Around 15,000 years ago - there weren't any cities in North America.

Instead the continent was covered with grassland and forest land.

And according to new research - there was a lot more poop from large animals all around.

And that poop nourished life on Earth.

New research from researchers at the University of Vermont shows that feces from animals are essential in moving nutrients up the food chain.

The research was published in the Proceedings of the National Academy of Sciences - and it points to a wide array of animals involved in the nutrient cycle - including whales - seabirds - animals that graze on land - and large predators.

The study is more than just a fun reason to talk scatological, though - it's also a dire warning about how human activities are leading to an impending breakdown of our planet's food web and nutrient cycles.

If we go back 15,000 years - the North American continent was home to mammoths and mastodons - elk and moose - and large carnivores like the saber tooth cat and the dire wolf.

All of these animals acted as a "distribution pump" with their feces - transporting nutrients from "hot spots" to other places.

And the distribution of nutrients works two ways - both up through the food chain biologically - and across the continent geographically.

Take my friend Stuart Pimm's example of a discovering how bear poop is essential to forest health in the Pacific Northwest.

Forests in the Pacific Northwest were lacking iodide in their soil - which trees in that area need to survive.

It was already interesting that the trees required iodide - which comes from the ocean - when the forests were very clearly inland.

But even more interesting was the fact that the forests suddenly started growing back - and that the soil was getting more and more iodide.

So what happened?

It turns out - some of the dams on the rivers had been torn out recently - and those dams had been blocking the Pacific Salmon populations from spawning upstream from the oceans.

With the dams removed - the salmon could once again spawn upstream - and the bears could once again eat the salmon.

Since the salmon would swim up the rivers from the ocean - they were rich in iodide - and once the bears had digested the salmon - they would poop that iodide in the forest where it would enrich the soils.

The iodide moved through the food-chain while it traveled from the Ocean to the inland forests.

That's how interconnected life on this planet is - and breaking just one link in the chain - can cause an entire ecosystem to collapse.

But it's not just iodide - let's look at another example with phosphorus.

Phosphorus is commonplace in households in the form of fertilizer - but it's also essential in the creation of DNA - and a building block for cell membranes.

It's essential for life on Earth.

Seabirds and some types of fish - like salmon - once carried more than 300 million pounds of phosphorus from the ocean onto land each year.

According to the study - that figure is down to about 4% of what it was - closer to 12 million pounds.

Marine mammals - like whales - used to move around 750 million pounds of phosphorus from the ocean's depths to the surface every year - that number is now closer to 165 million pounds.

And we've seen in American history what happens when we lose large grazing mammals and their poop.

Just look at the bison and how the collapse of the bison population in the great plains contributed to the dust bowl.

Likewise - researchers at UC Berkeley point to the loss of large animals in Alaska and the Yukon thousands of years ago as the reason that the region transformed from a productive mix of forest and grassland into the unproductive tundra that we know today.

And that lines up to what we're seeing in Africa - as elephants populations have dramatically declined, thorntrees have filled the landscape across the African savanna.

Both studies highlight the fact that it's particularly damaging when large animals and top predators are eliminated from the food chain - according to UC Berkeley study leader Anthony Barnosky:

"[I]f you pull out a top predator or a key herbivore today, you get dramatic change in the ecosystem. Our study makes it clear that in the past, such changes have lasted for thousands of years. These extinctions really do permanently change the dynamics. You can't go back."

So what's driven these extinctions historically?

Back 15,000 years ago, it was a naturally changing climate - combined with the appearance of Neolithic hunters.

Today the leading cause is humans - and the effects of climate change fueled by human activity.

According to research published at the end of August - our hunting and fishing habits are so out of control that scientists are now referring to humans as "superpredators."

We're wiping out adult populations of top predators at 9 times the rate that would naturally occur - and we're wiping out adult fish populations at 14 times the rate of what would naturally occur.

We're warming - polluting - and overfishing our oceans - we're making inland waterways toxic and fragmenting ecosystems with shortsighted and poorly conceived construction projects.

It's time to realize that human activity has real - lasting - effects on the planet - and to realize exactly how complex and interconnected the natural world is.

We need to get serious about cleaning up the environment and addressing runaway climate change - and we need to fundamentally change how we see our place in nature.

We need to get serious about living WITH nature - instead of living at the expense of nature.