Wonders of Life. Andrew Cohen

its varying path-length through the atmosphere.

      Light is an electromagnetic wave; energy from the Sun sloshing back and forth between electric and magnetic fields and driving itself through the vacuum of space at just over 299,792 km per second. Being a wave, light has a wavelength, and it is different wavelengths of light that we see as different colours. Human eyes are sensitive to wavelengths between around 400 nanometres and 700 nanometres (a nanometre is a billionth of a metre). We see 450 nm light as blue, 500 nm light as green, and 600 nm light as orangey-red. Beyond the red, at longer wavelengths, lies the infrared. This is the radiation that we feel as heat, but cannot see. Some animals, such as the pit vipers, have evolved the ability to detect infrared light, but humans need night-vision goggles to do so.

      At the short-wavelength end of the spectrum lies the ultraviolet. There are many animals that can see this light, from birds and bats to insects. Some flowers are intensely colourful in the ultraviolet part of the spectrum, yet their beauty remains hidden to us. The Sun is intensely active in the ultraviolet, but much of it is absorbed by the Earth’s atmosphere and never reaches the surface. This is a good thing, because short-wavelength UV is potentially extremely damaging to living things. To see why this is so, it is easier to think of light in another way. Light can also be viewed as a stream of particles called photons. The particulate nature of light was discovered at the turn of the twentieth century by Albert Einstein and others, and was one of the first steps in the development of quantum theory. The ‘quanta’ of light are known as photons, and the smaller the wavelength of the light, the higher the energy of the photons. High-energy UV photons are like little bullets, smashing into biological molecules with more than enough energy to break them apart. This is the reason why UV light can be dangerous. As the wavelength lengthens, however, the relationship of UV light with life becomes more ambiguous. Long-wavelength ultraviolet light, known as UVB, is beneficial to life (our bodies use it to produce vitamin D), but, just like the shorter wavelengths (UVC), it can also be damaging. UV light certainly poses a challenge to living things.

      The red light of sunrise, seen from the Chihuahua-Pacific railway.

      The train climbs through the Copper Canyon in Mexico’s interior.

      As early morning turns to afternoon and we rise into thinning air, the colours of the world shift from warm reds to harsher blues.

      As the hours pass on the Chihuahua-Pacific and we rise into the Mexican interior, two things happen: the Sun rises in the sky and the atmosphere becomes thinner. As the Earth’s atmosphere absorbs and scatters light of different wavelengths by different amounts, this changes the relative mixtures and intensities of wavelengths of light to which life is exposed. In particular, as the Sun climbs in the sky and the train climbs in altitude, the amount of potentially damaging UVB light rises dramatically. I measured the flux of UVB during the train journey with a small detector called a digital radiometer. In the early morning, with the Sun low in the sky at sea level, I measured a UVB flux of 22 microwatts per square centimetre. This is the power delivered to each square centimetre of a living thing by the ultraviolet light from the Sun. As the train climbed with the rising Sun, the UVB flux climbed to 260 microwatts per square centimetre, because the high-energy UVB photons had less atmosphere to pass through and were therefore less likely to be scattered or absorbed. My body’s response to this onslaught is to produce a pigment known as melanin. In simple terms, I get a suntan.

      LIGHT SPECTRA

      Coloured scanning electron micrograph of a section through human skin. The darker layer contains cells called melanocytes, which produce melanin.

      The use of pigments such as melanin evolved very early in the history of life on Earth – forming a fundamental component of life. They are the way that life interacts with light and protects itself from harm.

      Melanin is found in virtually all animals. In humans, this dark pigment is found in a type of cell under our skin called melanocytes. As high-energy ultraviolet photons rain down on our skin, they have the potential to damage the sensitive molecules that lie beneath. DNA is particularly susceptible to damage from UV, with potentially deadly consequences, and melanin is the first line of defence. The secret to its ability to shield cells from the damaging effects of high-energy photons lies in its molecular structure. Melanin is a complex molecule able to form polymers with varying structures depending on their location in the body. Its active heart, however, is a series of rings of carbon atoms bound together by a sea of mobile electrons. When a high-energy photon from the Sun hits one of the electrons, it doesn’t break the molecule apart. Instead, the energy is dissipated in around a pico-second, which is very fast indeed. In a million millionths of a second, the potentially threatening photon has been adsorbed and all its energy has been converted to heat. The melanin molecule survives intact to fight another day. Melanin is so efficient that over 99.9 per cent of the harmful UV radiation is adsorbed in this way, protecting cells from damage.

      Melanin in its many forms is ubiquitous in nature; it is even found deep in the human brain, where its function is unknown. Even microorganisms such as bacteria and fungi employ melanin to protect themselves from UV radiation. This suggests that the use of pigments such as melanin evolved very early in the history of life on Earth – forming a fundamental component of life. They are the way that life interacts with light and protects itself from harm. While this would probably have been irrelevant for the very first life forms on Earth, which most likely lived deep in the oceans around hot-water vents, the dangers of UV light would have been one of the first challenges faced by life as first it rose to the ocean surface and then eventually colonised the land.

      This image of the Sun, taken on 5–6 June 2012 by NASA’s Solar Dynamics Observatory, shows the transit of Venus, an event that will not happen again until 2117.

A CHILD STAR

      The Sun’s chromosphere is the source of ultraviolet radiation. It is thought that, in the first few billion years of its life, the Sun was seven times brighter in the ultraviolet.

      Four billion years ago our planet was under siege. Bombarded by the rocky remnants of the Solar System’s foundation, our world was a tortured land of barren rock and dust-filled skies.

      The early Earth was not a place that we would recognise as home – 4 billion years ago, our planet was under siege. Bombarded by the rocky remnants of the Solar System’s formation, our world was a tortured land of barren rock and dust-filled skies. The days were short, sweeping by in just five hours as the Earth spun frantically on its axis. Each morning this desolate landscape would have been met with the sight of a rising sun very different from the one we see today. Hanging in the sky was a sun in its infancy. If there had been human eyes to view it, it would have appeared only 70 per cent as bright as it is today, and Earth would have been in a kind of perpetual twilight. This raises an interesting question, because there is strong geological evidence that the temperatures on Earth were very similar to those today, and certainly permitted liquid water to exist on the surface. The reason for the relative stability of Earth’s climate as the Sun brightened is still a matter of research, although it is thought that a combination of higher concentrations of greenhouse gasses such as CO₂ in the atmosphere and, perhaps, less cloud cover resulting in less sunlight being reflected back out into space, kept surface temperatures