Reading — 2026 Jan–Apr Recall Set 22

The first plastics were developed as a substitute for natural rubber. Chemically, rubber is a polymer—a compound containing large molecules that are formed by the bonding of many smaller, simpler units, repeated over and over again. The same bonding principle—polymerization—is the basis of the creation of a huge range of plastics by the chemical industry. The first plastic was developed as a result of a competition in the USA. In the 1860s, $10,000 was offered to anybody who could replace ivory—supplies of which were declining—with something equally good as a material for making billiard balls. The prize was won by John Wesley Hyatt, with a material called celluloid. Celluloid was made by dissolving cellulose, a carbohydrate obtained from plants, in a solution of camphor dissolved in ethanol. This new material rapidly found other applications in the manufacture of everyday products such as knife handles and detachable collars and cuffs. But perhaps the best-known celluloid product was photographic film, without which the film industry could never have taken off at the end of the 19th century. Celluloid can be repeatedly softened and reshaped by heat, and is known as a thermoplastic. In 1907, Leo Baekeland (1863–1944), a Belgian chemist working in the USA, invented a different kind of plastic by causing phenol and formaldehyde to react together. Baekeland called it Bakelite, and it was the first of the thermosets—plastics that can be cast and moulded while hot, but cannot be softened by heat and reshaped once they have set. Bakelite was a good insulator, and was resistant to water and acid. With these properties it was soon being used in the manufacture of electrical switches as well as a variety of domestic items. As the century went on, the range of newly developed plastics increased. Chemists began looking for other small molecules that could be strung together to make polymers. In the 1930s, chemists in Britain discovered that the gas ethylene would polymerize under heat and pressure to form a thermoplastic they called polythene. Polypropylene followed in the 1950s. Both are used to make bottles, pipes and plastic bags. A small change in the starting material—replacing a hydrogen atom in ethylene with a chlorine atom—produced rigid PVC (polyvinyl chloride), a fireproof plastic suitable for drains and gutters. By adding certain chemicals, a soft form of PVC can be produced, suitable as a substitute for rubber in items such as waterproof clothing. A closely related plastic is Teflon or PTFE (polytetrafluoroethylene). It produces very little friction, making it ideal for products such as non-stick frying pans. Polystyrene, a hard, clear material like glass, was developed during the 1930s in Germany, and its applications included food containers and toys. Expanded polystyrene is rigid and is widely used in packaging and insulation. Polyurethane, developed in the same country, was commonly produced as a foam, which was very useful in the production of insulating materials. In the 1930s, the first of the man-made fibres was created—nylon. Its inventor was a chemist called Wallace Carothers (1896–1937), who worked for the Du Pont company in the USA. He found that under the right conditions two particular chemicals would form a polymer that could be pumped out through holes and then stretched to form long glossy threads that could be woven like silk. Its first use was to make parachutes for the US armed forces in World War II. In the postwar years, it completely replaced silk in the manufacture of stockings. Many other synthetic fibres joined nylon, including Orlon, Acrilan, and Terylene. Today most garments are made of a blend of natural fibres, such as cotton and wool, and man-made fibres that make fabrics easier to look after. Despite its enormous usefulness, plastic has its drawbacks. In fact, one of its great strengths—its indestructibility—is its greatest disadvantage. Beaches all over the world, even on the remotest island, are littered with plastic bottles that nothing can destroy. Nor is it very easy to recycle plastics, as different types of plastic are often found in the same items and call for different treatments. Plastics can be made biodegradable by incorporating into their structure a material such as starch, which is attacked by bacteria and causes the plastic to fall apart. Other materials can be incorporated that gradually decay in sunlight—although bottles made of such materials have to be stored in the dark, to ensure they do not disintegrate before they have been used.

A Dogs’ noses are renowned for sensitivity to smells, while human noses are thought to be poor by comparison, yet that might be a misconception. According to recent studies, our noses are in fact acutely sensitive instruments that guide our everyday lives to a surprising extent. Subtle smells can change your mood, behaviour and the choices you make, often without you realising it. Our own odours, meanwhile, indicate emotional states such as fear or sadness to those around us. The big mystery is why we aren’t more aware of our nasal activity. Noses have certainly never been at the forefront of sensory research, and were pushed aside until recently in favour of the seemingly more vital senses of vision and hearing. ‘There has been a lot of prejudice that people are not that influenced by olfactory stimuli, especially compared to other mammals,’ says Lilianne Mujica-Parodi, who studies the neurobiology of human stress at Stony Brook University in New York, in the United States. B One of the first people to assert the relative unimportance of human smelling was Pierre Paul Broca, an influential 19th-century anatomist. After comparing the proportion of the brain devoted to smell in different animals, he suggested that mammals can be classed into two broad groups: macrosmatic mammals, such as dogs, have a finely tuned sense of smell which they rely on to perceive the world, while we, along with other primates and the marine mammals, are microsmatic – we have small olfactory organs that we only rely on to a small extent. That idea seemed to fit with more recent studies which found that the majority of mammals have genes coding for about 1000 different types of smell receptor. Most of these genes aren’t expressed in humans, giving our noses just 400 different types of receptor. C Yet these findings may have been misleading. Brain scans now show that more of the brain is devoted to smell processing than Broca’s anatomical studies suggested. And although we may have fewer types of receptor than other mammals, Charles Greer at Yale University in the United States has shown that the human nose and brain are unusually well connected, with each group of receptors linking to many more neural regions than is the case in other animals. That should give us a good ability to process incoming scents. Once researchers began looking, they found the nose to be far more sensitive than its reputation suggested. One study, for example, found that we can detect certain chemicals diluted in water to less than one part per billion. That means that a person can detect just a few drops of a strong smell like ethyl mercaptan in an Olympic-sized pool. D ‘We are also exceptionally gifted at telling smells apart, even in the case of two molecules whose only difference is that their structures are mirror images of one another. That is fantastic sensitivity,’ says George Dodd, a perfumer and researcher at the olfaction group of the University of Warwick, in the United Kingdom. What’s more, it’s becoming clear that the brain’s olfactory centres are intimately linked to its limbic system, which is involved in emotion, fear and memory. That suggests a link between smell and the way we think. E The power of smell will be no news to estate agents, who often advocate the smell of baking bread or brewing coffee to promote the sale of a house. But there are more subtle and surprising effects too. When Hendrik Schifferstein and colleagues, from Delft University of Technology in the Netherlands, pumped the smell of orange, seawater or peppermint into a nightclub, the revellers danced more, rated their night as more enjoyable, and even thought the music was better than when there was no added scent. Meanwhile, Rob Holland, of the University of Utrecht in the Netherlands, found that the hint of aroma wafting out of a hidden bucket of citrus-scented cleaner was enough to persuade students in a hostel to clean up after themselves. F Other work has found that scent can influence our cognitive skills. A study by William Overman and colleagues at the University of North Carolina in the United States found that when men were subjected to a novel smell – either good or bad – during a task used to test decision-making skills, they performed significantly worse than normal. The researchers conclude the scent stimulated brain areas connected with emotion, making their decisions emotional rather than rational. G Smells are especially good memory evokers, but it is actually a myth that odours trigger more detailed memories than other stimuli. ‘The memory is not more accurate and you don’t remember more details,’ says Yaara Yeshurun at the Weizmann Institute of Science in Rehovot, Israel, ‘but it is unique in that it is more emotional.’ This isn’t surprising when you consider that there are certain brain areas dedicated to both emotion and olfaction, such as the amygdala, and there is a strong link between emotion and memory. In 2009, Yeshurun found that the link between a memory and a smell is stronger if the smell is unpleasant rather than pleasant. She also discovered that the very first time we attach a smell to an object, it evokes a much greater response in our brains than for any subsequent encounter with the smell or object, laying down stronger foundations for the memory. That doesn’t happen with any other sense. Since those first encounters with a smell would have happened at a young age, this explains why smells often transport us back to our childhood.

The New Zealand species of lizard, the tuatara, is firmly embedded in the national psyche: an icon for today which dates from the age of dinosaurs; an ancient reptile commemorated on the back of the five-cent coin. New Zealanders feel an affinity with the tuatara, and accept that active conservation management is required to ensure it will be among the legacies left to future generations. When European explorers reached New Zealand in 1769 they found two large islands, which together they called the mainland, and many tiny offshore islands around the coast. The naturalists who came with the explorers disregarded the tuatara, though it is improbable none were seen. Only several decades later did a tuatara specimen reach the British Museum, where it was eventually classified as just another type of lizard. One of the first scientists who realised that aspects of tuatara anatomy were odd—unchanged for tens of thousands of years—was Albert Gunther in 1876. Gunther believed the tuatara was one of the most valuable objects in zoological anatomical collections, and also noted, in passing, the reptile was likely to become extinct. From today’s perspective, it is striking that Gunther expressed no concern about the probable demise of the tuatara. He and his contemporaries were products of their age, strongly influenced by Charles Darwin’s theory, which had only recently been published. Their views were something like this: ‘Extinction is a natural process. It is sad that species disappear, but that is part of nature.’ There is a second important aspect of Gunther’s work. He recorded, correctly, that some of the mammals introduced by Europeans were predators of the tuatara—particularly rats. But what he did not realise was that New Zealand has two species of rat, both introduced, both with an appetite for tuatara: the ship’s rat came with European explorers and settlers; but the kiore rat had already been in the country for hundreds of years, brought by Polynesians from the Pacific Islands. Gunther failed to recognise the distinction, believing all rats to be a relatively recent introduction. Little further research was conducted until Ian Crook of the NZ Wildlife Service published his findings in 1973, which can be summarised as follows. Tuatara thrive on offshore islands with no rats. Tuatara never survived on islands with ship’s rats. On a few islands, small and declining populations of tuatara occur with the kiore. This should not be seen, however, as evidence that tuatara and kiore can coexist. Rather, Crook proposed, kiore probably only arrived recently on such islands, and thus the small populations represent extinctions in progress. Throughout the 1990s, Richard Holdaway and his colleagues at Victoria University in Wellington documented the surprising discovery that kiore probably arrived about 1,800 years ago, although the human population of New Zealand is thought to be no older than 800 years. How is this possible? Presumably, Holdaway argued, the kiore were brought by Polynesian explorers who visited the country but did not settle. Thereafter, the rats were agents of ecological warfare, exterminating perhaps 1,000–3,000 species. Thus, tuatara and many other species were already rare or extinct when permanent human inhabitants—the Maori—arrived around 1300. This hypothesis is still being debated, but the evidence continues to accumulate in its favour. Conservation practice has changed dramatically since Crook’s findings were published in 1973. Eradication of rats from any given environment was believed to be virtually impossible until about 1980, but since then has become routine. Enormous conservation benefits are accruing as newly rat-free offshore islands are providing sanctuaries for the country’s rarest species. In 1995, for example, Nicola Nelson of the Department of Conservation established 68 tuatara on Titi Island. Since then, four more populations of tuatara have been established elsewhere under similar conditions. Today, numbers of tuatara are still a fraction of what they once were, but for the first time in 1,800 years the decline has been reversed. While the recovery of rare species is itself a good thing, the truly significant outcome of this research is that it liberates the imagination. If we can remove predatory introduced mammals from islands, why not from the mainland too? Perhaps the questions we ask should demonstrate even more visionary ambition. Can non-mammalian pests also be removed from the mainland? Our rivers, for example, are full of surrogate rats, in the form of introduced species of fish called trout. Some day more people will understand that trout have replaced a whole native fauna in our waterways, just as rats replaced tuatara on the mainland. Will such knowledge lead to the creation of mainland ‘aquatic islands’ where we can once again establish those species of indigenous fish that used to live in our rivers? Similarly, can bellbirds and tuis replace birds like starlings and mynahs? The answers to such questions are uncertain, and opposing sides will doubtless be fiercely debated. But the role of scientific knowledge in illuminating the past will be crucial. Just as we no longer tolerate extinction, in the future we may no longer accept a mainland devoid of the biological wonders of our past such as tuatara. Conservation is thus not primarily about the past but about imagining and then creating the future we wish for our children and ourselves. For 80 million years until humans arrived, tuatara occurred throughout New Zealand—might they do so again?