Science: quotes, facts and reflections - Page 3

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Your lungs hold about 480 million alveoli, tiny air sacs where oxygen passes into the blood and carbon dioxide passes out. Opened out flat, their combined surface would cover 70 to 80 square metres.

Each alveolus is a cup-shaped pocket 0.2 to 0.5 millimetres across, wrapped in a mesh of capillaries, the body's finest blood vessels. The barrier between air and blood is usually 0.6 to 2 micrometres thick (a micrometre is a thousandth of a millimetre). Gases cross it by diffusion, simply moving from where they are more concentrated to where they are less, so no pump is needed. Special cells coat the inside of the sacs with surfactant, a detergent-like liquid that stops them collapsing when you breathe out. Babies born very early may not yet make enough of it, which is one reason they can struggle to breathe. The figure of about 480 million comes from a 2004 microscope study of adult lungs, and new alveoli keep forming until about the age of eight.

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Bone adapts to the loads it carries. In a 1977 study of professional tennis players, the hard outer wall of the upper-arm bone in the playing arm was about 35 per cent thicker in men, and 28 per cent thicker in women, than in the other arm.

The 1977 study, by H. H. Jones and colleagues, compared X-rays of both arms in the same players, so each person served as their own comparison. Later scans found the same gap in bone mineral content, a measure of how much bone there is. A 1995 study of 105 Finnish national-level tennis and squash players, all women, found that the gain was about twice as large in those who had started playing by the time of their first period as in those who started later, which suggests bone responds most while the skeleton is still growing. The principle is often called Wolff's law, after the German surgeon Julius Wolff, who argued in the 1890s that bone reshapes itself to match the forces on it. Cells called osteoblasts build new bone where loads are high, while osteoclasts remove it where it is not needed.

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After many infections and vaccines, the body keeps memory cells, types of white blood cell that recognise the germ involved. They can last for decades, so if the same germ returns, the immune system responds faster and more strongly.

There are two main kinds of memory cell. Memory B cells can quickly produce antibodies, proteins that latch onto a specific germ, while memory T cells direct the attack or kill infected cells. An early careful record of lasting immunity came from the Danish doctor Peter Panum, who studied a measles epidemic on the Faroe Islands in 1846. Measles had not reached the islands since 1781, and none of the elderly islanders who had caught it then fell ill again. Modern studies agree. In 2003, researchers found memory B cells against smallpox in people vaccinated more than 50 years earlier, and these cells responded strongly to a new dose. In 2008, survivors of the 1918 flu pandemic still had B cells making antibodies against that virus, nearly 90 years later.

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A fever is not the body overheating by accident: signals from the immune system tell the brain to raise its target temperature, rather like turning up a thermostat. The extra heat can help immune cells work and slow down some germs.

Body temperature is controlled by the hypothalamus, a small region at the base of the brain. During an infection, substances called pyrogens trigger the release of a molecule called prostaglandin E2, which raises the hypothalamus's target, or set point. Until the body reaches the new target it feels cold, so you shiver and get the chills; when the fever breaks, you sweat to shed the extra heat. Paracetamol and ibuprofen lower a fever by blocking prostaglandin production. Fever is an ancient defence: even cold-blooded animals such as lizards run one by moving somewhere warmer. In 1975, Matthew Kluger and colleagues infected desert iguanas with bacteria and kept them at different temperatures; the lizards kept warm enough to have a fever survived more often. A very high or long-lasting fever still needs a doctor.

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Identical twins start life with almost the same DNA, but they grow more different with age. Diet, smoking, illness, stress and chance leave chemical tags on their genes, called epigenetic marks, which turn some genes up and others down.

Epigenetics means changes in how genes are used that do not alter the DNA sequence itself. One common mark is a small chemical group, called a methyl group, attached to DNA, which tends to quieten the gene beneath it. In 2005, a team led by Manel Esteller in Madrid compared 40 pairs of identical twins aged 3 to 74. The youngest twins had almost identical marks, but older pairs differed much more, especially those who had spent more of their lives apart or lived differently. NASA's Twins Study offered a rare case: Scott Kelly spent 340 days on the International Space Station while his identical twin, Mark, stayed on Earth. Many changes in Scott's gene activity appeared in space, and most faded after he came home.

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Viruses cannot reproduce without getting inside a living cell, yet some have left permanent traces in us. Roughly 5 to 8 per cent of human DNA comes from ancient viruses that inserted their genes into the reproductive cells of our distant ancestors.

Retroviruses, the family that includes HIV, copy their genetic material into DNA and insert it into the chromosomes of the cells they infect. Usually this happens in ordinary body cells and ends with the host. But when it happened in a cell that went on to become an egg or sperm, the viral DNA was passed to the next generation like any other gene. These relics are called endogenous retroviruses, ‘endogenous’ meaning ‘arising from within’. Over millions of years most were broken by mutations and can no longer make viruses. Some, however, were put to work. The best-known example is syncytin, a protein made from an old viral gene that once helped the virus fuse with cells. In humans it helps build the outer layer of the placenta, where cells merge into a single sheet between mother and baby.

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Bacteria do not only pass genes from parent to offspring: they can also hand them directly to other bacteria, even of different species. This is one of the main ways antibiotic resistance spreads.

Biologists call this horizontal gene transfer. It happens in three main ways: bacteria can pass a plasmid, a small ring of DNA separate from their main chromosome, through direct contact (conjugation); they can pick up loose DNA left behind by dead cells (transformation); or viruses that infect bacteria can carry genes from one cell to the next (transduction). Its medical importance became clear in Japan, where a 1959 publication showed that resistance to several antibiotics could jump between different species of gut bacteria. A recent example is mcr-1, a gene that makes bacteria resistant to colistin, one of the last-resort antibiotics. It was reported in 2015 on a plasmid in bacteria from pigs in China and was soon found in many countries. This is why resistance can move through a hospital or farm far faster than inheritance alone would allow.

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Antibiotics kill bacteria or stop them multiplying by attacking parts that viruses do not have, such as the bacterial cell wall. Colds and flu are caused by viruses, so antibiotics cannot treat them.

Viruses are far smaller than bacteria and have no cell wall or machinery of their own; they multiply inside our cells, using the cells' own machinery, so antibiotics have nothing to hit. Flu does have antiviral medicines, but they are a different kind of drug. Taking antibiotics that are not needed still has costs: side effects such as diarrhoea and allergic reactions, and more chances for bacteria to evolve resistance. Alexander Fleming, who discovered penicillin in 1928, warned in 1945 that misusing it could breed resistant microbes. Bacterial resistance to antibiotics is now estimated to have directly caused 1.27 million deaths worldwide in 2019. Sometimes a cold or flu is followed by a bacterial infection, such as pneumonia, and then a doctor may prescribe an antibiotic.

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Sleep is more than rest: during the night the brain strengthens and sorts what it learned during the day and helps keep emotions in balance, while the body releases growth hormone and carries out repairs.

A night's sleep runs in cycles of about 90 minutes. Each cycle moves from light sleep into deep sleep and then into REM sleep, named after the rapid eye movements that come with it and the stage with the most vivid dreams. As early as 1924, the psychologists John Jenkins and Karl Dallenbach found that two volunteers remembered more of the nonsense syllables they had learned if they slept afterwards than if they stayed awake for the same time. In 2007, brain scans showed that after a night without sleep, the amygdala, an area involved in emotional reactions, responded far more strongly to upsetting pictures. In 2013, a study in mice found that the spaces between brain cells widen by about 60 per cent during sleep, helping fluid wash away waste. Regularly sleeping too little harms attention, learning and mood.

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Scientists still have no single accepted explanation for why we dream. Dreams are most vivid in REM (rapid eye movement) sleep but also occur in other stages, and research links them with memory, emotion and the brain's own activity at night.

In 1953, Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago noticed bursts of rapid eye movements (REM) during sleep and linked them to dreaming; people woken during REM sleep report a dream about 80 per cent of the time. Later studies found that people woken from other stages also report dreams, usually shorter and less story-like. Explanations differ. Sigmund Freud, in The Interpretation of Dreams (1899), saw dreams as disguised wishes. In 1977, Allan Hobson and Robert McCarley proposed that dreams are the brain's attempt to make a story out of random signals from the brainstem, the part of the brain that joins the spinal cord. Other researchers argue that dreams help us rehearse threats, sort memories or soften emotional experiences. Each theory explains some dreams, and none explains them all.

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Dopamine is not simply a pleasure chemical; it helps the brain learn, predict and pursue rewards before they arrive.

Dopamine is one of the brain's chemical messengers. In 1958 the Swedish scientists Arvid Carlsson and Nils-Åke Hillarp showed that it works as one, and Carlsson later shared the 2000 Nobel Prize in Physiology or Medicine for this work. In the 1990s Wolfram Schultz recorded dopamine cells in monkeys. An unexpected sip of juice made them fire; once a light reliably came before the juice, they fired at the light instead, and they dipped below normal if the promised juice never came. This ‘reward prediction error’ signal tells the brain when things turn out better or worse than expected, which is how it learns what is worth doing. Kent Berridge's experiments separated ‘wanting’ from ‘liking’: rats with almost no dopamine left still showed pleasure at sugar but made no effort to get it. The same system shapes habits and addiction, and its loss in Parkinson's disease affects movement.

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Pain is not a direct reading of damage: the nervous system produces it, weighing signals from the body against danger, attention, memory and context. That is why the same injury can hurt very differently from one person or moment to the next.

Nerve endings called nociceptors detect heat, pressure or chemicals that could harm tissue and send warning signals to the spinal cord and brain, but it is the brain that decides whether, and how much, it hurts. During the Second World War, the American anaesthetist Henry Beecher found that most of the badly wounded soldiers he treated did not ask for morphine, perhaps because surviving and leaving the battle outweighed the injury. In 1965, Ronald Melzack and Patrick Wall proposed the gate control theory: signals can be turned up or down in the spinal cord, which is partly why rubbing a knock helps. Phantom limb pain, felt in an arm or leg that has been amputated, shows that pain can exist where there is no tissue. In 2020 the International Association for the Study of Pain revised its definition, noting that pain and nociceptor activity are not the same thing.

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Some people cannot see pictures in their mind's eye. Asked to imagine an apple, they know what it looks like but no image appears. This is called aphantasia; a 2022 study found that almost 1 in 100 people have no mental imagery at all, and about 4 in 100 have none or only a faint one.

The British scientist Francis Galton first described the difference in 1880, after asking people to picture their breakfast table that morning: some saw it vividly, while others, to his surprise, said they saw nothing. The subject was then largely ignored until 2015, when the neurologist Adam Zeman and his team at the University of Exeter studied people who could not visualise and named the condition aphantasia, from Greek words meaning ‘without an image’. Many people only discover it as adults, when they realise that others mean ‘picture it’ literally. People with aphantasia can still remember, plan and create; Ed Catmull, a co-founder of Pixar, has said he has it. Many of them still dream in images. The opposite, unusually vivid imagery, is called hyperphantasia.

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In synaesthesia, one sensation automatically brings another with it: a letter may always appear in a particular colour, or a sound may bring a taste. About 4 per cent of people have some form of it, and their pairings stay remarkably stable over the years.

The word comes from the Greek syn, ‘together’, and aisthesis, ‘sensation’. The most studied type is grapheme-colour synaesthesia, in which letters and numbers (graphemes) have colours. Other forms link sounds with colours or words with tastes, or place sequences such as the months of the year at fixed points in space around the body. Researchers check it by testing people again: synaesthetes give the same colour for about 90 per cent of letters, even years later, while other people manage only 30 to 40 per cent after a few weeks, even when warned. It often runs in families. The novelist Vladimir Nabokov described his coloured alphabet in his memoir Speak, Memory, and the composer Olivier Messiaen saw colours when he heard chords and built them into his music.

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Confirmation bias makes people seek, notice and remember evidence that fits what they already believe, and give less weight to evidence that does not.

The English psychologist Peter Wason studied the habit in 1960. He told people that the numbers 2, 4, 6 followed a rule and asked them to find it by suggesting other sets of three. Many guessed something like ‘numbers going up by two’ and tested only sets that fitted their guess, such as 10, 12, 14, so they often announced a rule that was too narrow. The real rule was simply ‘any three numbers in rising order’. In a 1979 Stanford University study, supporters and opponents of the death penalty read about two studies, one for it and one against, both invented by the researchers. Each side rated the study that agreed with them as better run and more convincing, and found fault with the other. The bias shapes which evidence we look for, how we read unclear information and what we remember later.

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We often judge how common something is by how easily examples come to mind. Psychologists call this mental shortcut the availability heuristic, and it makes recent, vivid or frightening events seem more frequent than they really are.

Amos Tversky and Daniel Kahneman described the shortcut in 1973. In one test they asked whether more English words begin with the letter K or have K as their third letter. Most people chose the first, because words starting with K are easier to call to mind, yet words with K in third place are more common. The same effect shapes our sense of risk. Plane crashes and shark attacks make headlines, while everyday causes of death such as heart disease rarely do, so the dramatic dangers feel larger. After the attacks of 11 September 2001, many Americans drove instead of flying; the psychologist Gerd Gigerenzer estimated that about 1,500 more people died on American roads in the following year as a result. Kahneman later received the 2002 Nobel Memorial Prize in Economic Sciences for this line of work; Tversky had died in 1996.

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Crows, ravens, magpies and jays belong to the corvid family, among the cleverest birds known. New Caledonian crows shape twigs and leaves into hooked tools, and wild crows in Seattle scolded a mask for years after researchers wearing it had trapped them.

New Caledonian crows live on islands in the Pacific east of Australia. To pull grubs out of holes, they make stick tools and tear strips from the edges of pandanus leaves, whose natural barbs work as hooks. In 2002, researchers at the University of Oxford saw a captive crow called Betty bend a straight wire into a hook and use it to fish a small bucket of food out of a tube. The mask experiment took place in Seattle, where John Marzluff's team at the University of Washington wore a distinctive mask while trapping and ringing wild American crows. For years afterwards, crows scolded anyone wearing that mask, including birds that had never been caught and had learned the danger from others. Corvids lack the layered cortex that primates think with, but like other birds they pack their nerve cells two to four times as densely as mammals do.

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In a study published in 2006, an Asian elephant called Happy at the Bronx Zoo in New York passed the mirror test: after a white cross was painted on her head, she looked into a large mirror and kept touching the mark with her trunk. Few animals pass this classic test of self-recognition.

The mirror test was devised in 1970 by the American psychologist Gordon Gallup Jr. An animal is marked where it can see the mark only in a mirror; if it uses the mirror to inspect the mark on its own body, it seems to recognise itself. Chimpanzees, bonobos and orangutans pass, as do bottlenose dolphins and, in a 2008 study, Eurasian magpies. In the elephant study, Joshua Plotnik, Frans de Waal and Diana Reiss gave Happy, Maxine and Patty a mirror 2.4 metres square. All three used it to inspect hidden parts of their bodies, but only Happy touched the painted mark; the researchers suggest the others may not have cared about a small spot of paint. In 2022 Happy became a legal test case: New York's highest court ruled 5–2 that she could not be freed under habeas corpus, the right that protects people from unlawful detention.

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Whales and dolphins live in a world of sound. Sunlight fades within a few hundred metres of the surface, but sound carries far through seawater, more than four times faster than through air. They sing, whistle and click to keep in touch, to hunt and to find their way.

Sound moves through seawater at about 1,500 metres per second, and low-pitched calls lose little energy on the way. Christopher Clark of Cornell University, using US military underwater recordings, showed that whale calls can carry for thousands of kilometres. Toothed whales, such as dolphins and sperm whales, also use echolocation: they send out clicks and listen for the echoes to find prey in the dark. Each bottlenose dolphin develops its own signature whistle, which works much like a name; other dolphins copy it to call that individual. Humpback whale song became famous in 1970, when the biologist Roger Payne released the album Songs of the Humpback Whale, which helped start the Save the Whales movement. Today, noise from ships, sonar and seabed surveys worries scientists, because it can drown out the sounds these animals depend on.

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Mycorrhizal fungi grow in partnership with plant roots, trading water and minerals for sugar, and one fungus can link the roots of several trees. How often these shared networks move useful amounts of food or warning signals between trees in real forests is still uncertain.

Mycorrhiza means “fungus root”. Most land plants live in partnership with such fungi: the fungal threads reach far into the soil and bring back water, phosphorus and nitrogen, and the plant pays in sugars made by photosynthesis. The idea of a forest-wide network became famous after 1997, when the Canadian forest ecologist Suzanne Simard reported carbon passing between birch and Douglas fir trees through shared fungi. It was nicknamed the “wood wide web”, and later books described “mother trees” feeding their young. In 2023, Justine Karst, Melanie Jones and Jason Hoeksema reviewed the field studies. They found little or no evidence that these networks are widespread in forests, that transfers through them reliably help seedlings, or that big trees favour their own offspring. They also found that later papers often cited the original results more positively than the data allowed.

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A lichen is a partnership rather than a single organism. A fungus builds the body and shelters a green alga or a cyanobacterium, a bacterium that makes food from sunlight, and that partner feeds the fungus sugars in return.

For a long time lichens were thought to be single organisms. In 1867 the Swiss botanist Simon Schwendener proposed that a lichen is really a fungus living with algae, and many experts of his day rejected the idea. Lichens were the first partnership for which biologists used the word “symbiosis”, meaning different organisms living closely together. In 2016 a team led by Toby Spribille found that many lichens also contain a yeast, a second kind of fungus, as a third partner. About 20,000 species are known, growing on bark, bare rock, Arctic tundra and even in deserts. People have long made use of them: litmus, the dye in school acid tests, comes from a lichen, and “reindeer moss”, a winter food of reindeer, is a lichen too.

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Coral reefs are built by polyps, tiny animals related to jellyfish. Microscopic algae living inside their tissues turn sunlight into food for them, and together they build some of the richest habitats on Earth.

Each polyp lays down a skeleton of limestone (calcium carbonate) beneath it, and over thousands of years these skeletons pile up into reefs; most living reefs are less than 10,000 years old. The algae inside the polyps, called zooxanthellae, supply up to 90% of the coral's nutrients. Partnerships like this, which recycle nutrients instead of losing them, help explain a puzzle Charles Darwin described in 1842: how reefs can flourish in tropical seas that are poor in nutrients. When the water stays too warm for too long, corals expel their algae and turn white, which is called bleaching. Bleached coral is not yet dead, but it starves if the heat lasts. The fourth global bleaching event, which began in 2023 and was confirmed by the US agency NOAA in April 2024, has affected reefs in at least 82 countries and territories.

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Despite the name, horseshoe crabs are not crabs at all. They belong with spiders, scorpions and ticks, and animals very like them were already living in the sea about 445 million years ago.

Horseshoe crabs are chelicerates, the group of arthropods (animals with jointed legs and a hard outer skeleton) that also includes spiders, scorpions, mites and ticks. True crabs are crustaceans, a separate branch. Some genetic studies have even placed horseshoe crabs inside the arachnids, the spiders' own group, though a more recent analysis disagrees. Only four species survive: one along the Atlantic coast of North America and three in Asia. Their blood is blue, because it carries oxygen with a copper-based protein instead of iron-based haemoglobin, and an extract of it is used to test medicines for bacterial contamination. In spring and summer, Atlantic horseshoe crabs come ashore to spawn, usually at high tide around the full and new moons. On Delaware Bay in the eastern United States, migrating shorebirds called red knots refuel on their eggs, so catching the crabs there is restricted to protect the birds.

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A mass extinction wipes out a large share of Earth's species in a short time by geological standards. There have been five great ones. After the last, 66 million years ago, the dinosaurs other than birds were gone, and mammals spread into the ways of life they had left empty.

The “Big Five” were identified by the palaeontologists David Raup and Jack Sepkoski in 1982. The worst, the “Great Dying” about 252 million years ago, wiped out around 81% of marine species and cleared the way for the reptile group that later produced the dinosaurs. The most recent, 66 million years ago, removed about three-quarters of all plant and animal species. In 1980 Luis and Walter Alvarez and their colleagues found that rocks laid down at that moment contain a thin layer rich in iridium, a metal rare in Earth's crust but more common in asteroids. The impact crater, 180 km wide, was identified in 1990 at Chicxulub, buried under Mexico's Yucatán Peninsula. Recovery took millions of years. Biologists call the burst of new species that fills emptied roles adaptive radiation.

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