For decades scientists have grappled with one of our planet's greatest biogeographical mysteries: how the geologic history of the Amazon River basin has shaped its magnificent ecosystem. Now new research describes sedimentary evidence from eastern Colombia and northwestern Brazil that suggests the enormous basin was covered by ocean water at least twice in the past. The Amazon is famous for its river, which only started flowing around nine million years ago. Ideas about the structure of the preriver landscape include a flooded rain forest, a huge freshwater lake and a fan-shaped, continent-spanning network of streams. Understanding the shape of this ancient landscape is crucial to learning what spawned the staggering biodiversity of the Amazon rain forest.

One theory posits that marine waters inundated western Amazonia during the Miocene epoch, 23 million to five million years ago, possibly creating an environment where hosts of new species could evolve. Scientists agree that sections of the Amazon have been underwater in the past, but there is no consensus as to the mechanism or extent of the flooding.

A study published earlier this year in Science Advances indicates that the Amazon flooded during two separate intervals in the early and mid-Miocene (roughly 18 million and 14 million years ago, respectively). Study co-authors Carlos Jaramillo, a pollen expert at the Smithsonian Tropical Research Institute in Panama, and Jaime Escobar, a paleoclimatologist at the University of the North in Colombia, and their colleagues contend that the Caribbean Sea breached the South American coast by way of Venezuela and Colombia, covering massive tracts of ancient Amazonia with a tongue of saltwater that reached into the continent. As the water advanced, it lost depth and became a marine/brackish ecosystem, then a watery transitional zone, which finally gave way to dry land. Distinct layers in the sediment cores show that in Colombia, the flooding events lasted for 900,000 and 3.7 million years, respectively, while in western Amazonia, which is farthest inland, they endured for 200,000 and 400,000 years.

The researchers also describe how sediment cores have yielded fossil evidence supporting the occurrence of these marine inundations. The most interesting macrofossils found in the several-centimeter-wide core from Colombia's Saltarin region are a shark tooth, possibly from a blacktip or a hammerhead, and a mantis shrimp, a marine organism that burrows into the sandy and muddy bottom of tropical seas. “Finding a shark tooth in such a narrow core means there have to be many more, and that points to the extent of these saltwater incursions,” Jaramillo says. “The Amazon rain forest is a very dynamic system and not as old as previously thought. Today it covers an area as large as the continental U.S., but nearly 14 million years ago it was an ocean.”

The new findings agree with previous research led by geologist and pollen expert Carina Hoorn of the University of Amsterdam and Amazon Regional University IKIAM in Ecuador. Hoorn recently determined the age of the river but was not involved in the Science Advances study. “Taken together, the evidence for marine incursions into Amazonia is really overwhelming” and paves the way for further research on how this marine environment may have influenced the evolution of the region's biodiversity, she says.

But others are more cautious. “This paper provides important, though not absolutely conclusive, evidence of marine incursions,” says evolutionary biologist Christopher Dick of the University of Michigan, who studies plant diversification in several parts of the Amazon. “But most of the other scenarios are still possible, even with these new data.”

Jaramillo and his co-authors do not go so far as to say that the Miocene saltwater incursions alone are the reason for the Amazon's biodiversity, but they think several of the existing plant genres in today's rain forest could trace their lineages to species living in forests that were permanently flooded. Dick disagrees, however. From a terrestrial botanical perspective, he says, it would be hard to prove that any extant species diversity could be attributed to these floods.

It appears that the detective work in this mighty river and the forest it nurtures will stretch well into the future.