I find it hard to imagine a world without humans - maybe because we've had such an impact on the planet.
Now scientists think that we've had such an effect that we should define a whole new epoch of time as the human age - or the Anthropocene.
Millions of years in the future, how will scientists know when the age began? Today's experts think this 'marker' might be nuclear fallout from tests carried out in the 1950s. Other possible markers could be the appearance of plastics in the environment, or a spike in carbon dioxide production.