Milestones

Gregor Mendel’s experiments on pea plants, conducted in the 1850s and ’60s, are introduced to a wider audience thanks to three botanists independently studying inheritance.

German physicist Max Planck deduces that radiation is absorbed or emitted only in discrete packets, calling them “quanta.”

Albert Einstein introduces the special theory of relativity, merging matter with energy and implying the unity of space and time.

Albert Einstein proposes that light travels through space in the form of particles, later called photons.

Albert Einstein introduces the equivalence principle: Gravity is equivalent to acceleration.

Henrietta Leavitt discovers that brighter Cepheid variable stars blink slower than dimmer ones, a relationship that gave astronomers a way to measure cosmic distances.

Mathematician G.H. Hardy and physician Wilhelm Weinberg independently derive a formula for the frequency of gene variants in populations. Their work will become a foundation of the science of population genetics.

Fritz Haber demonstrates a method for making ammonia from nitrogen and hydrogen, which was scaled up by Carl Bosch over the next several years. The Haber-Bosch process made ammonia fertilizer widely available, thus helping to boost agricultural yields.

An amateur archaeologist reports finding fossils of a human ancestor near Piltdown, England. Piltdown Man is touted as evidence that a big brain evolved early in human evolution, but the fossils are later exposed as a hoax. 

Meteorologist Alfred Wegener suggests that Earth’s continents aren’t fixed in place, but drift around the globe.

Geologist Arthur Holmes publishes The Age of the Earth, presenting the first complete geologic timescale and arguing for using radioactive materials as geologic clocks.

Physicists Lawrence Bragg and William Henry Bragg demonstrate that X-rays can be used to accurately determine the position of atoms in a crystal. X-ray crystallography would prove essential in the discovery of the structure of DNA and many other biological molecules.
Connection: New vistas

Danish physicist Niels Bohr uses quantum theory, the notion that energy comes in discrete packets, to explain the structure of the hydrogen atom.

Albert Einstein submits four papers establishing the general theory of relativity, the foundation for today’s understanding of the cosmos.

A deadly strain of influenza spreads around the globe, ultimately killing an estimated tens of millions of people. 

After inventing the mass spectrograph, now known as the mass spectrometer, British physicist and chemist Francis Aston uses it to discover a large number of isotopes.

Harlow Shapley and Heber Curtis publicly debate whether “island universes” – what we now call galaxies – exist outside the Milky Way.

German chemist Hermann Staudinger proposes that the linking together of many small molecules can form materials with high molecular weights, such as natural rubber. He later calls them macromolecules.

X-rays can cause mutations that are inherited by the next generation — at least in fruit flies.

Frederick G. Banting finds that insulin, isolated from the islets of Langerhans in the pancreas, promises to cure diabetes.

Russian physiologist Ivan Pawlow (Pavlov) reports that mice learn to associate an electric bell with dinner after 300 lessons of the bell accompanying food.

The first successful heart valve surgery is performed, on a 12-year-old girl in Boston.

A new “vitamin X” that is key to animal reproduction is reported; the next year it would be formally named vitamin E.

American physicist Arthur Compton reports that X-rays lose energy when they are scattered by charged particles. The “Compton effect” indicates that light has a particle nature.

Edwin Hubble measures the distance to the Andromeda nebula, showing it is in fact another galaxy. The Milky Way is not the entire universe.

Karl von Frisch finds that bees report to hive mates where nectar has been found with a jazzy dance.

French physicist Louis de Broglie introduces the idea that particles, such as electrons, could exhibit the properties of waves.

The largest U.S. science society pledges its support of Tennessee teacher John T. Scopes, who has been arrested for teaching evolution.

German physicist Werner Heisenberg develops a way to describe the energies of electrons in atoms using matrix algebra. His approach became known as matrix mechanics.

William D. Harkins achieves transmutation of elements, converting nitrogen to fluorine and then to hydrogen and oxygen by bombarding the starting element with a helium nucleus. A German physicist later hits gold with hydrogen to make mercury.

The Ford Trimotor takes its first flight. The all-metal plane was reliable and comfortable for passengers, and commercial airlines quickly adopted it.

Physicist Erwin Schrödinger develops “wave mechanics,” a way to describe the energies of electrons in atoms by viewing electrons as waves. Wave mechanics was soon shown to be mathematically equivalent to the matrix mechanics proposed by Werner Heisenberg the previous year.

German physicist Max Born shows that Erwin Schrödinger’s wave equation can be used to calculate the probabilities for various possible outcomes of an atomic observation but does not allow prediction of a single specific outcome.

Based on a tooth found at China’s Zhoukoudian site, Davidson Black identifies a new hominin species named Sinanthropus pekinensis (now called Homo erectus) that lived several hundred thousand years ago.

The discovery of a flint point alongside the remains of prehistoric buffalo in Folsom, N.M., helps convince anthropologists that humans have lived in the Americas since the Ice Age.

Danish physicist Niels Bohr presents the principle of complementarity, arguing that both particle and wave views are necessary for a full description of the subatomic world.

Anthropologist Margaret Mead’s book Coming of Age in Samoa controversially argues that casual sexuality and other cultural practices on a South Pacific island make adolescence smoother for girls there than girls in Western cultures.

Geologist Arthur Holmes proposes that heat from the radioactive decay of elements in the Earth keeps the planet’s interior partially molten.

German psychiatrist Hans Berger reports the first human electroencephalogram, or EEG.

Ronald Fisher publishes a mathematical analysis of how natural selection can change the distribution of genes in a population, helping to synthesize Mendelian genetics with Darwin’s theory of evolution.

Albert Szent-Györgyi reports isolating hexuronic acid, later identified as vitamin C.

Sewall Wright begins to publish work showing that “random drift,” or chance fluctuations in a population’s gene frequencies, could be a significant factor in evolution.

Mayan glyphs are deciphered for the first time.

Heavy hydrogen atoms, now known as deuterium, are discovered by Harold Urey and George M. Murphy.

In his book The Moral Judgment of the Child, developmental psychologist Jean Piaget argues that children develop moral ideas in three broad stages largely via interactions with peers.

Pioneering memory researcher Frederic Bartlett publishes experiments showing that what people remember about past events consists of a mix of fact and culturally influenced fill-ins.

Physicist James Chadwick discovers an electrically neutral particle, the neutron, contained within atomic nuclei.

Images from physicist Carl Anderson’s cloud chamber reveal a positively charged particle with a mass equivalent to the electron: a positron. This antimatter partner of the electron had been foreshadowed by the work of theoretical physicist Paul Dirac.

Fritz Zwicky examines galaxies in the Coma cluster and determines that there is unseen mass, what scientists now call “dark matter.”

A strong electrical shock is found to restore a heartbeat to surgical patients whose hearts have begun fibrillating or have stopped.

British researchers isolate the virus that causes influenza for the first time, after using human throat washings to infect ferrets.

Henry Dale reports the discovery of acetylcholine, a chemical released by nerves to command a muscle to move.

Neurosurgeon Wilder Penfield reports on an epilepsy treatment, now known as the “Montreal Procedure,” that destroys areas of the brain where seizures begin.

Scientists use electro­encephalography to show that two types of electrical waves, labeled alpha and beta, occur in the brain.

Albert Einstein, Boris Podolsky and Nathan Rosen publish a paper asking “Can quantum-mechanical description of physical reality be considered complete?” Their answer: no. Yet Niels Bohr strongly disagrees.

Seismologist Inge Lehmann discovers that Earth has a solid inner core, distinct from its molten outer core.

Major new antibiotics, developed in Germany and later known as sulfonamides, are reported to show promise in U.S. tests against Streptococcus infections.

A new subatomic particle somewhere between an electron and a proton in mass, later termed the muon, is reported from debris of cosmic ray bombardments.

A vaccine for yellow fever is reported by Max Theiler and Hugh Smith. It’s based on a weakened (or attenuated) live virus.

Eugene Houdry’s process for using aluminum- and silicon-based catalysts to make gasoline is demonstrated at an industrial scale.

Nuclear physicist Hans Bethe describes how hydrogen atoms inside stars combine to form helium, releasing vast amounts of energy in the process.

In South Africa, Robert Broom identifies a robust form of hominin, named Paranthropus robustus, that had giant molar teeth and a skull built for heavy chewing. The species, now known to have lived 1.8 million to 1.2 million years ago, is evidence that more than one type of hominin once called the region home.

Claude Shannon’s master’s thesis “A Symbolic Analysis of Relay and Switching Circuits,” published in 1938, outlines the foundations for digital circuit design.

Engineer and amateur meteorologist Guy Stewart Callendar links a rise in average temperatures around the globe to the release of carbon dioxide from the burning of fossil fuels.

Chemist Roy Plunkett invents the superslick material polytetrafluoroethylene, more commonly known by its trademark Teflon.

Isidor Isaac Rabi discovers nuclear magnetic resonance in molecular beams. It would later be developed into a valuable tool for chemistry and medicine.
Connections: New Vistas

J. Robert Oppenheimer and Hartland Snyder describe how what we now call black holes could form as a massive star collapses under the weight of its own gravity.

Epidemiological data show that adding fluorine to drinking water cuts the risk of cavities.

French schoolboys call anthropologists’ attention to 30,000-year-old prehistoric cave art in Lascaux.

David G.C. Luck describes radar and it potential use in plane navigation.

Physicists Maria Goeppert Mayer and J. Hans D. Jensen develop a theory of the nucleus as composed of shells of protons and neutrons. It explains why nuclei with certain “magic numbers” of protons and neutrons are more stable.

After creating large amounts of carbon-14 using a cyclotron, or “atom-smasher,” researchers hail the isotope’s potential as a medical tracer. In 1949, Willard Libby and colleagues demonstrate that radioactive carbon can be used to measure the age of organic objects like fossils. The technique works on samples up to about 50,000 years old.

Archer Martin and Richard Synge report a new form of chromatography called “partition chromatography.” It formed the basis for much of the separation and analysis of complex mixtures today.

With the formation of the Manhattan Engineer District, the United States kick-starts a massive project to build an atomic bomb, which employed more than 120,000 people at its peak.

Details of an epidural nerve block that allows pain-free childbirth without putting women to sleep are reported.

Leo Kanner reports on his studies of a mental illness in 11 children that causes them to largely ignore the people around them, a disease that would come to be called autism.

A team at Rutgers University discovers the antibiotic streptomycin, the first drug to be effective against Mycobacterium tuberculosis. It remains a common treatment for tuberculosis today.

Oswald Avery, Colin MacLeod and Maclyn McCarty report evidence that DNA is the carrier of genetic information, though the result is not widely accepted at first.

Experiments in mice show that chemicals can — like radiation — induce mutations.

John Von Neumann describes a computing architecture that will become the standard for decades to come.

The first influenza vaccine is licensed for use. It’s an inactivated vaccine that contains both influenza A and B viruses.

Physicist Walter M. Elsasser and geophysicist Edward Bullard separately propose between 1946 and 1949 that Earth’s magnetic field is a self-sustaining dynamo generated by the movement of fluid in the liquid outer core, which produces an electric current.

The U.S. National Bureau of Standards reports timekeeping accurate to a millionth of a second using quartz crystals.

Claude Shannon’s “A Mathematical Theory of Communication” establishes the new field of information theory.

Scientists lay out the steps of photosynthesis.

Canadian psychologist Donald Hebb publishes his account of how neurons reinforce each other’s activity, a description that’s now shorthanded, “fire together, wire together.”

John Enders and colleagues grow poliovirus in cells that don’t originate in the nervous system, an important step toward developing polio vaccines.

A team led by meteorologist Jule Charney produces the first computer-driven weather forecast.

Lederle Laboratories scientists show that lacing animal feed with trace amounts of the antibiotic aureomycin can boost the growth of livestock.

William W. Morgan and colleagues present a model of the Milky Way’s shape made of cotton balls to the American Astronomical Society meeting, depicting the galaxy as a spiral. The team received a raucous ovation with stomping feet.

Doctors take samples of cervical cancer cells from a Black woman named Henrietta Lacks. The cells, which divide indefinitely, were shared and studied widely without Lacks’ consent, becoming a key tool in biomedical research. 

Physicians link atherosclerosis to the circulation of large fatty particles in the blood and suggest that a low-cholesterol diet could prevent the condition.

Phillips Petroleum chemists J. Paul Hogan and Robert Banks, while studying new ways to make gasoline, discover polypropylene, which also leads to a new way to make high-density polyethylene.

Rita Levi-Montalcini identifies chemicals from mouse tumors that stimulate growth in chick embryos — the discovery of nerve growth factor.

The United States tests the first hydrogen bomb, a thermonuclear weapon 1,000 times more powerful than the bomb dropped on Hiroshima.

A blender experiment by Alfred Hershey and Martha Chase demonstrates that DNA, not protein, is the genetic material.

Building on earlier work by French physicist Louis de Broglie, theoretical physicist David Bohm suggests a deterministic interpretation of quantum theory that incorporates “hidden variables.”

Physiologists Nathaniel Kleitman and Eugene Aserinsky report the discovery of rapid eye movement during sleep and link it to dreams.

Scientists report that chlorpromazine, developed to treat nausea and vomiting, also helps sedate mental patients.

Marguerite Vogt and Renato Dulbecco publish a method for purifying and counting poliovirus particles. Vogt and Dulbecco’s approach remains the gold standard for purifying and counting virus particles.

Though solar cells, which convert sunlight to electricity, date to decades earlier, scientists at Bell Laboratories announce the first practical silicon solar cell.

Psychologist George Miller reports that people can typically hold no more than about seven types or pieces of information in short-term memory, a capacity Miller dubbed “the magical number seven, plus or minus two.” He publishes the work the next year.

Physicians report that smoking harms the heart.

Donald Glaser reports on the first photographs taken with his new bubble chamber, a tool for recording collisions of subatomic particles.

Scientists detect the neutrino, an electrically neutral subatomic particle released in radioactive decay and other reactions. It had once been thought to be undetectable.

A workshop for a small group of scientists organized by John McCarthy at Dartmouth College is often considered the beginning of the field of artificial intelligence.

Meteorologist Norman Phillips develops the world’s first general circulation climate model, which captures how energy flows between the oceans, atmosphere and land.

The first full-scale commercial nuclear power plant, known as Calder Hall, switched on in Cumbria, England.

Scientists show how living things manufacture steroids, suggesting ways to block cholesterol formation.

The Soviet Union launches Sputnik 1, the first artificial satellite. Soon after, Sputnik 2 is launched with a dog on board.

Particles that are mirror images — those with opposite “handedness” or orientation of their spin — don’t necessarily behave identically. Chien-Shiung Wu and colleagues report the discovery of this phenomenon, called parity violation, in decays of cobalt-60 atoms.

Scientists Roger Revelle and Hans Suess report that oceans do not take up as much carbon dioxide as previously thought, which suggests much of the gas produced by human activities is going into the atmosphere.

A new influenza virus emerges in East Asia, ultimately killing 1.1 million people worldwide.

John Bardeen, Leon Cooper and Robert Schrieffer set out a theory explaining how electron pairs can flow without resistance through low-temperature materials.

President Eisenhower signs legislation establishing the National Aeronautics and Space Administration, better known as NASA.

Henry Dolger reports that diabetes is really two diseases: type 1 with little or no insulin made, and type 2 in which the body doesn’t use insulin well.

Physicist Roger Bacon, working at Union Carbide in Parma, Ohio, demonstrates the first high-performance carbon fibers.

At Olduvai Gorge in Tanzania, Mary Leakey finds the skull of a robust hominin. She and her husband, Louis Leakey, name it Zinjanthropus boisei (now called Paranthropus boisei). The discovery shifts the focus of hominin fieldwork to East Africa.

In a widely cited book review, linguist Noam Chomsky criticizes B.F. Skinner’s view of language as learned behavior and suggests that human speech springs from an innate grammar capacity.

David Hubel and Torsten Wiesel illuminate the visual system in the cat, opening up an area of inquiry for how brain systems handle information from the senses.

A team reports that a virus can hijack a cell’s machinery for reproduction, suggesting new ways to make vaccines.

Jack Kilby of Texas Instruments and Robert Noyce of Fairchild Semiconductor independently invent the integrated circuit, or microchip.

The U.S. Food and Drug Administration approves the first oral contraceptive, soon known simply as “the pill.”

Physicians report successfully transplanting bone marrow from one woman to another with Hodgkin’s-like disease.

Theodore Maiman demonstrates the first optical maser, or laser.

Soviet cosmonaut Yuri Gagarin becomes the first human to orbit the Earth.

Geologists use a radiometric dating technique to estimate a true date, rather than a relative age, for an ancient hominin fossil.

The first cochlear implants, which translate sounds into electrical stimulation, are implanted in deaf patients, in Los Angeles.

Geologist Harry Hess and geophysicist Robert S. Dietz independently argue in 1961 and 1962 that the seafloor is pulling apart at mid-ocean ridges.

Scientists reveal how the sequence of DNA’s four chemical subunits encode the instructions for the creation of the amino acids that make up proteins. One team including Francis Crick and Sydney Brenner discovers that it’s a triplet code, with each sequence of three subunits, or “genetic letters,” coding for one amino acid. In the same year, Marshall Nirenberg and Heinrich Matthaei report the three-letter sequence associated with one particular amino acid, jump-starting efforts to identify more.

Jacques Monod and François Jacob describe messenger RNA and its role in carrying genetic information from the cell nucleus to the ribosome for protein synthesis.

Albert Sabin’s oral polio vaccine is licensed in the United States. It’s an attenuated vaccine, which contains a weakened virus, that is taken as drops with a cube of sugar.

Geophysicist J. Tuzo Wilson suggests that volcanic island chains form as plates move over upwellings of magma in the mantle.

Mammography is shown to be a valuable gauge of the presence of breast tumors.

The first measles vaccine is licensed in the United States. In 1968, an improved measles vaccine that is still used today becomes available.

Military tests show that a high-altitude atomic bomb detonation could unleash a broad electro­magnetic pulse that would disrupt all electronics.

Louis Leakey and colleagues report that a collection of fossils found at Tanzania’s Olduvai Gorge are the earliest known remains from the genus Homo. The researchers fold the fossils into a new species, Homo habilis, thought to have lived about 2.4 million to 1.4 million years ago. The classification remains controversial.

Astronomers describe 12 strange celestial objects that appear a bit like stars and a bit like galaxies as quasars, for quasi-stellar radio sources.

Beginner’s All-purpose Symbolic Instruction Code, a computer programming language designed to be easy to use, successfully runs its first programs on a General Electric computer at Dartmouth College.

Murray Gell-Mann and George Zweig independently propose the existence of quarks — fractionally charged particles within protons and neutrons.

Physicist John Bell devises a mathematical theorem that would allow researchers to experimentally rule out any “hidden variables” that might explain the results of quantum entanglement experiments in classical, deterministic terms.

The NASA Mariner 4 spacecraft takes the first photos of another planet from space.

Cambridge Instrument Company markets the first commercial scanning electron microscope, which, according to Science News, “offers broad new avenues of biological research.”

The Soviet Luna 9 spacecraft makes the first soft landing on the moon.

Norwegian physiologist Terje Lømo observes that connections between nerve cells strengthen with use, a core principle of neuroscience called long-term potentiation.

By studying blood proteins, Vincent Sarich and Allan Wilson develop a “molecular clock” for primate evolution. The pair estimates that humans and African apes diverged about 5 million years ago. The latest estimates suggest that humans and chimpanzees, now known to be our closest living relative, diverged sometime between 9 million and 6 million years ago.

The great unifying theory of the earth sciences is born with the publication of two separate studies, one by geophysicist W. Jason Morgan in 1967 and the other by geophysicists Dan McKenzie and Robert Parker in 1968.

The first pulsar — stellar objects emitting beams of radiation that look from Earth like pulses — is discovered by Jocelyn Bell Burnell.

Meteorologists Syukuro Manabe and Richard Wetherald model connections between Earth’s surface and atmosphere and calculate how changes in carbon dioxide would affect the planet’s temperature.

Christiaan Barnard in Cape Town, South Africa, transplants a human heart into Louis Washkansky.

The first mumps vaccine, developed by Maurice Hilleman, is licensed in the United States.

Christiaan Barnard in Cape Town, South Africa, transplants a human heart into Louis Washkansky.

The idea that modern birds evolved from dinosaurs takes hold, with new evidence of warm-bloodedness and fossil discoveries such as the small, carnivorous Deinonychus antirrhopus.

American inventor Douglas Engelbart, at a meeting in San Francisco, demonstrates in one system most of the elements of modern personal computing.

Robert Heath Dennard receives a patent for dynamic random access memory that requires just one transistor, ultimately leading to big boosts in computer memory density.

A book by biologists Paul and Anne Ehrlich popularizes the notion of a “population bomb” in which global overpopulation would lead to mass starvation.

RCA Laboratories unveils the first liquid crystal displays, developed by electrical engineer George Heilmeier and his team.

Scientists report for the first time test-tube fertilization of human eggs.

The ARPANET is born, as two computers — one at UCLA and one at the Stanford Research Institute — link up and share a message.

The first rubella vaccine is licensed in the United States, following an epidemic during which 20,000 children were born with congenital rubella syndrome, which can include hearing loss and developmental problems.

Eric Kandel shows how synapses change in response to learning, early work that led to a greater understanding of short- and long-term memory.

Har Gobind Khorana and colleagues report creating the first synthetic gene.

Two research teams working independently discover that tumor viruses can use an enzyme, called reverse transcriptase, to transfer genetic information from their RNA into the DNA of a host cell.

Microbiologist Satoshi Ōmura discovers the antiparasitic drug ivermectin, still widely used today. The drug is based on compounds made by a bacterium found in a Japanese soil sample.

Using an electron microscope, physicist Albert Crewe takes the first photographs of individual atoms.

Mariner 9 orbits Mars, sending home pictures of a global dust storm.

Studying baker’s yeast, geneticist Leland H. Hartwell identifies genes that regulate how cells divide as well as proteins that halt division if a cell’s DNA is damaged.

Restriction enzymes, which cut DNA in specific locations, are successfully used for the first time. They would become an essential tool for molecular biology.

Federico Faggin, Ted Hoff and Stanley Mazor develop what’s widely considered the first microprocessor, the Intel 4004, a single-chip central processing unit, or CPU, with 2,300 transistors.

Experiments show that the properties of what was thought by many to be a new form of water, called “polywater,” come about from the presence of impurities in ordinary water.

Studies of radio emissions from Cygnus X-1 support claims that it is a black hole, the first detected black hole candidate.

Phytochemist Tu Youyou discovers artemisinin, used today along with its derivatives to treat malaria.

Michael E. Phelps and colleagues invent a technique for seeing radioactive tracers in the living brain called positron emission tomography, or PET.

A group of women in India lead a series of widely publicized protests against deforestation, linking environmental protection with the protection of human communities.

Two teams of physicists find a new subatomic particle, the J/psi, which, it soon became clear, could be explained only by a new type of quark, the charm quark. This discovery and others during the period, known as the November revolution, quelled lingering doubts that quarks were real constituents of larger particles such as protons, neutrons and the J/psi.

Researchers report evidence that Freon and other chloro­fluorocarbons destroy stratospheric ozone.

Astronomers William Hartmann and Donald Davis propose in 1975 the Giant Impact Hypothesis, suggesting the moon formed out of material ejected from Earth after a large collision. Space scientist Alastair Cameron and astronomer William Ward developed the same idea independently, proposing it in 1976.

At the Asilomar Conference in California, scientists for the first time develop rules restricting investigations in the nascent field of genetic engineering.

George Köhler and César Milstein report finding a technique for merging cells that produce a particular antibody with tumor cells to grow vast quantities of descendant cells that produce the original, single antibody.

The first spacecraft to survive on Mars for an extended time looked for signs of life, but its results were inconclusive.

Scientists find that Earth’s ice ages over the last 500,000 years correlate to three different orbital variations — cycles lasting about 23,000 years, 42,000 years (now said to be 41,000) and 100,000 years, confirming a hypothesis proposed decades earlier by astronomer Milutin Milankovitch.

The first Cray-1 supercomputer is installed at Los Alamos National Laboratory in New Mexico.

Two spacecraft head to the outer reaches of the solar system, eventually passing all the giant planets and leaving the solar system altogether.

Carl Woese and George Fox report identifying the third domain of life, adding the archaebacteria (now commonly called Archaea) alongside bacteria and eukaryotes.

Frederick Sanger and his team use a DNA sequencing technique that they developed to sequence the full genome of the virus phiX174.

A fiber-optic cable carries live, commercial telephone traffic for the first time, introducing an era of telephoning by light.

Work reported by Edward Lewis in 1978 and Christiane Nüsslein-Volhard and Eric Wieschaus in 1980 reveals which genes control early embryonic development in fruit flies. The discoveries help explain congenital defects and lead to research on similar genes that determine the human body plan.

Astronomers detect the first gravitational lens, seeing a single quasar appear as a double image.

Drillers begin extracting a deep ice core from the Dye-3 site in southern Greenland. The core would later offer evidence for how quickly climate has changed in the past.

The Three Mile Island nuclear power plant experiences a catastrophic accident.

Luis Alvarez and Walter Alvarez publish a report saying that a large space rock hitting Earth was responsible for the mass extinction event 66 million years ago that killed off all nonbird dinosaurs. The father-son team proposed the idea at American Geophysical Union’s annual meeting the year before.

Physicist Paul Benioff discusses the theoretical possibility of a quantum computer, an idea further popularized by Richard Feynman.

An outbreak of two rare and serious diseases among gay men — Kaposi’s sarcoma and Pneumocystis pneumonia — mark the discovery of what would come to be known as AIDS.

NASA reports satellite evidence that the stratospheric ozone layer is being depleted globally.

Neurologist Stanley Prusiner identified a misfolded protein, calling it a “prion,” as the infectious agent in the degenerative brain disorder Creutzfeldt-Jakob disease.

Thomas Cech and colleagues report that RNA can function like an enzyme.

Surgeons successfully implant the first permanent artificial heart into a human.

Following earlier experiments in the 1970s, French physicist Alain Aspect and colleagues report the strongest test to date of entanglement. The results agree with quantum mechanical predictions and show no signs of hidden variables.

Kary Mullis invents the polymerase chain reaction technique, which can make millions of copies of a DNA sequence in a short amount of time.

A genetic marker for Huntington’s disease was mapped to chromosome 4. It was one of the first of many human disease genes to be located.

Scientists including Carl Sagan publish a paper titled “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions” that describes how smoke from cities annihilated by nuclear weapons and from forest fires ignited by airbursts could bring on global cooling.

Françoise Barré-Sinoussi and Luc Montagnier discover the retrovirus that is eventually named Human Immunodeficiency Virus, the cause of AIDS.

Chuck Hull invents stereolithography, an early 3-D printing technique.

George Glenner and Caine Wong discovered a “novel cerebrovascular amyloid protein.” That was amyloid-beta, the key component of plaques in the brains of people with Alzheimer’s disease.

Walter Jakob Gehring identifies the homeobox, a cluster of genes expressed in early embryonic development that determine how an organism’s body develops.

Physicist Charles Bennett and computer scientist Gilles Brassard propose a theoretical system for quantum cryptography, which would use photons in a superposition of states to create a secure key.

Svante Pääbo reports isolating a snippet of DNA from a roughly 2,400-year-old Egyptian mummy. Although the sample may have been contaminated with modern DNA, the investigation kicks off the study of ancient DNA in paleoanthropology.

U.S. federal agencies approve the first two experimental releases of genetically modified organisms: antifrost bacteria for strawberries and tumor-resistant tobacco plants.

The space shuttle Challenger explodes, killing all seven crew members, including a high school teacher.

Margaret Geller, John Huchra and Valérie de Lapparent map a section of the observable universe, revealing a structure that encompasses large walls and giant voids.

Physicists Georg Bednorz and Alex Müller discover the first materials that superconduct at relatively high temperatures, kicking off a search for more, similar materials.

A controversial study of modern-day people’s mitochondrial DNA, a type of DNA passed down from mother to child, suggests that all humans can trace their maternal ancestry back to a population that lived in Africa roughly 200,000 years ago.

Geochemist Wallace Broecker describes a global system of ocean currents that transports heat and salt between surface and deep waters and around the globe, influencing regional climates. 

The drug fluoxetine, or Prozac, received approval from the U.S. Food and Drug Administration as a treatment for major depression.

Intelligence researcher James Flynn reports large jumps in IQ scores from one generation to the next in many Western nations, indicating that environment shapes performance on intelligence tests.

After inventing optical tweezers, which use laser light to capture small particles, Arthur Ashkin trapped living bacteria without harming them.

The Montreal Protocol to phase out stratospheric ozone-destroying chloro­fluorocarbons gets enough signatures; it goes into effect January 1, 1989.

NASA scientist James Hansen testifies to lawmakers about the consequences of global warming, vaulting climate change into the public eye in the United States.

The Intergovernmental Panel on Climate Change, a United Nations body that synthesizes and summarizes the literature of climate science, is born. It releases its first assessment report in 1990.

Alan Handyside of London’s Hammersmith Hospital is able to check an embryo for genetic defects before implanting it into a mother’s uterus. This testing, known as preimplantation genetic diagnosis or PGD, enables parents carrying a genetic or chromosomal defect to avoid passing it to their children.

Tim Berners-Lee, a researcher at the European laboratory CERN, writes the first proposal for what would become the World Wide Web.

Martin Fleischmann and B. Stanley Pons report on benchtop nuclear reactions that they describe as “cold fusion” but that are never confirmed.

A tanker accident dumps more than 11 million gallons of crude oil in Alaska’s Prince William Sound.

Seiji Ogawa and colleagues at Bell Labs invent functional magnetic resonance imaging, or fMRI, a method that reveals blood flow changes in the brain, a proxy for activity.

Based on just nine minutes of data, the Cosmic Background Explorer, or COBE, reveals that the cosmic microwave background radiation aligns with what is expected from blackbody radiation, good evidence that it is an afterglow of the Big Bang. 

In the first federally approved gene therapy trial, researchers inject genetically engineered cells into a 4-year-old patient with an inherited immune deficiency.

Delegates at the First National People of Color Environmental Leadership Summit in Washington, D.C., draft and adopt 17 principles of environmental justice, setting a foundation for a growing movement.

A series of research efforts compellingly link stomach ulcers to the H. pylori bac­terium.

David Jewitt and Jane Luu spot a population of small objects orbiting the sun beyond Neptune, which came to be known as the Kuiper Belt.

Neuroscientists discover that a protein can prompt mature nerve cells in adult mice to divide, dispelling the belief that adult mammals’ brain cells cannot reproduce. Whether this happens in human brains is still unclear.

Giacomo Rizzolatti and colleagues discovered a set of neurons in the brains of macaque monkeys that fire both when monkeys do something and see another monkey doing it. These neurons would later be called mirror neurons.

Scientists for the first time clone human embryos, raising a host of ethical questions.

Astronomers report evidence of Massive Compact Halo Objects at the outskirts of the Milky Way. These MACHOs account for some but not all of galactic dark matter.

Analysis of two ice cores drilled near each other in Greenland confirm the reality of abrupt climate change in the near geologic past.

Physicist Charles Bennett and collaborators propose that entanglement can, in principle, be used to transfer a particle’s quantum information from one place to another, a feat termed “quantum teleportation.”

Michel Mayor and Didier Queloz find a giant planet orbiting the star 51 Pegasi, the first exoplanet discovered around a star like the sun.

Tim White and colleagues report the discovery in Ethiopia of what is then the oldest known hominin, which lived 4.4 million years ago. In 2009, 15 years later, White and colleagues publish the first in-depth look at the species, Ardipithecus ramidus.

An academic battle breaks out among evolutionary psychologists and others over the nature of human decision making and what it means to be rational.

Astronomers report the most compelling evidence for the existence of a black hole, at the center of galaxy M87, about 50 million light-years from Earth.

A pair of genes, BRCA1 and BRCA2, appear to play a role in some breast cancers that cluster in families.

Mathematician Peter Shor discovers an algorithm that would allow a quantum computer to factor large integers quickly and so crack the toughest secret codes.

The Intergovernmental Panel on Climate Change establishes that there is evidence of a “discernible” human influence on climate.

Researchers for the first time sequence the genome of a bacterium, Haemophilus influenza.

Researchers at the National Institute of Standards and Technology demonstrate a quantum logic gate with trapped ions.

Carbon isotope measurements from Greenland rocks push back the history of life on Earth to 3.85 billion years ago.

Researchers report that a combination of three antiretroviral drugs is more effective against HIV than two-drug therapy and can lead to undetectable levels of virus.

Biologists isolate human embryonic stem cells, which have the potential to become nerves, blood or any other tissue.

An IBM supercomputer called Deep Blue defeats world chess champion Garry Kasparov.

The first-ever international treaty to reduce greenhouse gas emissions, the Kyoto Protocol, was adopted. It went into effect in 2005.

Physicists Dik Bouwmeester, Anton Zeilinger and colleagues harness quirks of quantum behavior to transfer one photon’s polarization state to a remote photon.

Astronomers uncover data indicating that the expansion of the universe is picking up speed.

Physicists discover that neutrinos, long thought to be massless, have a tiny amount of mass. The find raises a slew of still-unanswered questions, whether neutrinos are their own antiparticles, for example, or whether the particles can help explain the scarcity of antimatter in the universe.

Two independent research groups report deciphering the genetic blueprint of humans.

Physicists report getting light pulses to stand still, without destroying the photons.

A nearly complete skull found in Chad and dating to between 7 million and 6 million years ago may be from the earliest known hominin, Michel Brunet and colleagues announce. The team names the find Sahelanthropus tchadensis.

Astronomers put the age range of the universe at between 13 billion and 14 billion years.

Researchers find increasing numbers of genes that contain instructions to make RNA that is not used for making proteins.

Neurologist Helen Mayberg begins work testing deep brain stimulation on people with severe depression, targeting an area called the subcallosal cingulate. 

A deadly viral pneumonia that emerged in China, called SARS for severe acute respiratory syndrome, sickens more than 8,000 worldwide.
 

A surprising find is reported from the Indonesian island of Flores: A small-brained, short-statured hominin lived there about 100,000 to 50,000 years ago. Named Homo floresiensis, but often called the hobbit, the species may be a case of island dwarfism.

A magnitude 9.1 earthquake off the coast of Indonesia spawned a devastating tsunami that killed over 250,000 people in 14 countries.

A technique to control genetically engineered nerve cells (shown) in animal brains precisely using laser light, now known as optogenetics, is reported.

Physicists at Brookhaven National Laboratory smash nuclei of gold atoms together to recreate the phase of matter thought to have existed in the early moments of the universe. The quark-gluon plasma is a hot, dense soup of quarks intermingling freely with gluons, which under normal conditions bind quarks into larger particles like protons and neutrons.

With all the new bodies discovered in the Kuiper Belt, the International Astronomical Union controversially declares Pluto a dwarf planet, not a planet.

China passes the United States as the world’s biggest emitter of carbon dioxide, as measured by annual emissions.

Biologists turn human skin cells into stem cells, without embryos.

The Intergovernmental Panel on Climate Change declares that the warming of the climate is “unequivocal.”

A Mars lander definitively confirms the presence of water on Mars, after the rover “touched and tasted ice.”

A tumor suppressor protein turns out to have a previously unrecognized function: helping to slice stretches of RNA into regulatory molecules called microRNAs.

Researchers get a surprise when mitochondrial DNA extracted from a stray finger bone from Russia’s Denisova Cave matches neither Neandertal nor modern human DNA, suggesting a previously unknown hominin lived there.

The Neandertal is the first ancient hominin to have its genetic blueprint, or genome, pieced together. Studies of the ancient DNA reveal humans mated with Neandertals. 

Cross-cultural researchers led by anthropologist Joseph Henrich review evidence that members of Western, educated, industrialized, rich and democratic (WEIRD) societies are among the least useful populations to study in order to generalize about how resources get shared and other features of human nature.

A report claimed that a weird form of life incorporates arsenic in place of phosphorus in biological molecules. On second glance, the evidence didn’t hold up.

The biggest oil spill in the history of the United States dumps a mixture of crude oil and natural gas into the Gulf of Mexico for five months.

A magnitude 9.0 earthquake in Japan and the tsunami it spawned kill more than 15,000 people and damaged reactors at the Fukushima power plant, triggering the worst nuclear disaster since Chernobyl in 1986.

North Carolina sediment cores reveal that sea levels began rising precipitously in the late 19th century, a trend attributed to climate change.

Physicists report clocking neutrinos traveling faster than the speed of light, seeming to call into question Einstein’s theory of special relativity. In the end, a loose cable was to blame.

Research groups led by Susumu Tonegawa and Mark Mayford independently create false memories in the brains of mice.

Long-standing inabilities to confirm influential psychology findings in repeat studies spark an ongoing debate over research methods and practices.

Jennifer Doudna, Emmanuelle Charpentier and colleagues introduce the CRISPR/Cas9 gene-editing tool. The next year, Feng Zhang and his team would adapt the tool to cut DNA at precise locations in the human genome.

What’s known as a convolutional neural network makes a big leap in performance in the annual ImageNet Challenge, an object detection and image classification competition. Convolutional neural networks would become the standard architecture for computer vision.

Scientists attribute extreme weather events, including heat waves and extreme rainfall, to climate change.

Scientists amass substantial evidence that animals, including people, and their resident bacteria, archaea, fungi and viruses should be thought of as superorganisms.

Astronomers report detecting ripples in spacetime imprinted on the cosmic microwave background radiation, the flash of light released into space about 380,000 years after the Big Bang. It would have been the first direct evidence of cosmological inflation, but it turned out not to be true.

Lee Berger and colleagues report finding a small-brained species of Homo, named Homo naledi, in a South African cave. Although the team thinks H. naledi was an early member of the genus, later dating shows it lived only about 300,000 years ago, making it a contemporary of Homo sapiens.

Precise analyses of ancient Siberian lava provide the smoking gun, convicting ancient volcanic eruptions in the greatest known mass extinction on Earth 252 million years ago.

A United Nations climate conference in Paris produces an international agreement to try to limit global warming to 2 degrees Celsius, and preferably 1.5 degrees C, above preindustrial levels.

An outbreak of Zika virus in Brazil spreads elsewhere in North America. The infection is linked to microcephaly and other neurological disorders.

The first baby with three biological parents is born with DNA from his mother, father and the mitochondria of an egg donor. The birth raises hopes for preventing inheritance of mitochondrial disorders —and triggers fears of “designer babies.”

Scientists for the first time report the detection of cosmic ripples in spacetime, gravitational waves from colliding black holes, thus verifying one of general relativity’s predictions.

The Keeling curve, an iconic graph that tracks the rise in atmospheric carbon dioxide, passes an annual minimum of 400 parts per million.

Jean-Jacques Hublin and colleagues announce that excavations in Morocco have uncovered Homo sapiens fossils dating to about 300,000 years ago. The discoverers say the remains are the oldest known members of our species.

Gene therapy becomes a reality with the approval by the U.S. Food and Drug Administration of two personalized treatments that engineer a patient’s own immune system to hunt down and kill cancer cells.

Chinese scientist Jiankui He uses CRISPR/Cas9 to create the first gene-edited babies — twin girls — to reduce their risk of contracting HIV. His actions lead to the formation of an international commission on the clinical use of gene editing.

Investigators report using a public genealogy database to track down a man suspected of being the Golden State Killer, prompting a flurry of concerns about privacy and ethics. The man, Joseph James DeAngelo, pleaded guilty to 13 murders and 13 rape-related charges.

An outbreak of measles that began in 2018 threatens the United States’ claim of having eliminated measles.

A cluster of pneumonia cases in China with no known cause are reported in December. In 2020 the cases are linked to a novel coronavirus that will soon spread around the globe, killing millions of people and counting.

SpaceX launches three astronauts to the International Space Station, marking the first time a private company sent humans into orbit.

NASA’s Parker Solar Probe, launched in 2018, leaves interplanetary space and enters the sun’s atmosphere.

Surgeons successfully transplanted a heart from a genetically modified pig into a human patient. The patient survived for two months.