The Sixth Extinction

The preliminaries

• 200,000 years ago, a new species emerged from eastern Africa. Homo sapiens weren’t that strong but they were resourceful. Strangely, none of the usual geographical constraints checked them—they expanded, crossed paths with creatures twenty times their size, and conquered them.
  • This continued for thousands of years; meanwhile, Homo sapiens reproduced at an unprecedented rate.
  • Humans then discovered and used subterranean reserves of energy and change the atmosphere’s composition. This drastically altered the climate and ocean chemistry; extinction rates soared.
• Earth has undergone some changes so wrenching that the diversity of life plummets to nearly zero. Five were so catastrophic that they’re in their own category: the Big Five.
  • We are now witnessing, and causing, the Sixth.

Atelopus zeteki – Panamanian golden frog – Mass v. background extinction

• Toxic golden frogs were once easy to spot in Panama. Then, the frogs began to disappear and, by 2002, they were effectively wiped out.
  • Mystifyingly, frogs, rare and not rare, vanished not only from populated areas but also from pristine places.
• Researchers discovered a strange microorganism on their skin, which was eventually identified as a chytrid fungus.
  • Chytrid fungi can be found on treetops and deep underground, but this particular species had never been seen before.
  • The fungus interferes with the frogs’ ability to absorb electrolytes through their skin, which causes a heart attack. It’s likely that humans, through easy travel from continent to continent, brought the foreign fungus.

The take-away

• Any event that has occurred only five times in 500 million years is exceedingly rare.
  • Extinction is rare and it occurs at what’s known as the background extinction rate.
    • Often it’s in terms of extinctions per million species-years. For instance, with mammals, it’s around 0.25 per million species-years.
    • This means that of the 5,500 mammal species today, the background extinction rate would be one species to disappear every 700 years.
• Mass extinctions are different—they eliminate a significant proportion of the world’s flora and fauna in a geologically insignificant amount of time.
  • As for amphibians, they should go extinct every thousand years or so. The odds of an individual witnessing such an event should be zero, but almost every reptile/amphibian specialist working in the field has seen several go extinct.
  • Amphibians are the world’s most endangered class of animals; the group’s extinction rate is 45,000x higher than the background rate.
    • Extinction rates among other groups are approaching amphibian levels.
    • ⅓ of all reef-building corals, ⅓ of all freshwater mollusks, ⅓ of sharks and rays, ¼ of mammals, ⅕ of reptiles, and ⅙ of birds are headed to oblivion.
• Here, with the frogs, the human effect has caused foreign entities to travel as they never have before, short-circuiting their evolutionary ability to defend themselves.

Mammut americanum – American mastodon – Extinction as a concept

• A person found an enormous molar in New York in 1705: the “tooth of a Giant.”
  • People found similar molars, as well as other bones, and the teeth didn’t fit into existing categories, though the rest looked like they belonged to elephants.
• Naturalist Jean-Leopold-Nicolas-Frederic Cuvier taught at the Paris Museum of Natural History in 1795.
  • He determined that the mastodon had been wiped out 6,000 years ago when the mammoth (different species) died off. 
  • In fact, the mastodon disappeared around 13,000 years ago as part of a wave of disappearances known as the megafauna extinction. This coincided with the spread of Homo sapiens.

The take-away

• Extinction emerged as a concept in revolutionary France, largely thanks to Cuvier and the mastodon.
  • Cuvier had conceived of a whole new way of looking at life: species died out and this was a widespread phenomenon.

• Cuvier began collecting fossils and soon had 23 species he deemed extinct, including a pygmy hippo, an elk, a large bear, and a giant amphibian.
  • Many bore some relationship to species still alive.
• But what about a bizarre fossil from Bavaria? It looked like a flying reptile, which he named ptero-dactyle, meaning “wing-fingered.”

• The study of stratigraphy (the analysis of the order and position of layers of archaeological remains) was just beginning, but people understood that different layers of rocks had been formed during different periods.
  • The plesiosaur, ichthyosaur, and ptero-dactyle were all in Mesozoic-era deposits.
  • This led to another extraordinary insight: extinction had a direction.
    • Lost species whose remains could be found near the surface, like mastodons and cave bears, belonged to orders of creatures still alive. 
    • Farther back were creatures with no obvious modern counterparts. Keep digging and mammals disappeared; reptiles dominated the world.
• As Cuvier’s list of extinct species grew, he decided there had been multiple cataclysms.
  • The changes that had caused extinctions must have been of a great magnitude—so great that animals were unable to cope.

Pinguinus impennis – The great auk

• The great auk was a large bird, covered with brown feathers, that couldn’t fly. It ranged from Norway to Newfoundland to Florida and was regularly eaten for dinner. 
• They were great swimmers but during the breeding season they waddled ashore and were highly vulnerable.
• Humans plundered their breeding sites and eventually slaughtered them all – the last known pair were killed in 1844.

The take-away

• Charles Lyell, a young geologist, thought Cuvier’s vision of the earth was unpersuasive.
  • Lyell thought every natural feature was the result of very gradual processes operating over many millennia. He believed that all organisms had existed in all eras, and those that had vanished could pop up again.
• Charles Darwin read Lyell’s work while on the HMS Beagle.
  • Toward the end of the voyage, Darwin encountered coral reefs, which provided him with his first major breakthrough: the interplay between biology and geology.
    • If a reef formed around an island or along a continental margin that was slowly sinking, the corals, by growing slowly upward, could maintain their position relative to the water.
    • As the land subsided, the corals would form a barrier reef.
• To Darwin, natural selection eliminated the need for creative miracles. His theory of how species originated was also a theory of how they vanished. The struggle for existence rewarded the fit and eliminated the less so.
  • The theory of natural selection is grounded on the belief that each new species is maintained by having some advantage over those it competes with.
  • Auks, evolutionarily unprepared for humans, failed to survive.

Impact hypothesis

• In the 1970s, geologist Walter Alvarez discovered the first traces of the giant asteroid that ended the Cretaceous period.
  • Isabella Silva, another geologist, pointed out that the limestone from that period’s last stage contained diverse, abundant, and relatively large foraminifera (tiny marine creatures with calcite shells).
    • Above that was a layer of clay with nothing. And above that was a layer of clay with foraminifera, all very tiny and completely different from the larger ones below.
  • So, species appeared then disappeared then reappeared in an abrupt process. The large ones vanished when the last of the dinosaurs died off. How much time did that half-inch of clay represent?
• Alvarez’s father Luis (a Nobel Prize winner who’d developed the first linear proton accelerator and co-discovered tritium) came up with the idea using the element iridium.
  • Iridium is rare on earth but common in meteorites. Bits of it constantly fall on the planet. The longer it had taken the clay layer to accumulate, the more cosmic dust would have fallen.
    • When they tested the clay, the amount of iridium was off the charts.
• Soon, they formed the impact hypothesis. One day 65 million years ago, an asteroid six miles wide collided with Earth. It exploded, releasing a hundred million megatons of TNT worth of energy. Debris, including iridium from the asteroid, spread around the planet.
• The Alvarezes believed the main cause of extinction wasn’t the impact but the dust.
  • The asteroid was moving around 45,000 mph, and an enormous cloud of searing vapor raced over North America, incinerating anything in its path.
  • The asteroid blasted vast quantities of rock in the air, the particles lit up the sky everywhere at once, and generated enough heat to broil the surface of the planet.
  • The dust was rich in sulfur, and sulfate aerosols blocked sunlight.
  • Then came a long, severe winter that decimated forests and collapsed marine ecosystems. On land, every animal larger than a cat seems to have died out, including dinosaurs.

Anthropocene

• This current age is “Anthropocene,” a human-dominated epoch.
  • Human activity has transformed ½ of Earth’s land surface. Most of the world’s major rivers have been dammed or diverted.
  • Fisheries remove more than ⅓ of the primary production of the oceans. Humans use more than ½ of the world’s readily accessible fresh water runoff.
  • People have altered the composition of the atmosphere, and climate is likely to depart significantly from natural behavior.
  • Humans have rearranged the planet, transporting the flora and fauna of Asia to the Americas, of Oceania to Europe, and so on.
• The world is changing in ways that compel species to move, but it’s also changing in ways that create barriers that prevent them from doing so.

Ocean chemistry

• Castello Aragonese is a tiny island west of Naples.
  • In the sea around it, there are streams of gas bubbling out of vents, and the gas is almost 100% CO₂, which dissolves in water to form an acid. Because of all this CO₂, the waters provide a near-perfect preview of the future for oceans.
  • The vents produce a pH gradient. On the eastern edge of the island, the waters are “normal”—the present. Closer to the vents, the acidity increases and pH declines—the future.
  • In the waters far from the vents, there are lots of sponges, fish, sea urchins, seaweed, etc.
  • Closer to the vents, a common barnacle is gone. A common mussel is gone. Common seaweed, worms, coral, snails, and mollusks are all gone.
  • Around ⅓ of the species found in the vent-free zone were gone near the vents.
• Since the start of the industrial revolution, humans have burned enough fossil fuels (coal, oil, and natural gas) to add 365 billion metric tons of carbon to the atmosphere. Deforestation has added another 180 billion tons.
  • As a result, the concentration of CO₂ in the air today is higher than at any other point in the last 800,000 years.
  • This will produce an average global temperature rise of between 3.5 – 7 degrees Fahrenheit, which will trigger world-altering events, including the disappearance of most glaciers and the flooding of coastal cities.
• Oceans cover 70% of the earth’s surface. When water and air come into contact, there’s an exchange. The ocean absorbs gas from the atmosphere, and gas dissolved in the ocean is released into the atmosphere.
  • Roughly the same quantities are being dissolved as are being released. But if the atmosphere’s composition changes, the exchange becomes lopsided: more CO₂ enters the water than comes back out.
  • Because of all this extra CO₂ , the oceans are now 30% more acidic than they were in 1800, and will continue to acidify.
• No single mechanism explains all the mass extinctions in the record, although drastic changes in ocean chemistry has been a good predictor.
  • Oceans have absorbed roughly ⅓ of the CO₂  pumped into the air—around 150 billion metric tons.
  • Just as it makes a big difference to your blood chemistry whether you take a month to go through a six-pack or an hour, it makes a big difference to marine chemistry whether CO₂ is added over a million years or a hundred.
    • As it is, things are moving too fast for organisms to adapt.

Acropora millepora – Coral reefs – Calcification rates

• Coral reefs are part animal, part vegetable, and part mineral, at once teeming with life and mostly dead.
  • Researchers don’t think reefs will outlast the Anthropocene; it’s likely to be the first major ecosystem in the modern era to become ecologically extinct. 

The take-away

• Researchers have been trying to see how calcification rates on reefs have changed over the decades.
  • Corals are deeply sensitive to the saturation state with respect to calcium carbonate—a measure of the concentration of calcium and carbonate ions floating around.
    • When CO₂  dissolves in water, it forms carbonic acid, which eats carbonate ions, lowering the saturation state.
• There’s a linear relationship between the coral’s growth rate and water’s saturation state.
  • Before the Industrial Revolution, major reefs were in water with a saturation state between four and five. Today, there’s almost no place left where the saturation state is above four, and by 2100 there will be none above three.
  • Eventually, saturation levels may drop so low that corals quit calcifying altogether.
• When water temperatures rise past a certain point, the symbiotic relation between the corals and their dependants breaks down and they stop growing.

Species-area relationship

• One ecology rule is the “species-area relationship,” or SAR. The larger the area sampled, the greater the number of species encountered. The correlation is not linear, but it’s a curve that slopes in a predictable way.
  • Usually, the formula is S = cA^z where S is the number of species, A is the size of the area, and c and z are “constants” that vary according to the region and taxonomic group. The ratio holds no matter the terrain.
  • In terms of extinction, SAR is key—humans constantly change the value of A (e.g., taking land from animals or making it uninhabitable).
  • With minimum warming, 9% – 13% of all species currently alive will be extinct by 2050. With maximum warming, it will be 21% to 32%.
• Global warming restructures ecological communities. Different species will respond differently; some will die, some will thrive.
  • With well-placed reserves, many threats to biodiversity (illegal logging, mining, ranching) could be minimized.

Myotis lucifungus – Little brown bat

• In 2007, biologists in NY conducting a bat census in a cave found dead bats everywhere; it looked like their noses were dunked in powder. The same thing happened the following winter in 33 caves in 4 different states.
  • The theory is that the fungus eats at their skin, irritating them enough to wake them from hibernation, and they use up the fat that was supposed to last through the winter.
• When a new pathogen appears, new hosts have no defenses.
  • Scientists suspected that whatever killed the bats was something never encountered before because of the high mortality rate, so it was likely an introduced species.
  • The fungus has been traced to Euopre, where it is widespread. The bats there carry it unbothered, which suggests that they and the fungus evolved in tandem.

The take-away

• Without human interference, for most species, long-distance travel is practically impossible.
  • In the 19th century, paleontologists found odd fossils on different continents. For instance, Mesosaurus turned up in Africa and South America. How could it have crossed oceans? 
  • Alfred Wegener, a German meteorologist, thought that the continents must have shifted and separated over millions of years. At one time, continents formed a supercontinent, Pangaea.
• Now, global trade and travel limit the remoteness of even the remotest islands. 
• Two different things can happen when a new organism shows up.
  • First, nothing. The new species doesn’t survive.
  • Second, not only does the introduced organism survive, but it also gives rise to a new generation–”establishment.” And then it spreads.
    • Out of every 100 potential introductions, between 5 and 15 will establish.

Homo neanderthalensis

• In 1856, Neanderthal bones were discovered, which have since turned up all over Europe and the Middle East. They lived in Europe for at least 100,000 years. Then, around 30,000 years ago, the Neanderthals vanished.
  • Modern humans arrived in Europe around 40,000 years ago and as soon as they made their way to a region where Neanderthals lived, Neanderthals disappeared.
• Svante Paabo, an evolutionary anthropologist in Germany, basically invented the study of ancient DNA, and is intent on sequencing the entire Neanderthal genome.
  • When the project is completed, it should be possible to lay the human genome and the Neanderthal genome side by side and identify exactly where they diverge. They were probably our closest relatives, and somewhere in our DNA lies the key mutation.
  • Soon, researchers discovered something odd.
    • Some sequences were more similar to some humans than to others. Specifically, Europeans and Asians shared more DNA with Neanderthals than did Africans.
    • The “Out of Africa” theory states that all modern humans are descended from a small population that lived in Africa 200,000 years ago.
      • It appears that, before modern humans wiped out the Neanderthals, they had sex with them, and these children populated Europe, Asia, and the New World.
• One of the most basic ways modern humans differ from Neanderthals is how we treat obstacles. Neanderthals stopped when they reached water or a similar barrier, while humans pushed forward.
  • Paabo believes there must be a gene for this particular brand of madness: overcoming seemingly insurmountable obstacles.

Homo sapiens

• Life is extremely resilient but not infinitely so. The causes of these extinctions are varied: glaciation, global warming, and asteroids.
  • We are in the midst of the Sixth Extinction, this time caused solely by humanity’s transformation of the ecological landscape.
  • One possibility is that we will eventually be undone by our actions.
    • We freed ourselves from evolutionary constraints but remain dependent on Earth’s biological and geochemical systems. By disrupting these systems (altering the atmosphere, felling rainforests, acidifying oceans) we’ve put our own survival in danger.

• How biodiversity can be tracked via army ants, Eciton burchellii
• The disadvantages of being an enormous animal, and how small bands of Homo sapiens managed to wipe out nearly all enormous animals across many continents
• The Sumatran rhino and the very incredible things being done to save it from extinction
• How trees migrate in accordance with climate
• The story that ammonite fossils tell about life in the water millions of years ago
• The devastating glaciation that took place 444 million years ago and how it nearly wiped out all life on Earth
• How biodiversity drops off with isolation and why that matters in a world where humans have created barriers to creatures attempting to escape isolation

• Details about the end-Permian extinction: 252 million years ago, there was a massive release of carbon into the air—temperatures soared, the seas warmed by as much as 18 degrees, and when it was over, 90% of all species had been eliminated
• The immense damage that climate change has wrought on the tropics