Wonders of Life. Andrew Cohen

relative lack of brightness, however, was deceptive. Beyond the visible and into the UV, the infant Sun was dazzling. This is because the Sun’s outer layers were much hotter than they are today, energised by the star’s higher spin rate giving rise to intense electromagnetic heating. Hotter surfaces radiate more of their energy in the high-energy, short-wavelength part of the spectrum – in other words, they are brighter in the ultraviolet.

      It is thought that the young Sun was seven times brighter in the ultraviolet during the first few billion years of its life. The UV flux at the top of the Earth’s atmosphere would have been similar to that experienced by Mercury today, a planet around 100 million km closer to the Sun. The composition of the young Earth’s atmosphere is not well known, but it is unlikely that it was able to absorb such high levels of UV radiation. This suggests that it would have been necessary for life to deal with an intense UV onslaught, which may in turn have driven the evolution of pigments at a very early stage in its history.

      The young Sun was seven times brighter during the first few billion years of its life. It would have been necessary for life to deal with an intense UV onslaught, which may in turn have driven the evolution of pigments at a very early stage in its history.

      A coronal mass ejection blasts off the surface of the Sun in the direction of the Earth, and is deflected by Earth’s magnetic field.

      The aurora borealis (northern lights) occurs when the solar wind – charged particles from the Sun – is drawn by the Earth’s magnetic field to the polar regions.

A UNIQUE AND COLOURFUL WORLD

      On 18 March 2011, after a seven-year journey around our Solar System, NASA’s Messenger space probe became the first spacecraft to orbit the tortured inner planet of Mercury. Six days later it reactivated its dormant instrumentation, switching on its powerful cameras and returning the first photograph ever taken from Mercury’s orbit. This pioneering spacecraft has sent back thousands of images from the closest planet to the Sun, revealing in extraordinary high definition its complex surface, pitted with craters. But these images also reveal a monochrome world; there is little colour to decorate its dusty, damaged surface.

      On its journey to Mercury, the Messenger spacecraft also flew by another of the inner planets – Venus. Again, despite all the acuity of modern technology, this is a planet painted with a limited pallet. A yellow fug shrouds another monochrome planet; a surface with texture but little colour.

      Rocks on Mars (left, taken by the Curiosity Rover), compared with rocks on Earth (right). Both images exhibit rounded gravel fragments, suggesting that Mars, like the Earth, once had surface water.

      While Messenger was travelling to the inner Solar System, looping around planet after planet to lose enough energy to break its fall towards the Sun, another great adventure was playing out on our sister planet, Mars. Images taken by the now iconic Mars Rovers Spirit and Opportunity have revealed in spectacular detail a planet rich in geology and promise – perhaps even populated by simple, sub-surface life – but limited in shades.

      Messenger did photograph one planet that broke the monochromatic mould, however: our very own planet Earth. Side by side, these images of the rocky planets are quite startling in their contrast; it is only our planet that displays a consistent eruption of colour. Ours is a world painted in colour – a rainbow landscape of greens, blues, reds, yellows and violets. Colour, it seems, is a product of life.

      A false-colour image of Venus, made by the Galileo Probe, and showing the planet's monochromatic sulphuric-acid cloud formations.

      European Space Agency Meteosat image of the Earth, showing its remarkably vivid colours, which contrast with the far more monochromatic palette of the other planets of our Solar System.

THE ORIGIN OF LIFE’S COLOURS

      Isaac Newton was the first to demonstrate that white light is made up of a multitude of colours when he famously revealed the rainbow hiding in sunlight in 1671 using a simple glass prism. This multicoloured rain illuminates everything on Earth, but why is life so good at selecting only certain colours to reflect into our eyes?

      As a particle physicist, I feel I am permitted to think of everything in terms of the interactions between particles. This is a sensible thing to do, since every experiment conducted in the history of science has shown that the elementary building blocks of nature are particles. To be sure, these particles do not behave like little grains of sand or billiard balls; they are quantum particles, and this allows them to exhibit wave-like behaviour. But they are particles nonetheless, and this applies to light as well as electrons, quarks and Higgs bosons.

      I will therefore choose to picture the light from the Sun as a rain of particles – an endless stream of photons that rain down on the surface of the Earth after a 150-million km journey from the surface of the Sun. At a subatomic level, when a photon hits something – a leaf, for example – it hits an electron around an atom or molecule and, if the structure of the molecule is just right, the photon will transfer all of its energy to that electron. If the structure of the molecule isn’t right, the photon will not be absorbed. In this way, only photons of certain energies interact and are absorbed, and those energies are determined by the structure of the molecules themselves.

      As we have already seen, a photon’s wavelength is directly related to its colour. So, another way of saying that pigment molecules interact only with photons of particular energies is to say that they absorb only particular colours of light, reflecting the rest away. This is how pigment molecules work – they interact only with photons of particular energies, and therefore absorb only particular colours of light.

      There is a dazzling array of pigment molecules in nature, from carotenes that colour a carrot orange, to polyene enolates, a class of red pigments unique to parrots. In some cases the animals and plants produce the pigments themselves, but in many cases they are absorbed into the organism through its diet. If flamingos didn’t ingest beta-carotene from blue-green algae in their diet, their trademark pink colour would quickly turn white.

      The selective nature of pigment molecules’ interactions with photons is the reason for life’s rich and varied colour palette. Think about a green leaf. We see it as green because green photons do not interact with the molecules in the leaf. Red and blue photons do – they are both absorbed by a pigment called chlorophyll. If the rain of photons falls on a surface that reflects the majority of them back (such as the feathers of a swan or the sclera of an eye) we perceive the surface as white. If the light falls on a surface that absorbs photons of all energies (such as a raven’s feathers) the surface appears black. The Mexican tiger flower (Tigridia pavonia) absorbs all but the lower-energy red photons in sunlight, and so this flower is red. The feathers of the Mexican blue jay (Aphelocoma wollweberi) absorb low-energy photons but reflect the higher-energy blue photons back into your eye.

      A light micrograph of melanocytes (pigment cells), which produce the pigment melanin. It is melanin that absorbs the harmful ultraviolet rays found in sunlight.