Wonder: Something new to discover. - Page 5

Editor’s note

Wonder

For the pleasure of learning something you did not know, from an unfamiliar artist to a surprising discovery.

WonderScience

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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Earth's outer shell is broken into huge plates that move a few centimetres a year, about as fast as fingernails grow. Where they part, new ocean floor forms; where they collide, old floor sinks back into the mantle and mountains rise. This slow movement, plate tectonics, also drives most earthquakes and volcanoes.

The idea that continents move goes back to Alfred Wegener in 1912, but geologists accepted it only in the 1960s, when surveys of the ocean floor showed new crust forming along mid-ocean ridges and spreading away on both sides. Old ocean floor sinks back into the mantle at subduction zones, where one plate dives beneath another. That is why the oldest large areas of ocean floor are only about 180 to 200 million years old, while some continental rocks are about 4 billion years old. Turkey shows the process at work: most of the country sits on the small Anatolian plate, whose edges include the North and East Anatolian Faults. The 1999 İzmit earthquake struck on the first, and the 2023 Turkey–Syria earthquakes on the second.

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Earth's magnetic field deflects most of the solar wind, the stream of charged particles flowing from the Sun, but steers some of them down towards the polar regions. When they hit the thin upper air, it glows as an aurora.

The particles, mostly electrons and protons, are funnelled into two rings around the magnetic poles, called the auroral ovals, and strike oxygen and nitrogen in the upper atmosphere, most often around 100 km up. The atoms and molecules give off the energy they receive as light. Oxygen gives the common green glow and, higher up, a rarer red; nitrogen adds blue and purple lower down. Galileo used the name aurora borealis in 1619, after Aurora, the Roman goddess of the dawn, and Boreas, the Greek god of the north wind; its southern twin is the aurora australis. During strong solar storms the ovals widen and auroras appear far from the poles. In May 2024, in the strongest geomagnetic storm for more than 20 years, they were seen from Spain, Turkey and Cyprus, and even from the Canary Islands.

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Earth spins on an axis tilted about 23.4°, and the tilt gives us the seasons. Over a cycle of about 41,000 years it rocks only between about 22° and 24.5°. The Moon's pull is thought to keep it this steady; without a large moon, the tilt could swing far more, with much bigger shifts in climate.

Even this small rocking matters: it is one of the slow orbital cycles, worked out by the Serbian scientist Milutin Milanković, that help pace the ice ages. The Moon's pull on Earth's slightly bulging equator makes the axis wobble at a steady pace that stays out of step with the tugs of the other planets, so those tugs cannot build up. In 1993 the French astronomer Jacques Laskar and colleagues calculated that without the Moon the tilt could swing chaotically, at times approaching 90°. Later simulations, in 2011, found smaller but still large swings of about 20 to 25°. Mars, which has only two tiny moons, has seen its tilt vary between about 0° and 60° over millions of years.

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Saturn is 95 times as massive as Earth, yet its average density is about 30% lower than that of water: in an impossibly large bathtub, it would float.

Saturn is a gas giant, made mostly of hydrogen and helium. It is big enough to hold more than 700 Earths, so its mass is spread through an enormous volume, and its average density works out at about 0.69 grams per cubic centimetre, against 1 for water. It is the only planet in the Solar System less dense than water. The bathtub picture is only a comparison: Saturn has no solid surface, and deep inside, its hydrogen is squeezed into a liquid around a core rich in rock and ice. Saturn also spins fast, once in about 10 and a half hours, and the spin flattens it: it is about a tenth wider at the equator than from pole to pole.

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Olympus Mons on Mars is the largest known volcano in the Solar System: more than 21 km high and about 600 km wide, covering roughly the area of Italy. It is a shield volcano, built from countless flows of runny lava that spread into broad, gentle slopes.

Its slopes are gentle, about 5% on average. At the top lies a group of six overlapping calderas, collapsed craters that together measure about 60 by 80 km and reach 3.2 km deep, and around its base runs a cliff up to 8 km high. Mars helps volcanoes grow this big: its gravity is weaker, and its crust does not move in plates, so the ground stays over the same hot spot and lava keeps piling up in one place for a very long time. In the 19th century the Italian astronomer Giovanni Schiaparelli saw a bright spot there and called it Nix Olympica, “Olympic Snow”. When the Mariner 9 probe reached Mars in 1971, the summits of this region were among the first features to appear above a planet-wide dust storm, and Olympus Mons was confirmed as a volcano.

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