On a single afternoon in 1980, Walter Alvarez and his physicist father Luis Alvarez announced in Science that a thin layer of clay at Gubbio, Italy — the boundary separating the Cretaceous from the Paleogene — contained iridium at thirty times the normal crustal abundance. Iridium is rare in Earth's crust and abundant in meteorites. The Alvarezes argued that the end-Cretaceous mass extinction — the event that ended the long reign of the non-avian dinosaurs sixty-six million years ago — had been caused by a single asteroid impact. The hypothesis was widely ridiculed; in 1991 the Chicxulub crater on the Yucatán Peninsula was confirmed as the impact site, and one of the most contested proposals in modern paleontology became the textbook account.
Biodiversity — the variety of living forms at a given moment — is the standing balance of two opposing flows: origination by speciation and extinction. The total has grown gradually over the Phanerozoic but is far from uniform in space. The latitudinal diversity gradient, species richness rising sharply toward the equator, is the most consistent macroecological pattern in biology; the Amazon holds more tree species than all of temperate North America and Europe combined. The 36 biodiversity hotspots identified by Norman Myers — Madagascar, the tropical Andes, the Atlantic Forest, the Cape Floristic Region, the Caribbean — cover about 2.4% of Earth's land surface and contain roughly half its plant species. Whatever drives the gradient, the biosphere's diversity is spatially leveraged: clearing one hectare of Amazon forest carries a different biological cost from one hectare of European pasture.
That flow has been catastrophically interrupted at least five times in the last 540 million years. The rock record shows 75–96% of marine species disappearing within geologically brief intervals — the Big Five mass extinctions — and recovery taking five to ten million years each, with the replacing biota qualitatively different. Mammals before Chicxulub were nocturnal and shrew-like; mammals after radiated into vacated niches and produced everything from whales to humans. Mass extinctions are not setbacks; they are regime changes. Modern extinction rates, conservatively estimated, are 100 to 1,000 times the geological background — and by extrapolation, biodiversity loss within the coming centuries would reach mass-extinction magnitude on a vastly compressed timescale. Whether this is the opening phase of a sixth such event is the central question.
The 30×30 framework agreed at COP15 Montreal in December 2022 — 30% of land and ocean protected by 2030 — is the policy bet that the trajectory can still be deflected, with uneven implementation. The IPBES Global Assessment (2019) estimated that up to 1 million species are threatened with extinction in the coming decades; the figure depends on extrapolations from the small subset of life formally described, and what is securely demonstrable now is the direction rather than the magnitude. Harder to dispute is the spatial-leverage logic: most species live in a small fraction of the world's biomes, and decisions about land use in those biomes — the Amazon, the Congo Basin, Borneo, the Brazilian Cerrado — disproportionately determine the answer.