Q.Approximately how many mass extinction events had already occurred in Earth's geological past before the current, largely human-driven episode of species loss?

Extinction is the permanent disappearance of a species from Earth. Five mass extinction events are recorded in the geological past, generally attributed to natural causes such as asteroid impacts or major volcanic activity; many biologists consider the present, largely human-driven episode of biodiversity loss a possible sixth mass extinction, with current extinction rates estimated at roughly 100 to 1,000 times the natural background rate. The IUCN Red List classifies a species' extinction risk into standard categories, from most to least severe: Extinct (EX, no individual survives anywhere), Extinct in the Wild (EW, survives only in captivity/cultivation), Critically Endangered (CR), Endangered (EN), Vulnerable (VU, together the three 'threatened' categories reflecting decreasing but still serious risk), Near Threatened (NT), and Least Concern (LC, not currently at significant risk).