Although I suspect that intelligent life is rare in the cosmos, I’m playing little more than a hunch. So it’s interesting to see that Andrew Watson (University of East Anglia) has analyzed the chances for intelligence elsewhere in the universe by looking at the challenges life faced as it evolved. Watson believes that it took specific major steps for an intelligent civilization to develop on Earth, one of which, interestingly enough, is language. Identifying which steps are critical is tricky, but in the aggregate they reduce the chance of intelligence elsewhere.

A linguist at heart, I wasn’t surprised with the notion that the introduction of language marks a crucial transition as intelligence develops. But what are the other steps, and how do they feed into the possibility of life elsewhere? These interesting questions relate to how long the biosphere will be tenable for life as we know it. If, as was thought until relatively recently, Earth might support life for another five billion years, we would have emerged early in the history of our biosphere. But it is now believed that in perhaps a billion years, the era of complex macroscopic life will be ending, the victim of decreasing CO2 and increasing temperatures.

Startlingly, we’re faced with the fact that the Earth’s biosphere is even now in its old age. Here’s Watson’s take on the matter:

The question of the future life span of the biosphere has relevance to estimates of the likelihood that complex, perhaps intelligent, life evolves on a given planet. At present, Earth is the only example we have of a planet with life, and the fact that our own existence depends on Earth having developed complexity and intelligence introduces an anthropic “self-selection” bias into our sample of one… If we learned that the planet would be habitable for a set period and if we had evolved early in this period, then even with a sample of one, we might suspect that this suggested evolution from simple to complex and intelligent life was relatively likely to occur. By contrast, however, it is now believed that we evolved late in the habitable period; this suggests that our evolution is a comparatively unlikely occurrence.

The model Watson analyzes assumes that on planets where intelligence arises, its evolution is governed by the need to pass through a number of critical transitions, each of which are unlikely to occur in the time available. Critical steps might be events like the transition from unlinked replicators to chromosomes, or the differentiation of the eukaryotic kingdoms of plants, animals and fungi in the late Proterozoic. A number of essential evolutionary steps are suggested, the common thread being that they all involve increases in structural and genetic complexity.

An alternative to the critical step model would suggest that the evolution of intelligence is simply long and slow. The problem with that idea is our old friend Fermi, whose paradox would force us to ask why we see no signs of intelligent activity around us in the cosmos. For intelligence under the alternative model should have evolved on planets somewhat older than ours, whereas if the critical step model is right, then the passage through the steps becomes a tremendous roadblock to intelligence. The transition from biogenesis to observerhood is tightly constrained.

Why? Back to Watson, who uses the lottery analogy, explaining that each step in the process conditions what follows:

In terms of the lottery analogy, we need to condition our observations on winning not just the lottery of biogenesis, but several subsequent lotteries as well, in which tickets are only issued to those who have won in the previous round. In such a model, the constraint on absolute probability given by an early win in the first round becomes rapidly less useful as further rounds are added.

Is evolution a predictable movement toward intelligence? Watson doubts it:

There are numerous examples where complex traits have apparently been lost from organisms, and the question of whether increases in complexity are in fact any more intrinsically likely than decreases remains unresolved… From the perspective adopted here, this appearance of evolution as a monotonic “progress” toward ourselves results from “anthropic self-selection bias”… In this case, there is no need to postulate any directionality to evolution; and, in general, the kind of outcome seen on Earth may be vanishingly unlikely.

That, of course, has major implications for what we might expect to find around other stars. Vanishingly rare intelligence is the result of the evolutionary lottery taken through its repeated cyclings, and it’s noteworthy that in this model, even where intelligence does arise, it comes late in the history of the planet on which it appears. Thus civilizations find themselves in senescent biospheres, surrounded by other systems that may have some forms of life, but probably not intelligence.

The paper is Watson, “Implications of an Anthropic Model of Evolution for Emergence of Complex Life and Intelligence,” Astrobiology Volume 8, No. 1 (2008).