Evrim Yazgin
Cosmos science journalist
The extinction of the largest dinosaurs might have aided the evolution of the small, fruit-eating ancestors of primates, according to new research.
Early primates – ancestors of humans – would not have been able to flourish alongside sauropods, the largest animals to ever walk on Earth.
Sauropods were dinosaurs and are known for their long necks and tails, small heads and huge size. The largest, called titanosaurs, are estimated to have grown to 40m long and 100 tonnes.
These giant herbivores were “ecosystem engineers” – by virtue of their immense size, they would have shaped the habitat around them by knocking down trees and eating a lot of vegetation. Each animal could eat more than 1 tonne of food per day.
When dinosaurs went extinct 66 million years ago, the sauropods disappeared.
Forests grew back thicker and blocked the Sun’s rays from reaching the ground. This, in turn, led to the proliferation of plants with large seeds and fruit.
Such plants became the main food source for many animal species, including our primate ancestors.
The earliest known primate is Purgatorius. Its fossils date to right after the extinction of the dinosaurs. It was about 15cm long and looked more like a shrew than today’s monkeys.
From these humble beginnings, all primates evolved – including monkeys, apes, lemurs and humans.
The idea that fruit-bearing plants’ success after the extinction of dinosaurs led to the evolution of primates is not a new one. But scientists have struggled to demonstrate this in the fossil record until now.
New research published in the journal Palaeontology supports this theory.
“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,” says first author Christopher Doughty from Northern Arizona University (NAU), US.
Doughty’s team modelled seed and fruit size trends over the last 66 million years, compared to the body size of herbivores. They included new knowledge about how large animals effect forest structures and how seeds grow into saplings.
Their model matched up with seed and animal size over time.
But they were surprised to find their model pointed to a mysterious change about 35 million years ago. Seed and animal size both reversed course and started getting smaller.
This is because land animals had once again become sufficiently large to have a similar effect on the forests as the dinosaurs did, though not as great an impact.
They say that these trends could be explained by the evolutionary arms race between herbivores and plants rather than external factors.
“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,” Doughty explains. “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.”
“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,” adds second author Benjamin Wiebe, a PhD student at NAU.
Another change occurred about 50,000 years ago when another extinction event saw the beginning of the demise of the large ice age mammals. Like in the aftermath of the dinosaur extinction, this megafauna extinction saw a darkening of the understory and an increase in seed size.
More recently, human activity such as logging is having an impact on forest growth similar to that of the sauropods.
