Adaptive radiation
Adaptive radiation sounds like a term from a physics laboratory. Yet it tells one of biology’s quietest stories: how one becomes many. This story begins with a single bird.
In 1981, a large cactus finch reached Daphne Major, a small island in the Galápagos. He had travelled more than 100 kilometres from Española. The team of Peter and Rosemary Grant, who had studied the island’s finches since 1973, noticed the newcomer. He was larger than the local birds and sang an unfamiliar song.
He mated with a female of the resident medium ground finch. His sons learned his song, which did not attract the island’s other females. Within two generations, the “Big Bird” lineage was breeding exclusively within its own group. Researchers tracked it for six generations. In 2017, they reported in Science what they had observed: a new species arising from a hybrid, witnessed as it happened.
How can so many species of one kind seem to appear so suddenly? Biology’s answer is more gradual than it first appears. It begins small.
How one becomes many
No population is entirely uniform. Some individuals are slightly larger, others have stronger beaks, and some withstand drought better. When groups become separated — by water between islands or simply by feeding on different foods — each encounters its own selective conditions, and small differences accumulate. Eventually, the groups stop interbreeding; on Daphne Major, a song was enough. When a single ancestor branches into many forms occupying different niches, biologists call the process adaptive radiation.
Darwin’s finches provide the classic example: 18 species have developed from one common ancestor in no more than about two million years. The cichlids of Lake Victoria in East Africa diversified even faster. The lake dried out between roughly 20,000 and 16,000 years ago, then filled again around 15,000 years ago. Since then, more than 500 cichlid species have emerged there — a moment in the history of life.
From a distance, this resembles an explosion. Up close, it looks like a tree that continues to branch. But every tree has roots. If one can become many so readily, where did the first one come from?
And before that?
In 1953, Stanley Miller filled a flask with water, methane, ammonia and hydrogen, then passed electric sparks through it to simulate lightning. A week later, it contained amino acids, the building blocks of proteins. Simple gases and energy had produced some of life’s raw materials.
How chemistry became biology remains an open question. One widely supported hypothesis, the “RNA world”, suggests that early life depended on RNA, a molecule able both to store information and to accelerate chemical reactions. One of the strongest clues is found in every cell today: the catalytic core of the ribosome, the machinery that produces proteins, consists of RNA. Nevertheless, it remains a hypothesis.
Researchers trace all life today, from bacteria and redwood trees to humans, back to one common ancestor known as LUCA. A study published in 2024 estimates that it lived around 4.2 billion years ago and was probably not alone. Other early organisms simply left no descendants.
Why does life not begin anew today? Charles Darwin suggested an answer in 1871. Writing to the botanist Joseph Dalton Hooker, he imagined life originating in a “warm little pond”. He added that such material would now be “instantly devoured, or absorbed” by organisms already present. Once life exists, it occupies the ground. For a very long time, that was all it did.
How many became one
For more than two and a half billion years, Earth’s life consisted solely of microbes. The next major advances came through cooperation.
Mitochondria, the small structures within our cells, were once free-living bacteria. In 1967, Lynn Margulis argued that another cell had taken them in and that they had remained there. Today, this idea is widely accepted. Later, cells came together to form multicellular organisms, solitary insects formed colonies, and humans developed societies connected by language. Evolutionary biologists John Maynard Smith and Eörs Szathmáry described these steps as evolution’s “major transitions”. They also noted that there is no theoretical reason why life should become more complex at all.
Martin Nowak of Harvard University captured the common thread in 2006: “Cooperation is needed for evolution to construct new levels of organization.”
There is a complication, and it matters for what comes next. In these transitions, partners often surrendered their independence. Smaller units became parts of a larger whole and frequently cannot reproduce without it. Mitochondria have lost too many genes to return to a free-living existence.
Cooperation created successive levels of organisation, from the cell upwards. The next level was never guaranteed. How likely, then, was any of this: a planet of microbes becoming a planet with someone capable of asking that question? One physicist raised it over lunch.
Where is everybody?
In the summer of 1950, physicist Enrico Fermi made a quick estimate at Los Alamos. It suggested that the galaxy should be filled with civilisations. Where, then, were they? According to one recollection, he simply asked: “Where is everybody?” Many answers have since been suggested. None has been settled.
Our own planet provides a sobering clue. Life emerged early, yet remained microscopic for most of its history; multicellular life is only a little over a billion years old. Evolution never guaranteed the arrival of complex life, much less intelligence.
Chance mattered too. About 66 million years ago, an asteroid struck what is now Mexico’s Yucatán Peninsula. Roughly three quarters of all species became extinct, including every dinosaur except the birds. Afterwards, mammal diversity expanded dramatically: in less than 20 million years, new species evolved to inhabit the air, oceans and treetops. This was, in effect, another adaptive radiation and the start of the age of mammals, to which we belong.
Back at the lake
This is the furthest point of our journey. From here, we return to Earth, to the lake and to ourselves.
Diversity develops slowly and can disappear quickly. In 1954, the Nile perch, a large predator, was introduced into Lake Victoria. Alongside pollution and other pressures, it contributed to the loss of around 200 of the lake’s unique cichlid species in about half a century. What had developed over thousands of years disappeared within a human lifetime.
And us?
The science ends here. What follows is our interpretation.
Here, people differ from mitochondria. Mitochondria had no choice. People do. Experiments show that human altruism is a powerful force, unique in the animal world. We cooperate not only out of necessity; we can choose to do so.
Researchers have examined what these choices can achieve. Elinor Ostrom, awarded the Nobel Prize in Economic Sciences in 2009, demonstrated through field studies that local communities sharing pastures, fisheries and forests can establish their own rules and manage these resources sustainably, without central authority or privatisation.
For the global society, this long journey from finch to cell to society offers two lessons. Life gained richness by branching into diverse forms and reached new levels by bringing forms together. The global society needs both, without paying the price that mitochondria paid. The Global Consent, eleven commitments of the global society set down in January 2026, expresses this too. One commitment is to act through cooperation: “Sustainable solutions emerge from cooperation, shared responsibility, and mutual respect. We reject conquest and domination as principles of progress.” Another calls for protecting the differences of societies from “domination, exploitation, and forced uniformity”.
The alternative would be a social monoculture: a single crop where life calls for many. History offers stark examples. Totalitarian states and racist movements have sought one people, one creed and one way of life, relying on force to impose them.
Some even invoked evolution to justify their ambitions, as though nature rewarded domination by the strong. Nature’s history tells a different story: its richness arose through branching, its great advances through cooperation. A monoculture may be efficient for a season but remains fragile; one blight can destroy the field. Nor is the attraction of uniformity limited to dictators. It appears wherever one model is proclaimed the only correct one: in markets, bureaucracies, well-intentioned movements and ourselves.
Cooperation that preserves differences among its members is harder than uniformity and harder than acting alone. But unity can be chosen; uniformity must be imposed. Only the former can endure without force.
And you?
A journey that started with one bird ends with one person. If evolution was never predetermined, neither is the future. That offers no guarantee, but it leaves room to act.
Life’s great transitions began with small units acting together. Ostrom’s commons began with neighbours agreeing on rules. The Global Consent makes the same request of everyone: “We take responsibility where we stand.” Nothing heroic is required. It might mean organising a meeting, finally speaking to a neighbour or choosing to join a group.
In 1981, nobody on Daphne Major could have known that a wandering bird with an unfamiliar song would begin something new. Most beginnings look much like that.
Written by a human author, edited with AI assistance.
Further information
Uppsala University — The origin of a new species of finches observed (2017), on the Big Bird lineage (Lamichhaney et al., Science 2017).
The Linnean Society of London — Icons of Evolution: Why are there so many Darwin’s finches? (2024).
Knowable Magazine — One fish, two fish, 3,000 fish: the evolution of East Africa’s cichlids (2024).
Eawag — Cichlids: watching speciation in real time (2018).
IUCN Global Invasive Species Database — Lates niloticus (Nile perch) in Lake Victoria.
The Scientist — Miller-Urey amino acids, circa 1953 (2009).
Chemistry World — How RNA reveals clues to life’s origins on Earth.
University of Bath — Insight into one of life’s earliest ancestors (LUCA), Nature Ecology & Evolution (2024).
Darwin Correspondence Project — Letter from Charles Darwin to J. D. Hooker, 1 February 1871 (“warm little pond”).
Embryo Project Encyclopedia (Arizona State University) — “On the Origin of Mitosing Cells” (1967), by Lynn Sagan (Margulis).
Nature — Szathmáry and Maynard Smith, The major evolutionary transitions (1995).
Science — Martin A. Nowak, Five rules for the evolution of cooperation (2006).
BBC Sky at Night Magazine — Where are all the aliens? The Fermi Paradox (2020).
American Museum of Natural History — Impact: The End of the Age of Dinosaurs (2025).
Nature — Ernst Fehr and Urs Fischbacher, The nature of human altruism (2003).
The Nobel Prize — Elinor Ostrom, Prize in Economic Sciences 2009.
Encyclopaedia Britannica — Social Darwinism.
New England Journal of Medicine / United States Holocaust Memorial Museum — Susan Bachrach, In the Name of Public Health: Nazi Racial Hygiene (2004).
Wikimedia Commons — Darwin’s finches by John Gould (1845), public domain.
