Asteroid That Killed the Dinosaurs Gave Rise to the Rainforests

What do you think of when you think of a tropical rainforest? Brilliant flowers? Lush, leafy canopies? Dense, dark understories where predators and prey play hide and seek?

It turns out, none of these things were true of northern South American rainforests before the asteroid that wiped out the dinosaurs rammed into Earth some 66 million years ago. A new study, published in Science this month, examined plant fossils from present-day Colombia to show how one catastrophic event transformed tropical rainforests.

“[A] single historical accident (a meteorite falling on a morning of a day 66 million years ago) changed the tropics so much that the forest we have today is the product of that day,” study co-author and staff paleontologist at the Smithsonian Tropical Research Institute (STRI) Carlos Jaramillo tells Treehugger in an email. “It seems like magic reality in the best style of Gabriel Garcia Marquez!”

Before the Asteroid Hit

Before STRI conducted this research, scientists didn’t know how different the tropical rainforests of Central and South America had once been. 

“For a very long time, biologists had just assumed that flowering-plant-dominated tropical rainforests (as we know them today) existed since about 130-120 million years ago when flowering plants diversified,” Mónica Carvalho, first author and joint postdoctoral fellow at STRI and at the Universidad del Rosario in Colombia, tells Treehugger in an email. 

So the STRI team spent years gathering and examining more than 6,000 leaf fossils and more than 50,000 individual pollen spores from both before and after the asteroid hit, as Carvalho explained in a press release. This was complicated, time-consuming work. 

“It's not easy finding fossils in the tropics,” Carvalho tells Treehugger. “There are deep soils almost everywhere and you can only find exposed rocks in limited places where it's relatively dry most of the year.”

The researchers had to visit coal and siltstone mines in search of leaf fossils, asking permission from the operators to enter each mine and sometimes not finding anything at all. Jaramillo says the hardest data to track down were leaf fossils with their cuticles intact. 

“[It] took years of sampling effort to find enough of them,” Jaramillo says. 

But the persistence paid off. The researchers were able to paint a picture of Cretaceous-era forests that looks completely different from the contemporary tropical woodlands. 

The forests of 70 to 66 million years ago were not dominated by flowering plants and legumes like today’s, Carvalho explained. Instead, the flowering plants that did exist were mixed with ferns and conifers like monkey-puzzle trees, kauri pines, and Norfolk Island pines. These trees grew far apart, allowing plentiful light to filter down to the forest floor. Flowering plants grow faster and have higher rates of photosynthesis, while legumes are adept at fixing nitrogen. The comparable reduction of flowering plants and the absolute absence of legumes meant that pre-impact forests were probably less productive, slower at cycling nutrients, and less successful at storing carbon.

“The rainforests that lived just prior to the extinction were functionally and ecologically different from modern rainforests,” Carvalho says. 

How the Impact Transformed the Rainforests

At the end of the Cretaceous period, an asteroid the size of Manhattan rammed into what is now the Yucatan. The destruction went beyond the initial impact, as the study authors explained in a video.

Scorching fragments of the asteroid fell to the ground and sparked wildfires. The resulting cloud of dust and ashes obscured the sun for years afterward. The fallout drove three-fourths of then-living species to extinction including, famously, the dinosaurs. Also wiped out were 45% of the plant species then living in contemporary Colombia. 

How exactly did this devastation give rise to today’s vibrant rainforests? The researchers have three hypotheses:

1. Dinosaurs had kept forests open by moving their large bodies through the vegetation. When they disappeared, the forests could grow denser. 
2. The ash from the impact enriched the soil, favoring faster-growing flowering plants. 
3. The extinction of tropical conifers enabled flowering plants to take over their niche. 

Whatever the reason, the study is evidence that life eventually finds a way, but also that we shouldn’t take the biodiversity of contemporary rainforests for granted.

“Life on Earth carries on,” Carvalho says. “The planet has seen thousands of species come and go, and eventually, new species will evolve, but we know this takes millions of years. The real question is whether we, as humans will be able to survive the drastic changes we have created on our own planet.”

Human Impact on the Amazon Rainforest

Today’s rainforests are under serious threat from human activity. The Amazon, for example, saw its highest rate of deforestation in 12 years during 2020. There are concerns that if enough trees are felled, much of the forest would pass a tipping point at which it would no longer be able to make its own rain and would degrade into grassland.

Worldwide, biodiversity is also under threat to such an extent that scientists have said we are in the midst of the sixth mass extinction. Carvalho says that the 45% of plant species that were wiped out when the asteroid hit is roughly equivalent to the number of species predicted to go extinct by the end of the century if habitat destruction continues.

A loss like that cannot be recovered easily. Jaramillo says it took around seven million years for tropical forests to regain the amount of biodiversity it had before the asteroid hit. We can expect a similar lag if we wipe out the unique species now flourishing in the Amazon.

“The forest may come back but the diversity is gone forever,” he says.