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Space Topics: Search for Extraterrestrial Intelligence

Response to "The Abundance of Life-Bearing Planets"

by Ernst Mayr

I fully appreciate that the nature of our subject permits only probabilistic estimates. There is no argument between Carl Sagan and myself as to the probability of life elsewhere in the universe and the existence of large numbers of planets in our and other nearby galaxies. The issue, as correctly emphasized by Sagan, is the probability of the evolution of high intelligence and an electronic civilization on an inhabited world.

Once we have life (and almost surely it will be very different from life on Earth), what is the probability of its developing a lineage with high intelligence? On Earth, among millions of lineages of organisms and perhaps 50 billion speciation events, only one led to high intelligence; this makes me believe in its utter improbability.

Sagan adopts the principle "it is better to be smart than to be stupid," but life on Earth refutes this claim. Among all the forms of life, neither the prokaryotes nor protists, fungi or plants has evolved smartness, as it should have if it were "better." In the 28 plus phyla of animals, intelligence evolved in only one (chordates) and doubtfully also in the cephalopods. And in the thousands of subdivisions of the chordates, high intelligence developed in only one, the primates, and even there only in one small subdivision. So much for the putative inevitability of the development of high intelligence because "it is better to be smart."

Sagan applies physicalist thinking to this problem. He constructs two linear curves, both based on strictly deterministic thinking. Such thinking is often quite legitimate for physical phenomena, but is quite inappropriate for evolutionary events or social processes such as the origin of civilizations. The argument that extraterrestrials, if belonging to a long-lived civilization, will be forced by selection to develop an electronic know-how to meet the peril of asteroid impacts is totally unrealistic. How would the survivors of earlier impacts be selected to develop the electronic know-how? Also, the case of Earth shows how impossible the origin of any civilization is unless high intelligence develops first. Earth furthermore shows that civilizations inevitably are short-lived.

It is only a matter of common sense that the existence of extraterrestrial intelligence cannot be established by a priori arguments. But this does not justify SETI projects, since it can be shown that the success of an observational program is so totally improbable that it can, for all practical purposes, be considered zero.

All in all, I do not have the impression that Sagan's rebuttal has weakened in any way the force of my arguments.

Now Read Carl Sagan's final comments
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