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Abruptly Warming Climate Triggered Megabeast Revolutions

Around 34,000 years ago, woolly mammoths went extinct from parts of Europe, only to be replaced by… woolly mammoths. The two groups—the disappearing individuals and their substitutes—belonged to the same species. If you looked at their fossils, you probably couldn’t tell them apart. Their genes, however, reveal them to be part of two genetically distinct lineages, one of which suddenly displaced the other.

Alan Cooper, who specialises in studying the DNA of extinct animals, first noticed this pattern ten years ago, and not just among mammoths. The steppe bison—a huge cow—was quickly replaced by a different bison species. Later, one population of cave bear was swapped out for another genetically distinct population. Later still, the giant short-faced bear disappeared and the modern grizzly took its place. “This all happened very abruptly,” says Cooper. “They’re weren’t all happening at the same time, but the patterns were there, and many of these changes were only detectable through DNA.”

These giant beasts—the woolly mammoth, the cave and short-faced bears, the steppe bison, the Eurasian cave lion, the woolly rhino, and the Irish elk—are now extinct. There have been some two centuries of debate over what killed them. Was it a blitzkrieg of incoming humans, with our insatiable appetites and sharp spears, or was it a change in climate?

The turnovers that Cooper saw were extinctions of a kind, and they certainly didn’t seem to be due to hunting. At least six of them took place tens of thousands of years after humans arrived on the scene. If we hunted these animals to extinction, we sure took our sweet time over it, and in some cases, we seemingly ignored a virtually identical group or species that then took the place of the vanquished. An environmental event seemed more likely. But what kind of event?

To find out, the team used ancient DNA to discover cases of turnover that are hard to observe from fossils alone. They then carbon-dated the bones of the ingoing and outgoing animals to work out when these turnovers happened. Finally, they plotted these dates against an accurate record of North Atlantic climate over the past 60,000 years, gleaned from Greenland ice and Venzeuelan sediments.

Most of the replacement events lined up with sudden bursts of warming called Dansgaard-Oeschger (D-O) events, which happened a lot between 12,000 and 60,000 years ago. In each one, average temperatures abruptly rose by anywhere from 4 to 16 degrees Celsius in just a few decades, before gradually falling again over several millennia. If you plot the temperatures on a graph, you see a distinctive sawtooth pattern—sharp upward spikes of warming and then gentle downward slopes of cooling. “They’re the most abrupt changes in climate that you got in the Pleistocene—those transitions from cold to warm,” says Cooper.

When one group of large beasts cycled into another (and, eventually, into nothing), it usually happened during the warm periods, or interstadials, that followed the D-O events. “Periods of cold are often held up as a key reason for megadeath, but we see that our extinctions and genetic transitions didn’t fall in those periods of time,” says Cooper. During the cold, the animals may have retreated into warmer refuges, and their populations may have contracted, but they didn’t die out entirely. That only happened after sudden warmth.

“In the last two and a half million years, ice ages have been the rule for the earth’s climate system — the warm periods are the exception,” says Jacquelyn Gill from the University of Maine. “Given that, it absolutely makes sense that the authors found evidence for more turnover during warmer climates, rather than cold events.”

“What we don’t know is whether it’s the warming that’s doing the damage or the pace of change,” Cooper adds. It’s probably not as simple as cold-adapted species losing out to warm-adapted ones. The sudden temperature rises would also have caused dramatic changes in rainfall and other weather patterns. They might also have altered the ranges of different animals, bringing separate populations or species into new contact and new conflict. Finally, humans could still have played a role in finishing off these giant beasts, after changing climates had weakened them. Supporting that idea, Cooper’s team found that transition events were less common during earlier parts of the Pleistocene when D-O events were common but humans were not.

“These and previously published data argue convincingly that, sometimes, extinctions happened in the absence of humans—especially local extinctions followed by re-establishment of populations from elsewhere, as more ideal climate conditions returned,” says Beth Shapiro from the University of California, Santa Cruz. But “it’s hard to imagine that humans did not at least contribute somewhat,” she adds. If climate change corralled animals into ever narrower ranges, those “refuges” would have been fantastic hunting grounds—more like slaughterhouses than sanctuaries.

“As animals became stressed due to rapid changes in climate, and consequent reduction in habitat and loss of connectivity between whatever patches of habitat remain, humans are poised to have the biggest potential negative impact on these populations,” says Shapiro. “It’s about timing—poor timing, if you’re a mammoth.”

The synergistic threats of rapidly warming climate and relentlessly destructive humans has lessons for us in the present, says Cooper. “We should really be aware that if you go back 10,000 years, the climate goes apesh*t,” he says. “People generally have no idea about this, but these warm-cold-warm-cold patterns are standard for the Pleistocene. The stability of the Holocene, which human civilisation was built upon, is totally anomalous. That makes me very concerned about prodding the global climate system with a stick”, as we are now doing.

“I think these results are most sobering when applied to the present, where the few remaining megafauna survivors are some of our most threatened,” adds Gill. “When it comes to the conservation of elephants, rhinos, or tigers, it’s clear that we need to be conserving the genetic diversity that may be critical to their survival through the coming centuries of warming.”

Reference: Cooper, Turney, Hughen, Brook, McDonald, Bradshaw. 2015. Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science http://dx.doi.org/10.1126/science.aac4315

21 thoughts on “Abruptly Warming Climate Triggered Megabeast Revolutions

  1. If 34,000 years ago there was a species die off due to a warming climate at a time when humans were not an influence then why is humans the primary cause today?

  2. Curly4, you question is answered in the article…
    “The stability of the Holocene, which human civilisation was built upon, is totally anomalous. That makes me very concerned about prodding the global climate system with a stick”, as we are now doing.”

  3. Because…
    1) There are 7 Billion more of us
    2) We altered the environment to suit us (cities, dams, etc…) all of which reduce the available habitat for other species
    3) We have specifically altered other animals to suit our needs (breeding different types of dogs, fatter chickens, etc) and introduced animals into environments that were not prepared to handle them (Google “dodo birds”)
    4) As a species we are burning fossil fuels which release a lot of greenhouse gases. Which traps heat. Which increases the Earth’s temperature. Which will inevitably disturb the natural process we have grown accustomed to (warmer oceans will change flow patterns – this might make it easier/harder for certain weather patterns to exist. El Nino is an example I believe)

    Multiple reasons. More than I care to list.

  4. In reference to Curly4’s comment: Humans are primary causes for species die off today as we are removing the ranges and refuges where threatened species live. In addition, we are intentionally killing species where their ranges still exist, e.g. rhinos, elephants, tigers, etc. 300 years ago the grasslands of North America were covered with massive herds of bison and the skies were darkened with flocks of passenger pigeons. Humans destroyed the entire species of what was the most abundant bird population in the world within three generations. It’s really not hard for us to wipe out a species of over 3 billion animals. We did that without even trying.

  5. Pertinent to this story, these days where would they repopulate from? We’ve succeed up the landscape so much that species are no longer as mobile

  6. Curly4 – I don’t know that humans ARE the primary cause today, but given the current and rapidly growing population of humans, we certainly have to be making an impact. I don’t think that any individual is creating too much CO2, but there are simply too many of us. The combining of oxygen and carbon is the engine that drives life. The basic underlying problem is overpopulation. Simple. But not easy to fix. But the environment will take care of it over time. We just won’t like the method.

  7. Curly: Because we’re dumping carbon dioxide into the atmosphere at a massive rate. It’s not like they’re like “oh there’s warming, must be humans”.

  8. I think it likely that exotic warm-climate diseases and parasites, like those that are spreading today as temps rise, are likely culprits here. I am sure that Ed has considered this, but methinks his scientific rigor kept him from speculating without hard data, because unlike mining DNA from widespread bones, soft tissues are usually needed to find bugs and parasites. However, perhaps new studies on ancient dung can shed some light on this.

  9. Curly, there’s these people called “scientists” who look for reasons for climate change. They examine every scenario they can think of. They eliminate them one by one if there’s no evidence to support them. Currently, the only workable evidence for our current warming is the increase in atmospheric CO2. The usual suspects – change in ocean currents, spike in solar activity, quirks in earths orbit – have all been eliminated. These folks are very good at what they do. I would recommend reading the actual abstracts of published studies, because most science writers (You are definitely an exception, Mr. Yong) screw things up and are more interested in inflammatory headlines than actually educating the reader.

  10. In 1956, William L. Thomas, Jr., edited a ecological text (now considered a classic) entitled,
    Man’s Role in Changing the Face of the Earth. I suggest that Curly4 read it and his question will be answered.

  11. The reason that humans are doomed is simply because we forgot that we are megafauna as well. Climate change is now irreversible and the only people who believe otherwise are those who believe we are above Nature and therefore technology can fix the problems that technology created. That has never happened in the history of Mankind and it should be noted that nature builds from the bottom up not top down. Bacteria and cockroaches will be the building blocks of the next era, us – not so much.

  12. The climate responds to whatever the dominate forcing function is at the time. Today that happens to be humans emitting trillions of tons CO2 and increasing the concentration to 400ppm in mere decades, the highest in millions of years.

  13. Humans are the primary cause today due to the rapid increase in (human produced) carbon dioxide and other gases. Higher levels of these increase (rapidly) temperatures. Not really that complicated. Some just don’t want to think about it.

  14. Yeah, so burning huge quantities of coal and oil has absolutely no effect on the very thin layer of gas we live in…this month Mauna Loa records 402PPM. You and all your kind are FI.

  15. There are different causes of global warming, that has indeed happened before, sometimes even before humans had evolved into being humans. But humans are causing this one this time. And causing it to happen more quickly than ever before. This article calls these warmings 34,000 years ago “sudden bursts”, but they still took thousands of years to rise a few degrees – we are causing a rise at least that much in just a few hundred years.

    We know humans are the primary cause, because we’ve known for over a century how much carbon dioxide (CO2) insulates the atmosphere and warms it, and we know how much CO2 we’ve pumped into the atmosphere, especially in the last few generations. We see the warming, we see the sealevel rising, we see the species migrating and going extinct, all the way they would if we were right about the CO2.

    Look, people die every day from many causes. But when we find someone dead with bullet holes, we know the cause, and we know we shouldn’t shoot people.

  16. what about the possibility of the animals loved the cold and sought colder climates during the warmngs and thusly not in area anymore. ???

  17. Curly4, in case you’re sincerely interested in knowing the answer to your question … there have been many natural causes of climate change in the past: changes in the earth’s tilt, volcanoes, etc. etc. etc.

    The difference is that now we can rule out all those “natural” causes. The only thing that explains the data now is *us*.

    If you think I’m wrong then I would suggest you provide a natural explanation that explains the current warming without invoking us as the cause … and don’t provide it to me or in these comments. Provide it to the Koch Brothers … I’m sure they’d pay you millions, if not billions, of dollars if you could provide such an explanation. As for me, I won’t hold my breath…

  18. Matthew you have a good point but you seemed to have missed the statistics in the article that stated. “In each one, average temperatures abruptly rose by anywhere from 4 to 16 degrees Celsius in just a few decades, before gradually falling again over several millennia” This is not the gradual change you posted but a very sudden change. Far worse than what we are seeing now even.

  19. what about HYPERTROPHISM? that is, any living thing which has reached a certain degree of success, tends to reproduce the elements of that success until that same factor finally causes its own downfall. best example, the sabre-tooth ‘tiger’ which finally found its own fangs to be an encumbrance rather than an advantage.
    im always a bit uncomfortable to read that everything can be attributed to the most fashionable theory. yes climate change is a real concern now, but we shouldn’t conflate it with every crisis past or present.

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