Conventional wisdom held for at least a century that the very earliest humans to set foot in North America did so about ten thousand years ago. Starting in the 1970s, discoveries in South America and along the west coast of North America began unraveling this theory, pushing the "first inhabitants" further and further back. However, these were "pushbacks" measured in the thousands of years. Now a new discovery has the potential to push that date into the very distant past:

Human footprints discovered beside an ancient Mexican lake have been dated to 40,000 years ago. If the finding survives the controversy it is bound to stir up, it means that humans must have moved into the New World at least 30,000 years earlier than previously thought.

The footprints were found in a fossilized volcanic deposit, which is especially fortuitous. Volcanoes spew out all sorts of nifty radioactive compounds that decay at regular rates over various time periods. Using these and other available dating resources should allow a firm triangulation. In other words, there will be controversy over this, but if the team did their job correctly they should have a very big gun indeed to use against potential critics.