Its opening phase appears to have started about fifty thousand years ago, when the first humans migrated across Australia and America.

NEWYORKER: The Sixth Extinction?

While 1km-size and bigger asteroids are thought to hit only once every 500, 000 years, on average, rocks 50-100 metres across, like the Tunguska object, are thought to hit the planet once every thousand years.

ECONOMIST: A large asteroid sweeps by Earth. Where are the others?