Whenever the Earth experiences a mass extinction of animals, it is the smaller animals that survive and re-populate the world, according to a new study and a report on csmonitor.com.

Researchers, looking at a mass extinction know as the Hangenberg event, have concluded the larger species all died off, while leaving the smaller ones, which began to thrive and continue.

In a news release, Lauren Sallan, and environmental scientist at the University of Pennsylvania, said that after a mass extinction, such as the Hangenberg event that occurred about 350 million years ago, you may have one surviving giant species, but everything else is about the “size of a sardine.”

The research suggests the smaller species had a distinct advantage over the much larger ones, in that they were able to breed much faster than their larger relatives.

Dr. Sallen added that you wound up with a few sharks in the ocean at about three feet in length, but most fish were less than about five inches.  Still, these small creatures became the ancestors of everything that dominates the world from then on, including humans.

Debates have ranged over the years about the reason that species become larger and larger.  A theory known as Cope’s rule, says animals become larger to become more proficient hunters and to avoid being hunted themselves.

Another suggests that the presence of increased oxygen or colder climates leads to species becoming larger over time.

This news is relevant because many scientists are predicting that we are on the verge of what is called the sixth mass extinction, and saying human activity is the chief cause for this latest round of massive species loss, with some saying it is already underway.

But many of the same scientists are also saying it can be prevented by being aggressive in our efforts to conserve endangered species, citing the recent recommendation for removing the humpback whales from the endangered species list.

The research team listed habitat loss, over-exploitation of species for economic gain and climate change as factors that must be addressed to turn the trend around.  And research shows that if the larger animals are lost, evolution takes a very long time to replace them.

Dr. Sallen also said that whatever the reason for the loss, the smaller faster-reproducing fish are more likely to keep the ecosystems going, and it could be a really long time before the larger fish return.