The Sixth Mass Extinction, also known as the Sixth Extinction or the Holocene extinction event, is an ongoing extinction event perpetrated by human beings. It began about 50,000 years ago, when modern man first left Africa. Since then, it seems to have been nothing but havoc. At least 20,000 species have gone extinct at the hands of humans, and possibly far more. Terrestrial species have had it the worst, as that is where human influence is strongest.
First, from about 50,000 to 20,000 years ago, dozens of species of megafauna -- large animals such as mammoths, saber-toothed tigers, the Dire Wolf, cave bears, and many others -- went extinct shortly after humans started moving in on their territory. These are called the Pleistocene megafaunal extinctions, and represent the first phase of the Sixth Mass Extinction. Climate change is sometimes advanced as a cause for their extinctions, but this argument bears little weight, as the animals in question survived numerous glacial and interglacial periods before going extinct soon after the arrival of humans, which happened to occur shortly after the end of the last glacial period.
The Sixth Mass Extinction occurred as humans spread from continent to continent. First in Eurasia, then in Australia, then the Americas. Precious species, such as the Elephant Bird of Madagascar and Haast's Eagle in New Zealand went extinct relatively recently, around 1500 BCE, after the arrival of European explorers. Even animals that had unpalatable flesh, like the Dodo of Mauritus, were killed, mostly for sport. Unhelpful especially for island animals is their docile temperament, a result of living for thousands or even millions of years without major predators.
This event has especially accelerated in the last 60 years, from the middle of the 20th century and into the 21st. Destruction of habitat and the introduction of invasive species such as rats, dogs, and cats have resulted in the mass extinction of numerous animals, especially island species. These invasive species, bred in the intense competition of Continental life, easily dispatch island species. Eradication programs of invasive species undertaken very recently, only in the last few decades, have preserved some species on the brink of extinction.
In many ways, the Sixth Mass Extinction is different than the previous five mass extinctions. For instance, it is the first known mass extinction to be caused by a species, especially a single species, rather than through abiotic natural causes such as volcanism or asteroid impact. It is also characterized by its relatively fast timescale of action, especially when referring to the last 60 years. Though some mass extinctions occurred in 10,000 years or less, and the Cretaceous-Tertiary extinction that wiped out the dinosaurs occurred very rapidly, the Sixth Mass Extinction is faster than most of the others.