LONDON (AP) — James Lovelock, the environmental scientist whose influential Gaia theory sees the Earth as a living organism imperiled by human activity, has died on his 103rd birthday.

Lovelock’s wife and children said Wednesday that he died the previous evening “in his home surrounded by his family,” from complications related to a fall. The family said until six months ago Lovelock “was still able to walk along the coast near his home in Dorset and take part in interviews, but his health deteriorated after a bad fall earlier this year.”

His contribution to environmental science included developing a device to measure ozone-depleting chlorofluorocarbons in the atmosphere and pollutants in air, soil and water.

The Gaia hypothesis, first proposed in the 1970s, saw the Earth itself as a complex, self-regulating system that created and maintained the conditions for life on the planet. Lovelock said human activity has thrown the system dangerously out of kilter.