In just seven decades, the scientists say, humans have brought about greater changes than they did in more than seven millennia. Never in Earth’s history has the world changed this much, this fast. Never has a single species had the capacity to wreak so much damage — or the chance to prevent so much harm. “It’s a line in the sand,” said Francine McCarthy, a professor of Earth sciences at Brock University in Ontario, who has led research on Crawford Lake. “The Earth itself is playing by a different rule book. And it’s because of us.”