This week we’re looking at nothing less than the state of life on earth. The planet has seen mass extinctions before, periods of widespread and dramatic species loss. Some now fear human activity is driving another one: land cleared for farms, homes and roads; waters filled with pollution and emptied of fish; skies choked with gasses causing climate change. But does it add up to a mass extinction? In the first of a two-part series, we examine the evidence of species loss and compare it with the geological record.

(Image: Dinosaur skeleton, Credit: Getty images)