It used to be thought that they came 13,000 years ago across the then-existing land bridge connecting Siberia and Alaska, during what is known as the ‘Clovis’ period.
A paper published today in the journal Science has measured with high precision (with new techniques) the age of a mastodon fossil bone with a weapon point embedded in it that was found in 1970. It found that it is 13,800 years ago, with an uncertainty of only 20 years, suggesting that humans were here earlier than thought, supporting other evidence that there was human hunter activity here as early as 15,000-16,000 years ago.
A large number of mammals (mastodons, woolly mammoths, sabre-toothed cats, giant sloths, camels) disappeared rapidly around 12,700 years ago and it was thought that this must have been due to rapid climate change as the Ice Age ended, since Clovis hunters were not thought to have been around for that long.
But the new earlier date for humans in the Americas suggests that mammal extinction may have been accelerated by humans hunting them with weapons.