have been exterminated, of course. Since that time, the Moon has kept the Earth’s axis pretty stable. Here is a rather nice video depicting the event:
However, recent measurements indicate that Moon rock is the same as the Earth’s mantle and not made part from Earth and part from some other planet or giant meteorite, so unless Orpheus was a comet made mostly of ice, some other explanation for its formation will have to be found.
Either way, it still seems likely to have been the product of a collision, so how likely is it that this will happen elsewhere in the Universe? Well, with such a vast Universe, it has to happen a lot, of course. But look at all the elements that have to come together:
- Collision with a planet in the habitable zone, not too far from or too close to a star;
- Resultant planet is suitable for life to develop and survive long enough to develop civilization (size, day length, atmosphere type, plenty of water, right mineral composition and numerous other factors);
- Collision was early enough for life to develop afterwards;
- At such an angle that the subsequent orbits are stable;
- A glancing blow, not a direct hit, so enough material is ejected and retained in orbit;
- Creates a large enough satellite to stabilise the resulting planet’s axis;
- Satellite is far enough away to be able to coalesce and not fall back down;
- Satellite is close enough to be able to stabilise the planet’s axis for billions of years;
- Climate is stable enough that evolution has the time to develop complex life forms and civilization;
- It is also argued by some that the large tides caused by the early Moon helped speed up early molecular evolution.
Will we discover that the majority, if not all, alien civilisations originate on worlds with stable climates – that is, worlds with big satellites? A strange conclusion to reach, but probably correct! Doesn’t that make it seem more likely that intelligent life is a rarity in the Universe?
On the other hand, some scientists think that a large moon is not so vital. A simulation carried out at Pennsylvania State University suggests that in many circumstances, the axial tilt can be stable enough even without a large moon, depending on what other influences are around. If the planet is rotating ‘backwards’ it tends to be more stable, and if its day length is less than 12 hours it tends to be more stable as well (a bit like a fast spinning top is more stable, I suppose). They also found the earlier study to be over pessimistic in its predictions… so it is all up in the air at this stage!
Up until about 620 million years ago (620Ma), life on Earth was primitive by today’s standards. The most advanced life forms were single-celled plants and animals known as Archaebacteria and Eubacteria (such as blue-green algae). They are hardy organisms with small cells and no cell nucleus. The Archaebacteria could, and still do, survive in boiling water, ice, acids, alkalis, and darkness. They didn’t need oxygen to breathe – indeed some have recently been discovered that “breathe” iron! Fossil records indicate that this had been the situation throughout most of the Earth’s history, from about 3.8 billion years ago up until that time. Before these primitive forms, there was no life at all as far as is currently known: just chemicals.
Just what the difference is between life and chemicals is a philosophical question that I don’t want to get into here, but suffice it to say that life is defined as reproducing entities: replicators, in the modern terminology. Living things exhibit the “four F’s” of behaviour, nameley fight; flight; feeding and er… reproduction.
620Ma, in the Vendian period, that all changed, and the first multi-cellular organisms appeared in a sudden and unprecedented explosion of variety and indeed quantity: the Ediacarans. What had happened?
It has long been known that most of the land masses in the Vendian times was clustered over the poles, indeed, most of it in a huge continent called Pannotia over the South pole. Evidence also shows widespread glaciation as well. Very widespread glaciation, in fact.
It appears that the entire Earth, oceans and all, may have been covered in an average of 1km of ice! You can read the original papers on this theory: Kirschvink and Hyde, if you like. It seems that the Earth went into an ice age, but it ran out of control: too much ice formed, perhaps because the continents were near the poles, and land masses cool easily. The ice reflected sunlight, further cooling the Earth. Eventually, the entire planet was covered with a thick layer of ice.
How did life survive? After all, ice is not very nutritious, and although the sky must have been mostly cloudless, photosynthesising plants would have been stuck under the ice, not on top of it!
Life must have survived around hot springs and geothermal vents. Also, it is thought that in a few places the ocean ice will have frozen slowly rather than rapidly. When this happens, the ice that forms is transparent rather than opaque – so some sunlight could have passed through it to the photosynthesisers in the ocean beneath.
Nevertheless, nearly all life will inevitably have been killed. It is thought that the snowball effect lasted only for a few tens of millions of years, and was eventually ended because volcanoes, over millions of years, pumped carbon di-oxide into the atmosphere. Eventually, this built up to around 10% of the atmosphere, and the global cooling would have gone into sharp reverse, with the average temperature of the Earth rising from around -30C to around +50C in a matter of just a few years. What followed was massive melting of the ice and a century-long rainstorm (the first rain in at least 10M years) that washed the excess CO2 out of the atmosphere again. By now the continents had moved, and the ice didn’t re-form.
Life found itself with a warm, indeed rather hot, wet world, and hardly any competition. It is thought that this lack of immediate competition allowed all sorts of mutations and changed creatures to survive, in effect causing the explosion of forms subsequently discovered in the fossil record.
Here is an episode of Miracle Planet about the Snowball Earth theory [50m long].
So, returning to the theme of alien life, what do we have? Complex life on our world appears to have evolved recently, only because of an accident of continental mass and alignment, climate feedback, and volcanic activity. There might be many worlds out there with simple life. But how likely is it that the above history has also happened many times elsewhere in the Universe, to give life the competition-free spurt it apparently needed? Maybe continental drift isn’t so common, and maybe the way it happened here is not the way it necessarily happens generally.
Of course, you could argue that a snowball is just one possible kick-start. Massive meteoroids can also cause mass extinctions, and apparently have done more than once even on this planet. Either way, we will be looking for planets with varied histories: nice stable places are no good for encouraging the development of complex life forms, it seems. But, as we saw above with the Orpheus collision, the planet can’t be too unstable either! It has to be just right… The climate has to be mostly stable over periods of hundreds of millions of years. The real question, which is unanswered by science or space exploration so far, is how likely is this?
Climate stability is of course influenced by more than just the prevalence of ice ages, and indeed by more than just the planet’s distance from its star and the luminosity of that star. A recent study suggests that the planet’s axial tilt is also a vital factor.
The Earth has a mild tilt of about 23 degrees off from the vertical, resulting in mild but distinct seasons, like Summer and Winter. However, if the tilt was near to zero, so there were no seasons, for example, then it is thought that the polar regions would become much colder (as they don’t warm up or experience any ice melting in the Summer), and at the same time, equatorial regions would become much hotter as they wouldn’t get to cool down in the off-seasons. Indeed it is possible that equatorial regions would become hot enough for vital gases like Oxygen to begin evaporating off into space rather too quickly for complex life as we know it to be able to develop. On the other hand, it is possible that very violent weather systems would be too common, as the atmosphere redistributed heat from the equator to the poles at great speed.
Another possibility would be on a planet with a large axial tilt. This would mean that for half a year one side of the planet would be facing its star, and for the other half of the year it would be facing away: it would alternately roast and freeze, and tremendous weather systems would distribute heat (and anything that wasn’t screwed to the ground, or underground, pretty much). Such violent swings of temperature over such time periods would not be livable for most Earth-life. How well native life could adapt is an open question at the moment, of course.
Another factor could be length of day. A mild axial tilt may be no good on a planet that always keeps one face to its star, or in any case has a very long day length, for much the same reasons as the last example: overheating on one side, overcoooling on the other, and storms inbetween…
Now I argue here and on the previous Alien Life page that civilizations are so unlikely that really it is no surprise that a) we can’t find them and b) they haven’t found us. The above two examples make it clear that complex life is hard to achieve, and intelligent life must surely be even harder, given the enormous length of time it took even though life began on Earth as soon as it possibly could have.
Some argue that with so many stars and so many planets out there, life must in fact be pretty common, and intelligent life could be pretty common too. By “common” they mean maybe a few civilizations per galaxy – not the millions that might exist in a Star Trek style Universe, but still common enough. Well, what if they’re right? If there are a few civilizations out there, why can’t we find them or why, since they would most probably be much more advanced than us, haven’t they apparently found us or used our planet for their own purposes already?
There are a number of standard explanations for this.
- (As I’ve argued) Advanced life and advanced civilizations are just too difficult to develop. We know that:
- It took a long time for multicellular life to develop on Earth
- That development required a chain of unlikely coincidences in the construction of the Earth’s habitat
- Despite a number of flowerings of more complex life on Earth, we seem to be the first civilized species here
- Perhaps all advanced species destroy themselves after a short time
- This is part of what is known as the Great Filter theory: something eliminates civilizations – themselves, or some force of Nature, or maybe just one hostile alien species that got there first – and will show up here as soon as it detects our radio waves. Certainly we know of enough possible disaster scenarios that could finish us off completely, from nuclear and biological warfare, to asteroid impact, environmental degredation, hostile AI, rogue nanotechnology, and on and on. Maybe we’ll discover some new technology that will finish us off before we realise the danger. Not that we’ve ever retreated from dangerous technology before.
- Some argue that they are here but keep their presence secret (UFO’s and the like)
- There is no hard evidence for this theory. Lots of sightings, but no evidence. That just doesn’t add up. If the aliens are so careless as to be seen so often, there certainly would be some hard evidence too. We can be almost completely certain that UFO sightings are delusions, mistakes, or lies.
- The Zookeeper Scenario: the aliens are here but we are in quarantine
- This possibility can’t be scientifically falsified, so is not a scientific theory as such. If true, it only takes one exception, one Zookeeper to break the rules, out of possibly millions of alien species, to render the quarantine useless. Would all species and all individuals really keep to the rules?
- Humans are descendants of aliens
- Doesn’t answer the question: where are these other aliens? Where are the signs of them in the Universe at large? Where are the other civilizations or is there only one, in which case we are back where we started: we’re all there is.
- They are out there but haven’t reached us yet
- According to the supposed numbers, the odds are that at least one civilization would have formed millions of years ago and populated the galaxy already, even if they were limited by the speed of light, and even if the vast majority of alien civilizations had no interest in populating the galaxy – it takes only one.
- We don’t know how to detect their signals or they don’t want to chat
- They would probably know how to be detected by more primitive species if they wanted to be (maybe the Spiritualists are speaking to them – telepathy or mind contact would be the most sensible way for an advanced species to contact a more primitive one that lacks appropriate technology). Also, even without detecting their signals, we should be able to detect their presence: used up resources, signs of things going on, artefacts on Earth, inhabitable planets (maybe one day) and so on. But there seems to be nothing whatsoever, so far.
- Out of millions of species, some would want to chat. It only takes one.
- Maybe some future technology will help us detect them or their signals, if they are in fact there.
- Maybe we are the first in the galaxy
- Well, I would agree with that!
- Maybe supernovae, gamma-ray bursters, asteroids and so on exterminate life too frequently;
- Maybe technology and science are rare even for intelligent species; but again, it takes only one;
- But if intelligent life isn’t so difficult to produce, then we would almost certainly not be the first: there has been far too much time already, at least so it appears.
Is it possible that we are alone in the entire (observable) Universe? Well, yes, but science has proceeded very effectively by assuming that we are nothing special: that what goes on here is more or less what goes on everywhere. That the laws of Nature are the same here and everywhere, more or less. Our Sun is a typical star, our solar system is typical (and Kepler bears this out so far), and so on… so there should be at least a few others out there. It all depends on those numbers in the Drake Equation.