Wonders of Life. Andrew Cohen

class of pigments is unique to parrots, and the brightly coloured rainbow lorikeet (Trichoglossus haematodus) dramatically illustrates life’s varied palette.

      Pigment molecules perform a large variety of functions in living things. Some, as in the case of melanin, evolved to absorb light for protection. It is not known whether the first pigments were used for protection, although many biologists think this was the case. Protection is a simple function, needing no additional complexity such as a nervous system to respond to light’s stimulus. There are also pigments that simply make organisms colourful, in order to attract a mate, warn off a predator, entice insects to nectar or invite animals to consume vivid-coloured fruit. But some pigments do not simply dissipate the energy from the Sun as harmless heat or reflect it for display. Chlorophyll, the pigment that lends the natural world its verdant hue, is such a molecule, and its ability to absorb photons and, when integrated into a complex set of molecular machines, use their energy to do something useful, changed the world.

      Flamingos are pink because they ingest beta-carotene from blue-green algae in their diet.

FROM THE SMALLEST BEGINNINGS…

      Van Leeuwenhoek’s development of the single-lens microscope led to the possibility of viewing microbial life. This image shows merismopedia – a genus of cyanobacteria whose cells are arranged in perpendicular rows one cell thick to form rectangular colonies.

      Antonie van Leeuwenhoek was a draper who sold cloth in the Dutch city of Delft in the 1660s. He had no scientific training, was notoriously bad-tempered and could speak only Dutch. And yet this grumpy man literally changed the way we look at the world: he discovered microbes, the most ancient life forms on the planet.

      Although the microscope had been invented at the beginning of the seventeenth century, little of much importance had been discovered with it. Then, in Holland, people began to use single-lens microscopes, in which the lens was a tiny ball of glass about 1 millimetre across. Like other Dutch microscopists, such as Jan Swammerdam or the great philosopher Benedict de Spinoza, Van Leeuwenhoek both ground his lenses and pulled them from thin rods of heated glass. But he had an extra trick to produce large quantities of high-quality tiny glass spheres and was so protective of his technique that he kept it secret. What exactly it was is still unknown: his refusal to share his technique may have held back the development of science. This is why, ultimately, the ability to keep secrets is not a common or desirable trait in a scientist – something that legions of conspiracy theorists would do well to remember.

      Despite its simplicity, the single-lens microscope gave unparalleled access into the world of the small and revealed amazing wonders – they could magnify up to 500X, revealing incredible detail in the anatomy of insects and plants. In 1672, Van Leeuwenhoek had been introduced to the Royal Society in London, which invited the Dutchman to turn his powerful device to all sorts of substances, including pepper.

      The single-lens microscope gave unparalleled access into a previously unseen world. This image is a micrograph of the bacterium Clostridium tetani, a rod-shaped, anaerobic bacterium of the genus species Clostridium that causes tetanus.

      Bacteria are organisms known as prokaryotes, which do not have a cell nucleus. The twisted, thread-like spirochaete bacterium shown in this image (here magnified 4,000 times actual size) causes syphilis in humans.

      Bacteria have existed for almost the entire history of life on Earth. This computer-generated image depicts a small group of the bacteria Treponema pallidum, which causes diseases such as syphilis, bejel, pinta and yaws.

      Bacteria are found in even the most inhospitable places on Earth. This image shows psychrophilic (cold-loving) bacteria, discovered in Ace Lake, Antarctica, in 1992.

      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