The origins of modern tropical rainforests, such as those in the Amacayacu National Park, in Guaviare, Colombia, can be found in the aftermath of the asteroid attack that caused mass extinction at the end of the Cretaceous 66 million years ago.
(MAYELA LOPEZ / AFP via Getty Images)
- An asteroid impact wiped out most of the life on the planet 66 million years ago.
- Before the asteroid, Colombian forests were airy and light, with long conifers.
- Researchers have used plant and pollen fossils to track the changes over millions of years.
The dense, dark rainforests in what is now Colombia looked very different 66 million years ago.
Sunlight streamed through tall conifers to reach ferns and flowering shrubs with enough space to grow along the forest floor. The air was much less humid. And yes, there were dinosaurs.
Then a giant asteroid falls into the earth, destroying 75% of life on the planet and ending the Cretaceous.
“One single historic accident has changed the ecological and evolutionary trajectory of tropical rainforests,” Carlos Jaramillo, of the Smithsonian Tropical Research Institute in Panama City, told Science News. “The forests we have today are actually the by-product of 66 million years ago.”
Jaramillo is a paleopalinologist, which means he studies ancient pollen. He was part of a team that for 20 years collected and analyzed 72 million to 58 million years ago thousands of fossils of pollen, spores and leaves. Their study was published in Science last week.
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“It took us a long time to gather enough data to get a clear picture of what was going on during the extinction,” Mónica Carvalho, 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-shaped conifers have died out. Over the next 6 million years, the remaining flowering plants and ferns took over the rainforests.
They grew close together and created the thick canopy that blocked 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 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.
(GUILLERMO LEGARIA / AFP via Getty Images)
The researchers have some ideas as to why this change occurred.
Without hungry herbivorous dinosaurs stomping the vegetation and walking through the bushes, the plants can be left untouched. Conifers may just not be suitable for the tropics anymore.
Lastly, remember that asteroid? It could release tsunamis that have deposited carbon-rich sediment in the forests. Ash from wildfires caused by the explosion could serve as fertilizer. Conifers do not do as well in nutrient-rich soil as flowering plants.
“This is something we are continuing to explore as we search for more fossil sites and study the tropics,” Carvalho said.
Bonnie Jacobs, a paleobotanist at Southern Methodist University in Texas who was not part of the study, told New Scientist: ‘We love the way it ended up, this incredibly diverse, really structurally complex forest, but currently we are living through a mass extinction caused by humans, and again, whole ecosystems are being put on a different path. In the case of the rainforest, we may like the final product, but not all the animals that lived in the Cretaceous. ”
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Jaramillo said that the path that the rainforests have taken for millions of years has lessons for us because we see extensive deforestation in the Amazon and around the world.
“In some of the places we studied, I could see right before my eyes how this forest that took 66 million years to build passed within a day, and the rate of deforestation is staggering,” Jaramillo told New Scientist said. ‘We know from this study that it takes a long time to rebuild these diverse forests: you can not cut down the forest and think,’ Oh, tomorrow I’ll still plant trees. ‘”
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