Timeline
of human history
version 2 - by
Finn Sivert Nielsen
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Timeline
4 - 600,000,000 BP to Present
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Pangea
and the History of Complex Life. Trilobites, dinosaurs and primates.
The great extinctions
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| We
now take a closer look at the Phanerozoic itself, zooming in on the last
600 million years before the present. At this magnification we discover
that the Cambrian Explosion had a precursor, which produced a similar multiplicity
of new species, nearly all of which died out when the Cambrian Explosion
itself came around. During the latter event, which lasted a mere 20 million
years, nearly all modern animal and plant phylae originated, along with
fundamental organic capacities such as image-forming eyes. The two "explosions" following each other seem to indicate that something was brewing under the surface that sought release. The sudden and comprehensive nature of the Cambrian Explosion once it came further strengthens this impression. To my knowledge, no-one knows what triggered the Cambrian event. I was struck, however, to discover that the Earth's ozone layer was only formed 600 million years ago (how and why it formed, I do not know, but it came just in time for the two successive "explosions"). I do not have the expertise to judge if there is any connection here, but it seems just possible that complex life simply could not survive without the ozone-layer's UV protection. There's a lot of information in this timeline - note, for example, the origin of seed plants: it was seeds that allowed vegetation to spread inland from the coast. Or note the recurrence of ice-ages throughout the Phanerozoic, but on a less than Snowball Earth scale - is this "damping" a result of the ubiquity of life? The main lesson to be drawn from this timeline, however, is that of the recurrent mass extinctions that punctuate the Phanerozoic. Five of these are shown - known as "the big five" - but in fact extinctions, large and small, have been common throughout the Phanerozoic. The great grand-daddy of all extinctions occurred in the Permian and Triassic ages, at 252 million BP. It was probably caused by a very massive and long-term volcanic incident, in which an area in Siberia the size of Western Europe, known as the Siberian Traps, cracked open into a vast roiling magma field from which, over the years, was spewed enough lava to cover the entire continental US with a layer a mile thick. All this eruptivity disseminated very large amounts of Carbon Dioxide as well, which warmed the atmosphere - and then the oceans: sufficiently to release vast underwater deposits of frozen methane, which escaped into the atmosphere, warming conditions still further. The +10° C heat wave lasted several thousand years, and, along with ocean acidification, obliterated nearly all marine species and more than 90 percent of all land species as well. It took some 30 million years for life to return to its previous richness. Though not quite as destructive as this, the remaining four of the "big five" were serious enough. The "K-T" extinction, at 66 million years ago, occurred closest to our time and is considered the second-most destructive event. It killed off all non-avian dinosaurs and ushered in the age of primates. The extinctions are paradoxical. On the one hand, they seem to have a sort of limping, half-assed periodicity to them, on the other hand, they have no common cause, springing equally from glaciation, vulcanism, meteor impact or other issues, and are thus wholly unpredictable. The ubiquity of extinctions (one speaks of a "background extinction rate") and their seemingly random occurrence may present something of a challenge to current Darwinian orthodoxy. For how can the "survival of the fittest" be assured in a world where utterly unpredictable and indiscriminately deadly catastrophes threaten us equally, no matter how "fit" we seem to be? |
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© 2018 Finn Sivert Nielsen (fsnielsen.com)