The Panama seaway begins to close

Convergence between the Caribbean and Pacific plates begins narrowing and shallowing the Central American Seaway from about 15 million years ago, and by roughly 9 million years ago the deepwater connection is gone, leaving a chain of islands separated by shallow straits. Full closure comes later, around 2.8 million years ago, and the timing is genuinely contested — some molecular datasets argue for a much earlier effective barrier. The shoaling already begins to redirect Atlantic circulation and to let a few animals island-hop between the continents.