Life is a precariously robust process.
This might seem like a contradiction. After all, precarious means "dangerously likely to fall or collapse". Robust, on the other hand, means strong and vigorous.
How can something be both in danger of collapsing while being strong and vigorous?
Life, as we know it, has existed on this planet for at least 3 billion years. It began at a time when the planet was routinely bombarded by asteroids, meteors, and other material leftover from the formation of the solar system.
It evolved at a time when the Earth's atmosphere wasn't anything like it is today. Indeed, it more closely resembled the atmosphere of Venus rich in carbon dioxide with only traces of nitrogen and hydrogen gas.
Life evolved when the Earth's crust when volcanic eruptions were common. The oceans, lakes, rivers, and all of the other components of the water cycle were still settling into place. The atmosphere lacked the protective ozone layer blasting the surface with ultraviolet light.
On the whole, the early Earth was a miserable place.
But in this environment, living organisms emerged. They found a way to survive. They found a way to adapt and evolve
They even found a way to change the world around them.
The early atmosphere changed over time to the modern atmosphere. The carbon dioxide dissolved into the oceans forming carbonates which combined with various metal ions to give massive carbonate beds. We see these as the uplifted ridges in the Rocky Mountains, among other places.
However, just scrubbing the carbon dioxide would not have given rise to the present atmosphere. Oxygen needed to be added. It was the rise of the blue-green algae and cyanobacteria that eventually gave us our rich oxidizing atmosphere.
This change in the composition was catastrophic for many early forms of life. They had evolved in a reducing atmosphere and free oxygen was deadly. The poisonous byproducts of one form of life nearly wiped out all of the others. A mass extinction occurred.
This is why life is both precarious and robust. Living organisms find a way to survive and adapt to their surroundings. However, they sit on the precipice of extinction. Changes in the environment - either sudden or protracted - can lead many species over the edge.
There have been five mass extinctions recorded in the geological record of the planet. In this case, an extinction is a massive and rapid loss in biodiversity. But rapid in geological terms is not the same as rapid in our daily lives. A thousand years is a blink of the eye in geologic time.
The first mass extinction took place around 450 million years ago at the end of the Ordovician period. Over the course of time, some 60 to 70 per cent of all species alive went extinct.
Similarly, the second mass extinction is considered to be the Devonian-Carboniferous transition, a prolonged series of extinctions events that eliminated 70 per cent of all species over the course of 20 million years.
The third mass extinction ended the Permian period. It represents the closest that life on this planet came to being completely wiped out when approximately 95 per cent of all species disappeared. Whole new taxa became dominant after the "Great Dying".
Life came perilously close to being wiped out. And yet it survived.
The Triassic-Jurassic extinction event, some 200 million years ago, and the Cretaceous-Paleogene extinction event, some 66 million years ago, each wiped out about 75 per cent of the species present on the planet. The latter was the end of the dinosaurs and likely caused by the impact of a giant asteroid. Each set the diversity back millions of years.
And yet, in each case, life on this planet hung on. Over time, the number and diversity of organisms that filled the ecological niches after each extinction event increased. It seems that every time life on this planet gets knocked down, it comes back swinging even harder.
Of course, if you happen to be a member of one of the species that goes extinct, then that is small comfort.
Many scientists believe that we are presently engaged in the "Sixth Extinction". The rate of species loss is significantly higher right now - and for the past several thousand years - than it has been for millions of years. This is seen in the extinction of a number of species, such as frogs and other amphibians.
The fossil record suggests that we should be losing a species of frog every 1000 years or so. Over the past few decades, it has been closer to a species per year.
Will it wipe out life on this planet?
No. Life is robust and will survive. But what creatures will take over remains to be seen. In the meantime, life seems to be precariously perched on the edge of another great extinction.
