As long ago as 1993, Harvard biologist E.O. Wilson estimated that Earth is currently losing around 30,000 species per year — which breaks down to the even more daunting statistic of 3 species per hour. This has led many biologists to believe that the current biodiversity crisis is leading into 'the Sixth Extinction' which is more imminent than previously thought.
The fifth major extinction occurred at the end of the Cretaceous period which wiped-out the terrestrial dinosaurs and marine species as seen from fossil records of this period. It is believed that this event was caused by collisions between the Earth and extraterrestrial bodies. There is also the theory of a volcanic event that produced the Deccan traps of India as part of the chain of physical events that disrupted ecosystems so severely that many species succumbed to extinction.
The sixth extinction however, is different from anything that has previously occurred because for most part it is human-induced. There is little doubt that humans are the direct cause of ecosystem stress and species destruction in the modern world through their activities.
The first phase of the sixth extinction began shortly after Homo sapiens evolved and migrated out of in Africa. Humans reached the middle east 90,000 years ago. They were in Europe starting around 40,000 years ago. Neanderthals, who had long lived in Europe, survived our arrival for less than 10,000 years, and then abruptly disappeared. So modern humans have a long history of destroying native species wherever they migrate and this can be traced to even pre-historic times with the help of fossil records.
The second phase of the sixth extinction began around 10,000 years with the invention of agriculture. Agriculture represents the single most profound ecological change in the entire 3.5 billion-year history of life. Once agriculture became main-stream, humans could manipulate other species for their use. This also meant a departure from the 'hunter-gatherer' stage of history which means the species could overpopulate. Around this time it is believed that there were about 1 billion people populating the Earth. Currently, there are 6 billion people, by 2020, there could be 8 billion people. There is an upper limit to the carrying capacity of humans on earth, that number is usually estimated at between 13-15 billion.
The ecosystems of today's world has been plunged into chaos. There isn't a single face of the planet untouched by the human hand. Conservation measures, sustainable development, and ultimately, stabilization of human population numbers and consumption patterns seem to offer some hope that the Sixth Extinction will not be as drastic as some of the previous extinction events.
Life is incredibly resilient and it has always recovered but only when environmental stress has dissipated or reduced. In the case of the Sixth Extinction, the cause is ourselves. So this we have a choice: to continue on the path to our own extinction or modify our behaviour. Behaviour modification is incredibly challenging but in order for Life to rebound, it is now supremely essential.