Mass Extinction of Life on Earth
Most people are familiar with the mass extinction of life on earth during the late Cretaceous era, when the dinosaurs that roamed the earth were killed off, and only smaller scale animals were left.
What is not widely known is that the mass extinction of the dinosaurs was not the first mass extinction to happen during the Earth's history - and may not be the last.
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A brief history of the earth
The first thing to understand is that the earth didn't always look like it does today. We believe it was formed some 4.5 billion years ago, as molten debris rotating around the sun coalesced into planets, the earth being one of them. It took the first few billion years just to cool down and form land and water.
The position of the continents on earth was not fixed either. The surface of the earth is formed from a series of plates, which are constantly either rubbing against each other (the Pacific plates) or moving away from each other (the plates in the middle of the Atlantic are moving apart thanks to a fault in the middle from which new molten lava is constantly pushed up and then flows sideways, making the plates larger and making existing land on them move apart). Basically the Pacific is narrowing and the Atlantic is widening.
In addition the earth's orbit around the sun varies, sometimes becoming more elliptical - during those periods, the earth cools, which increases the ice at the caps, and reduces the size of the oceans, which in turn exposes more land.
The combination of continental drift and the ebb and flow of the size of the oceans means that the size and positions of the continents changes over time. Indeed they are moving very slowly as we speak.
Permian Mass Extinction
We know from the fossil record that from about 600 million years ago, life starts to thrive on earth.
We can date the fossils thanks to uranium-lead dating. Tiny naturally occurring particles of uranium are present everywhere on earth including in bones. Living bone (such as the bone in humans and animals that are alive) has very small amounts of uranium in it. But during fossilisation, bone gets enriched with uranium from the rocks it is fossilised in, at the moment of fossilisation. From that moment on, the uranium then proceeds to decay at a set rate into lead. So by measuring the proportions of uranium particles to lead particles in fossilised bones, you can tell how long the process of uranium decay has been going on, and hence the date the bone got fossilised. As the bone would have got fossilised at some point after the death of the animal, you can date approximately when it lived.
From about 280 million years ago, at the start of the Permian era, the fossil record shows therapsidas, like the one whose fossil is depicted in the image above. These are animals with mammal like characteristics - they have hair, produce lactation and are erect and are the first mammal-like animals on earth.
For about 30 million years, they dominate the earth, some eating vegetation, and being preyed upon by other therapsids (like the carnivore in the picture above).
Then suddenly at the 250 million year mark, they disappear from the fossil record. The earth continues to deposit sediment, and there are faint traces of plant and other small life in these deposits, which we can date, but over a period of 100,000 years, the bulk of the life that existed before (95%) has just gone. So what happened?
We know from deposits of basalt in what is now Siberia (and was the north eastern land mass in the image above) that there was a series of fierce and ongoing eruptions of volcanoes across that entire section of land, about 250 million years ago. Basalt is produced when lava is spewed out in the air and cools rapidly, leaving a smooth black dense mineral deposit behind. When huge quantities of basalt are produced, it is known as a flood basalt event.
From the extent of the basalt deposits, the eruptions would have been fierce enough to raise the earth's temperature by 5 degrees - enough to kill some animals, but not all. But it was enough to alter weather systems drastically, and the rise in temperature also raised the temperature of the sea. We can see from marine cores taken from the bottom of the ocean, that marine life fossils also start disappearing at this time. And the rise in sea temperature would have had a deadly secondary effect - it would have released methane trapped at the bottom of the sea in the form of frozen methane hydrate. Once the methane got into the air, it would have raised the earth's temperature by a further five degrees. The cumulative ten degree rise in temperature is enough to kill most of the remaining land animals.
Only some plants and a few small mammals survive - and reptiles survive. High temperatures favour cold-blooded creatures.
Triassic mass extinction
As the earth recovered from it's shock, the Triassic period got underway 250 million years ago, and it was dominated by reptiles. The beginnings of the dinosaurs develop as do the first flying animals, the pterosaurs. And mammals also struggle to recover.
At the end of the Triassic period, 200 million years ago, the record shows another mass extinction. During the triassic era, the earth's land mass had melded together to form one giant continent that scientists call Pangaea (Greek for "all the earth").
But at the end of the period, the continent breaks up, forced apart by violent volcanic activity along the plate lines.
The sediment shows what scientists refer to as the "coal gap". Coal is formed from fossilised vegetation, and there is a sudden gap where there is very little coal, so something drastic must have happened to kill off most of the vegetation during this period. Marine deposits show a drastic drop in marine life and amphibians, and land sediments show a drop in the remaining therapsidas.
This extinction wasn't as drastic as the Permian extinction. The Permian extinction showed a drop of 95% of life on earth, but the late Triassic era showed a 50% drop. However, reptiles survived again, to become the dominant species.
The Jurassic and Cretaceous Era
The Jurrasic and Cretaceous eras are dominated by reptiles, in particular, dinosaurs, and all their relatives (tyranosauras rex and so on).
The earth's continents continue to drift apart and there is no ice on the poles, this is a very hot tropical era and reptiles flourish in these conditions (being cold-blooded creatures).
But this 100 million year period of stability is brought to end by the mass extinction known officially as the Cretaceous-Paleogene extinction.
Extinction of the Dinosaurs - the late Cretaceous Extinction Event
This event happened 64 million years ago.
It is believed that the cause was an asteroid crashing into the earth at the point that is present day Mexico. The sediment from this era shows massive quantities of iridium, which is extremely rare on earth but abundant on asteroids (which we can verify by using spectrum analysis of asteroids using telescopes. There is physical evidence of an asteroid too - the Chicxulub crater on the Yucatan peninsula.
The impact of the asteroid would have been horrendous - a dust cloud would have blocked sunlight and threatened plants and anything that depended on plants for food (like dinosaurs). There would have been firestorms in the atmosphere and radiation. The firestorms would have raged across the planet, burning everything in it's path and increasing CO2 and thus increasing the temperature enough to kill those organisms that survived the initial onslaught.
As it happens, we know that at the same time the asteroid struck, there was another massive eruption of volcanoes on the Deccan plateau of what is now India, large and sustained long enough to produce flood basalt deposits, raise the temperature of the planet and alter weather systems.
The combination of the asteroid impact, and the volcanoes finished off really large animals. The animals that survived were small enough to burrow to safety, and small enough so their reproductive cycles were short - so they could rapidly evolve their way to adapt to the new environment.
The late Cretaceous mass extinction wiped out 75% of life - not as bad as the Permian mass extinction, but worse than the Triassic mass extinction.
Earth Today
Homo Sapians (human beings) have only been around for about 170,000 years. We are creatures of the Ice Age - throughout our existence there has been ice on the poles, with global temperatures at historic lows when you look at the earth's history. At some periods we've experienced colder climate than now, for example, when the ice advanced some 10,000 years ago, life in Europe and northern asia almost got wiped out, as did life in northern Africa (an increase in ice means a decrease in rainfall, so advancing ice in the north paradoxically causes the Sahara desert to increase).
Yet it's hot weather that really threatens us, because we are mammals and struggle to regulate our body temperature when the climate is above 40 degrees centigrade (this is the reason the warm periods in the earth's history are always dominated by cold-blooded creatures and the cold periods are dominated by warm blooded mammals).
We don't know how we'd fare if there was another mass eruption of volcanoes raising the temperature as in the Permian extinction and late Cretaceous extinctions, let alone how we'd fare if man-made secondary effects caused the temperature to rise. We don't evolve as fast as other living creatures because we have comparatively long lifespans and have children only every 20-40 years.
Yet, we are probably the most evolved creatures ever to roam the earth. We've managed to not only get to the moon (and build spacesuits that withstands the Moon's 140C temperature) but to go to the bottom of the oceans. There is no doubt that we as a group have the ingenuity to survive whatever is thrown at us - but may not be willing to pay the cost. For instance, each spacesuit cost millions of dollars - would we be willing to make one for every person on the planet, all 6 billion of us, plus also safeguard our food supplies?
Part of the urgency in ensuring that we make sure we can offset any rise in the earth's temperatures is down to the fact that in an emergency, only a few would probably be given protection of mankind's brilliance. We say "mankind's brilliance" but in reality our whole society depends on the brilliance and scientific understanding of a few people, who number a few million (the portion of the earth's population that understands physics and other sciences - the "man in the street" is ignorant of the physics of how his smartphone works, let alone how to build a spacesuit), which is then shared amongst the rest of the population. In a mass extinction emergency, those few would probably protect themselves but not have the resources to protect everyone else. We can survive en masse only if the majority are prepared to press governments to plan to prevent mass extinctions from happening in the first place.