How to Make a Human Being: A Body of Evidence

2 | For reasons not entirely understood, a patch of energy escapes the eternally inflating quantum landscape and expands into a universe of space and time. The energy evolves into energy of different kinds. An array of different kinds of particles come into existence spontaneously and randomly out of nothing, among them quarks, gluons and photons. As the universe expands the particles evolve into other kinds of partlcles. Under gravity the first light atoms – hydrogen and helium mostly – are drawn together as quasars and stars. Gravity arranges the stars as galaxies and galaxy superclusters. The stars catch fire as great furnaces that transmute light atoms into heavier atoms. When some of the stars of the right size die and explode the first heavy atoms are spewed into space. Gravity acts on the light atoms and the newly forged heavier atoms to make a second generation of stars. Some of these stars of the right size explode and release even heavier atoms into space. And repeat. Simple compounds like water, carbon dioxide and some amino acids are also produced. How the simple compounds evolve into DNA is not yet known (it may never be known), but few doubt that evolution is how it happened. Human beings tell a plausible, if patchy, tale of how hydrogen was woven into flesh. It may be the same tale aliens tell. Assuming that aliens are made of flesh. And tell stories.

3 | Physicists used to criticise biology for being too descriptive and not testable. All that changed with the advent of genetics and molecular biology. Less than a century ago natural selection alone could not account for the complexity of living forms, but genetic theory turned natural selection into a rigorously testable theory.

4 | Life used to distinguish biology from physics. For centuries, biology was plagued with the notion of vitalism, a mysterious – perhaps even mystical – force that was invoked to account for the seemingly unbridgeable gap between living and non-living forms. In his book What is Life? (1944) Erwin Schrödinger wondered if life might emerge out of some simple physics of self-replication and a single molecule. His book influenced many biologists. Francis Crick swapped physics for biology when he read it.

Genetics killed vitalism. Once we could explain how genes determine biological function, vitalism was redundant. The golden age of molecular biology was ushered in. The discovery of the gene revealed the astonishing relatedness of all living forms. It came as a great shock to biologists to discover how much genetic information is conserved across all life forms. Our gene sequences are 95 per cent identical to those of yeast. The difference between yeast and a human being is largely a matter of organisation. We share 50 per cent of our genetic code with a banana. Sixty per cent of genes in humans that are associated with disease have a homologous gene – i.e. the same gene making the same proteins and doing a similar job – in the fly. The Krebs cycle – a vital part of the metabolic process – is common to all forms of life, from single-celled bacteria to flies and kangaroos. Within the Krebs cycle is a tiny epicycle that generates ATP, the chemical that powers muscle contraction and nerve impulse. ‘Where there is life,’ writes chemist P.W. Atkins, ‘there is ATP.’ These observations can be proclaimed either derisively or with wonder. Do we have more in common with flies and bananas than we have allowed ourselves to imagine, or do genes only take us so far? Or both?

5 | How did life on earth first emerge?

Nobody has a freakin’ clue.

Steen Rasmussen, physicist specialising in artificial life

I cannot myself see just how we shall ever decide how life originated.

Francis Crick

If you study life and you’re not flabbergasted by the hypothesis of evolution, you just haven’t looked carefully enough. I think it’s a tragedy that nobody walks into a class and expresses that wonder … We have no evidence that life is not a miracle, i.e. a very low probability event. And we should give that one away and say we don’t know the hell where it came from. If you want to believe in God at that point, it’s as good a theory as any other, but no one will say that, and that is a pity. It all has to be in the running.

From a conversation between a scientist and the author

The universe has the curious property of making living beings think that its unusual properties are unsympathetic to the existence of life when in fact they are essential for it.

John Barrow, theoretical physicist

The more I study the universe and study the details of its architecture, the more evidence I find that the universe in some sense must have known we were coming.

Freeman Dyson, theoretical physicist

6 | If any of the constants of nature – the mass of the electron, say – were different, so would the whole universe be.

7 | The early solar system was a violent place not conducive to life as we know it. A disk of dust orbiting our sun accreted under gravity and multiple collisions into numerous objects larger than dust-sized. After many such collisions and over a long period of time these numerous objects came together as planets and asteroids. Once these large objects had settled into fixed orbits and got out of the way of each other, the solar system gradually became a place where collisions are rare events rather than the norm.

Life on earth would not be possible if the earth’s orbit was a little more elliptical than it is. The earth would repeatedly travel outside the narrow zone in which life as we know it here is sustainable.

Life on earth was not possible until the earth’s temperature stopped varying so wildly. It took most of the earth’s 4,500-million-year history before this was the case. The gradual stabilisation of the amount of carbon dioxide (and other so-called greenhouse gases) in the atmosphere has played an important role in the stabilisation of the earth’s temperature. (Through the interference of humans, that stabilisation is being tested.)

Life would not be possible without water. At some point in the earth’s deep history the atmosphere begins to fill with water vapour. There is a day when it rains for the first time. Some of the oldest fossil remains are of the indentations of rain made in rocks discovered in India. They are at least three billion years old, though by then it had been raining for at least a billion years.

Life would not be possible if water did not have the unusual property of being less dense in its liquid state than in its solid state, otherwise the oceans would have frozen from the bottom up and killed all the first life of the seas.

A fine day is rare enough even in the Cotswolds, and certainly it is not the default state of the world anywhere very much. If it is an accident that the earth is conducive to life it is only because of innumerable accidents. Innumerable accidents begin to look suspiciously like no accident at all.

What we don’t know is if these conditions, and many others, would rule out life, or only life as we currently understand it.

8 | The numbers game doesn’t work for life. With our best current measuring devices we are beginning to find other solar systems with proto-earths, and so the probability of there being life elsewhere goes up. But the probability also goes down, because we have not yet found life. On all the proto earths discovered so far the conditions are hellish. For as long as we do not find life elsewhere, what we find instead are the ever-finer conditions needed for life here. The more closely we investigate the conditions that did not result in life elsewhere, the more finely tuned those conditions become for life as it appeared here. There is no evidence that life is not a miracle; or, in scientific language, a very low probability event.

How you answer the question: Is there life elsewhere? is a matter of taste.

There is nothing that God hath established in a constant course of nature, and which therefore is done every day, but would seem a Miracle, and exercise our admiration, if it were done but once.

John Donne (1572–1631), metaphysical poet

9 | Science doesn’t do miracles: better to believe that there are many universes with many different conditions, and that we necessarily inhabit one island universe – ideally one of many island universes – in which the conditions for life are as they need to be.

Science is the question, what does a world without the mediation of the supernatural look like?, repeated over and over again. If science were to accept irreducible mystery, or irreducible complexity, that would be to give up and admit defeat. Science moves into the unknown with the assumption that there is always something more that can be found out. To allow our universe special properties is one way of bringing scientific investigation to an end. To allow humans a privileged perspective is another. If humans have anything of the God-like about them they move beyond the reach of the scientific method. Instead, try to imagine a more encompassing universe in which the specialness disappears. And carry on. As science progresses, what the universe can be gets subtler and more mysterious. And as a by-product, the gods, God, and human beings become subtler too. If as scientists we were pragmatically to ignore what happens only once in the lifetime of the universe we would disavow ourselves and our planet home. Out of the scientific method, we humans never quite come into view. We can always look further out, but we humans seem always to be just over the horizon.