CLOSE
Original image
Kevin Walsh via Flickr // CC BY 2.0

5 Extinctions That Wiped Much of Life off Planet Earth

Original image
Kevin Walsh via Flickr // CC BY 2.0

by Aliya Whiteley

The more we get to know about the history of the Earth, the more incredible it becomes. Our planet formed about 4.5 billion years ago, and for the first billion years it was without life. Then organic molecules began to form simple cells.

It’s tempting to think that from those first cells the business of evolution took hold and created the plants and animals we see today, but this simplified version overlooks some of the most catastrophic developments that happened along the way. Five mass extinction events have wiped out nearly every living thing on this planet. So the next time you’re feeling less than brave, remind yourself that you are descended from some seriously tough survivors. You’re already one of nature’s great success stories.

1. ORDOVICIAN-SILURIAN EXTINCTION—445 MILLION YEARS AGO

Most life forms were still living in the oceans at the time of the first mass extinction. There are many theories as to how that happened: global cooling that brought on an ice age, volcanic gases, or maybe changes in ocean chemistry. Whatever the cause, about 85 percent of species were wiped out.

2. LATE DEVONIAN EXTINCTION—375 MILLION YEARS AGO

The oceans recovered and teemed with life once more, and that diversity had begun to spread onto land at the time of the second mass extinction, when 79–87 percent of all species died due to environmental change. A series of several extinction events spread over approximately 40 million years wiped out most of the life on earth. The cause is unclear, but some scientists have theorized that the sudden increase in plant life could have triggered a period of anoxia (lack of oxygen). Other suggestions include volcanic eruptions on a huge scale, or another ice age.

Although plants may have triggered the destruction, it was the marine life that was hardest hit. Armored fish died out completely. Reef ecosystems vanished from the seas and were not seen again for the next 100 million years. But there were some who benefited: Into these gaps in the oceans’ ecosystems came some of nature’s hardiest survivors—the sharks.

3. PERMIAN-TRIASSIC EXTINCTION—250 MILLION YEARS AGO

This is also known as the Great Dying, and with good reason: 70 percent of land species and 90 percent of marine species disappeared, including half of all marine families. Plant life also suffered; only a few forests remained. It’s the only event in which insects also died out en masse. The devastation to life was so thorough, this mass extinction event is known as the Great Dying.

The culprit was, once again, environmental change. An enormous volcanic event in an already hot, dry climate led to a massive increase in carbon dioxide, and as ice sheets melted, methane escaped into the atmosphere, adding to the problem. These greenhouse gases led to the creation of anoxic conditions in marine habitats once more.

4. END-TRIASSIC EXTINCTION—200 MILLION YEARS AGO

After the Great Dying, it took approximately 20 million years for the Earth to recover. Unfortunately, soon after the Earth returned to its previous level of diversity, the next mass extinction came along and nearly wiped out the dinosaurs just as they were getting started. But it was the mammal groups who really suffered this time around, along with large amphibians: 76 to 84 percent of all species died out. The culprit may have once again been volcanic activity.

But dinosaurs managed to recover remarkably well, becoming the dominant creatures on the planet after this particular extinction event. And so they might well have remained, if it wasn’t for what happened next …

5. END-CRETACEOUS MASS EXTINCTION—66 MILLION YEARS AGO

This is the event we all know about. Many experts theorize that a large asteroid hit the Earth and contributed to rapid environmental changes. Sea levels plummeted, volcanic activity threw ash and poisonous gases into the air, and 71 to 81 percent of all species died. All non-avian dinosaurs perished, leaving the way clear for the small mammals that managed to survive.

BONUS: HOLOCENE EXTINCTION—10,000 BCE to ONGOING

And here we are today, having evolved from those small mammals. Are we in the grip of the sixth mass extinction of life on our planet? It's unclear how many species we're losing annually—one widely cited estimate is 140,000 species per year [PDF]—but it’s difficult to be sure of the size of the problem, as less than 3 percent of species on the planet are thought to have been formally assessed for risk.

The growth of humanity may be causing a loss of biodiversity, but the good news is that we have developed to the point where we might be able to do something about our own impact on the planet. We’re already aware of the problem—and there might even still be time to fix it.

Original image
PiccoloNamek, Wikimedia Commons // CC BY-SA 3.0
arrow
Animals
How Just One Nasty Winter Forced This Lizard to Evolve
Original image
PiccoloNamek, Wikimedia Commons // CC BY-SA 3.0

Scientists say a single unusually cold season altered the course of history for one American reptile. The green anoles who survived the winter of 2014 were those who could stand the cold—a trait they passed on to their offspring. The researchers published their findings in the journal Science.

The green anole, Anolis carolensis, also known erroneously as the American chameleon, is a vibrant little lizard that makes its home in the southeastern U.S. and Caribbean. Its range extends from Texas as far north as Oklahoma. This is unusual for reptiles, whose cold-blooded bodies typically restrict them to balmier climates.

Map of the geographic range of the green anole.
Graphic by Julie McMahon

To find out how the anoles were managing it, Shane Campbell-Staton, now of the University of Illinois, paid visits to five scattered populations in 2013. He collected samples and a few live lizards from each group to test their DNA, gene expression, and tolerance for low temperatures.

He found a fair amount of variation between lizard communities. Those in Oklahoma had clearly evolved to handle the weather there, while specimens from further south couldn't take the cold.

Satisfied with his data and findings, Campbell-Staton prepared to wrap up the project.

Then winter came. You may remember the winter of 2014, when a polar vortex created record-breaking low temperatures and wrought terrible storms across the U.S., including in anole territory. Campbell-Staton couldn't help but wonder how—or if—the cold-intolerant lizards had survived.

The next spring and summer, he and his colleagues made another circuit through anole country and collected more samples. The Oklahoma families hadn't fared too poorly. But down south, things had clearly changed. The genetic code of Texan lizards looked more like their northern cousins, and individuals were far better at handling a chill. 

The research team realized that the brutal winter had killed off most of the cold-intolerant lizards, leaving behind only those who happened to have genes more like their northern cousins'. Those lizards reproduced, creating new generations of cold-ready individuals.

But that's not necessarily a good thing.

"One might think, 'Oh, they responded! They're better now,'" Campbell-Staton said in a statement. "But selection always comes at a cost, which is death, basically. It may be that the animals that did not survive this storm had the genetic variants to survive a heat wave, or a drought, or some other extreme event. And now those lineages are essentially gone." 

Original image
iStock
arrow
science
Scientists Think They Know What Causes Trypophobia
Original image
iStock

Picture a boat hull covered with barnacles, a dried lotus seed pod, milk bubbles on a latte, or a honeycomb. Images of these objects are harmless—unless you're one of the millions of people suffering from trypophobia. Then they're likely to induce intense disgust, nausea, and fear, and make your skin crawl.

Coined fairly recently, the term trypophobia describes the fear of clusters of holes. The phobia isn’t recognized by the Diagnostic and Statistical Manual of Mental Disorders, but its visibility on the internet suggests that for many, it’s very real. Now, scientists in the UK think they've pinpointed the evolutionary mechanism behind the reaction.

Tom Kupfer of the University of Kent and An T. D. Le of the University of Essex shared their findings in the journal Cognition and Emotion. According to their research, trypophobia evolved as a way to avoid infectious disease. Thousands of years ago, if you saw a person covered in boils or a body covered in flies, a natural aversion to the sight would have helped you avoid catching whatever they had.

But being disgusted by skin riddled with pathogens or parasites alone doesn't mean you're trypophobic; after all, keeping your distance from potential infection is smart. But trypophobia seems to misplace that reaction, as the authors write: "Trypophobia may be an exaggerated and overgeneralized version of this normally adaptive response."

Lotus pod.
Lotus seed pods are a common trigger of trypophobia.

This explanation is not entirely new, but until now little research has been done into whether it's accurate. To test their hypothesis, the scientists recruited 376 self-described trypophobes from online forums, and another 304 college students who didn't claim to have the affliction. Both groups were shown two sets of images: The first depicted clusters of circle-shaped marks on animals and human body parts (the "disease-relevant cluster images"); the second showed clusters of holes on inanimate objects like bricks and flower pods ("disease-irrelevant cluster images"). While both groups reported feeling repulsed by the first collection of photographs, only the trypophobes felt the same about the pictures that had nothing to do with infection.

Another takeaway from the study is that trypophobia is more related to sensations of disgust than fear. This sets it apart from more common phobias like arachnophobia (fear of spiders) or acrophobia (fear of heights). And you don't have to be trypophobic to be disgusted by a video of Suriname toadlets being born through holes in their mother's back. We can all be grossed out by that.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios