The enormous celestial body that rammed into Earth some 66 million years ago didn't just wipe out all non-avian dinosaurs; it also triggered long-term change that completely transformed the evolutionary and ecological trajectory of tropical rainforests.

In a study published April 1 in Science, researchers based primarily in Colombia examined thousands of pollen and leaf fossils to show how tropical rainforests shifted from having open canopies to having closed canopies, replete with significantly different plant populations, thanks to environmental changes set off by the Cretaceous-Paleogene extinction event. 

"When we started this study 14 years ago, we thought this was just a hiccup with a few species going extinct, and that the forest basically was the same before and after," co-author Carlos Jaramillo, a researcher at the Smithsonian Tropical Research Institute, said in an interview with The Academic Times. "What we found was a story quite different. The rainforest we have today is really a byproduct of this historical action that happened 66 million years ago and changed the tropics forever."

On the morning of the Chicxulub impact, the rainforest of central Colombia shared some similarities with today's: It was a hot, humid landscape, densely populated with trees and other plants. 

However, while today's rainforest is dominated by flowering plants and trees, the Cretaceous rainforest contained a mix of these angiosperms as well as ferns and conifers. The latter, such as monkey puzzle trees, mostly from the Araucariaceae family, were once common in the tropics but disappeared from rainforests at the end of the Cretaceous period.

"The well-supported findings of Carvalho et al. reveal that angiosperms were well on the way to becoming dominant and diverse members of forest communities by the latest Cretaceous — a previously accepted consensus," wrote professor Bonnie F. Jacobs, of Southern Methodist University, and associate professor Ellen Currano, of the University of Wyoming, in an accompanying Perspective article in Science. "But it is now clear that in the tropics, as is true at higher latitudes, gymnosperms (cone-bearing plants, for example) and ferns were important members of forested communities before the [Cretaceous-Paleogene] boundary."

These plants cohabited in bright, open-canopy forests, where sunlight was able to reach the forest floor. In the closed-canopy rainforest of today, flowering plants vie for maximum potential to photosynthesize, so they grow as many leaves as they can near the top of the canopy. 

According to Jaramillo, trampling, gnawing and gap-clearing dinosaurs probably played a significant role in shaping Cretaceous rainforests, while their extinction likely helped allow for closed-canopy forests to grow later on.

After the Chicxulub impact, plant diversity cratered by about 45%, and took 6 million years to recover. Conifers largely vanished, but at least one lineage of angiosperms survived, which, after millions of years, grew into the thousands of plants in the legume family that populate tropical rainforests today. This lineage of legumes appeared after the Cretaceous period, but because the legumes produce nitrogen in soil, their presence helped foster a nutrient-rich environment for angiosperms to flourish. Falling ash from the impact helped add phosphorus to the soil, as well. 

"Imagine that you have a brand new garden, you put down the soil and then you add nitrogen and phosphorus. Under these conditions, the angiosperms just took over. They grow very fast," Jaramillo said. "They reached the canopy, and the conifers were out of luck because they couldn't reach the light anymore, producing these new kinds of forests."

To be able to draw this timeline, Jaramillo and his colleagues examined fossilized pollen and leaves from 39 different sites in the Colombian rainforest, ranging from 56 million to 72 million years old. 

Pollen, according to Jaramillo, is useful because it is resilient — some pollen samples can be up to 300 million years old — and can be analyzed to determine when a species originated and when it went extinct. Analyzing leaves, on the other hand, allows researchers to determine the taxonomic affinities of plants, such as the plant families that produced a given plant. 

"It's almost impossible that a full tree will become fossilized with everything together," Jaramillo said. "Here, we study the parts of the tree, and then we try to put them back together. Each part is a different part of the story."

Aside from always hunting for new fossil sites across the tropics, Jaramillo will be participating in a couple of future research projects. One is the TADP, or Trans-Amazon Drilling Project, a $5 million venture that will involve drilling holes 2 kilometers long at three different sites across the Amazon rainforest, creating fresh sites for Jaramillo and his colleagues to plumb for ancient pollen. 

These samples will help Jaramillo track climate and diversity in the Amazon across past warming events, including that in the middle Miocene. But, like so much scientific research over the past year, this project has been delayed due to COVID-19.

The study, "Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests," published April 1 in Science, was authored by Monica R. Carvalho, Smithsonian Tropical Research Institute and Universidad del Rosario; Carlos Jaramillo, Smithsonian Tropical Research Institute, University of Montpellier and University of Salamanca; Felipe de la Parra, Instituto Colombiano del Petróleo; Dayenari Caballero-Rodríguez and Camila Martínez, Smithsonian Tropical Research Institute; Fabiany Herrera, Smithsonian Tropical Research Institute and Chicago Botanic Garden; Scott Wing, National Museum of Natural History; Benjamin L. Turner, Smithsonian Tropical Research Institute and University of Florida; Carlos D'Apolito, Smithsonian Tropical Research Institute and Universidade Federal de Mato Grosso; Millerlandy Romero-Baez, Smithsonian Tropical Research Institute and ExxonMobil Corporation; Paula Narváez, Smithsonian Tropical Research Institute and Instituto Argentino de Nivología; Mauricio Gutierrez, Smithsonian Tropical Research Institute and Universidad de Chile; Conrad Labandeira, National Museum of Natural History, University of Maryland and Capital Normal University; German Bayona, Corporación Geológica Ares; Milton Rueda, Paleoflora Ltda; Manuel Paez-Reyes, Smithsonian Tropical Research Institute and University of Houston; Dairon Cárdenas, Instituto Amazónico de Investigaciones Científicas SINCHI; Álvaro Duque, Universidad Nacional de Colombia; James L. Crowley, Boise State University; Carlos Santos, BP Exploration Operating Company Limited; and Daniele Silvestro, University of Fribourg and University of Gothenburg.