Darwin's universal appeal

In anticipation of Darwin's bicentennial next year, Martin Rees praises the accessibility of the scientist's ideas and asks whether the natural selection he described on Earth points to life among the stars

We celebrate the 200th anniversary of Charles Darwin’s birth in 2009. The only other British scientist with comparable fame and influence is Sir Isaac Newton and, in terms of sheer IQ, he would have ranked way ahead - he had a singular intellect.

But I’m happy that it is Darwin, not Newton, who is being feted. There are two reasons for this: first, Darwin was an engaging and admirable character. He had a fulfilled life – adventurous early on, he spent five transformative years on the HMS Beagle, followed by serene and productive decades at Down House in Kent. Cosseted by his family, he was surrounded by his plants, barnacles and other creatures, and was in contact with a worldwide network of naturalists. In contrast, Newton was solitary and unbalanced when young, and vain and vindictive in his later years.

Darwin’s second attraction is the accessibility of his ideas. Newton wrote in Latin and his mathematics were challenging to the greatest scholars of his time. Darwin, in contrast, is neither arcane nor impenetrable. His great unifying idea, natural selection – called by Dan Dennett with only slight hyperbole “the best idea anyone ever had” – can be grasped by anyone. Darwin’s influence pervades contemporary culture more than any other figure in science.

Darwin’s work, like all great science, was based on hard slog and a focus on detail. But in The Origin of Species it’s distilled into a broad vision, conveyed in elegant prose. The book was published in 1859, so 2009 marks not only Darwin’s birth but also the 150th anniversary of his greatest work. The impact of this bestselling book on general culture was immediate and profound - and resonates even more today.

It is a scientific tragedy that Darwin never learnt about Gregor Mendel’s work, which laid the foundations for modern genetics. But Mendel’s 1866 paper, 'Experiments with plant hybrids', which reported the classic experiments he conducted in his monastery garden, appeared in an obscure learned journal and wasn’t properly appreciated for decades.

Darwin's concept of natural selection was, when first promulgated, 'just a theory'. Now, however, it is underpinned by a vast body of evidence and deeper understanding of genetics. Inferences from traditional palaeontology are now complemented by studies of ancient DNA. And natural selection, which in animals and plants operates over ‘geological’ time spans, can be directly observed - on a timescale of years - in, for instance, HIV.

We can trace the key stages in life’s development on our planet. The simplest organisms seem to have emerged within 100 million years of the final cooling of the Earth’s crust, about four billion years ago. But about two billion years seems to have passed before the first eukaryotic (nucleated) cells appeared, and a further billion before multi-cellular life. Most of the standard body types seem to have first appeared during the ‘Cambrian explosion’ a little more than half a billion years ago. The immense variety of creatures on land have emerged since that time – punctuated, of course, by major extinctions such as the event 65 million years ago that wiped out the dinosaurs.


The Origin of Species closes with these famous words: "There is a grandeur in this view of life ... Whilst this planet has been cycling on according to the fixed law of gravity, from so simple a beginning, forms most wonderful, forms most beautiful, have been and are being evolved."
As always in science, each advance brings into sharper focus a new set of questions.

Pre-eminent among them is the origin of the very first life: the transition from the non-living into the first entities that were able to reproduce and evolve. Darwin himself speculated about chemical processes in a “warm little pond”. There is still no consensus; however, many now favour a less quiescent origin in hot springs or volcanic vents.

The “simple beginning” of Darwin's great story - the young Earth, with its complex chemistry and geology, cycling on in its orbit - is itself a complicated system. It is the role of astronomers to probe back still further and to set our entire solar system in a broader context of space and time. Here, there has been immense progress since Darwin’s day. We are starting to understand how, starting from some still-mysterious ‘genesis event’ nearly 14 billion years ago, atoms, galaxies, stars and planets emerged. On one planet, our Earth, life began. Darwinian selection led to the emergence of creatures able to ponder their origins.
Next year is an important anniversary for astronomers as well: it marks the 400th anniversary of Galileo’s first telescope.

So 2009 is both Darwin Year and the International Year of Astronomy. It is therefore a year when we should surely focus on the great question that links the two themes and one that perennially fascinates the public: is there life elsewhere?

Does the Darwinian process operate not just on Earth but elsewhere in the cosmos? Would we expect life to emerge on any planet in the right temperature range, where there is water, along with other elements such as carbon? At present, this question is open. As often in science, lack of evidence leads to polarised opinions in the community, but agnosticism is really the only rational attitude while we know so little about how life began, how varied its forms and habitats could be and what evolutionary paths it might take.

There is nothing new about speculation about a ‘plurality of worlds’. In 1584, Giordano Bruno, a Dominican monk, published a book called On the Infinite Universe and Worlds. In February 1600, he was burnt at the stake in the Campo de Fiori, Rome, where he is now commemorated by a fine bronze statue. Among Bruno’s conjectures was that: "There are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours.” In the last years of the 20th century, his prescient belief was vindicated: powerful modern telescopes have revealed that there are, assuredly, planetary systems orbiting many other stars. Indeed, astronomers have strong reason to suspect that there are billions of planets, orbiting Sun-like stars, which resemble the young Earth.

Bruno had a further conviction: "No reasonable mind can assume that heavenly bodies that may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human earth." Our conception of the physical universe has been transformed since Bruno’s time, but we still cannot gauge the likelihood of extraterrestrial intelligence.

I think there is a real chance of understanding the origin of life so that we will know whether life is a ‘fluke’ or whether it is near-inevitable in the kind of initial ‘soup’ expected on a young planet. But there is a second separate question that may be even more difficult: even if simple life exists, what are the odds against it evolving into something that we would recognise as intelligent?

Even if simple life existed on many planets around nearby stars, complex biospheres like Earth’s could be rare: there could be some ‘bottleneck’, some key stage in evolution, that is hard to transit or bypass. Perhaps it is the transition to multi-cellular life. The fact that simple life on Earth seems to have emerged quite quickly, whereas even the most basic multi-cellular organisms took nearly three billion years, suggests that there may be severe barriers to the emergence of any complex life. Or the ‘bottleneck’ could come later. Even in a complex biosphere, the emergence of intelligence isn’t guaranteed. If, for instance, the dinosaurs hadn’t been wiped out, the chain of mammalian evolution that led to humans may have been foreclosed and we cannot predict whether another species would have taken our role.

Some evolutionists regard the emergence of intelligence as a contingency - even an unlikely one - and that, if the history of life on Earth were to be ‘re-run’, it might not have appeared. Others dissent from this line, however. In the latter camp is my Cambridge colleague Simon Conway Morris, co-discoverer of the extraordinary variety of Cambrian-era life forms in the Burgess Shale in Canada. He is impressed by the evidence for ‘convergence’ in evolution (for instance, the fact that Australasian marsupials have placental counterparts in other continents) and argues that this might almost guarantee the emergence of something like us. He writes: "For all of life’s plenitude, there is a strong stamp of limitation imparting not only a predictability to what we see on Earth but by implication elsewhere."

Perhaps more ominously, there could be a ‘bottleneck’ at our own present evolutionary stage – the stage when intelligent life starts to develop technology. If so, the future development of life depends on whether humans survive this critical evolutionary phase. This does not mean that the Earth has to avoid a disaster, only that, before this happens, some humans or advanced artefacts have spread beyond their home planet.

Searches for life will justifiably focus on Earth-like planets orbiting long-lived stars. But science fiction authors remind us that there are more exotic alternatives. Perhaps life can flourish even on a planet flung into the frozen darkness of interstellar space, whose main warmth comes from internal radioactivity (the process that heats the Earth’s core). There could - as Fred Hoyle envisaged in his classic novel The Black Cloud – be diffuse living structures, freely floating in interstellar clouds; such entities would live (and, if intelligent, think) in slow motion, but nonetheless may come into their own in the long-range future.

Life on a planet like ours will (if it has not by then escaped) be doomed when its central Sun-like star exhausts its fuel and, in its final convulsions, blows off its outer layers. Such considerations remind us of the transience of inhabited worlds and also that any seemingly artificial signal could come from super-intelligent (though not necessarily conscious) computers, created by a race of alien beings that had already died out.

It would in some ways be disappointing if searches for alien intelligence were doomed to failure. On the other hand, it would boost our cosmic self-esteem: if our tiny Earth were a unique abode of intelligence, humanity would then have greater cosmic significance than it would merit if the galaxy already teemed with complex life. Darwin himself wrote that: "Judging from the past, we may safely infer that not one living species will transmit its unaltered likeness to a distant futurity." And artificial genetic modifications can now induce far faster changes than natural selection. There is abundant time for post-human intelligence (organic or silicon-based) to spread through the entire galaxy. Even if life were now unique to Earth, we shouldn’t conclude that it was a trivial afterthought in the universe. There is still plenty of time. Astronomers may learn, before this century is out, whether Darwinian evolution is a unique feature of the ‘pale blue dot’ in the cosmos that is our home - or whether Darwin’s writ runs, as Newton’s does, in the wider universe. 

Martin Rees is Astronomer Royal and president of the Royal Society