How dinosaur extinction influenced fruit evolution

The natural environment is repeatedly reinventing itself with dynamic processes which have a tendency to continue on for millions of years. Extinction events are among the strongest forces which not only kill specific life forms but also offer the basis for fresh avenues of evolution.

A recent study evokes a fascinating image of the process, revealing how the death of the largest dinosaurs indirectly created the fruit-rich habitats which were so important to our primate ancestors.

The research was led by Christopher Doughty, an ecoinformatics professor at Northern Arizona University.

A prehistoric legacy

Sauropods, the long-necked, plant-munching dinosaurs of the ancient world, had a massive influence on the world around them. These hulking dinosaurs knocked over trees on a routine basis and munched through tons of plant matter.

When Sauropods went extinct some 66 million years ago, their loss profoundly changed ecosystems. Forests rebounded and closed up, which decreased the amount of sunlight on the ground and paved the way for an impressive evolutionary shift.

Sauropods and ecological change

The extinction of Sauropods enabled trees to evolve without interruptions. The forests became denser, and tree competition for light intensified.

Trees developed larger fruits and seeds to dominate others, and these provided their offspring with an advantage over others by supplying them with additional energy and greater survival chances in shaded conditions.

“At first glance, the darker forest understory caused by dinosaur extinctions may seem unimportant, but it could have directly led to the evolution of our fruit-eating primate ancestors,” said Professor Doughty.

The dinosaur-fruit connection

Before the dinosaurs went extinct, fruits were scarce, and seeds were small. But with the ecological niche left open by the sauropods, plants evolved immediately.

Stronger seedlings, with larger seeds, were able to compete for limited sunlight. In addition, fleshy fruits evolved as an incredibly efficient way of enticing animals to scatter seeds and reproduce.

Even though this evolution scenario was previously theorized, no fossil records existed to conform the hypothesis.

To investigate, Professor Doughty and his team developed a cutting-edge model of seed and fruit size evolution after the dinosaurs became extinct. The model precisely replicated fossil records for the last 65 million years and decisively substantiated the hypothesis.

Reversal of dinosaur-driven fruit trends

Interestingly, the model also forecasted a future reversal of seed evolutionary patterns.

Seeds began to shrink about 35 million years ago. This coincided with the appearance of new large mammals that were successful in duplicating, though on a smaller scale, the forest-clearing behavior previously shown by sauropods.

“Our model predicted these animals would open the forest enough that sufficient light began to enter the understory, and larger seeds were no longer successful over smaller seeds,” explained Doughty.

“The evolutionary pressure for seed size to increase began to diminish. Thus, we were able to explain the trends in seed size over time without resorting to external influences such as climate change.”

Megafauna extinction and ecological reset

About 50,000 years ago, large mammals like mammoths went extinct. These animals previously opened forests by knocking down trees and clearing vegetation. Their extinction allowed forests to become dense and dark again.

With less sunlight reaching the forest floor, plants adapted by producing larger seeds. This highlights how big animals shape their environment, influencing the evolution of plants.

Humans as ecosystem shapers

Modern human activity mimics the prehistoric impacts of the giants. Current selective forest harvesting mimics the prehistoric pressure imposed by dinosaurs on seed size and dispersal.

But if humans stop modifying forest structure and are not substituted by other megafauna, forests will shift back to more dense, dark structures, and this may trigger another evolutionary adjustment towards larger seeds.

“These results provide a striking example of how large dinosaurs – and their eventual extinction – not only shaped their contemporary environment but also triggered cascading effects on ecosystems for millions of years,” said Benjamin Wiebe, an NAU researcher.

“The next time you’re eating fruit or pondering, ‘why am I here,’ consider the impact of the dinosaur extinctions.”

The study is published in the journal Palaeontology.

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