Wonders of Life. Andrew Cohen

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      The single-lens microscope was the forerunner of the scanning electron microscope (SEM), which provides a far greater level of magnification. This image shows Clostridium botulinum, the cause of botulism in humans.

      In the spring of 1676, Van Leeuwenhoek tried to discover why pepper was hot; he assumed that there must be some tiny spikes on the surface of the pepper that would explain the tongue-tingling sensation of heat generated by peppercorns. His initial attempts to confirm the ‘spiky pepper hypothesis’ failed when he used dried pepper, so he put a handful of peppercorns in water and let them soften for three weeks. On 24 April, he drew some of the ‘pepper-water’ into a glass capillary tube with an extremely fine bore, fixed the tube in front of the metal plate that held his tiny lens in place, and held the apparatus up to the light. To his amazement he saw that the water was full of an incredible number of ‘animalcules’, or tiny ‘animals’. As he wrote to the Royal Society, they ‘were incredibly small, nay so small, in my sight, that I judged that even if 100 of these very wee animals lay stretched out one against another, they could not reach to the length of a grain of coarse sand.’ These were in fact protists and bacteria. The scale of life had just become almost infinitely smaller than had been imagined.

      When the Royal Society got news of Van Leeuwenhoek’s astonishing discovery, they instructed their resident microscopist, Robert Hooke, to replicate the ‘pepper-water’ experiment. He failed, because Van Leeuwenhoek had neglected – perhaps deliberately – to make clear that he had used a capillary tube. Eventually, Hooke realised what was missing from his set-up and was able to confirm the observation. For many years it was thought that the use of pepper infusion was necessary to observe the animalcules – some classes of bacteria are still called infusoria. Of course, the bacteria and protists were in the water all along.

      Although he didn’t realise it at the time, Van Leeuwenhoek was the first to observe bacteria, the most numerous and ancient life forms on the planet. He went on to explore and detail many uncharted aspects of the biological world – a year later he made the momentous discovery of spermatozoa – but it is rightly for his discovery of the bacteria that he is remembered.

      It is estimated that there are around 10³¹ bacteria alive on Earth – a hundred million times the number of stars in the observable Universe.

      Bacteria have been around for almost the entire history of life on Earth. The oldest known bacterial fossils are almost 3.5 billion years old. Typically just a few millionths of a metre in length, these single-cell life forms come in a multitude of forms, from spheres and rods to spirals or even cuboidal shapes. A single drop of water contains, on average, a million bacteria; a gram of soil may be home to 40 million; in your body there are ten times as many bacteria as there are human cells. It is estimated that there are currently around 10³¹ bacteria alive on Earth – a hundred million times the number of stars in the observable Universe. By mass, they are comfortably the dominant organisms on our planet.

      Bacteria are organisms known as prokaryotes, which means that they do not have a cell nucleus. They share this trait with another group of single-celled organisms known as archaea. The lack of a nucleus, and indeed virtually any complex structures inside their cells, distinguishes them from all other forms of life, which are known as eukaryotes. All animals, plants, fungi and algae – in fact, anything that we would regard as ‘complex’ – are eukaryotes. The overwhelming majority of biologists today believe that eukaryotes emerged from prokaryotes around 2 billion years ago, and that this fundamental and revolutionary change happened only once. We’ll return to this quite remarkable claim later in this chapter. For now, we’ll remain in the domain of the prokaryotes, and explore a more ancient yet no less epochal leap in life’s capabilities, achieved purely by the seemingly lowly bacteria, that turned the planet green and paved the way for the eukaryotes to flourish.

EATING THE SUN

      For a plant, one of the purposes of oxygenic photosynthesis is to capture energy from the Sun. This coloured micrograph of the leaf of a Christmas rose (Helleborus niger) shows the vertical cells of chloroplasts, which perform this function.

      If you made it through school biology lessons, you will have heard of photosynthesis. Indeed, you may well be able to recite the famous chemical equation from memory:

      6CO₂ + 6H₂o → C₆H₁₂O₆ + 6O₂

      Energy from the Sun

      Photosynthesis uses carbon dioxide and water to produce sugars and oxygen in a process powered by the energy of the Sun. But the use of the term photosynthesis to describe this particular process is a colloquialism. Specifically – and this is most definitely not a pedantic distinction – the above equation refers to oxygenic photosynthesis, and this makes all the difference in the world.

      Perhaps the best way to unravel the evolutionary origins of photosynthesis, and explain the significance of the term oxygenic, is to look at it from the perspective of a plant. The purpose of photosynthesis, if you are a plant, is twofold. One is clearly visible in the famous equation: it is to make sugars, which is done by forcing electrons onto carbon dioxide. The other, which is hidden in the detail, is to capture energy from the Sun and store it in a usable form. All life on Earth stores energy in the same way, as a molecule called adenosine triphosphate, or ATP. This suggests strongly that ATP is a very ancient ‘invention’, and the details of its production and function could provide clues as to life’s origin 4 billion years ago.

      PHOTOSYNTHESIS

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