To the editors:

H. G. Wells—once a pupil of “Darwin’s bulldog” Thomas Huxley—was perhaps the first modern thinker to consider feedback between cultural and biological evolution. In his Time Machine (1895), the two posthuman species, the Eloi and the Morlocks, encountered by the Time Traveler in AD 802,701 represent the biological outcome of processes caused by cultural forces. A bifurcation occurred in the biological morphospace, the space of all possible lifeforms, creating two species adapted to their physical and cultural surrounding—entirely consistent with the previous biological, chemical, and cosmological history. Two years later in The War of the Worlds, Wells suggested that a perfectly advantageous evolutionary strategy for the Martians threatened by extinction would be to expand into a new ecological niche by colonizing other planets, notably Earth.

What led Wells to these majestic astrobiological speculations, inspiring not only hordes of science fiction buffs, but also many scientists around the world to this day? The answer is simple: firm beliefs in both the Copernican principle and the universality of evolution. Over the incredible spans of both space and time, conceptually simple regularities discovered by Charles Darwin and Alfred Russel Wallace would hold, producing an unimaginably rich variety of outcomes. Some of these outcomes we may find morally offensive, but that is only an expression of anthropocentrism, nothing more. The Copernican assumption is needed in order to weigh the typicality of a particular outcome. In his excellent essay, Lawrence Krauss argues that the inference of design on the basis of fine tuning is unwarranted, and that scientists should be careful in employing fine-tuning arguments to explain observed regularities in the universe.

Krauss’s dig hits the spot—but it should go deeper. The topic must be approached from both cosmological and astrobiological perspectives. Seeking nonexistent design in everything is a symptom of an underlying malady of our age, which will remain dangerous even if the symptoms are successfully removed by palliative measures. When Krauss writes, “If Darwin and Wallace have captured the allegiance of the scientific community, their triumph has not been entirely complete beyond it,” he is undoubtedly correct. Before Darwin and Wallace, there were Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Christiaan Huygens. Their lessons have not been entirely taken to heart either; their triumph—arguably the greatest in the history of science—has not been entirely completed beyond the halls and labs of science. To a large degree, most of human life unfolds today on the background of an anti-Copernicanism as strong as any member of the Inquisition that tried Galileo could desire. Today’s social, legal, and political institutions are heavily anthropocentric and hence anti-Copernican, as apparent in the uphill battle waged by proponents of animal rights. Modern arts and literature are to a large degree anthropocentric and hence anti-Copernican, as demonstrated in the resistance encountered by those authors, mostly relegated to underappreciated genre fiction, who dared to openly question or reject the dogma.¹ Naked geocentricism often crawls into discussions on the future of humanity, in particular coupled with anti-humanist deep ecology, but the response to these ideologies should be no, Earth is not necessarily the only home for humanity and yes, we should seek other habitats for our descendants in the Solar System and in the Milky Way. The appalling lack of enthusiasm for human spaceflight in most of the world and the recent increase in opposition to cosmic exploration and colonization can also be partly explained by anti-Copernican sentiment.²

The development of modern astrobiology since 1995 has shown how tightly evolution is connected with Copernicanism. It is no coincidence that Wallace, Darwin’s co-discoverer of natural selection, was a pioneer of astrobiology as well. In two books published late in his life, Man’s Place in the Universe (1904) and Is Mars Habitable? (1907), Wallace argued for specific positions regarding astrobiological issues on the basis of evolution. While his conjecture that terrestrial life is enabled by the unique position of Earth in the galaxy has been disproved by subsequent astronomical progress, his negative verdict on Martian life remains to be decisively confirmed or falsified. Wallace’s more fundamental point remains salient, nonetheless: researchers should apply our terrestrial understanding of biological evolution to other habitats in the universe, in the same manner as astrophysicists apply laboratory data on, say, spectral line widths of ions to the study of distant quasars—that is, in a profoundly Copernican manner.

But what about the fish? When Krauss asserts that “the fish might ask why its world is made of water,” he is talking about a rather primitive, pre-Copernican fish, which sees itself as a pinnacle of creation. Clearly, that is a parochial, not universal, construal. As Hercule Poirot would say, “It is not important who the suspect is. It is important who the suspect is.” We must think about the emphasis: it is unimportant whether the observer is a human, or a fish, or an AI, or an alien from the Zeta Reticuli planetary system. It is important what the essence of their observership is, what makes them observers.

A more sophisticated construal could emphasize, for instance, how similar fish and humans are in the context of Earth’s biosphere. In the tree of life the rather large branch of fish is indeed very close to the small branch of mammals, which includes humans. Both are very distant to the major parts of the biosphere, branches corresponding to microorganisms, Arthropoda, and Mollusca. Humans have many things in common with fish, many more than we would have with any realistic extraterrestrial intelligence. And yet, even the most alien of possible alien intelligences still benefits from the same universal desiderata that determine the survival of both humans and fish: complexity, stability, and a degree of predictability. That these desiderata are prima facie improbable is a fact. Their origin remains a puzzle to be resolved even after researchers discard naive design inferences.

This is indirectly demonstrated by those anthropic arguments which are not cosmological in scope. There are many such instances in the generalized domain of astrobiology, including planetary science and macroevolutionary trends, as well as in unrelated fields such as risk analysis or economics. Recent developments in planetary sciences, both within the solar system and as applied to the large number of extrasolar planets, have been fueled by the quest for habitability. According to NASA’s Astrobiology Roadmap:

We must move beyond the circumstances of our own particular origins in order to develop a broader discipline, “Universal Biology.” Although this discipline will benefit from an understanding of the origins and limits of terrestrial life, it also requires that we define the environmental conditions and the chemical structures and processes that could support life on other habitable planets. Thus we need to exploit universal laws of physics and chemistry to understand polymer formation, self-organization processes, energy utilization, information transfer, and Darwinian evolution that might lead to the emergence of life in planetary environments other than Earth. ... Some conditions that support chemistry that is sufficiently rich to seed life might be detrimental to self-organization of biological structures. Conversely, conditions that promote the emergence of biological complexity might be unfavorable to organic chemistry. Thus an integrative, interdisciplinary approach is necessary to formulate the principles underlying universal biology. The perspectives gained from understanding these principles will markedly improve our ability to define habitability and recognize biosignatures.³

The probability of observing specific properties of an environment—any environment in the universe—is linked with the number of observers, whose density is a consequence of complex evolutionary processes which could not be reasonably accounted for in the conventional causal manner. While planetary parameters play the determining role in the degree of habitability,⁴ they cannot be said to cause it in the proximate sense. The relationship is similar to the one between fundamental physical constants like the Planck constant, the coupling constants of forces, or masses of quarks and leptons in the Standard Model, on one side, and properties of the ¹²C nucleus, exhibiting the famous fine tuning discovered by Fred Hoyle and mentioned by Krauss, on the other side. In theory, fine tunings of energy levels in the ¹²C nucleus are reductively explained by properties and, consequently, fine tunings of fundamental constants. If one accepts ontological reductionism, there is no other mystical factor there: the ¹²C nucleus consists of quarks and gluons obeying dynamical laws prescribed by quantum physics and nothing else, so its properties, including controversial ones, are reducible to the properties of the constituents. In practice, however, the ¹²C nucleus is orders of magnitude too complex and a too strongly interacting quantum system to be understood in terms of these basic ingredients. We cannot track the fine tuning in the ¹²C nucleus down to the fine tuning of fundamental constants. We are thus justified in treating the fine tuning in ¹²C as a distinct phenomenon. We have reached that conclusion exactly by discarding any mystical or nonphysical ingredient.

As a further example along the same lines, one might argue that the first anthropic argument in planetary sciences appeared centuries ago, in the work of French philosopher and naturalist Bernard Le Bouyer de Fontenelle. His Conversation on the Plurality of the Worlds was published in 1686, a year before the revolution inaugurated by Isaac Newton’s Principia.⁵ In a single paragraph , Fontenelle deals with the habitability of the Earth:

In the next places, the reason why the planes of their [comets’] motions are not in the plane of the ecliptic, or any of the planetary orbits, is extremely evident; for had this been the case, it would have been impossible for the Earth to be out of the way of the comets’ tails. Nay, the possibility of an immediate encounter or shock of the body, of a comet would have been too frequent; and considering how great is the velocity of a comet at such a time, the collision of two such bodies must necessarily be destructive of each other; nor perhaps could the inhabitants of planets long survive frequent immersions in the tails of comets, as they would be liable to in such a situation. Not to mention anything of the irregularities and confusion that must happen in the motion of planets and comets, if their orbits were all disposed in the same plane.⁶

To the question, “Why are (observed) orbits of comets highly inclined, in contrast to the coplanar planetary orbits?” Fontenelle offers a deceptively simple answer: humans would not be here to contemplate the peculiarities of cometary trajectories if these orbits were different—that is, similar to those of planets. Our existence in a stable environment selects some particular planetary and cometary configuration out of the entire set of all such configurations possible under the dynamical laws. There is no reason to believe that the observed configuration will be typical or average. This passage of Fontenelle was published eight years prior to Edmond Halley’s celebrated suggestion of 1694, that comets may crash into planets with catastrophic consequences. Today, it is clear that the frequency of impacts is a major component of the habitability of all planets, including Earth, Mars, or even the Mars of Wells’s novel.

Fontenelle’s argument for a high inclination of cometary orbits is an excellent illustration of the entire paradigm of regarding the anthropic principle as selection effect, having nothing to do with the alleged metaphysical notion of design. Anthropic arguments and the wider field of anthropic reasoning is deeply Copernican and hence anti-anthropocentric. The similarity of the words should not fool us. Anthropic arguments are both evolutionary and valid irrespectively of whether observers are human beings, intelligent fishes, superintelligent AIs, Eloi, Morlocks, or invading Martians. They highlight instances of dysteleological selection effects and point scientists toward research into islands in the parameter space—created by spontaneous symmetry breakings and unknowable boundary conditions—rather than toward particular points. It is the sailing among those islands which is the truly important and intriguing part of this emerging confluence between cosmology and astrobiology.

Milan Ćirković

Lawrence Krauss replies:

I thank Milan Ćirković for this thoughtful and illuminating letter, which I found quite fascinating. There is much that I agree with here, and little to elaborate on. The notion of the anthropic principle as a selection effect rather than evidence of design is of key importance. Clearly in some sense the inherent connection between anthropic arguments and astrobiology is quite relevant, and may in the future become more important. I would only suggest that at present there is a great deal more known about cosmology than astrobiology. The latter is a nascent field with very little in the way of observational data, and much theoretical speculation. At present it is uncertain whether any other planets are truly habitable by biological systems like ours, or if not, what different types of life, much less intelligence, may exist in other systems. Finally, scientists do not yet fully understand the origin of life on Earth, so it is difficult to more than vaguely speculate about the origin of life elsewhere. Clearly there is a lot to learn, and it is important that we not be distracted by metaphysical gobbledygook in the process, a subject on which the writer of this letter and I are in full agreement.