Wonders of Life. Andrew Cohen

water on the balmy Earth – no oceans, no rivers and lakes, no raindrops and no life.

      It is also water’s strong hydrogen bonds that explain the pond skater’s ability to walk on water. To understand why, it is necessary to think just a little about the nature of chemical bonds themselves. The reason a bond forms, at the most fundamental level, is because it is energetically favourable for it to do so. This means that a clump of water molecules loosely attached to each other by a network of hydrogen bonds is a lower energy configuration than a swarm of water molecules freely whizzing around ignoring each other.

      The influence of water on biology is immense; indeed, perhaps we cannot truly understand biology until we understand water.

      Think about what it means to boil water. You have to put energy into the water to boil it and produce steam; steam is gaseous water, which means that the molecules are whizzing around ignoring each other. When you heat water up, some of the energy goes into breaking the hydrogen bonds between the water molecules. If you have to put energy in to break the bonds, then it must mean that you get energy out by letting the hydrogen bonds re-form and allowing the steam to condense back into water again. This is why steam burns you easily – when it touches your skin and condenses into water, a large amount of energy is released and this hurts! Part of what you are feeling is the energy released as the network of hydrogen bonds re-forms, turning the steam back into liquid.

      Because the hydrogen-bonded liquid state of water is a lower energy configuration than the non-hydrogen-bonded gaseous state, this has an interesting effect at the water’s surface. Hydrogen-bonding lowers the energy of a collection of water molecules, so every water molecule wants to hydrogen bond to others if it can. The molecules at the surface, however, don’t have as many molecules to bond with, as above them there is only air. This means that it is always energetically favourable for water to minimise its surface area; less surface means more hydrogen bonds.

      When a pond skater puts its hairy, hydrophobic legs onto the water’s surface, it bends the surface and therefore increases the surface area. This increases the energy of the water, which pushes back, trying to flatten its surface and thereby reducing its energy. This force is known as surface tension, and it keeps the pond skater afloat. This is also, by the way, the reason why raindrops are spherical. A sphere is the shape that minimises the surface area of a water drop, and it is therefore the most energetically favourable shape for a collection of water molecules to assume.

      Water’s high boiling point and surface tension are just the beginning, as far as biology is concerned. Water’s polar nature doesn’t only allow the formation of hydrogen bonds between water molecules, it also allows it to break up other weakly bonded molecular structures and disperse them. In other words, it is a superb solvent, able to dissolve salts and other nutrients which in turn allows them to be dispersed around the body and made available for chemical reactions to take place. It is also highly structured in its liquid phase. We now know that water behaves more like a gel than a liquid, with complex networks of hydrogen-bonded water molecules forming giant, fleeting structures. These structures, it is thought, play a vital role in the complex biological reactions within cells. In a sense, water acts like scaffolding around which biology can happen. It is known that the activity of proteins depends both on their chemical structure and their precise orientation and shape, and hydrogen bonding between water molecules and the protein molecules plays an important role in orientating these complex molecules so that they can carry out their biological functions correctly.

      Water is a fascinating and unique substance – so much so that its influence on biology and its own internal structure, both created by hydrogen bonding, are still extremely active areas of research. This is why it is said that we won’t truly understand biology until we understand water. It is also the origin of the strong suspicion, shared by many biologists, that water is one of the essential ingredients for life, not only on Earth, but anywhere in the Universe.

INTO THE LIGHT

      Life on Earth is a dazzling continuum of organisms of dramatically varying sizes and complexities. There are some ingredients that all living things share: water, plus a handful of chemical elements vital for life, such as the constituents of DNA – hydrogen, oxygen, nitrogen, carbon and phosphorus. Other ingredients and conditions are necessary for the particular biosphere we find on Earth today. Without them, human beings would certainly not exist, but whether they are fundamental to the development of complex life is an open question.

      Virtually every living thing on the planet today is ultimately powered by sunshine. Every mouthful of every meal has its origins in the Sun, from the fruit and vegetables created by plants that absorb sunlight directly, to the meat and fish that deliver their sunshine second- or third-hand as part of the complex food chain.

      It appears today as if the Sun is a truly fundamental ingredient for life, a provider without which life couldn’t exist. Yet this intimate relationship with our nearest star is not a simple one. The Sun is a far from benevolent companion. Its radiant rain has a dark side that is as dangerous as it is nourishing, and early in the development of life on Earth it is likely that the Sun was a presence to be avoided rather than cherished. To understand how life transformed its relationship with light, we have to go back billions of years, to a time when life sheltered in the darkness. For many biologists, life on Earth didn’t begin in the light, but rather in the darkness of the deep oceans. The transformation of light from threat to food required one of life’s most extraordinary inventions: oxygenic photosynthesis. The evolution of this biological process ultimately resulted in the capture of carbon and the release of large amounts of oxygen into the atmosphere, which in turn played a key role in triggering the explosive evolution of life from the simple to the complex and conscious.

      Without light, the process of oxygenic photosynthesis would not be possible. It was this biological process that resulted in the release of oxygen into the Earth’s atmosphere.

A TRAIN JOURNEY THROUGH TIME

      The Chihuahua-Pacific railway has a vertical climb of 2,400 m (8,000 ft), and, as it climbs, the air thins and the colours of the world shift from warm reds to harsher blues.

      As a rule, I don’t enjoy filming in jungles. Humidity and DEET combine, in my view, to create discomfort, and it is unfortunate that biodiversity implies lots of animals, some of which are best avoided. It was with some relief, therefore, that the crew and I left the verdant but challenging beauty of the Yucatan behind for the freshening altitudes of the north. With 37 bridges, 86 tunnels and a vertical climb of 2,400 m along 673 km of track, the Chihuahua-Pacific railway is one of the great train journeys of the world. The old train leaves the coastal town of Los Mochis at 6 am, shortly before the reddening sky delivers stripes of warming light into the wooden carriages through slatted blinds. As the train rattles away from the coastal plane, the landscape shifts from towns and villages to pine forests and mountains, and we head inwards and upwards towards Mexico’s mountainous interior and the Copper Canyon, a network of gorges to rival its more famous northerly neighbour, the Grand Canyon.

      One shouldn’t need a reason to ride the Chihuahua-Pacific. It’s one of those things in life that’s worth doing, simply because it’s there. But, being a film crew, we have a reason; to observe the shifting nature of the Sun’s light as it arrives at the surface of the Earth. As early morning turns to afternoon and we rise into thinning air, the colours of the world shift from warm reds to harsher blues. These are real, physical changes picked up by our eyes as the quality of the