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The Industrial Revolution put the clock in charge of the working day. Before factories, most people worked by daylight, the seasons and the task in hand; from the 1770s, mills such as Richard Arkwright's at Cromford in Derbyshire ran day and night in twelve-hour shifts.

The historian E. P. Thompson traced this change in a famous essay of 1967, “Time, Work-Discipline, and Industrial Capitalism”. Farm and craft work, he argued, had been measured by tasks: milking, harvesting, finishing a piece. Factory work was measured in hours. Cromford Mill, opened in 1771, was the world's first water-powered cotton-spinning mill. It started with about 200 workers, most of them women and children, the youngest only seven. Bells rang at 5 in the morning and 5 in the evening, the gates were shut at exactly 6, and anyone late lost that day's pay and was fined another. In industrial towns, where alarm clocks were not yet cheap or reliable, “knocker-ups” were paid a few pence a week to tap on upstairs windows with long poles.

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Less than 66 years, well within a single lifetime, separate the first powered aeroplane flight from the first Moon landing. Orville Wright flew for 12 seconds and 37 metres on 17 December 1903; Apollo 11 landed on the Moon on 20 July 1969.

The Wright brothers, bicycle makers from Dayton, Ohio, flew near Kitty Hawk, North Carolina, and that first hop was shorter than the wingspan of a Boeing 747. What followed came fast. In 1919 John Alcock and Arthur Brown flew non-stop across the Atlantic; in 1927 Charles Lindbergh did it alone, from New York to Paris; and in 1947 Chuck Yeager flew faster than sound. Orville Wright died in 1948, nine years before the Soviet satellite Sputnik opened the space age. Yuri Gagarin orbited the Earth in 1961, and in July 1969 Neil Armstrong took a scrap of the 1903 Flyer's wood and fabric with him to the Moon.

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The Cold War between the United States and the Soviet Union (1947–1991) was fought mostly without direct battles between the two. They competed instead through weapons, science, sport, films, propaganda and the race into space.

George Orwell used the phrase “cold war” in 1945, and the American journalist Walter Lippmann made it popular in 1947. Because nuclear weapons made open war between the superpowers too dangerous, the rivalry spread into other arenas. After the Soviet Union launched the Sputnik satellite in 1957, the United States passed the National Defense Education Act of 1958 to fund science, maths and language teaching. In 1959, at an American exhibition in Moscow, Vice-President Richard Nixon and the Soviet leader Nikita Khrushchev argued about capitalism and communism in a model kitchen, an exchange remembered as the Kitchen Debate. In 1972 in Reykjavík, the American Bobby Fischer beat the Soviet champion Boris Spassky for the world chess title. The Olympics were boycotted in turn: the United States and many allies stayed away from Moscow in 1980, and most of the Soviet bloc from Los Angeles in 1984.

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The Apollo Guidance Computer, which helped steer astronauts to the Moon, was one of the first computers built from integrated circuits: small silicon chips that each carry several electronic components. Until 1965, Apollo was the biggest user of these new chips.

The computer was designed at the MIT Instrumentation Laboratory from 1962 and built by Raytheon, and one flew in every Apollo command module and lunar module. The chip was still a new invention: Jack Kilby and Robert Noyce had made the first ones in 1958 and 1959. MIT used almost a single type throughout, a simple logic circuit designed by Fairchild Semiconductor, repeated thousands of times. According to the Computer History Museum, Apollo bought some 200,000 of them at 20 to 30 dollars each, and by 1965 the Minuteman missile programme had become the bigger buyer. Orders like these kept the young industry going until prices fell: the average chip cost 50 dollars in 1962 and 2.33 dollars in 1968. The Space Race is often credited with GPS too, but GPS was a separate US Department of Defense project, begun in 1973.

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On 12 April 1961 the Soviet cosmonaut Yuri Gagarin, aged 27, became the first human in space. His Vostok 1 spacecraft circled the Earth once, and the flight lasted 108 minutes from launch to landing.

Gagarin, who had trained as a foundry worker before becoming an air force pilot, was launched from Baikonur, in today's Kazakhstan. As the rocket lifted off he called out “Poyekhali!” (“Let's go!”). His orbit took him as high as 327 kilometres. On the way back he ejected from the capsule about 7 kilometres above the ground and came down by his own parachute near Engels, in the Saratov region, where a farm worker and her granddaughter were the first to meet him. Because the record rules of the day required pilots to land inside their craft, the Soviet Union claimed he had, and admitted the truth only in 1971. Gagarin became famous around the world. He died in a training-jet crash in 1968, aged 34. Since 2011 the United Nations has marked 12 April as the International Day of Human Space Flight.

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When Neil Armstrong radioed “Houston, Tranquility Base here. The Eagle has landed” on 20 July 1969, his words reached Earth about 1.3 seconds later. Radio waves travel at the speed of light, and the Moon is about 384,000 km (239,000 miles) away.

Eagle was the lunar module that carried Armstrong and Buzz Aldrin down to the surface. Light, and so radio, travels at almost 300,000 km per second. At the Moon's average distance of about 384,400 km, a signal takes roughly 1.28 seconds each way, so every exchange with Mission Control in Houston had a built-in pause of about 2.6 seconds before an answer could arrive. Large dish antennas at Goldstone in California and at Honeysuckle Creek and Parkes in Australia picked up the signals; the clearest pictures of the first steps came through Australia and were watched live by an estimated 600 million people. Before leaving, the crew set up a panel of reflectors on the surface. Scientists still fire lasers at it and time the light's return, which is how we know the Moon is drifting away from Earth by about 3.8 cm a year.

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From the late 8th century, Abbasid Baghdad became a great centre of translation, where scholars turned Greek, Persian and Indian works of science, medicine and philosophy into Arabic. The Bayt al-Hikma, or House of Wisdom, was one part of this wider effort, not its only home.

Baghdad was founded in 762 by the caliph al-Mansur as the capital of the Abbasid dynasty. Caliphs, courtiers and wealthy families paid for translations, often by Syriac-speaking Christian scholars who knew Greek. The best known, Hunayn ibn Ishaq (809–873), is said to have translated more than a hundred works, including much of the Greek physician Galen. Papermaking, learned according to tradition from Chinese prisoners in 751, made books cheaper. The House of Wisdom began as a palace library under al-Mansur or Harun al-Rashid and flourished under al-Ma'mun (reigned 813–833). The mathematician al-Khwarizmi worked in Baghdad in this period: the word “algorithm” comes from his name, and “algebra” from the title of his book. Some historians now argue that its image as a university-like academy was largely created by scholars of the 19th and early 20th centuries. It was destroyed when the Mongols sacked Baghdad in 1258.

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Ibn Sina, known in Europe as Avicenna, completed his Canon of Medicine in 1025: a five-book encyclopaedia of the medical knowledge of his time. Translated into Latin in the 12th century, it became a standard textbook in European universities for centuries.

Ibn Sina (c. 980–1037) was born near Bukhara, in today's Uzbekistan, into a Persian family, and served as a physician at several courts in Iran. The Canon's five books cover the basic principles of medicine and anatomy; simple remedies, listed alphabetically; diseases of each part of the body; conditions affecting the whole body, such as fevers; and compound medicines. It combined the Greek medicine of Hippocrates and Galen with Aristotle's philosophy and knowledge from Persian and Indian medicine. Ibn Sina stressed testing remedies by experience and warned that a mixture can act more strongly than its ingredients suggest. The Latin version, Canon medicinae, entered university courses from the 14th century, was first printed in 1472 and was still being taught at Padua in 1715. It remains a key text of Unani (“Greek”) medicine, a traditional system practised in India.

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Between the 1860s and the 1880s, Louis Pasteur and Robert Koch showed that many diseases are caused by microbes, living things too small to see, such as bacteria. This germ theory taught doctors to clean their hands and sterilise their instruments, and cities to protect their drinking water.

For centuries many doctors blamed disease on miasma, “bad air” rising from filth. There were clues against it even before Pasteur and Koch. In 1847 Ignaz Semmelweis cut deaths among new mothers in a Vienna hospital by making doctors who came straight from dissecting bodies wash their hands in chlorinated lime, a bleaching disinfectant, and in 1854 John Snow traced a London cholera outbreak to a single water pump. Pasteur's experiments with swan-necked flasks showed that broth stays fresh unless microbes from the air reach it. The surgeon Joseph Lister read his work and in 1865 began cleaning wounds with carbolic acid to kill germs, the start of antiseptic surgery. Robert Koch then identified the bacteria behind anthrax in 1876 and tuberculosis in 1882. Vaccination is older: Edward Jenner's smallpox vaccine dates from 1796.

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A 2025 Finnish study found that men who had suffered severe abuse or neglect as children showed some differences in the chemical tags on their sperm DNA and in the small RNA molecules their sperm carry. In mice such changes can affect offspring; in humans that has not been shown.

Epigenetics studies chemical tags and molecules that help switch genes on or off without changing the DNA code itself. One such tag is DNA methylation, small chemical groups attached to DNA; small RNAs are short molecules that fine-tune which genes are used. The study, by researchers at the universities of Turku and Copenhagen, compared sperm from fathers in FinnBrain, a Finnish study that follows families from pregnancy, who reported high or low levels of childhood maltreatment. It was small, with fewer than 60 men, and found differences in three regions of DNA, including areas near genes that help control brain development, and in 68 small RNAs. The idea comes from animal work: in 2014 Swiss researchers injected RNA from the sperm of mice stressed early in life into fertilised eggs, and the pups showed changes in behaviour and metabolism.

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Deliberately smiling can make people feel slightly happier. In 2022 a study of nearly 4,000 people in 19 countries found that posing a smile, or copying a smiling face, raised reported happiness a little; a subtler method, holding a pen between the teeth, gave less clear results.

The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

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In a cave in Jujuy, northern Argentina, archaeologists found two pipes carved from puma bone, with seeds of cebil, a tree whose seeds are psychoactive, lying beside them. Chemical traces inside the pipes point to plant smoking more than 4,000 years ago, among the oldest evidence of such use in South America.

Cebil, Anadenanthera colubrina, is a South American tree whose seeds contain tryptamines, compounds that strongly alter perception; further north in the Andes it is called vilca. The pipes come from Inca Cueva 7, a small rock shelter near the Humahuaca valley at the edge of the high Andean plateau, where seeds of cebil and of mesquite lay in the same layer. Radiocarbon measurements, which gauge how much of the carbon-14 in once-living material has decayed, give ages of about 4,000 years, roughly 2100 BC. The residue shows that a tryptamine-bearing plant was smoked, but it cannot prove exactly which one. Cebil use continued in the region for thousands of years: Wichí healers in northern Argentina still used its seeds, which they call hatáj, in the 1990s.

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A 2016 estimate found that a typical adult man weighing 70 kg has about 30 trillion human cells and 38 trillion bacteria. That overturned the old claim that bacteria outnumber our own cells ten to one: the two numbers are roughly equal.

The estimate comes from Ron Sender, Shai Fuchs and Ron Milo of the Weizmann Institute of Science in Israel. Their reference man is 20 to 30 years old, 170 cm tall and weighs 70 kg. The ten-to-one figure, they found, went back to a single rough calculation from 1972 that had been repeated for decades. Almost all the bacteria live in the colon, the last part of the gut, and together they weigh only about 0.2 kg. Among human cells, red blood cells dominate the count, making up about 84 per cent, while fat and muscle cells, far fewer but much larger, make up most of the body's cell mass. The balance also shifts through the day: a single trip to the toilet empties a quarter to a third of the colon's contents.

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An adult body replaces about 330 billion cells every day, around 80 grams of cells, according to a 2021 estimate by scientists at the Weizmann Institute of Science. Close to nine in ten of those cells are blood cells.

Ron Sender and Ron Milo combined the number, mass and lifespan of every major cell type in the body. Short-lived cells dominate the count. Red blood cells live about four months, and neutrophils, the most common white blood cells, only days, so the bone marrow must make vast numbers of them every day. By mass, the largest shares come from blood cells and from the cells lining the gut, which are shed and replaced within days. At this pace, a number of cells equal to all the roughly 30 trillion human cells in the body is replaced in about three months. That does not mean the whole body is renewed: most nerve cells in the brain last a lifetime, and heart muscle cells are replaced only very slowly.

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Bone is living tissue. In adults about a tenth of the skeleton is broken down and rebuilt each year, so most of it is renewed over roughly ten years; in a baby's first year, almost all of it is.

The work is done by two kinds of cells. Osteoclasts dissolve small patches of old or damaged bone, and osteoblasts then fill the gap with new bone, which hardens as calcium and phosphate are laid down in it. This cycle, called remodelling, repairs the tiny cracks caused by everyday movement and lets bone adapt to the loads it carries: in the 19th century the German surgeon Julius Wolff described how bone grows stronger where it is stressed. Bone is also the body's calcium store: bones and teeth hold about 99 per cent of it. In space, without gravity loading the skeleton, astronauts lose more than 1 per cent of their bone mass a month. From middle age, breakdown slowly outpaces rebuilding, which is why osteoporosis, the thinning of bone, becomes more common with age.

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Mitochondria, the parts of your cells that turn food into usable energy, very probably began as free-living bacteria. Well over a billion years ago one was taken inside another cell, where it went on living and dividing, and the partnership became permanent.

This is the endosymbiotic theory: endo means inside, and symbiosis means two different living things living closely together. Early versions of the idea date from the early 1900s, but it was the American biologist Lynn Margulis who gathered the microbiological evidence in a 1967 paper, and many scientists at first rejected it. The evidence is now strong. Mitochondria have their own DNA, a small ring like a bacterium's, which in humans holds 37 genes. They have two membranes, make some of their own proteins with bacteria-like machinery, and multiply by splitting in two, as bacteria do. Their closest living relatives are a group of bacteria called alphaproteobacteria. Because a baby's mitochondria come from the mother's egg, mitochondrial DNA is used to trace ancestry through the female line.

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Your gut and brain communicate through nerves, hormones, immune signals and microbes, which is why stress and digestion can affect each other.

Scientists call this link the gut–brain axis. The gut has its own network of about 500 million nerve cells, the enteric nervous system, sometimes called the “second brain”, which can run digestion largely by itself. It is connected to the brain mainly through the vagus nerve, a long nerve running from the brainstem down to the abdomen, and most of its fibres carry messages upwards, from the gut to the brain. More than 90 per cent of the body's serotonin, a chemical messenger, is found in the gut. Gut bacteria play a part too: in a 2004 Japanese study, mice raised entirely free of germs overreacted to stress. Stress can speed up or slow down the gut, one reason it often worsens irritable bowel syndrome, a common gut disorder. Most research on the microbes' role has been in animals, and human evidence is still limited.

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A placebo is a dummy treatment, such as a sugar pill. In pain studies, placebos can reduce the pain people report and also dampen activity in the brain areas that process pain, even though they contain no active drug.

The word placebo is Latin for “I shall please”. One of the first clues that the effect is physical came in 1978, when Jon Levine and colleagues studied patients recovering from wisdom tooth removal. They gave them a placebo and naloxone, a drug that blocks the effect of endorphins, the body's own painkilling chemicals. In patients who had responded to the placebo, naloxone increased the pain, which suggested that the placebo relief came from endorphins. In 2004 Tor Wager and colleagues used brain scans to show that a placebo painkiller reduced activity in pain-processing regions during a painful stimulus. Expectation works the other way too: in the nocebo effect, from the Latin for “I shall harm”, expecting harm makes symptoms worse. None of this means the pain was imaginary, or that placebos can replace effective treatment.

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Memory does not work like a recording. Each time we recall an event, the brain rebuilds it from stored fragments, filling gaps with what we know and what we have heard since, so a memory can change without our noticing.

Psychologists call this reconstructive memory. In his 1932 book Remembering, the British psychologist Frederic Bartlett described asking people to retell “The War of the Ghosts”, a Native American folk tale unfamiliar to them; each retelling drifted towards what felt familiar in their own culture. In 1974 Elizabeth Loftus and John Palmer showed people films of car accidents and asked how fast the cars were going when they “hit” or “smashed” into each other. Those asked with “smashed” gave higher speeds, and a week later were more likely to remember broken glass that was never there. This is known as the misinformation effect. It matters in court: according to the Innocence Project, mistaken eyewitness identification played a part in most US convictions later overturned by DNA evidence.

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The Dunning–Kruger effect describes how people who do worst at a task tend to overestimate their own performance the most. The psychologists David Dunning and Justin Kruger described it in 1999, after testing students on logic, grammar and humour. It does not mean that every beginner is confidently wrong.

Dunning and Kruger, then at Cornell University, published the study as “Unskilled and Unaware of It”. Students who scored in the bottom quarter did better than only about 12 per cent of their peers, yet guessed that they had beaten 62 per cent. The authors called this a dual burden: the knowledge needed to do something well is often the same knowledge needed to notice that it has been done badly. In one experiment, a short lesson in logic helped the weakest scorers judge their own results more accurately. Later researchers argued that much of the pattern may be statistical: when almost everyone rates themselves a little above average, the lowest scorers are bound to show the biggest gap, and computer simulations produce a similar curve. Dunning replies that the misjudgements are real, whatever their cause.

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Predictive processing, an influential theory in neuroscience, says the brain does not simply wait for information from the senses. It keeps guessing what is out there and uses the incoming signals mainly to check and correct those guesses.

The idea goes back to the German physicist Hermann von Helmholtz, who argued in the 1860s that seeing involves “unconscious inference”: the brain fills in what the eyes cannot fully tell it. In 1999 the researchers Rajesh Rao and Dana Ballard turned this into a computer model of vision. Higher brain areas send predictions down, and lower areas send back only the difference between prediction and input, called the prediction error. The philosopher Andy Clark and the neuroscientist Karl Friston later helped develop it into a broad theory of the mind. A well-known illustration is the hollow-mask illusion: the inside of a mask still looks like a face bulging outwards, because the brain's expectation of a normal face outweighs the evidence. The theory is widely used but still debated, and critics ask how some of its claims could be tested.

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Going over a problem again and again can feel like working on it. Psychologists call this rumination: dwelling on why something went wrong and what it means, without reaching a decision or a next step. Studies link it to low mood and to poorer problem-solving.

The American psychologist Susan Nolen-Hoeksema made rumination a major research topic from 1991, describing it as passively and repeatedly focusing on distress and its possible causes and consequences. In a 1995 study with Sonja Lyubomirsky, people in a low mood who were asked to dwell on their feelings came up with less effective solutions to problems with other people than those who were first given something else to think about. Edward Watkins at the University of Exeter later showed that the style of thinking matters. Abstract “why” questions, such as “Why does this always happen to me?”, tend to keep people stuck, while concrete “how” questions, such as “How did this happen, and what can I do next?”, helped depressed patients solve problems better. Some therapies now teach people to notice when they are asking why and to switch to how.

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In a 1999 experiment, people asked to count basketball passes in a short video were so focused that about half failed to notice a person in a gorilla suit walking through the middle of the scene. Psychologists call this inattentional blindness: missing something in plain sight because attention is busy elsewhere.

The psychologists Daniel Simons and Christopher Chabris ran the study at Harvard and published it in the journal Perception as “Gorillas in our midst”. It built on experiments by Ulric Neisser in the 1970s, in which viewers often missed a woman with an umbrella strolling through a similar ball game. The term inattentional blindness was coined by Arien Mack and Irvin Rock in 1992. Expertise does not protect against it. In 2013 researchers at Harvard Medical School inserted a gorilla, 48 times the size of the small nodules the doctors were looking for, into a lung scan. Of 24 radiologists, 83 per cent did not see it, although eye tracking showed that most of those who missed it had looked straight at it. A British road-safety advert later used the same trick, with a moonwalking bear, to warn drivers that they can miss cyclists.

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Emotions are not only feelings in the head. Fear speeds up the heart and breathing, grief can take away appetite and sleep, and joy changes posture and expression. These bodily changes are part of the emotion itself, and sensing them may be part of how we come to feel it.

In 1884 the American philosopher and psychologist William James turned the usual order around in an essay called “What Is an Emotion?”. We do not tremble because we are afraid, he argued; we feel afraid because we tremble. The physiologist Walter Cannon attacked the idea in 1927, and the debate ran for decades, but modern research keeps finding the body at the heart of emotion. In 2014 Lauri Nummenmaa and colleagues in Finland asked 701 people to colour in body outlines to show where they felt activity rise or fall with different emotions. Most emotions showed up in the chest, where breathing and heartbeat change; anger and happiness also lit up the arms, sadness appeared as weakened arms and legs, and disgust centred on the throat and stomach. The maps were similar for Finnish, Swedish and Taiwanese participants.

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