When was the Amazon rainforest created?

We know that nowadays dust from the Sahara feeds the Amazon rainforest, but exactly when the Amazon jungle was created was unclear. Now we know.

65 million years ago a large asteroid impacted the Yucatan peninsula. It killed the dinosaurs and heralded a nuclear winter that lasted several years. But the 11 to 81 kilometers wide asteroid didn't 'just' erase the dinosaurs from the face of the earth, the impact had a profound effect on the fauna too.

Recently, researchers used fossil pollen and leaves from Colombia to investigate how the impact changed South American forests[1]. They found that cone-bearing plants called conifers and ferns were common before the huge asteroid struck what is now the Yucatan Peninsula in Mexico.

But after the devastating impact, plant diversity declined by roughly 45% and extinctions were widespread, particularly among seed-bearing plants.

While the forests recovered over the next six million years, flowering plants, came to dominate them.

The structure of tropical forests also changed as a result of this transition. During the late Cretaceous Period, when the dinosaurs were still alive, the trees that made up the forests were widely-spaced. The top parts did not overlap, leaving open sunlit areas on the forest floor.

But post-impact, forests developed a thick canopy that allowed much less light to reach the ground.

So how did the impact transform the sparse, conifer-rich tropical forests of the dinosaur age into the rainforests of today, with their towering trees dotted with multi-coloured blossoms and orchids?

The scientists have formulated some theories as to why this could happen. Firstly, the foraging dinosaurs could have kept the forest from growing too dense by feeding on and trampling plants growing in the lower levels of the forest. A second explanation is that falling ash from the impact enriched soils throughout the tropics, giving an advantage to faster-growing flowering plants. The third explanation is that the preferential extinction of conifer species created an opportunity for flowering plants to take over.

These ideas aren't mutually exclusive, and could all have contributed to the outcome we see today.

[1] Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests in Science - 2021