Authors: Bill Bryson
In spite of this new-found optimism, we still lack an accepted theory of life’s origin. In 1859, Charles Darwin gave a convincing theory of how life has evolved over billions of years from simple microbes to the richness and diversity of the biosphere we see today, but he pointedly left out of his account how life got started in the first place. ‘One might as well speculate about the origin of matter,’ he quipped. Nevertheless, he did outline the germ of an idea, by referring to ‘a warm little pond’ in which all manner of chemicals might accumulate and, driven by the energy of sunlight, would react to form ever more complex molecules. Over an immense period of time sufficient chemical complexity might eventuate that the ‘soup’ would make the transition from non-living to living (whatever that transition may be – nobody knows).
Darwin’s casual suggestion became the ‘primordial soup’ theory of life’s origin, developed by J.B.S. Haldane and Alexander Oparin in the 1920s. The theory was put to an interesting experimental test in 1952, when Stanley Miller, then a student of Harold Urey at the University of Chicago, sought to re-create the conditions on the primeval Earth by putting methane, ammonia, hydrogen and water in a flask and sparking electricity through it for a week. Miller was delighted to discover a red-brown sludge of organic gunk in the flask, from which many amino acids were identified. Amino acids are the building blocks of proteins, and some scientists saw the Miller–Urey experiment as the first step on the road to life down which a simple chemical mixture would be inexorably conveyed by the passage of time. Many pre-biotic soup experiments have since been performed under various conditions (we now know that the early Earth did not have an atmosphere quite like that assumed by Miller). It turns out to be easy to make amino acids; in fact, they are even found in meteorites. Much harder, however, is to produce long proteinous chains (peptides), or the building blocks of RNA and DNA. Some scientists are still hopeful that ‘more of the same’ would create life given enough time, but others are sceptical that simply zapping chemicals willy-nilly with energy will turn a non-living mixture into a living cell. It is often remarked that we may soon be able to make life in the laboratory using existing microbes as a blueprint and reconstructing a new organism piecemeal. (Viruses have already been made that way, but viruses do not satisfy some definitions of life because they lack the ability to reproduce unaided.) While that may be true, and is clearly possible in principle, it would not solve the problem of how Mother Nature performed the trick without fancy equipment, trained biochemists and a clear plan of action.
From the point of view of the Copernican principle, we do not need to know the details of biogenesis, only how probable it is given plausible pre-biotic conditions. Is life on Earth the result of a freak chemical accident, or are there general principles that favour the emergence of organised complexity, and thereby facilitate the formation of life ‘against the raw odds’ computed from random shuffling of building blocks? Such a ‘life principle’ (essentially Copernicus’ principle for biological systems) is often mooted, but there is no hint of how it might be derived from the known laws of physics and chemistry. Nevertheless, the science of complexity is in its infancy, and it may be that there are general principles of complex organisation that are not yet understood. It is frequently pointed out that the elements needed for life – primarily carbon, but also oxygen, nitrogen, hydrogen, phosphorus and sulphur – are common in the universe, and that even simple organic molecules have been found in interstellar clouds. Sometimes this is used to argue that life must therefore also be common, but that is to confuse a necessary with a sufficient condition. To be sure,
these substances are necessary for life, but it may require all sorts of other materials, and special conditions, before the basic building blocks self-assemble into the hugely elaborate structure of a living cell. It’s easy to make bricks, but making houses requires far more than throwing a pile of bricks in the air.
If life were discovered on another planet, it would offer support for a life principle. There is, however, a caveat. By common consent, Mars offers the best hope for finding extraterrestrial life in the near future. Unfortunately, it may not settle the matter. Mars and Earth trade rocks blasted off their surfaces by asteroid and comet impacts, and hurled into orbit. A couple of dozen Mars meteorites have been found on Earth so far. During geological history, a prolific traffic of material has taken place between the two planets, mostly Mars to Earth on account of Mars’ lower gravity, but some the other way too. It has become clear in recent years that microbes could hitch a ride this way. Cocooned within a rock, a microbe would be shielded from the harsh conditions of interplanetary space, especially the radiation, and could remain viable even after a sojourn of some millions of years orbiting the Sun. It seems inevitable that living terrestrial microbes will have been delivered to Mars this way, especially before 3.5 billion years ago when the bombardment by cosmic debris was far higher than it is today. Conversely, if there was once life on Mars, it will have spread to Earth. The intermingling of the two biospheres complicates the story of life’s history though. It may be that life started on Mars and later came to Earth, or vice versa, or that life started from scratch independently on both planets, but became cross-contaminated. Only if there is clear evidence for two independent origins would the discovery lend support to the life/Copernican principle.
While we wait (possibly a very long time) for Mars to be explored for life, and perhaps evidence for a second genesis, there is a way that the life principle can be tested right here on Earth. No planet is more earthlike than Earth itself, so if life does pop up on cue in earthlike conditions, it
should have emerged many times over on our home planet. Biologists have long assumed that all life on Earth has descended from a single common origin. Gene sequencing confirms that all known organisms are genetically linked and can be positioned on a universal tree of life. However, the vast majority of species are microbes, and only a tiny fraction of these has even been characterised, let alone sequenced. You can’t tell by looking what they are made of. It is entirely possible that some terrestrial microbes are the products of different biogenesis events, in effect ‘alien organisms’, constituting a type of shadow biosphere. The universal tree of life on Earth might actually be a forest. The identification of a single microbe that is sufficiently alien for us to rule out a common origin with standard life, would have sweeping consequences. It would establish the Copernican principle for biology and point to a universe teeming with life.
And that brings me to the tantalising question of whether we are alone in the universe, as Monod claimed. When it comes to
intelligent
life, the status of the Copernican principle is very uncertain indeed. Even if life has got going on many planets, there is no known law or principle that compels it to evolve intelligence or sentience. The Darwinian mechanism implies that evolution is blind; nature cannot ‘look ahead’ and strive for the goal of intelligence, or any other trait. So there will be no progressive trend towards sentient beings like ourselves, unless it comes about because natural selection strongly favours certain features and structures, or if there are yet-to-be-discovered principles of organisation at work in nature.
Nevertheless, as always experiment must be the arbiter, and fifty years ago that experiment began with the inception of SETI – the Search for Extraterrestrial Intelligence. A small band of astronomers have been sweeping the skies with radio telescopes in the hope of stumbling across a radio signal from an alien civilisation elsewhere in the galaxy, so far without success. At the time SETI began in 1960, the general feeling was that life, let alone intelligent life, was exceedingly atypical for a planet. The sentiment
was summed up by the biologist George Gaylord Simpson in a 1964 article entitled ‘On the non-prevalence of humanoids’, in which he described SETI as ‘a gamble at the most adverse odds with history’. Today, SETI receives far more scientific backing, although the basic facts have changed little since Simpson wrote his article. We still don’t know whether the origin of life on Earth was a freak event and whether the evolution of human intelligence was a statistical fluke.
What, then, is our place in the universe as currently understood? As far as we can tell, our planetary system, galaxy and galactic environs are unexceptional out as far as our most powerful instruments can penetrate, over twelve billion light years. But our biological situation remains unresolved. The universe might be teeming with life, or it may turn out that life is very rare – intelligent life more so. It is even conceivable that we are alone in the vastness of space. If so, history will have turned a curious full circle. Before Copernicus, people believed that humans and their planet occupied pole position in the universe. It may yet be that we are privileged after all, in being the only place in the universe with intelligent life.
Is that as far as we can take the Copernican principle, to the edge of the observable universe? As I have commented, each new advance in astronomy has unveiled a universe even larger and more majestic than previously realised, but with instruments like the Hubble Space Telescope we are approaching a fundamental limit due to the finite speed of light. When we see a galaxy 12 billion light years away, we see it as it was 12 billion years ago. Light can have travelled at most 13.7 billion light years since the big bang, so if that explosive event represented the true origin of the universe, then there is an ultimate horizon beyond which we cannot see. That does not mean the universe comes to an end there, any more than a horizon at sea signals the edge of the world. But it does mean we cannot directly observe what lies beyond. An uncritical application of the Copernican principle would suggest that if by some magic we could be transported across the horizon we would find a region of the universe that looked much the same as our region, with stars, galaxies and galactic clusters uniformly distributed on the largest scale of size. But inevitably this raises the question of how far we can extrapolate. Does this pattern continue to infinity, or is there some variation?
The attempt to construct proper mathematical models of the universe based on the best understanding of gravitation began shortly after Einstein published his general theory of relativity in 1915. For many
decades the default assumption was that the Copernican principle applied all the way to infinity (it is called the cosmological principle when applied to gravitational models of the universe). But in the 1970s this conventional wisdom was challenged. The basis for the challenge was the development of a theory of the big bang based on the application of quantum mechanics to the very early stages of the universe. Quantum mechanics is normally reserved for microscopic systems like atoms and molecules, but the theory predicts that, at a sufficiently early time, it would affect the evolution of the universe too. That time is about a hundred trillion-trillion-trillionths of a second after the big bang. According to some variants of the theory, there would not be a single big bang, but a countless number of them scattered randomly throughout space and time. Each quantum event would nucleate a universe with a big bang, ‘like bubbles in an uncorked bottle of champagne’, to use the words of the physicist Leonard Susskind. The space between the bubbles would expand so rapidly that, even though the bubbles themselves expand, they would rarely intersect. Our own universe would be just one of those bubbles. The entire collection is known as the multiverse. In the most popular multiverse theory, the size of the bubbles is stupendous – about 10
10,000,000,000
km across. So once again, the scale of the universe has leapt dramatically, but by a far larger factor than the jump from pre-Copernican cosmology to the time of Hubble. Now we confront the same Copernican principle on a mega-scale: do we live in a typical bubble? Will the other bubbles be similar to ours?
The evidence from theory suggests no. Physicists are convinced that many features of the laws of physics, such as the masses of subatomic particles, the nature and number of forces, and the density of dark energy (the mysterious stuff that seems to be making the expansion of our universe accelerate) are ‘frozen accidents’ locked in when the universe cooled from the searing heat of the big bang. If the experiment were done again, so to speak, the masses and forces would come out differently;
there might even be a different number of spatial dimensions. Einstein once famously expressed his distaste for quantum mechanics by declaring that ‘God does not play dice with the universe’. In the multiverse theory He plays dice with
universes
(I am tempted to say He plays at randomly blowing bubbles). Taking a God’s-eye-view, the multiverse is a patchwork quilt, featuring bubble universes of all hues and textures, distributed across a fantastic range of possibilities. What we had taken to be universal immutable laws of physics turn out to be more like ‘local bylaws, valid only in our cosmic patch’, to use Martin Rees’ evocative description.
A key feature of the multiverse’s cosmic smorgasbord is that only a tiny fraction of bubble universes will possess the right laws of physics to permit life and observers to arise. Many prerequisites needed for life, such as abundant carbon, stable stars and a universe neither too hot or chaotic, but cool and inhomogeneous enough to permit galaxies to form, depend very sensitively on the precise values of the parameters that characterise the laws and the initial conditions of the quantum universe-nucleation process. The ‘Goldilocks enigma’ – why our universe’s laws and initial conditions are, amazingly, just right for life – has been a source of puzzlement for a long time. The multiverse theory could explain what otherwise looks suspiciously like a cosmic fix, in terms of an observer selection effect. It is no surprise that we find ourselves living in one of those very rare universes that have bio-friendly laws; we obviously could not inhabit a bio-hostile one.