As the British naturalist marks a milestone birthday, we take a look at how his work has shaped science. Deep in the rainforest of the Virunga Mountains in Rwanda, David Attenborough watches as a troop of mountain gorillas ( Gorilla beringei beringei ) play in the undergrowth around him. One — a youngster — crawls onto his lap and tugs at his clothes. It is January 1978, and the cameras are rolling on a wildlife series called  Life on Earth . Credits: John Sparks/Nature Picture Library; BBC/Everett Collection; Nick Upton/Nature Picture Library Credits: John Sparks/Nature Picture Library; BBC/Everett Collection; Nick Upton/Nature Picture Library Although not his first on-camera appearance, it would prove to be a defining one for the British naturalist: around 15 million viewers in the United Kingdom tuned in to watch the television series when it aired on the BBC in 1979.…