Mass extinction appears to favor not the big and the bold, but the small and seemingly weak-bodied. But in these disastrous events, what makes small species thrive and become dominant?

New research from a team out of the University of Pennsylvania, published in the journal Science, studied the Hangenberg event, a mass extinction that occurred 359 million years past and caused lasting impact on Earth’s vertebrates.

Researchers found that for a minimum of 40 million years after the die-off, notably smaller fish dominated the oceans – unlike the pre-event scenario when large creatures led the picture.

"[Y]ou may have one gigantic relict, but otherwise everything is the size of a sardine," said Lauren Sallan, assistant professor in Penn's Department of Earth and Environmental Science in the School of Arts & Sciences, she added that it was no long a thriving ecosystem of massive things post-wide scale instabilities like mass extinction. 

The study concluded that small and fast-multiplying fish owned an “evolutionary advantage” over their larger counterparts.

What caused this unexpected shift in power and, in line with the Lilliput Effect, populating the post-instability oceans with smaller organisms that reproduced faster and more effectively? Sallan pointed to natural selection, regardless of what was eliminating large fish or making the ecosystems unstable.

“These disturbances are shifting natural selection so that smaller, faster-reproducing fish are more likely to keep going, and it could take a really long time to get those bigger fish back in any sizable way,” she explained.

These entities with newfound power and dominance were, in fact, likely less than 10 centimeters big –yet they were the ancestors of all species that ruled the world from then on, including human beings.

The researchers added that this pattern reflects the biological rule taking place in plants. For instance, after a forest fire, fast-multiplying grasses could colonize the affected areas first, and then followed by shrubs and large trees much later on.

This biological process in plants occurs on a much smaller scale and may take only decades, they added, but matches what took place in the planet’s oceans millions of years ago.

Research of this kind is highlighted at a time when many global fish populations are hurtling toward destruction due to overfishing and climate change, and scientists believe Earth is on the verge of a sixth major extinction due to human acts.