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A mass extinction wipes out a large share of Earth's species in a short time by geological standards. There have been five great ones. After the last, 66 million years ago, the dinosaurs other than birds were gone, and mammals spread into the ways of life they had left empty.

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The “Big Five” were identified by the palaeontologists David Raup and Jack Sepkoski in 1982. The worst, the “Great Dying” about 252 million years ago, wiped out around 81% of marine species and cleared the way for the reptile group that later produced the dinosaurs. The most recent, 66 million years ago, removed about three-quarters of all plant and animal species. In 1980 Luis and Walter Alvarez and their colleagues found that rocks laid down at that moment contain a thin layer rich in iridium, a metal rare in Earth's crust but more common in asteroids. The impact crater, 180 km wide, was identified in 1990 at Chicxulub, buried under Mexico's Yucatán Peninsula. Recovery took millions of years. Biologists call the burst of new species that fills emptied roles adaptive radiation.

Türkçe oku
Kitlesel yok oluş, Dünya'daki türlerin büyük bölümünün jeolojik ölçüyle kısa bir sürede ortadan kalkmasıdır. Bugüne kadar beş büyük kitlesel yok oluş yaşandı. 66 milyon yıl önceki sonuncusunda kuşlar dışındaki dinozorlar yok oldu; memeliler de onlardan boşalan yerleri doldurdu.

“Büyük Beşli” diye anılan bu yok oluşları 1982'de paleontologlar David Raup ile Jack Sepkoski belirledi. En ağırı, yaklaşık 252 milyon yıl önceki “Büyük Ölüm”dür: Deniz türlerinin yaklaşık yüzde 81'ini yok etti ve dinozorların da içinden çıkacağı sürüngen grubunun önünü açtı. Sonuncusu 66 milyon yıl önce yaşandı ve bitki ile hayvan türlerinin yaklaşık dörtte üçünü ortadan kaldırdı. 1980'de Luis ve Walter Alvarez ile ekip arkadaşları, o döneme ait kayaçlarda ince bir katman buldu. Katman, yer kabuğunda çok az bulunan ama asteroitlerde daha bol olan iridyum metaliyle zengindi. Asteroit çarpmasının bıraktığı 180 km çapındaki krater ise 1990'da, Meksika'nın Yucatán Yarımadası'nın altına gömülü hâlde, Chicxulub'da saptandı. Toparlanma milyonlarca yıl sürdü. Boşalan yerleri dolduran yeni türlerin böyle hızla çoğalıp çeşitlenmesine biyologlar adaptif radyasyon der.

Sources and further reading

Background reading: Extinction event
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