The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas.
Long ago, one great ocean flowed between North and South America. When the Isthmus of Panama joined the continents, it also separated the Atlantic from the Pacific Ocean. If this took place much earlier than the accepted date of 3 million years ago as recently asserted by some, the implications for both land and sea life would be revolutionary. Estimates of rates of evolutionary change, models of global oceans, the origin of modern-day animals and plants of the Americas and why Caribbean reefs became established all depend upon knowing how and when the isthmus formed.
An international team of researchers from 23 institutions now reevaluated all of the available lines of evidence, such as geologic, oceanographic, genetic and ecological data to come up with a firm estimate of when the Isthmus formed. Their work provided three key pieces of evidence that define when the land bridge was finally in place:
- Analysis of the family trees of shallow-water marine animals such as fish and sand dollars from the Pacific and Caribbean (Atlantic) sides of the isthmus show genetic mixing until after 3.2 million years ago.
- Surface waters from the Pacific and Caribbean mixed until about 2.8 million years ago, as seen in deep-ocean sediments.
- Massive migrations of land animals between North and South America began sometime before 2.7 million years ago.
These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.