How useful is the Copernican principle of mediocrity?

Reading John Gribbin's History of Western Science (The Folio Society 2006) recently, I was struck by his ringing endorsement of the Copernican principle (CP). This prompted me to ponder CP again, a principle widely held to be true, even essential, in science.

CP is a principle of mediocrity holding that neither we nor our earthly home occupy a privileged position in the universe and, on a cosmic scale, humans should not be considered to be special creatures. I have written before to shed doubt on the usefulness of CP and my current ponderings bolster that doubt.

CP is named after Polish astronomer and cleric Nicolaus Copernicus (1473- 1543) who initiated modern science with the publication of his famous book On the Revolutions of the Celestial Spheres in 1543. Copernicus proposed that the Earth is not at the centre of the universe but revolves around the sun.

This “demotion” of our locality was later amplified by the discovery that our sun is an ordinary star residing in the Milky Way galaxy which contains 1011 stars, many orbited by planets with environments suitable for life, in a universe containing 2x1012 galaxies.

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Evolved

It was also discovered that earthly life is not particularly special, being fully explicable in terms of interactions of matter and energy under the laws of physics and chemistry. And it was discovered that all life on Earth has evolved from a simple life form that seemingly arose spontaneously on Earth 3.5 billion years ago from lifeless chemicals, and that we human animals recently evolved from an apelike like ancestor.

The CP is not a scientific principle but rather a philosophical guideline. Although useful in some respects, I believe the assertion that humans are not special creatures is unwarranted. There are reasons also to suspect Earth could be a unique planet, or at least very special. Astronomical evidence published in 2016 indicates that Earth-like planets are far rarer in the universe than we had hitherto thought.

The search for extra-terrestrial intelligence (SETI) programme rests firmly on CP. SETI scans the cosmos for radio-signal broadcasts from extra-terrestrial civilisations. The reasoning is that since life arose spontaneously on Earth from non-living matter and the universe contains countless planets whose environments could support life, then life almost certainly arose on some of these planets and evolved into intelligent beings who wish, as we humans do, to broadcast this news. Some SETI enthusiasts assume that intelligent life is common in the universe.

However, the fact is we don’t yet know if any life, let alone intelligent life, exists elsewhere in the universe. And if life does exist elsewhere, we cannot assume that evolution of intelligence is inevitable.

Evolutionary life

Indeed, the evolution of human intelligence on Earth may well have been an enormously improbable fluke. The famous evolutionary biologist Stephen Jay Gould plausibly contends in his book Wonderful Life that evolution is such a contingent process that if we re-wound and replayed Earth's evolutionary life tapes we would end up with entirely different results. And the father of 20th century evolutionary biology, Ernst Mayr (1904-2005), although conceding that life could arise elsewhere in the universe, strongly asserted that the probability we will ever encounter intelligent alien life is effectively zero.

The most complex known physical object in the universe is the human brain. About half the 35,000 genes in human DNA are expressed in the brain. The brain has about 100 billion nerve cells, each connected with about 10,000 others. The number of interconnections in the human brain (1015) is vastly greater than the number of stars in our Milky Way galaxy (1011). It is unwise to assume such complexity will commonly evolve in the universe. It may happen, but we mustn’t assume.

Another principle, the Uniformity Principle (UP) holds that the same natural laws and processes that operate in our present-day scientific observations apply everywhere in the universe, now and in the past. This principle is closely akin to CP but more universal and science could scarcely operate without assuming uniformity in nature.

The existence of habitable planets elsewhere in the universe is within the scope of UP but to say that planets inhabited by intelligent life are necessitated would be an unwarranted stretch. Enthusiasts for alien life always quote CP. In my opinion, CP should not be accorded the same respect as UP.

William Reville is an emeritus professor of biochemistry at UCC