We can consider certain attributes of organisms a “niche” - a way life responds given environmental circumstances. So for example, grazing is a niche that is filled in Africa by zebra, gazelle and countless other large animals, the same niche is filled in Australia by Kangaroos and in North America by Buffalo. So grazing really is something that seems to crop up again and again. Or consider burrowing - that too appears to be something life seems to do again and again: Meercats in Africa, Tasmanian Devils in Australia and Hedgehogs in North America. Independently, in different places, separated widely by geography and time - grazing and burrowing, flight and eyesight evolved again and again. So what about intelligence? Is it a niche?
50 million years ago, the continents looked roughly as they do now. We know they were separated from one another by the same oceans we are familiar with on the planet today. If you wanted to see how life would evolve in slightly different environments, the 50 millions years ago you effectively had separate continents, effectively isolated from one another on which to conduct your experiments in biology. Let's say you wanted to see if human-like intelligence would be inevitable in the same way, say: flight seems to be (flight evolved in insects, fish, birds and mammals to name just some).
So here's what you do: in Africa, set the experiment running for some 5 million years after the common ancestor of Humans and Gorillas split into those two families and observe that homo sapiens eventually evolve. Left alone, for well over 50 million years (it can be argued that the continents were essentially separate up to 200 million years ago) we find that unlike in Africa where you get human-level intelligence in another otherwise similar continent Australia, the most complex organisms that evolve are koalas and kangaroos, while on Madagascar we have Lemurs, in South America and India we have Monkeys and in the Oceans we have dolphins. A person watching this grand experiment unfold from a god-like perspective might well ask: “how much longer should we wait for human-like intelligence to evolve in Australia and South America? We've waited 200 million years. Those humans in Africa are going to start migrating to all the other continents soon now too.”
To put it more plainly: if human beings did not evolve in Africa, how long would one need to leave all the life on Australia alone, for any of the species currently inhabiting that continent to evolve something akin to human-like intelligence? What is the likelihood that kangaroos would, eventually, evolve the capacity to do what humans can do and build a radio telescope? The question is not meant to be rhetorical. We know that out of the millions of species extant and extinct only one of them ever developed human-like intelligence. Namely - humans. Perhaps there were people besides homo sapiens (like the Neanderthal) that evolved intelligence as well - but they too evolved in Africa from a very closely related - likely intelligent - common ancestor. This surely is an indication that human-like intelligence is anything but a convergent feature of evolution. Given the experiments of the separate continents here on Earth, we find that life seems not to find the characteristic of intelligence as useful an attribute for survival as, say, wings or bipedalism which again and again evolved independently. Features that genuinely are convergent.
Philosopher Peter Slezak from UNSW makes an important point about the number of independent, but necessary, mutations that were selected for in the chain of evolution that led from simple bacteria to human-like intelligence. It seems that if there were many evolutionary paths to human-like intelligence (such as there are with wings) then the odds would be lower – but as far as we can tell, there seems to be a single (or at least a very small number of) evolutionary path(s) that works – namely the path that has led to us. Such a path contains millions, maybe billions of steps. But let us be ridiculously conservative and assume that only 100 such steps are required. Now further, each of those independent, but necessary steps had a certain probability of occurring. Possibly the probability of each step was far far less than 1/100. Let us be conservative once more and assume each step had a ½ chance of occurring (Slezak actually uses the figure of 1/10 – but I am being even more conservative than he). Now if each step has a ½ chance of actually happening and there are 100 such steps, this means that the probability of replicating that chain of events required to reach an organism with our intelligence is going to be (1/2)^100 = 7.9 x 10^-31. This number is difficult to appreciate - far more difficult to appreciate than the number of stars in the universe – it essentially suggests that even if all the Earth like planets in the universe were covered in bacteria, the chance of finding another sample of intelligent life would still be essentially zero. In other words, no matter how astronomical the number of stars and planets in the universe may be, the biological odds of recreating some evolutionary pathway to intelligence is so small as to blow the astronomical number out of the water.
Carl Sagan however believed that the number of independent pathways towards intelligence are likely to be not simply more than just one, but numerous. It would seem however, that unless the number of pathways to intelligence is absolutely staggering then it is unlikely to make much of a dent in the number calculated above which it must be emphasized is ridiculously conservative – and, given the evidence we have been looking at life on Earth and in the fossil record, is the best model we have for the probability of life arising. In other words, as far as we can tell, given the best evidence on hand, intelligence has only one way of evolving: namely that leading to homo sapiens. If the number of pathways were large as Sagan thought and others think: what reasons are there for believing this? No evidence on Earth suggests that there is anything other than a very small number of pathways to intelligence.
Astrobiologist Charley Lineweaver recalls iconoclast Frank Drake of the Drake Equation stating that the reason he, like many people believe that the Search for Extra Terrestrial Intelligence t is a worthwhile venture likely to yield results is that if you look at the fossil record, what you find is evidence of increasing complexity. Importantly, what you find is that the relative size of the brain over time increases.
But Lineweaver has what I would consider a complete knockdown argument against that idea and he has gone to some length to explain why it is that arguing there is a trend towards increasing “complexity” or “intelligence” in the fossil record is flawed. He points out that starting from any sufficiently extreme feature and working back will always seem to indicate a trend towards that feature. A common asserted phenomena here is what is referred to as “increasing encephalisation quotient or EQ”. EQ is a measurement of the size of the brain cavity compared to body mass. It appears that looking back through the fossil record, EQ increases. So it seems that evolution is selecting for greater and greater relative brain sizes and therefore intelligence. However Lineweaver asks us to consider what would happen if we were not humans but elephants in which case we would be more interested in what he calls the the “nasalation quotient” (NQ) which the fossil record shows there has been a gradual trend of increase in as well. All of the ancestors of modern elephants had shorter trunks, showing a trend in evolution towards towards long trunks. The point is that the trend towards longer noses is not a trend at all. Incrased NQ is an illusion. We can see that easily for our trunks are no longer than those of our ancestors and it seems there is no reason why trunks of modern elephants should continue to get longer, or shorter or remain the same. That is the point of random mutations and natural selection. The “trend” of long noses in elephants is not a general trend that occurs across species and certainly cannot be regarded as a convergent feature of evolution. Similarly human like intelligence cannot be convergent despite the insistence that there seems to be a trend.
So although there might be a lot of planets out there - and habitable ones - the question of whether there is intelligent life out there isn't so much a question of planets and astronomy. It's a question of evolution and here the biological numbers just swamp the astronomical ones.
This post is just an edited part of a much larger article on life elsewhere in the universe you can find here.