We are the products of our environment — and that goes for egrets and elephants as much as human beings. The history of all life on this planet has been one of change and adaptation. The environment changes, and life adapts. That’s evolution in a nutshell.
So it shouldn’t be a surprise that as the planet’s climate has changed through the geologic past — and it’s changed severely, from the hot and humid earth of the Triassic period to the ice ages that ended just 20,000 years ago — life has changed along with it. In a new paper published in the Proceedings of the National Academy of Sciences (PNAS), a group of researchers plot out just how the changing climate has impacted mammalian evolution in North America over the past 65 million years. They find that there have been six distinct waves of species diversity, and that the driving force of those waves has likely been climate change.
Here’s Brown University evolutionary biologist Christine Janis — a co-author on the paper with a group of Spanish researchers — on how changes in the climate beat out other factors like migration:
Although we’ve always known in a general way that mammals respond to climatic change over time, there has been controversy as to whether this can be demonstrated in a quantitative fashion. We show that the rise and fall of these faunas is indeed correlated with climatic change — the rise or fall of global paleotemperatures — and also influenced by other more local perturbations such as immigration events.
Of those six “waves” that Janis and her colleagues identify, four show statistically significant correlations with major changes in temperature, while the other two show a weaker correlation, most likely because those patterns corresponded to times when mammals from other continents invaded North America. Even today, invasive species are a leading cause of species extinction and ecosystem change — keeping in mind the fact that humans are, in a sense, an invasive species. But the PNAS study shows how relatively rapid changes in the planet’s temperature led to changes in ecosystems — woodland vegetation shifting to grasslands, for example — which in turn led to evolutionary changes in species themselves. Life adapts.
Of course, as the climate changes today — much more quickly than it has in the past — the question again is how life will adapt to a warmer world. Though the PNAS study doesn’t make any projections, other research has — and it doesn’t paint a pretty picture. It’s difficult to get a firm idea of how wildlife might adapt — or not — to rapid climate change, which makes it hard to project actual numbers of extinctions in a warmer world. The PNAS study shows that temperature change in the past has led to changes in species diversity; when it comes to man-made global warming though, we’re embarking on an unplanned experiment without a control group.
The good news — of sorts — is that the earth has experienced massive climate change and massive species die-off through its 4.5 billion-year history, and every time, life eventually bounces back. Climate change — even drastic climate change — isn’t new for the planet. But something else is: us. The earth has never seen a species as numerous or as demanding as the modern Homo sapiens, spread to every corner of the world, using up resources and transforming the planet through agriculture, mining and deforestation. Science looks to the past to try to understand the future, but nothing like us has ever happened to the earth before.