The Quiet Galaxy

Explanations · They were never there

Rare intelligence: evolution has no goal, and of the billions of species that have lived on Earth only one built anything

Proposed by George Gaylord Simpson, 1964 fi smaller

Evolution is not a ladder with us at the top. It is a branching process with no direction, and intelligence of the kind that builds telescopes is one outcome among millions, not the destination. Earth has carried life for about four billion years and complex animals for over half a billion; in all that time, among perhaps billions of species, human-level intelligence arose once, recently, and nearly died out. If that is typical, then even a galaxy full of living, animal-bearing worlds would have almost no one on it who could notice us. The silence would be the silence of forests and oceans, which are full of life and have nothing to say.

The case was made first by palaeontologists, the people who know the fossil record best. George Gaylord Simpson, one of the architects of modern evolutionary theory, wrote in 1964 that the search for other intelligences rested on a misunderstanding of evolution: there was no tendency toward minds, only toward whatever worked in a particular place at a particular time, and the long chain of accidents that produced humans would not repeat. Ernst Mayr, another founder of the field, said the same thing more bluntly thirty years later in a published argument with Carl Sagan. Of the roughly fifty billion species that Mayr estimated had lived on Earth, one had developed the kind of intelligence that could build a civilisation, and it had done so in the last few hundred thousand years. Sagan replied that if Earth is a fair example, planets teem with species and, given enough time, at least one of them will reach intelligence and technology, and that Mayr could not count Earth as evidence for one point and set it aside for another. Mayr answered that life on Earth refutes the idea that being smarter is always better: bacteria, single-celled organisms with nuclei, fungi and plants have never needed intelligence, and among the many groups of animals it arose in only one.

The modern form of the argument points at how slowly and narrowly intelligence appeared even once the ingredients were present. Large brains are expensive to run and risky to be born with. The human line nearly went extinct more than once. Other animals with large brains and complex societies, such as dolphins, elephants, crows and octopuses, have had as long as we have and show no sign of heading toward tools, fire or writing. Intelligence of the kind the Drake Equation cares about seems to have needed a particular mix of hands, social life, diet, climate stress and luck, and there is no reason to think that mix is common.

The argument bears on one factor: the share of living planets on which intelligence appears. It accepts that stars, planets and life may all be plentiful.

Against it, three points are usually raised. First, intelligence in the broad sense has evolved many times on Earth, in groups as distant as mammals, birds and molluscs; what arose once was not intelligence but technology, and the gap between the two may be a separate question. Second, half a billion years of animal life is not long compared with the age of the galaxy, and a planet that stays habitable for several billion years gives evolution many more chances. Third, once intelligence appears and starts to help its owners survive, it tends to be reinforced, so the first appearance may be the hard part and the rest may follow. The reply is that the first appearance is exactly the step being argued about, and one example of it is no basis for calling it likely.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

The test: what would strengthen or weaken it

Would strengthen it: Discovery of a second living world whose life has stayed simple or non-intelligent for billions of years; further evidence that intelligence on Earth depended on an unusual chain of events; a finding that other large-brained animals have been at their present level for tens of millions of years without change.

Would weaken it: A detected technological civilisation anywhere; evidence that intelligence on Earth arose independently in several lineages that were converging on tool use; a clear evolutionary argument that problem-solving intelligence is strongly favoured once animals exist.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Logged, no change By hand

    Rueda-Alaña and others (Science, 2025-02-13), with companion papers in the same issue, find that birds and mammals built similar brain circuits for complex cognition by separate developmental routes, which is the weakening test's 'intelligence arose independently in several lineages', with the caveat that the two share deep ancestry and common pressures. Mills and others (2025) remove the hard-steps support for the explanation. Logged both; the argument that no other Earth species built technology is untouched.

    Rueda-Alaña, 2025Mills, 2025 source 1source 2

How seriously it is taken

A mainstream position among evolutionary biologists, many of whom regard the optimism of astronomers about alien minds as naive; astronomers tend to reply that biologists are generalising from one planet. Neither side has evidence beyond Earth.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Checked 2026-10-16. Simpson's paper is correctly cited: Science 143 (3608), 769-775, 21 February 1964 (Crossref record for https://doi.org/10.1126/science.143.3608.769). The Mayr and Sagan exchange ran in 1995 in two consecutive issues of Bioastronomy News (the newsletter of the International Astronomical Union's commission on bioastronomy, published in the United States by The Planetary Society), volume 7, numbers 3 and 4, and Mayr's figure is there: "perhaps 50,000 million" (fifty billion) species since life began (the Spanish reprint at https://arxiv.org/abs/1012.2591, which gives the original citation). Corrected 2026-10-16: the body had Mayr answering Sagan that the dinosaurs had a hundred and fifty million years and never became intelligent. That argument does not appear in the exchange as reprinted (the dinosaurs come up there only as victims of the end-of-Cretaceous impact, in Sagan's part); it has been replaced with the reply Mayr actually gave, and Sagan's reply is now stated as it appears in the text. No change to the id, family, bears_on or the would_strengthen and would_weaken lines.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/rare-intelligence.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.