There is a growing school of scientific thought that the Earth is in the middle of the sixth great extinction event in its history. A mass extinction event refers to a period when an abnormally large number of animal species simultaneously die out over a time frame much shorter than that which could account for the ‘natural’ rate of extinction through the process of evolution, which refers to the process by which older species are replaced by newer variations which inhabit ecological niches. A mass extinction event can, therefore, take place over hundreds, thousands or millions of years, as in terms of evolutionary history such timeframes are minute.
Mass extinction events are caused by drastic changes to the conditions on the planet, which can occur for a number of reasons, from alterations to the atmosphere and volcanic activity to asteroids smashing into the surface. Through fossil records scientists have identified five previous mass extinction events in the history of the Earth.
The first mass extinction took place at the end of the Ordovician age. This was approximately 440 million years ago. At that time most animal species lived in the sea. Massive glaciations occurred which locked up most of the water on the planet. This caused sea levels to drop precipitously. It is thought that as much as 60 percent of animal species on Earth perished during the Ordovician mass extinction.
The second mass extinction event occurred over a period of around 20 million years, starting about 360 million years ago, during the later stages of the Devonian age. Successive glaciations meant that approximately 70 percent of all marine species were wiped out, particularly in the shallow areas of the seas. This is where coral reefs live and coral didn’t return to much of the ocean until over 100 million years later when a new species evolved. Insect and some formative amphibian animals had by this time inhabited the land, and many species of these terrestrial creatures were made extinct as well during this time.
The third extinction is the most devastating in the planet’s history – as many as 96 percent of all animal species died out during it, across the board, from terrestrial vertebrates to marine animals and insects. It is for this reason that the event is often referred to as ‘The Great Dying’. This event took place at the end of the Permian age, around 225 million years ago. The cause of the third extinction is not known for certain, but it thought that either a comet struck the Earth or massive volcanic activity unsettled the surface of the Earth, causing a sudden release of methane from the sea floor, which would have triggered massive climate change. The likelihood is that all played a part, causing a number of ‘pulses’ of extinction events during this period. All life currently on Earth is descended from the 4 percent of species that survived this extinction.
Around half of all marine invertebrates as well as many land animals, especially amphibians, were wiped out during the fourth mass extinction event, which took place over approximately 18 million years at the end of the Triassic age, beginning 200 million years ago. As with the third event, the likeliest causes are volcanic activity or a comet or asteroid hitting the Earth. About half of all animal species on the planet were wiped out by this event, including not only the dinosaurs but also marine reptiles and many plant species. Mammals were less affected, and this extinction event provided the historical niche through which the mammals, and eventually humans, came to dominate the planet.
The fifth mass extinction event is arguably the one most people are aware of, as it was during this time that the dinosaurs all became extinct. Occurring about 65 million years ago, a massive asteroid hit the planet (it is thought that a crater found off Mexico’s Yucatan Peninsula shows the point of impact), triggering huge changes in temperature, sea levels and altering the chemical makeup of the atmosphere. It is thought that global temperatures were caused to rise 10 degrees higher than they are today, while the oceans were thought to have risen by as much as 300 meters in comparison to today’s levels. This would have meant as much as 40 percent of the land mass that is currently above sea level would have been submerged.
The notion that we are currently in the midst of a sixth mass extinction event is a sobering one, particularly as we, humans, are the cause of it. It is not simply something that is happening now; the sixth extinction takes in much of human history. We have been responsible for the extinction of single species by our own hands, such as the hunting of mastodons to extinction by our ancestors, or the willful destruction of the carrier pigeon, but the rate at which humans have killed off animal species has increased exponentially as we have become more industrialized as a society. As international travel became possible we transported – inadvertently or deliberately – invasive species from one part of the world to another that wiped out native animals. Diseases that had not previously been active on one continent were bought by travelers and decimated wildlife. Modern chemicals have poisoned seas, killing coral reef species and causing the fish species that depend on them to decline dramatically. And as the global human population has grown the pressure on natural resources of food has become intense, so that many species, particularly of fish, are now close to extinction. And then there is climate change. The pollution caused by the burning of fossil fuels is changing the atmosphere if the planet, causing extreme weather events, increased temperatures, rising sea levels and fundamental changes to the make up of the planet. This will have catastrophic effects on animal species, but also, unless a radical shift in attitude and process occurs, could well cause the downfall of the human race eventually as well.