Aside from supercontinent formations, North and South America were unconnected up until several million years ago; then a tiny slat of land linked the continents, triggering impacts that affected the entire planet. This small stretch of land, known as the Isthmus of Panama, transformed global climate and launched a huge natural experiment in migration for plants and animals.
But exactly how many million years ago did the Americas link up? The "standard model" dates it to about 3 million years ago, while some more recent studies say 6 million to 15 million years, with "an initial land bridge" as early as 23 million years ago, according to a 2016 report in the journal Science Advances.
