Extreme evolutionary changes

Throughout the history of life environmental changes, and the evolutionary changes that result, have sometimes been so extreme as to cause massive extinctions. Nonetheless, given enough time, our planet's biosphere can recover and regain its former diversity. Consider the disaster that hit the Earth 65 million years ago. A brief description of what happened can hardly begin to convey its severity.

One day approximately 65 ± 1 million years ago, without warning, a 10 km wide asteroid plunged into the atmosphere above the Yucatan peninsula at a steep angle from the southeast. It traversed the distance from upper stratosphere to the shallow sea in 20 seconds, heating the air ahead of it to a blue-hot plasma. The asteroid hit the ocean and penetrated through the sea bottom, first into the Earth's crust and then into the molten mantle beneath the crust. As it did so, it heated up and exploded. The energy released, equivalent to 100 million megatons of TNT, was at least a million times as great as the largest hydrogen bomb that we humans have ever exploded. The atmospheric shock wave moved at several times the speed of sound across North America, incinerating all the forests in its path. Crust and mantle material erupted towards the sky, cooling and forming an immense choking cloud as it spread outwards. Shock waves raced through the crust, triggering force-10 earthquakes around the planet, and a 300 m high tsunami spread further destruction across a wide swath of all the Earth's coastal regions. Volcanoes erupted along the planet's great fault lines, adding their own noxious gases and dust to the witches' brew that was accumulating in the atmosphere.

Most of the animals and plants that lived in southern North America and the northern part of South America were killed by the direct effects of the impact. As the great cloud of dust blanketed the Earth over the next six months, blocking out the sun, many more animals and plants succumbed. There was no safe place on land or sea. Carbon dioxide released from the bolide impact caused a spike in temperatures worldwide (Beerling et al., 2002). All the dinosauras perished, along with all the large flying and ocean-going reptiles and all the abundant nautilus-like ammonites that had swum in the oceans; of the mammals and birds, only a few survived.

When the dust eventually began to clear the landscape was ghastly and moon-like, with only a few timid ferns poking out of cracks in the seared rocks and soil, and a few tiny mammals and tattered birds surviving on the last of their stores of seeds. It took the better part of a million years for the planet to recover its former verdant exuberance. And it took another 4 million years before new species of mammals filled all the ecological niches that had been vacated by the ruling reptiles.

Asteroid impacts as large as the one that drove the dinosaurs to extinction are rare and have probably happened no more than two or three times during the last half billion years. But at least seven smaller impacts, each sufficiently severe to result in a wave of extinction, have occurred during that period. Each was followed by a recovery period, ranging up to a few million years (though many of the recovery times may have been less than that (Alroy et al., 2001)). During these recovery periods further extinctions took place and new dades of animals and plants appeared.

Asteroids are not the only source of environmental devastation. Massive volcanic eruptions that took place some 251 million years ago were the probable cause of the most massive wave of extinctions our planet has ever seen, the Permian-Triassic extinction (e.g., Benton, 2003). As befits the violence of that extinction event, the resulting alterations in the biosphere were profound. The event set in motion a wave of evolutionary change leading to mammal-like therapsid reptiles (although therapsids with some mammalian characteristics had already appeared before the extinction event). It also gave the ancestors of the dinosaurs an opportunity to expand into vacant ecological niches, though the earliest dinosaurs of which we have records did not appear in the fossil record until 20 million years after the extinction event (Flynn et al., 1999).

Completed catastrophic events are characterized by both mass extinctions and sufficient time for recovery to take place. In general, the more severe the extinction event, more are the differences found to separate the pre-event world from the recovered world. The characteristics of the recovered world are largely shaped by the types of organisms that survived the catastrophe, but complex and unexpected subsequent interactions can take place. Therapsids with some mammalian characteristics survived the Permian-Triassic extinction, and the descendants of these surviving therapsids dominated the world for much of the Triassic period. Nonetheless, halfway through the Triassic, the therapsids began to lose ground. Dinosaurs began to dominate the niches for large land animals, with the result that the mammalian lineages that survived this conflict were primarily small herbivores and insectivores. However, some mammals were able to occupy specialized niches and grow quite large (Ji et al., 2006), and others were sufficiently big and fierce that they were able to prey on small dinosaurs (Hu et al., 2005). The later Cretaceous-Tertiary extinction provided the mammals with the opportunity to take over from the dinosaurs once more.

Continue reading here: Ongoing evolutionary changes

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