Evolution: quotes, facts and reflections

Related subjects

Browse this subject and connected topics.

Keep reading below, or choose a related topic to explore something different.

DriveReading mode

StillnessReading mode

WonderReading mode

WonderEvolution

Archaeopteryx, an animal about the size of a magpie that lived around 150 million years ago in what is now Germany, had feathered wings like a bird but also teeth, clawed fingers and a long bony tail like a small dinosaur. It was one of the first clues that birds are related to dinosaurs.

In 1861 the German palaeontologist Hermann von Meyer described a single fossil feather from the Solnhofen limestone of Bavaria, and the same year a nearly complete skeleton turned up nearby. The limestone was so fine-grained that it was quarried for printing plates, and it preserved even the outlines of feathers. The skeleton was bought by the British Museum and described by Richard Owen in 1863, four years after Darwin's On the Origin of Species. About a dozen specimens are now known, including the fine Berlin specimen found in the 1870s. Archaeopteryx is still usually placed at or near the base of the bird family tree, probably on a side branch rather than as the direct ancestor of modern birds.

ArchaeopteryxRead card

WonderEvolution

Darwin's finches, more than a dozen bird species of the Galápagos Islands, all descend from a single ancestor. Their beaks range from thick nutcrackers for hard seeds to fine tweezers for insects, each suited to the food on its island.

Darwin collected some of these birds in 1835 during the voyage of HMS Beagle but did not realise at the time how closely related they were; the ornithologist John Gould pointed it out after the voyage. Today they are a textbook case of adaptive radiation, in which one ancestral species splits into many, each suited to a different way of life. Despite the name, they are not true finches but belong to the tanager family. Since 1973 Peter and Rosemary Grant have studied them on the small island of Daphne Major. After a severe drought in 1977 left mostly large, hard seeds, birds with bigger beaks survived better, and the next generation's beaks were measurably larger: evolution seen within a few years.

Darwin's finchesRead card

WonderOcean

Hydrothermal vents, hot springs on the deep ocean floor, support dense communities of animals in total darkness. Their food chain starts not with sunlight but with microbes that make food from chemicals in the vent water, a process called chemosynthesis.

In February 1977 the geologist Jack Corliss and colleagues took the submersible Alvin down about 2,500 metres to the Galápagos Rift, a crack in the Pacific seafloor, expecting bare rock. Instead they found warm vents ringed with clams, mussels, crabs and tube worms. Vent water is rich in hydrogen sulphide, a gas poisonous to most life, but some bacteria can draw energy from it to build sugars, much as plants use sunlight. Giant tube worms, which have no mouth or gut, house these bacteria inside their bodies and live on what they make. The find changed ideas about where life can exist, on Earth and possibly beyond it.

Hydrothermal ventsRead card

WonderMigration

European eels live in the rivers of Europe and North Africa but begin life some 5,000 kilometres away in the Sargasso Sea, in the western Atlantic. Adults swim back there to breed, yet no one has ever seen them spawn.

How eels breed puzzled people for more than 2,000 years; Aristotle thought they arose from mud. In the early 1900s the Danish biologist Johannes Schmidt spent almost two decades netting eel larvae across the Atlantic and found that they grew smaller the further west he went, until the tiniest turned up only in the Sargasso Sea. The larvae drift to Europe with the Gulf Stream and turn into transparent 'glass eels' as they near the coast. In 2022 researchers reported the first direct evidence of adults reaching the Sargasso Sea, after tracking satellite-tagged eels released in the Azores. No eggs or spawning adults have yet been found. The species is now critically endangered.

European eel migrationRead card

WonderEvolution

In the Cambrian explosion, which began about 538 million years ago, most of the major animal groups alive today appeared in the seas within roughly 20 million years, among them the first arthropods, molluscs and early relatives of vertebrates.

The name makes it sound sudden, but the Cambrian explosion unfolded over millions of years and built on the simpler life of the preceding Ediacaran period. What changed was the sheer variety: eyes, legs, shells and hard skeletons appear, along with burrows and the first clear signs of predators hunting prey. Two exceptional fossil beds preserve even soft tissue: Chengjiang in China, about 518 million years old, and the Burgess Shale in Canada, about 508 million years old. Trilobites, armoured sea-floor arthropods, appear early in the period and survived for some 270 million years. Leading explanations for the burst include rising oxygen in the oceans, new genetic tools for building bodies, and an arms race between predators and prey.

Cambrian explosionRead card

WonderEvolution

In industrial Britain, the dark form of the peppered moth, first recorded in Manchester in 1848, became common where soot blackened the trees and hid it from birds. When the air was cleaned up in the twentieth century, the pale form returned.

The peppered moth became a textbook example of natural selection. By the 1890s nearly all peppered moths around Manchester were dark. In the 1950s Bernard Kettlewell's field experiments suggested that birds ate more of the moths that stood out against the bark, though critics later questioned his methods. After clean-air laws such as the UK's Clean Air Act of 1956 cut soot, the dark form became rare again. The geneticist Michael Majerus answered the critics with a six-year experiment in Cambridge involving 4,864 moths; published after his death, in 2012, it confirmed that birds took more of the conspicuous moths. In 2016 scientists traced the dark colour to a piece of mobile DNA inserted into a gene called cortex around 1819.

Peppered moth evolutionRead card

WonderEvolution

Most adults in the world make little lactase, the enzyme that digests milk sugar, after childhood. In several peoples with a long history of dairy farming, genetic changes that keep the enzyme working for life spread through natural selection, a trait called lactase persistence.

In Europe the most common variant sits in a stretch of DNA that switches the lactase gene (LCT) on and off; a widely cited estimate puts its first spread at roughly 7,500 years ago. Different mutations with the same effect arose independently among cattle herders in East Africa and pastoralists in the Middle East, so evolution reached one solution several times. Scientists call this gene-culture coevolution: a cultural habit, keeping animals for milk, changed which genes helped people survive. The story is more complicated than it first looked. A 2022 study in Nature found that Europeans used milk for thousands of years while the variant stayed rare, and suggested it spread fastest in famines and epidemics, when milk could make people without it seriously ill.

Lactase persistenceRead card

WonderEvolution

At Laetoli in Tanzania, footprints pressed into volcanic ash about 3.6 million years ago show that early human relatives already walked upright on two feet. They are among the oldest known footprints of hominins, the group that includes humans and our extinct close relatives.

The hominin prints were found in 1978 by a team led by the palaeoanthropologist Mary Leakey, two years after animal tracks were first noticed at the site. Ash from an eruption, probably of the nearby Sadiman volcano, was dampened by light rain, took the prints of animals and people walking across it, and set hard before more ash buried it. The main trail is about 27 metres long and was left by at least two individuals, most likely of the species Australopithecus afarensis, to which the famous skeleton nicknamed Lucy belongs. The feet show a humanlike arch and a big toe in line with the other toes, so upright walking was established long before large brains or stone tools.

Laetoli footprintsRead card

WonderEvolution

Convergent evolution happens when unrelated organisms independently evolve similar traits, such as streamlined bodies in sharks, dolphins and extinct ichthyosaurs.

Sharks are fish, dolphins are mammals and ichthyosaurs were marine reptiles, yet all three evolved a similar streamlined, fin-driven body built for fast swimming. The same pattern shows up elsewhere: camera-like eyes, with a lens that focuses light onto a retina, evolved independently in both vertebrates and octopuses, and the extinct Australian thylacine looked strikingly like a wolf despite being a marsupial with no close relation to dogs. Convergent evolution does not mean life is aiming for a fixed set of designs. Natural selection simply favours whatever body plan works best under a given set of physical pressures, so unrelated lineages can arrive at strikingly similar answers.

Convergent evolutionRead card

WonderEvolution

Maize, sugarcane and sorghum grow well in heat and strong sun partly because they use C4 photosynthesis, a way of concentrating carbon dioxide inside the leaf. This method has evolved independently more than 60 times in different groups of plants.

Most plants use C3 photosynthesis, which relies on an enzyme called RuBisCO to capture carbon dioxide. In heat and drought, plants close the pores in their leaves to save water, carbon dioxide inside runs low, and RuBisCO grabs oxygen instead, wasting energy in a process called photorespiration. C4 plants first trap carbon dioxide in their outer leaf cells as a compound with four carbon atoms, which gives the pathway its name, then carry it to inner cells where RuBisCO works and release it there, so the enzyme is surrounded by carbon dioxide. With more than 60 separate origins, it is one of the most often repeated adaptations in plant evolution. Scientists are now trying to engineer the trait into rice.

C4 photosynthesisRead card

WonderNature

Welwitschia, a plant of the Namib Desert in Namibia and Angola, grows only two leaves in its whole life. They keep growing from their base for centuries, and some plants are thought to be more than a thousand years old.

The Austrian botanist Friedrich Welwitsch came across the plant in southern Angola in 1859; he wrote that he knelt and gazed at it, half afraid that touching it would prove it imaginary. It is a gymnosperm, like conifers, which means its seeds form in cones rather than inside fruit, and male and female cones grow on separate plants. Over the years the wind tears the two leaves into many ribbons, so an old plant looks like a heap of leaves. The Namib is one of the driest places on Earth. The leaves catch fog and dew, which drip onto shallow roots, although a 2019 study suggests that rain soaking into the ground is its main water source. The plant appears on Namibia's coat of arms.

WelwitschiaRead card

WonderNature

The kākāpō, a flightless, night-active parrot of New Zealand, courts with sound: in the rare years when it breeds, males dig shallow bowls in the ground and boom from them through the night, a deep call that can carry for more than a kilometre.

The kākāpō is the world's only flightless parrot and the heaviest, with males weighing up to about 4 kilograms. It once lived across New Zealand, but hunting and the dogs and rats brought by Polynesian settlers, then the cats, stoats and rats that came with Europeans, drove it close to extinction. Today every surviving bird has a name and lives on predator-free islands or in fenced sanctuaries, watched over by the Kākāpō Recovery programme. Females breed only in years when rimu and some other native trees produce a huge crop of fruit, known as masting, which comes every two to four years. In those seasons rangers check nests closely and sometimes raise chicks by hand.

KākāpōRead card

WonderNature

Some lodgepole pines keep their seeds inside cones sealed shut with resin. The heat of a forest fire melts the seal and releases the seeds onto freshly cleared, ash-rich ground.

Botanists call this trait serotiny, from a Latin word meaning 'late': the cones open late, once heat has melted their resin, typically at around 45 to 60 degrees Celsius. Sealed cones can keep seeds alive on the branches for many years, so a stand carries its own seed bank long before any fire arrives. The strategy became famous after the Yellowstone fires of 1988, when many burnt slopes filled with young lodgepole pines within a few years. Not every tree does it. In some populations most cones stay sealed, while in others most open each year like ordinary cones, a mix that tends to follow how often fire has struck the area.

SerotinyRead card

WonderNature

Rafflesia arnoldii lives almost entirely inside a tropical vine as a parasite, emerging briefly to produce a vast carrion-scented flower.

Rafflesia arnoldii grows in the rainforests of Sumatra and Borneo. It is named after Sir Stamford Raffles and the surgeon and naturalist Joseph Arnold, who recorded it in Sumatra in 1818; local people knew it long before. It is a holoparasite: it has no green tissue at all and takes all its water and food from its host, a Tetrastigma vine of the grape family, living as threads of cells inside the vine's stem. A bud swells for many months before opening into a flower up to about a metre across and weighing several kilograms, the largest single flower of any plant. Its smell of rotting meat draws the carrion flies that pollinate it, and the bloom lasts only about a week. Several Rafflesia species are threatened as forests are cleared.

Rafflesia arnoldiiRead card

WonderNature

The hooded pitohui, a songbird of New Guinea, can carry batrachotoxin, a nerve poison also found in some poison frogs, in its skin and feathers. Levels vary from bird to bird and are thought to come from its diet.

People in New Guinea have long known the bird is unpleasant to eat, and Western naturalists recorded this as early as 1895; 'pitohui' is said to be a Papuan term for 'rubbish bird'. Its poison drew scientific attention in 1990, when researchers preparing the birds' skins for museum collections felt numbness and burning in their hands. In 1992 a team including the ornithologist Jack Dumbacher reported homobatrachotoxin, a close relative of batrachotoxin, in the skin and feathers, making pitohuis among the first poisonous birds documented by science. The likely source was later traced to small Choresine beetles, found in pitohuis' stomachs and carrying the same family of toxins. The poison probably helps deter predators and parasites such as feather lice.

Hooded pitohuiRead card

WonderBiology

Tortoiseshell cats are almost always female. Early in development, each embryo cell randomly switches off one of its two X chromosomes, and all its descendants, pigment cells included, keep that choice. One X carries an orange coat-colour gene, the other a non-orange version, so the switched-off X sets each patch's colour.

The process is called X-inactivation, or lyonisation, after the British geneticist Mary Lyon, who proposed it in 1961 from her studies of coat colour in mice. Female mammals have two X chromosomes and males one; switching one X off in each female cell keeps the dose of X-chromosome genes the same in both sexes. Because the choice is made while the embryo is still a small ball of cells and is then copied to every descendant cell, the colour appears in patches rather than a fine speckle. A male cat can be tortoiseshell only if he carries an extra X chromosome (XXY), a rare condition that usually leaves him sterile. Calico cats show the same mechanism, plus a separate gene that adds white patches.

X-inactivationRead card

WonderBiology

Cultivated sweet potatoes carry DNA that passed into them naturally, long ago, from a soil bacterium called Agrobacterium, and some of those bacterial genes are still active. No laboratory was involved: the crop is a naturally genetically modified plant.

Agrobacterium is known for inserting part of its own DNA into plants, making them produce nutrients for it; since the 1980s, genetic engineers have used the same trick to add genes to crops. In 2015 a team led by Tina Kyndt and Godelieve Gheysen of Ghent University, working with the International Potato Center, reported bacterial DNA in the sweet potato genome. One insertion, containing four bacterial genes, was present in all 291 cultivated varieties they tested but not in the plant's close wild relatives. That pattern suggests early farmers may have unknowingly favoured plants carrying it, perhaps for a trait it affected, although which trait, if any, is still unknown. The case is often mentioned in debates about genetically modified food.

Sweet potatoRead card

WonderOcean

Some sea slugs keep the chloroplasts of the algae they eat, the tiny green parts that turn sunlight into food, working inside their own bodies. The stolen chloroplasts can go on photosynthesising for days or, in some species, months.

These slugs belong to a group called the Sacoglossa, and the habit is known as kleptoplasty, from the Greek for 'thief' and 'formed'. They pierce algal cells, suck out the contents and store the chloroplasts in cells lining the gut. The best-known example, the emerald green Elysia chlorotica of North America's Atlantic coast, can keep chloroplasts from the alga Vaucheria litorea working for months. Scientists once thought the slug had taken algal genes into its own DNA to maintain them, but later genetic studies did not confirm this, and how the chloroplasts last so long is still debated. So is how much they feed the slug: in some experiments, starved slugs kept in the dark survived about as long as those kept in light.

KleptoplastyRead card

WonderBiology

The closest living relatives of animals are single-celled organisms called choanoflagellates. Their cells look strikingly like certain cells in sponges, and they offer clues to how the first animals evolved from single cells.

A choanoflagellate beats a single whip-like tail, a flagellum, to draw water through a collar of tiny finger-like projections that trap bacteria to eat. Choanoflagellates are not our ancestors but a sister lineage that split from the animal line hundreds of millions of years ago. Their genomes, first sequenced in 2008, contain genes for proteins such as cadherins, which animals use to stick cells together, and tyrosine kinases, which pass signals between cells. One species, Salpingoeca rosetta, can switch from living alone to forming small rosette-shaped colonies, and in 2012 Nicole King's laboratory at the University of California, Berkeley, found that a molecule released by a particular bacterium triggers the switch.

ChoanoflagellateRead card

WonderOcean

A siphonophore, a relative of jellyfish, grows from a single fertilised egg into a colony of connected parts. Each part, called a zooid, takes on one job: swimming, feeding, defence or reproduction.

Siphonophores are cnidarians, the group that includes jellyfish and corals. The fertilised egg becomes a single founding zooid, which buds off all the others, so every part is genetically identical. Each zooid corresponds to a whole polyp or small jellyfish in related animals, but here it is specialised and cannot live alone, so the colony works like one body with organs. The best-known siphonophore is the Portuguese man o' war, whose painful sting comes from its feeding and defensive zooids. Many others are fragile, glassy animals of the deep sea. In April 2020 researchers filming in the Ningaloo Canyons off Western Australia found a giant Apolemia colony coiled in a spiral whose outer ring was estimated at about 47 metres, among the longest animals ever recorded.

SiphonophoraeRead card

WonderOcean

When attacked, hagfish release mucus and microscopic protein threads that expand through seawater into extraordinary volumes of defensive slime.

Hagfish are eel-shaped, jawless fish that live on the seafloor. More than 70 species are known, and fossils about 300 million years old already look much like them. They often feed by burrowing into the carcasses of dead fish and whales. Their slime comes from glands along the body, which release mucus and tightly coiled protein threads. In seawater the threads unravel in a fraction of a second, and a small amount of secretion can turn litres of water into slime, enough to clog the gills of an attacking fish; underwater cameras have filmed sharks letting go and swimming off. Each thread is far thinner than a human hair yet strong and stretchy, so researchers study it as a model for new fibres. Afterwards the hagfish ties itself in a knot and slides the knot along its body to wipe the slime off.

HagfishRead card

WonderEvolution

Three-spined sticklebacks, small fish of northern seas, have moved into lakes and rivers again and again. Each time they tend to evolve lighter armour, often using a gene variant their sea-going ancestors already carried.

Sea-going sticklebacks carry 30 to 40 bony plates along each side, protection against predatory fish; most freshwater populations have between none and a dozen. A single gene called Eda (ectodysplasin) largely decides how many plates grow, and the low-armour version survives at low frequency in sea populations, ready whenever fish settle in fresh water. The change can be fast. In 1982, Loberg Lake in Alaska was poisoned to clear it for trout and salmon, and sea-going sticklebacks recolonised it. In the twelve years from 1990, the fully armoured form fell from the whole population to about one fish in ten, while a low-plate form rose to three in four. Because thousands of lakes were colonised separately after the last ice age, the same change has happened independently again and again.

Three-spined sticklebackRead card

WonderEvolution

In Hawaii, silverswords, shrubs, cushion plants, vines and trees all descend from a single tarweed, a sticky plant of the daisy family from North America's Pacific coast, that reached the islands millions of years ago.

The ancestor belonged to the tarweeds, sticky, resinous relatives of sunflowers that grow along the Pacific coast of North America. DNA studies suggest it reached Kauaʻi about five million years ago, perhaps as a seed caught in a bird's feathers. With few competitors, its descendants spread into dry scrub, wet forest, bogs and bare lava fields, and today about 34 species in three genera make up what botanists call the silversword alliance. The best known, the silverswords themselves, form globes of silvery, sword-shaped leaves on the high volcanic slopes of Maui and Hawaiʻi Island; some species flower only once, sending up a tall stalk of flower heads before dying. Many members are now endangered, chiefly by grazing animals and invasive plants brought by people.

Silversword allianceRead card

WonderEvolution

Lake Victoria's cichlids, small and often colourful fish, split into more than 500 species with different jaws, colours and diets in only about 15,000 years. Much of the raw genetic material came from an ancient cross between two long-separated lineages.

Lake Victoria dried out almost completely about 17,000 years ago and refilled about 14,700 years ago, as the climate turned wetter. Its cichlids belong to a larger family group of more than 700 closely related species in the region's lakes, which scientists call the Lake Victoria superflock. A genetic study led by Joana Meier, published in 2017, found that the group descends from hybrids between two lineages that had long been apart, one from the upper Nile and one from the Congo basin. The mix gave natural selection a large stock of gene variants to work with as fish specialised in scraping algae, crushing snails, catching insects or hunting other fish. Since Nile perch were introduced to the lake in the 1950s, at least 200 of its cichlid species are thought to have died out.

CichlidRead card