Five times before in the geological record we can detect catastrophic declines in species across the globe, the most recent being that of 65 million years ago when the dinosaurs vanished. The so-called "Cretaceous-Tertiary extinction" event.
Before that, the others we recognise are (from Space.com):
End Triassic extinction, roughly 199 million to 214 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean. The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear.
Permian-Triassic extinction, about 251 million years ago. Many scientists suspect a comet or asteroid impact, although direct evidence has not been found. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction. The Permian-Triassic catastrophe was Earths worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals.
Late Devonian extinction, about 364 million years ago, cause unknown. It killed 22 percent of marine families and 57 percent of marine genera. Erwin said little is known about land organisms at the time.
Ordovician-Silurian extinction, about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.
The main difference with the present biodiversity crisis (half of all mammal species in decline, a quarter of them in serious danger of extinction, worldwide commercial fish species collapse, bees, birds, reptiles and plants all under pressure) is that it seems to be the result of the massive expansion of human population over the last 100,000 years with an acceleration in the last 10,000 probably linked to the development of agriculture.
Or maybe it's completely natural and "nothing to do with us, mate" - but somehow I doubt that.
Irrespective of the cause, the effect is unarguable - we are living through a time of massive change in the biosphere of the Earth.
What I'm sad about is that we will continue to spend time arguing about the cause rather than doing whatever we can to adapt to or mitigate the situation.