The demise of dinosaurs around 66 million years ago sent major ripples throughout ecosystems, and it may have even paved the way for fruit to evolve. New research has scoured through the evolutionary history of fruit and seeds to determine when and how they changed in size over time, confirming that dinosaur extinction led to the growth of large fruit that fueled our primate ancestors’ diets. 

A recent study published in the journal Palaeontology shows the ebbs and flows of seed size throughout history, largely dictated by changing conditions of forests. The results — covering the fall of dinosaurs and the rise of mammals — demonstrate how certain species play a major role in influencing the world around them as ecosystem engineers. 

Fruit’s Fight For Light

Fruits represent such an important part of the human diet, yet they haven’t always looked the same as they do now. The origin of fruit is somewhat murky; the earliest known angiosperms, or flowering plants, are dated to the Early Cretaceous (around 135 million years ago) based on fossil evidence, but some scientists believe the first angiosperms may have arisen even earlier, in the preceding Jurassic period. During the Cretaceous period, the seed size of plants was small, and fruits were rare.

Fruits and seeds, however, would eventually change after the widespread disappearance of dinosaurs due to the Cretaceous-Paleogene extinction event. Before their extinction, towering sauropods — the largest land animals to ever exist — would alter the environment as they flattened trees and devoured copious amounts of vegetation. 

After the dinosaurs were wiped out, forests began to grow back, and the thick tree cover blocked the sun from reaching the ground layer. Scientists have theorized that competition for light in denser forests promoted the success of taller trees that had to grow faster than others, and that trees grown from larger seeds got a head start in this race. Fruits also became larger, and animals like early primates would ingest them and disperse seeds, guiding the spread of plants.

Read More: How Sauropods Evolved to Their Enormous Size

Prehistoric Seed Sizes

In the new study, researchers confirmed this theory with a model they created to see how seed and fruit size would respond to a darker forest understory following dinosaur extinction. The results closely replicated seed size trends in prehistoric times. 

Seeds grew once dinosaurs were out of the picture, but as the model continued, the researchers came across a surprising find in the evolutionary timeline. Around 35 million years ago, at which point land animals had become large enough to have a similar destructive effect on forests as dinosaurs once did, seeds shrunk back down to smaller sizes. 

“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,” said Christopher Doughty, an ecoinformatics professor at Northern Arizona University who led the study, in a statement. “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.”

The New Ecosystem Engineers

The model charted yet another fluctuation in seed size at around 50,000 years ago, when the Late Pleistocene extinctions caused megafauna like mammoths to begin vanishing. Megafauna extinction led forest understories to darken once again, with the model predicting a long-term increase in seed size as a result. 

In the modern age, humans have taken up the role of ecosystem engineer as deforestation continues at a rapid pace; logging practices have created openings for sunlight to reach the surface, with light levels in modern forests starting to resemble those present when dinosaurs existed. If humans were to stop these actions, and if megafauna don’t become the predominant species again, forests could theoretically darken again and set the stage for larger seeds to make a reappearance.

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Jack Knudson is an assistant editor at Discover with a strong interest in environmental science and history. Before joining Discover in 2023, he studied journalism at the Scripps College of Communication at Ohio University and previously interned at Recycling Today magazine.