The Amazon is a region of astounding natural beauty and incredible biodiversity. And its history might be just as colorful. According to a new study, published in Science Advances, the Amazon sank twice, with the Caribbean flooding the region and creating a shallow sea.

The latest finding will definitely create an intense debate, as there is no consensus on how the Amazon came to form. The team from the Smithsonian Tropical Research Institute have analyzed over 50,000 pollen grains from oil drilling cores and have found a tiny shark tooth, part of a mantis shrimp, and marine microorganisms. The team is confident that this is enough to make their case.

"Pollen records from oil wells in eastern Colombia and outcrops in northwestern Brazil clearly show two short-lived events in which ocean water from the Caribbean flooded what is now the northwest part of the Amazon basin," said lead author Carlos Jaramillo in a statement. "Geologists disagree about the origins of the sediments in this area, but we provide clear evidence that they are of marine origin, and that the flooding events were fairly brief."

The two flooding events happened between 18 and 17 million years ago and between 16 and 12 million years ago. They established this using the pollen. The team found two layers of marine pollen types separated by a layer of non-marine ones. They examined 900 types of pollen from the sediment to make that assessment.

There are, of course, other hypotheses. The region could have had a gigantic freshwater lake or a complex, shifting system of rivers. Sea water incursions might have been common and caused contaminations now seen in the fossil record. Data from more locations is probably necessary to reach a consensus.  

These important geological changes appear to have happened just before (in geological terms) the formation of the Andes, which formed between 10 and 6 million years ago. Scientists think that the secret to the incredible diversity of life forms in the region is due to its complex past.  

"It's important to understand changes across the vast Amazonian landscape that had a profound effect, both on the evolution and distribution of life there and on the modern and ancient climates of the continent," Jaramillo added.

The Amazon rainforest is about the size of the United States and is the largest rainforest on Earth. It holds 10 percent of all species living on the planet.