Dinosaurs went extinct after a single catastrophic event, when a massive asteroid smashed into Earth off the coast of Mexico.
But not all extinctions are so clearly defined. As Beth Shapiro explains in her recent book, "How to Clone a Mammoth: The Science of De-Extinction," scientists still don't agree on exactly how mammoths went extinct just thousands of years ago.
Humans are getting closer to the point that we might actually be able to resurrect some version of a mammoth — or really, much more likely, modify a modern animal so that it's more like one, as Shapiro explains.
That ability to somehow reverse a single extinction would be an amazing technological achievement, but with a new wave of mass extinctions currently transforming the world, there's something disturbing about our inability to settle on an answer to what ended the existence of these large mammals, which thrived until humans spread far and wide across the globe.
We know because of recent research that abrupt global warming came at the start of the extinction of mammoths and other ice-age megafauna, but scientists have yet to agree on how much of a role humans played at the end. "The question remains open," Shapiro writes, "perhaps because we are not particularly comfortable with the answer."
That answer may be particularly relevant as we look at how humans are affecting and changing the world around us today.
The last of the mammoths
The last of the woolly mammoths died approximately 3,700 years ago, on Wrangel Island, off the coast of Siberia. This was shortly after the death of Hammurabi, the Babylonian king whose code of laws is remembered as one of the first in history. The Great Pyramid of Giza was built almost 1,000 years before. The world had warmed enough that humanity probably arrived on this icy island around the same time.
But by then, mammoth populations had been on the decline for more than 16,000 years, ever since the peak of the last ice age. Most died about 10,000 years ago — only a few isolated island populations lasted longer.
As Shapiro writes, we have essentially two main theories about their demise: Either it got too warm to support the giants that dominated the Arctic, or our human ancestors hunted them to death.
Likely, it was some combination of factors.
Shapiro writes that it's hard to pin the mammoth's end on warmth alone. While mammoths were the most abundant large mammals in the North American Arctic for almost 100,000 cold years (with two ice ages), they were around for nearly 400,000 years total — including at a time when Earth was as warm or warmer than it is today. The fossil record indicates they were much less dominant during warm spells, with creatures like giant sloths, camels, and giant beavers thriving in the heat.
It was only at the beginning of the Holocene period, about 12,000 years ago, that many of the large species around the world started to disappear. The world got warmer and at the same time, humans spread to almost every continent — which could mean there's some connection.
And yet, people lived alongside and hunted mammoths in Asia for thousands of years before the Holocene began. In North America, large mammal populations had begun to decline significantly before humans arrived.
"The role of humans will become increasingly clear"
Shapiro says that in the case of the mammoth, it was the combination of a warming climate, changing vegetation, and the emergence of human hunters together that finally caused these creatures to disappear from most of the Earth 10,000 years ago.
"As we learn more about the timing and pattern of these and other recent extinctions, there is little doubt that the role of humans will become increasingly clear," Shapiro writes. If climate change forced mammoths into smaller and smaller cold territories, growing human populations looking for food quite likely took advantage of those fertile hunting grounds.
This fits into the latest research on what drove mammoths extinct. A recent study in the journal Science says that the best evidence is that natural climate change was the initial driver of mammoth extinction, shrinking their food supplies and forcing the dwindling mammoth populations into whatever cold refuges they could find.
Humans probably came in at the end of the process, making conditions for mammoths increasingly untenable. "The rise of humans applied the coup de grace to a population that was already under stress," Chris Turney, a paleoclimatologist at Australia’s University of New South Wales, told onEarth. This time, with humans around, mammoth populations didn't recover.
Why an old mystery matters today
Adam Cohn / Flickr
Back then, humans weren't necessarily powerful or dominant enough to be the main cause of an extinction. Now we might soon be advanced enough to bring mammoths back — though that's still a controversial and uncertain proposition.
Whatever the role humans played in the end of the mammoth, Shapiro writes that looking at the disappearance of species in general should make us pause when we look at the ways we transform the environment.
We need to take care with our power to shape the world and even push the climate in a warmer direction. The ecological consequences of changing the world are unpredictable, and the fact that we still don't quite understand our role in a major extinction — that of mammoths — should trouble us.
The woolly mammoth first appeared almost 400,000 years ago and disappeared from most of the world just 10,000 years ago. Humans, who first evolved 200,000 years ago, have not been on this planet nearly as long as mammoths were, but we are changing the world more than any species ever known.
Today, scientists estimate that close to half of Earth's living species could go extinct by the end of this century alone. These include not just large, charismatic animals, but also tiny reef invertebrates that might hold a cure for cancer or a way to slow aging. Most creatures cannot be brought back, and their disappearance may have a far greater impact than we think or are willing to admit so far.
As Shapiro writes, "this future could be one in which so many changes have occurred to the terrestrial and marine ecosystems that we, ourselves, are suddenly vulnerable to extinction."