Evolution's Biggest Losers

Major extinction events in Earth's history

               

Meteor Crater, Flagstaff, AZ

Supercontinents collide; dust and debris turn day into night; behemoth glaciers lumber over once fertile seas. In The Origin of Species, Charles Darwin bemoaned the “extreme imperfection” of the fossil record, but today, improved understanding of the physical sciences and breakthrough discoveries such as carbon-14 dating have painted a strange and vibrant portrait of the events that led up to Earth’s current population of flora and fauna. Contemporary theories posit that five great extinction cycles over the course of hundreds of millions of years greatly influenced which species survived and flourished, and which were relegated to the fossil record.

The earliest of these major extinction events occurred approximately 440 million years ago during the Ordovician-Silurian period, a time before organisms had made the transition from ocean dwelling to terrestrial. As the second-largest extinction on record, it managed to kill a staggering 85 percent of all Ordovician species. Attributed to rapid cooling and glaciation, the event essentially wiped the biological slate clean, with only the hardiest organisms surviving.

Olduvai Gorge, Tanzania

The next 100 million years passed without any great disruptions in biodiversity, until the Late Devonian extinction about 364 million years ago. By this time, primitive root plants, amphibians, and insects had begun populating land masses. While scientists have yet to determine the primary cause, they agree that it mainly affected marine creatures, particularly those living in warm waters. Terrestrial plants and animals seem to have emerged relatively unscathed, but the event eliminated about 70 percent of all species and 19 percent of the families of life present at that time.

The most destructive extinction event was the Permian-Triassic, 251 million years ago. During the early Permian, fusion of continental plates created Pangaea, a development which allowed land animals to roam freely, breeding and diversifying over a fecund period of millions of years. Eventually, however, climate change, supplemented by a possible meteorite impact and an increase in volcanism, caused the destruction of approximately 95 percent of marine and 70 percent of terrestrial species.

Following this cataclysm, it took nearly 30 million years for some of the more complex ecosystems and organisms to recover. Then, around 200 million years ago, rapid climate change produced the End Triassic extinction, which wiped out an entire class of marine life—small invertebrates called conodonts—and annihilated 76 percent of extant species. Lucky for us, several groups of synapsids, the ancestors of modern mammals, managed to survive. The resulting ecological gaps also allowed dinosaurs to evolve and become the dominant land animals.

The final massive extinction is also the best known. About 65 million years ago, major asteroid impacts or volcanic activity triggered the Cretaceous-Tertiary (K-T) extinction. Its catastrophic effects on climate and atmospheric conditions ultimately led to the decline of dinosaurs and the rise of mammals. Mammalian omnivores, insectivores, and carrion eaters thrived, with food sources abundant as larger animals died and decayed. The ancestors of crocodiles, turtles, and sharks also flourished. When all was said and done, every species of non-avian dinosaur disappeared, while the major mammal lines continued to diversify and change, setting the stage for both the evolution of modern humans and the creatures with which we share the planet.