Yesterday’s blog idea was triggered by week 3 in the series of scientific mysteries, as published by The Economist (week 1, 2, 3 and 4). Week 2 – the Multiverse – has already been covered in my blogs of 11 June and August 28. Week 1 – the origin of life – has already been covered in my blogs of 22 June (Ice Ages) and April 26 (Africa). Week 4 – mass extinctions – is quite an interesting addition.
In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions: the Cretaceous–Paleogene extinction event: 66 Mya, the Triassic–Jurassic extinction event: 201.3 Mya, the Permian–Triassic extinction event: 252 Mya, the Late Devonian extinction: 375–360 Mya, and the Ordovician–Silurian extinction events: 450–440 Mya. (Wikipedia)
The most common theories for the causes of mass extinctions are: impact events (e.g., asteroids, comets, meteors), climate change (read: (inter)glacial periods of Ice Ages), volcanism, gamma ray burst, plate tectonics. Other hypotheses, such as the spread of a new disease (read: viruses) or simple competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth’s history are too sudden and too extensive to have resulted solely from biological events. (NewWorldEncyclopedia)
Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history of life. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral disease might have been responsible for mass extinctions, such as the extinction of the dinosaurs and other life forms. This theory is hard to test but seems unlikely, since a given virus can typically cause disease only in one species or in a group of related species. Even a hypothetical virus that could infect and kill all dinosaurs, 65 million years ago, could not have infected the ammonites or foraminifera that also went extinct at the same time. (UCMP)
Actually, that period was also the end of the Mesozoic Era that began 252.2 million years ago and ended 66 million years ago. The Cenozoic Era is the third in the major eras of the Earth’s history and began about 65.5 million years ago and extends to the present. It was the interval of time during which the continents assumed their modern configuration and geographic positions and during which the Earth’s flora and fauna evolved toward those of the present. The most recent mass extinction of 65 mya is often thought to have been triggered by a meteor impact in Mexico. However, the start of the Cenozoic Era – and its massive plate tectonics – may even be a much more likely explanation.
In general, the Earth’s mass extinctions seem to match reasonably well with (the glacial periods of) known Ice Ages and major geological periods (plate tectonics). The Economist’s scientific mystery of the end Cambrian extinction (488 mya) also marks the beginning of the so called Ordovician Period which “ushered in significant changes in plate tectonics, climate, and biological systems. Rapid seafloor spreading at oceanic ridges fostered some of the highest global sea levels in the Phanerozoic Eon. As a result, continents were flooded to an unprecedented level, with North America almost entirely underwater at times”. (Britannica)
“Mass extinctions occur repeatedly, though irregularly, throughout Earth’s history, and occasionally these extinctions have been devastating to life on our planet – or have they? Extinction events have sometimes accelerated the evolution of life on earth by eliminating old dominating species and making room for new ones. A new study takes this idea a step further, showing that life may have never achieved the complexity necessary for the development of advanced multi-cellular organisms without recurring extinction events”. (Phys)
What we call the beginning is often the end. And to make an end is to make a beginning. The end is where we start from. T. S. Eliot, American poet (1888-1965)
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