The origins of modern tropical rainforests, like the one at Amacayacu National Park, in Guaviare, Colombia, can be found in the aftermath of the asteroid strike that caused mass extinction at the end of the Cretaceous Period 66 million years ago.

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At a Glance

An asteroid impact wiped out most life on the planet 66 million years ago.Before the asteroid, Colombian forests were airy and light, with tall conifers.Researchers used plant and pollen fossils to trace the changes over millions of years.

The dense, dark rainforests in what is today Colombia looked vastly different 66 million years ago.

Sunlight streamed through tall coniferous trees to reach ferns and flowering shrubs with plenty of room to grow along the forest floor. The air was much less humid. And, yeah, there were dinosaurs.

Then a giant asteroid slammed into the Earth, wiping out 75% of the life on the planet and ending the Cretaceous Period.

“A single historical accident changed the ecological and evolutionary ,” Carlos Jaramillo, of the Smithsonian Tropical Research Institute in Panama City, told Science News. “The forests that we have today are really the byproduct of what happened 66 million years ago.”

Jaramillo is a paleopalynologist, which means he studies ancient pollen. He was part of a team that spent 20 years gathering and analyzing thousands of fossils of pollen, spores and leaves from 72 million to 58 million years ago. Their study was .

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“It took us a long time to gather enough data that we could have a clear picture of ,” Mónica Carvalho, the study's lead author and postdoctoral fellow at the Smithsonian Tropical Research Institute, told Ars Technica.

After the asteroid, almost half of the plant species in Colombia disappeared. The cone-bearing conifers died off. Over the next 6 million years, the flowering plants and ferns that remained took over the rainforests.

They grew close together, creating the thick canopy that blocked out the sun. The water that evaporated from their leaves increased the humidity.

The Chiribiquete National Park in Colombia is the largest tropical rainforest national park in the world. “The forests that we have today are really the by-product of what happened 66 million years ago,” said Carlos Jaramillo, who studies ancient pollen at the Smithsonian Tropical Research Institute in Panama City.

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The researchers have some ideas about why this change occurred.

Without hungry herbivore dinosaurs chomping on the vegetation and traipsing through the bushes, the plants could grow undeterred. Conifers may have simply no longer been suited for the tropics.

Finally, remember that asteroid? It could have unleashed tsunamis that deposited carbon-rich sediment in the forests. Ash from wildfires triggered by the blast could have acted as fertilizer. Conifers don't do as well in high-nutrient soil as flowering plants.

“This is something we continue to explore as we search for more fossil sites and when we keep on studying the tropics,” Carvalho said.

Bonnie Jacobs, a paleobotanist at Southern Methodist University in Texas who was not part of the study, told New Scientist, “, this incredibly diverse, really structurally complex forest, but right now, we are living through a mass extinction caused by humans and, again, whole ecosystems are being set on a different path. In the case of the rainforest, we might like the final product, but all those animals that were alive in the Cretaceous did not.”

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Jaramillo said the path the rainforests took over those millions of years has lessons for us today as we're seeing extensive deforestation in the Amazon and around the world.

“At some of the places we studied, I could see right in front of my eyes how this forest that has taken 66 million years to build was gone in a day, and the rate of deforestation is staggering,” Jaramillo told New Scientist. “We know from this study that it takes a long time to build these diverse forests back: you can’t chop down the forest and think, ‘Oh, tomorrow I’ll plant more trees.’ ”

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