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5 Extinctions That Wiped Much of Life off Planet Earth

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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.

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Scientists Study the Starling Invasion Unleashed on America by a Shakespeare Fan

On a warm spring day, the lawn outside the American Museum of Natural History in Manhattan gleams with European starlings. Their iridescent feathers reflect shades of green and indigo—colors that fade to dowdy brown in both sexes after the breeding season. Over the past year, high school students from different parts of the city came to this patch of grass for inspiration. "There are two trees at the corner I always tell them to look at," Julia Zichello, senior manager at the Sackler Educational Lab at the AMNH, recalls to Mental Floss. "There are holes in the trees where the starlings live, so I was always telling them to keep an eye out."

Zichello is one of several scientists leading the museum's Science Research Mentoring Program, or SRMP. After completing a year of after-school science classes at the AMNH, New York City high school students can apply to join ongoing research projects being conducted at the institution. In a recent session, Zichello collaborated with four upperclassmen from local schools to continue her work on the genetic diversity of starlings.

Before researching birds, Zichello earned her Ph.D. in primate genetics and evolution. The two subjects are more alike than they seem: Like humans, starlings in North America can be traced back to a small parent population that exploded in a relatively short amount of time. From a starting population of just 100 birds in New York City, starlings have grown into a 200-million strong flock found across North America.

Dr. Julia Zichello
Dr. Julia Zichello
©AMNH

The story of New York City's starlings began in March 1890. Central Park was just a few decades old, and the city was looking for ways to beautify it. Pharmaceutical manufacturer Eugene Schieffelin came up with the idea of filling the park with every bird mentioned in the works of William Shakespeare. This was long before naturalists coined the phrase "invasive species" to describe the plants and animals introduced to foreign ecosystems (usually by humans) where their presence often had disastrous consequences. Non-native species were viewed as a natural resource that could boost the aesthetic and cultural value of whatever new place they called home. There was even an entire organization called the American Acclimatization Society that was dedicated to shipping European flora and fauna to the New World. Schieffelin was an active member.

He chose the starling as the first bird to release in the city. It's easy to miss its literary appearance: The Bard referenced it exactly once in all his writings. In the first act of Henry IV: Part One, the King forbids his knight Hotspur from mentioning the name of Hotspur's imprisoned brother Mortimer to him. The knight schemes his way around this, saying, "I'll have a starling shall be taught to speak nothing but 'Mortimer,' and give it him to keep his anger still in motion."

Nearly three centuries after those words were first published, Schieffelin lugged 60 imported starlings to Central Park and freed them from their cages. The following year, he let loose a second of batch of 40 birds to support the fledgling population.

It wasn't immediately clear if the species would adapt to its new environment. Not every bird transplanted from Europe did: The skylark, the song thrush, and the bullfinch had all been subjects of American integration efforts that failed to take off. The Acclimatization Society had even attempted to foster a starling population in the States 15 years prior to Schieffelin's project with no luck.

Then, shortly after the second flock was released, the first sign of hope appeared. A nesting pair was spotted, not in the park the birds were meant to occupy, but across the street in the eaves of the American Museum of Natural History.

Schieffelin never got around to introducing more of Shakespeare's birds to Central Park, but the sole species in his experiment thrived. His legacy has since spread beyond Manhattan and into every corner of the continent.

The 200 million descendants of those first 100 starlings are what Zichello and her students made the focus of their research. Over the 2016-2017 school year, the group met for two hours twice a week at the same museum where that first nest was discovered. A quick stroll around the building reveals that many of Schieffelin's birds didn't travel far. But those that ventured off the island eventually spawned populations as far north as Alaska and as far south as Mexico. By sampling genetic data from starlings collected around the United States, the researchers hoped to identify how birds from various regions differed from their parent population in New York, if they differed at all.

Four student researchers at the American Museum of Natural History
Valerie Tam, KaiXin Chen, Angela Lobel and Jade Thompson (pictured left to right)
(©AMNH/R. Mickens)

There are two main reasons that North American starlings are appealing study subjects. The first has to do with the founder effect. This occurs when a small group of individual specimens breaks off from the greater population, resulting in a loss of genetic diversity. Because the group of imported American starlings ballooned to such great numbers in a short amount of time, it would make sense for the genetic variation to remain low. That's what Zichello's team set out to investigate. "In my mind, it feels like a little accidental evolutionary experiment," she says.

The second reason is their impact as an invasive species. Like many animals thrown into environments where they don't belong, starlings have become a nuisance. They compete with native birds for resources, tear through farmers' crops, and spread disease through droppings. What's most concerning is the threat they pose to aircraft. In 1960, a plane flying from Boston sucked a thick flock of starlings called a murmuration into three of its four engines. The resulting crash killed 62 people and remains the deadliest bird-related plane accident to date.

Today airports cull starlings on the premises to avoid similar tragedies. Most of the birds are disposed of, but some specimens are sent to institutions like AMNH. Whenever a delivery of dead birds arrived, it was the students' responsibility to prep them for DNA analysis. "Some of them were injured, and some of their skulls were damaged," Valerie Tam, a senior at NEST+m High School in Manhattan, tells Mental Floss. "Some were shot, so we had to sew their insides back in."

Before enrolling in SRMP, most of the students' experiences with science were limited to their high school classrooms. At the museum they had the chance to see the subject's dirty side. "It's really different from what I learned from textbooks. Usually books only show you the theory and the conclusion, but this project made me experience going through the process," says Kai Chen, also a senior at NEST+m.

After analyzing data from specimens in the lab, an online database, and the research of previous SRMP students, the group's hypothesis was proven correct: Starlings in North America do lack the genetic diversity of their European cousins. With so little time to adapt to their new surroundings, the variation between two starlings living on opposite coasts could be less than that between the two birds that shared a nest at the Natural History Museum 130 years ago.

Students label samples in the lab.
Valerie Tam, Jade Thompson, KaiXin Chen and Angela Lobel (pictured left to right) label samples with Dr. Julia Zichello.
©AMNH/C. Chesek

Seeing how one species responds to bottlenecking and rapid expansion can provide important insight into species facing similar conditions. "There are other populations that are the same way, so I think this data can help [scientists],” Art and Design High School senior Jade Thompson says. But the students didn't need to think too broadly to understand why the animal was worth studying. "They do affect cities when they're searching for shelter," Academy of American Studies junior Angela Lobel says. “They can dig into buildings and damage them, so they're relevant to our actual homes as well.”

The four students presented their findings at the museum's student research colloquium—an annual event where participants across SRMP are invited to share their work from the year. Following their graduation from the program, the four young women will either be returning to high school or attending college for the first time.

Zichello, meanwhile, will continue where she left off with a new batch of students in the fall. Next season she hopes to expand her scope by analyzing older specimens in the museum's collections and obtaining bird DNA samples from England, the country the New York City starlings came from. Though the direction of the research may shift, she wants the subject to remain the same. "I really want [students] to experience the whole organism—something that's living around them, not just DNA from a species in a far-away place." she says. "I want to give them the picture that evolution is happening all around us, even in urban environments that they may not expect."

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Meet the Tully Monster, the Prehistoric Beast That Defies Categorization
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During the Carboniferous Period—a geologic period that lasted from about 359.2 to 299 million years ago, during the late Paleozoic Era—the world was filled with frightening, foreign, and funny-looking creatures. One of the most puzzling of these prehistoric animals may have been Tullimonstrum, better known as the Tully monster.

The Tully monster was a sea creature that resembled a giant worm crossed with a small squid. Out of its head jutted a long, skinny appendage with a hook that may have been a mouth, and its eyes were attached to fleshy stalks.

As bizarre as these features might appear to the modern eye, scientists in recent years have focused more on whether or not the Tully monster had a backbone. This defining feature could help them figure out the creature's evolutionary relationship to its ancestors and descendants. But so far, as PBS Eons explains below, experts are still at a loss for where the Tully monster fits into the tree of life.

Learn more about the mysterious Tully monster below:

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