We all know about the dinosaurs that once roamed the planet, but long after they went extinct,
great beasts we call the megafauna lived on every continent.
In the Americas, ground sloths the size of elephants pulled down trees with their claws.
Saber-toothed cats the size of brown bears hunted in packs,
but they were no match for short-faced bears, which stood thirteen feet on their hind legs,
and are likely to have driven these cats away from their prey.
There were armadillos as big as small cars, an eight foot beaver, and a bird with a 26 foot wingspan.
Almost everywhere, the world's megafauna were driven to extinction, often by human hunters.
Some species still survive in parts of Africa and Asia.
In other places, you can still see the legacy of these great beasts.
Most trees are able to resprout where their trunk is broken to withstand the loss of much of their bark
and to survive splitting, twisting and trampling, partly because they evolved to survive attacks by elephants.
The American pronghorn can run so fast because it evolved to escape the American cheetah.
The surviving animals live in ghost ecosystems adapted to threats from species that no longer exist.
Today, it may be possible to resurrect those ghosts, to bring back lost species using genetic material.
For instance, there's been research in to cloning woolly mammoths from frozen remains.
But even if it's not possible, we can still restore many of the ecosystems the world has lost.
How? By making use of abandoned farms.
As the market for food is globalized, infertile land becomes uncompetitive.
Farmers in barren places can't compete with people growing crops on better land elsewhere.
As a result, farming has started to retreat from many regions, and trees have started to return.
One estimate claims that two-thirds of land in the US that was once forested but was cleared for farming has become forested again.
Another estimate suggests that by 2030, an area in Europe the size of Poland will be vaccated by farmers.
So even if we can't use DNA to bring back ground sloths and giant armadillos,
we can restore bears, wolves, pumas lynx, moose and bison to the places where they used to live.
Some of these animals can reshape their surroundings, creating conditions that allow other species to thrive.
When wolves were reintroduced to the Yellowstone National Park in 1995, they quickly transformed the ecosystem.
Where they reduced the numbers of overpopulated deer, vegetation began to recover.
The height of some trees quintupled in just six years.
As forests returned, so did songbirds.
Beavers, which eat trees, multiplied in the rivers,
and their dams provided homes for otters, muskrats, ducks, frogs and fish.
The wolves killed coyotes, allowing rabbits and mice to increase,
providing more food for hawks, weasels, foxes and badgers.
Bald eagles and ravens fed on the carrion that the wolves abandoned.
So did bears, which also ate the berries on the returning shrubs.
Bison numbers rose as they browsed the revitalized forests. The wolves changed almost everything.
This is an example of a trophic cascade,
a change at the top of the food chain that tumbles all the way to the bottom, affecting every level.
The discovery of widespread trophic cascades
may be one of the most exciting scientific findings of the past half century.
They tell us that ecosystems that have lost just one or two species of large animals
can behave in radically different ways from those that retain them.
All over the world, new movements are trying to catalyze the restoration of nature in a process called rewilding.
This means undoing some of the damage we've caused,
reestablishing species which have been driven out, and then stepping back.
There is no attempt to create an ideal ecosystem, to produce a heath, a rainforest or a coral reef.
Rewilding is about bringing back the species that drive dynamic processes and then letting nature take its course.
But it's essential that rewilding must never be used as an excuse to push people off the land.
It should happen only with the consent and enthusiasm of the people who work there.
Imagine standing on a cliff in England,
watching sperm whales attacking shoals of herring as they did within sight of the shore until the 18th century.
By creating marine reserves in which no commerical fishing takes place, that can happen again.
Imagine a European Serengeti full of the animals that used to live there: hippos, rhinos, elephants, hyenas and lions.
What rewilding reintroduces, alongside the missing animals and plants, is that rare species called hope.
It tells us that ecological change need not always proceed in the same direction.
The silent spring could be followed by a wild summer.