The Universe popped into being with the Big Bang 13.7 billion years ago (we can round that off to 14,000,000,000 years--I've spelled that out with 9 zeros to help you keep track; there are some pretty mind-numbing time comparisons coming up in the next few paragraphs).
About 5 billion years ago the Earth was born. Then life appeared on Earth about 4 billion years ago. (We're still talking in 9-zero numbers here.)
It took a couple of billion years for life to get very diverse, that is, to evolve into millions of species to fill ever smaller and more varied ecological niches. But life on Earth had its ups and downs. Every few million years some ecological catastrophe would wipe out vast numbers of the species that had evolved. Sometimes these were due to massive volcanic eruptions that filled the atmosphere with soot and noxious gases. Sometimes these widespread extinctions were associated with rising or falling of the oceans. And at least once a giant asteroid smashed into the Earth, spewing clouds of fumes and particles that blocked out the Sun.
The most staggering of these mass extinctions is called "The Great Dying" (or the "Permian-Triassic extinction event"). It occurred 251 million years ago (251,000,000 years--with 6 zeros--that's about 1/20th the age of the Earth, in other words quite recently in Earth-age terms). Some 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals) were wiped out. Although this is called an "event," it wasn't exactly sudden. It actually occurred over several million years--only a scientist steeped in zeros like a geologist could think of it as an "event."
There have been quite a number of extinction events, some widespread (even global), as the hundreds of millions of years of life on Earth rolled by. But there are six in addition to the "Great Dying" that were so catastrophic as to take one's (or most of life's) breath away. The one 65 million years ago (65,000,000 years; the "Cretaceous-Tertiary extinction event") that wiped out the non-avian dinosaurs killed off about 75% of all species on the planet. Other major extinction events occurred--
205,000,000 years ago--the "Triassic-Jurassic extinction event"
but 46,000,000 years earlier
251,000.000 years ago--the "Permian-Triassic extinction event"
and 109,000,000 years earlier than that
360,000,000 to 375,000,000 years ago--the "Late Devonian extinction event"
and 65,000,000 years before that (give or take a few million years)
440,000,000 to 450,000,000 years ago--the "Ordovician-Silurian extinction event" (which was actually two events)
and barely 38,000,000 years before that
488,000,000 years ago--the "Cambrian-Ordovician extinction event."
Again, although these are called "events," most actually took place over millions of years.
What about the present? Starting about 100,000 years ago when the incursion of humans began to be a significant worldwide ecological factor, a major evolutionary catastrophe called the "Holocene extinction event" got under way. This has not been developing for millions of years like most of the extinction events referred to above. A hundred thousand years is ONE TENTH of a million years--barely the twinkling of a geological eye. Yet in that brief time there have been extensive species extinctions comparable to those that occurred over millions of years at the time of The Great Dying.
And this extinction process is accelerating. It has been estimated that the present rate of species extinction is 100 to 1,000 TIMES the normal, background biological rate. Scientist speculate that at the current rate approximately one half of the species now alive on Earth will become extinct within the next 100 years (that's not one thousand years--not one million years as with prior major extinction events).
There have been four main causes for this.
First, widespread agriculture and the attendant deforestation, desertification, and general habitat simplification and destruction.
Second, pollution of the atmosphere, oceans, and land with billions of tons of diverse bioactive chemicals--fertilizers, cattle excrement, pesticides, solvents, detergents, etc.
Third, land use by exploding populations of humans for cities, but also for suburban and rural habitations, and for recreational and industrial purposes.
Fourth, hunting and fishing, often with clever technologies (such as the spear or arrow or gun) and with relentless thoroughness.
The major extinctions of millennia gone by often took millions of years to devolve. Ours, the Great Holocene Dying, is killing off comparable catastrophic percentages of living species, but this time barely in the twinkling of a geologic eye.
Our species, Homo sapiens, is also sure to go extinct--soon, in geologic terms. And one can imagine the tired old Earth heaving a sigh of nostalgia, but also of relief, and going back to plodding through its millennia at a more normal geologic pace.
Bun Gladieux, president of the Presssure Positive Company, has a blog with an interesting series of topics.
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