Natalie Angier

GAITHERSBURG, Md. — On a laboratory bench at the National Institute of Standards and Technology was a square tray with two black disks inside, each about the width of the top of a Dixie cup. Both disks were undeniably black, yet they didn’t look quite the same.

Solomon Woods, 49, a trim, dark-haired, soft-spoken physicist, was about to demonstrate how different they were and how serenely voracious a black could be.

“The human eye is extraordinarily sensitive to light,” Woods said. Throw a few dozen photons its way, a few dozen quantum-sized packets of light, and the eye can readily track them.

Woods pulled a laser pointer from his pocket. “This pointer,” he said, “puts out 100 trillion photons per second.” He switched on the laser and began slowly sweeping its bright beam across the surface of the tray.

On hitting the white background, the light bounced back almost unimpeded, as rude as a glaring headlight in a rearview mirror.

The beam moved to the first black disk, a rondel of engineered carbon now more than a decade old. The light dimmed significantly, as a sizable tranche of the incident photons were absorbed by the black pigment, yet the glow remained surprisingly strong.

Finally Woods trained his pointer on the second black disk, and suddenly the laser’s brilliant beam, its brash photonic probe, simply disappeared. Trillions of light particles were striking the black disk, and virtually none were winking back up again. It was like watching a circus performer swallow a sword or a husband “share” your plate of french fries: Hey, where did it all go?

NIST disk number two was an example of advanced ultra-black technology: elaborately engineered arrays of tiny carbon cylinders, or nanotubes, designed to capture and muzzle any light they encounter. Blacker is the new black, and researchers here and abroad are working to create ever more efficient light traps, which means fabricating materials that look ever darker, ever flatter, ever more ripped from the void.

The NIST ultra-black absorbs at least 99.99% of the light that stumbles into its nanotube forest. But scientists at the Massachusetts Institute of Technology reported in September the creation of a carbon nanotube coating that they claim captures better than 99.995% of the incident light.

“The blackest black should be a constantly improving number,” said Brian Wardle, a professor of aeronautics and astronautics and an author on the new report. “Folks will find other materials that are blacker than ours.”

It’s not a mere ego-driven dance of the decimal point. The more fastidious and reliable the ultra-black, the more broadly useful it will prove to be — in solar power generators, radiometers, industrial baffles and telescopes primed to detect the faintest light fluxes as a distant planet traverses the face of its star.

The Color of Cleverness, and Rage

Blacker beauties canter through the natural world, too. Biologists lately have identified cases of superblack coloration in birds, spiders and vipers that go far beyond the standard melanin-based pigments of a crow’s plumage or a black cat’s fur, and vie with lab-grown carbon nanotubes in their structural complexity and power to conquer light.

Psychologists have gathered evidence that black is among the most metaphorically loaded of all colors and that we absorb our often contradictory impressions about black at a young age.

Reporting earlier this year in the Quarterly Journal of Experimental Psychology, Robin Kramer and Joanne Prior of the University of Lincoln in the United Kingdom compared color associations in a group of 104 children, aged 5 to 10, with those of 100 university students.

The researchers showed subjects drawings in which a lineup of six otherwise identical images differed only in some aspect of color. The T-shirt of a boy taking a test, for example, was switched from black to blue to green to red to white to yellow. The same for a businessman’s necktie, a schoolgirl’s dress, a dog’s collar, a boxer’s gloves.

Participants were asked to link images with traits. Which boy was likeliest to cheat on the test? Which man was likely to be in charge at work? Which girl was the smartest in her class, which dog the scariest?

Again and again, among both children and young adults, black pulled ahead of nearly every color but red. Black was the color of cheating, and black was the color of cleverness. A black tie was the mark of a boss, a black collar the sign of a pit bull. Black was the color of strength and of winning. Black was the color of rage.

“We have strong opinions about black and red,” Kramer said, “and that doesn’t seem to be true for any other color.”

The contrariness of black has long been expressed in our clothing. As the color best able to hide stains and dirt, black was the color of the laboring classes and of the pious: people who sought to signal their disinterest in personal vanity and worldly affairs.

“Black was the color of modesty,” said Steven Bleicher, author of “Contemporary Color: Theory and Use” and a professor of visual arts at Coastal Carolina University. “You still see that today across cultures, in Hasidic Judaism, where people wear all black, or the Amish.”

Black took on an air of cultured urbanity beginning in the Renaissance, when so-called sumptuary laws limited the wearing of rich colors like red and purple to the aristocracy. Newly prosperous merchants, lawyers, scholars and other professionals responded by donning luxurious black outfits of velvet, silk and fine wool, which also proved ideal for the display of gold accessories and brocade. Before long, aristocrats were wild for black clothing, too.

As clothes lost their drapiness and began hugging the body, people discovered another benefit of black. “It’s slimming,” Bleicher said. “And so we have the little black dress.” Not to mention James Bond’s Euro-cut black tuxedo and Peter Fonda’s black leather pants.

For artists, black is basal and nonnegotiable, the source of shadow, line, volume, perspective and mood. “There is a black which is old and a black which is fresh,” Ad Reinhardt, the abstract expressionist artist, said. “Lustrous black and dull black, black in sunlight and black in shadow.”

So essential is black to any aesthetic act that, as David Scott Kastan and Stephen Farthing describe in their scholarly yet highly entertaining book, “On Color,” modern artists have long squabbled over who pioneered the ultimate visual distillation: the all-black painting.

Was it Russian constructivist Alexander Rodchenko, who in 1918 created a series of eight seemingly all-black canvases? No, insisted American artist Barnett Newman: Those works were very dark brown, not black. He, Newman, deserved credit for his 1949 opus, “Abraham,” which in 1966 he described as “the first and still the only black painting in history.”

But what about Kazimir Malevich’s “Black Square” of 1915? True, it was a black square against a white background, but the black part was the point. Then again, English polymath Robert Fludd had engraved a black square in a white border back in 1617.

Clearly, said Alfred H. Barr Jr., the first director of the Museum of Modern Art, “Each generation must paint its own black square.”

Or its own superblack polyhedron. Artists today are experimenting with the new carbon nanotube coatings, to plumb such evergreen themes as the nature of light, absence, perception, deception and jewelry.

Diemut Strebe, an artist in residence at MIT, collaborated with Wardle on a project that would merge carbon at its most absorptive configuration, in the form of carbon nanotubes, with carbon in its most reflective and refractive state, as a diamond. How about if we smother a diamond in a layer of ultra-black carbon nanotubes, Strebe suggested, and watch its facets disappear?

“It was an exploration of a Heraclitean principle,” Strebe said. “The extreme opposites of how carbon behaves on exposure to light.”

One of their biggest challenges: finding a jeweler willing to lend them a chunky diamond that would be plastered with what amounts to high-tech soot.

“I tried many companies, Tiffany, others,” Strebe said. “I got many no’s.” Finally, L.J. West Diamonds, which specializes in colored diamonds, agreed to hand over a $2 million, 16.78-carat yellow diamond, provided the process could be reverse-engineered and the carbon nanotube coating safely removed.

The resulting blackened bling is on view at the New York Stock Exchange, which Strebe calls “the holy grail of valuation.”

Scamming the Eye

The key to true ultra-blackness is creating a material that absorbs light across the electromagnetic spectrum — not just visible light, but out to the far infrared, too.

To manage the task, explained John Lehman, an applied physicist and master ultra-black-smith at NIST’s campus in Boulder, Colorado, you take a carbon source like graphite and a metal like iron or nickel to serve as template and catalyst, and you cook them together in an oxygen-free setting to a temperature of about 1,400 degrees Fahrenheit.

As the graphite heats up, it saturates the ringlike structure of the metal and starts to push upward into a vertical array of hollow cylinders, each some billionth of an inch thick — the carbon nanotubes.

The final height, density and distribution of those nanotube trees in your nanotube bosk will determine how effectively your material can imprison photons and incorporate their energy into its constituent parts, and hence how extravagantly black it will appear.

“We start with the intrinsic properties of graphite, which already is pretty black,” Lehman said. “Then we essentially make a lot of little cavities for the light to bounce around in so the photons have a chance to be absorbed by the graphite.”

A similar interplay between chemistry and physics explains the newly discovered ultra-blacks in nature.

As Dakota McCoy of Harvard University and her colleagues have reported in Nature Communications and the Royal Society Proceedings B, the feathers of some species of birds-of-paradise and the decorative patches on peacock spiders rival the luxurious blackness of a lab-grown carbon nanotube jacket, reflecting well under 0.5% of the light cast upon them.

The researchers determined that, in addition to being flush with the dark pigment melanin, the superblack body parts in both the birds and the spiders had an unusual microstructure.

“If you’re a biologist, you know what a feather should look like,” said McCoy, who is completing her doctorate. But on examining a black feather from a bird of paradise under a microscope, “I almost fell out of my chair.”

Rather than lying in a flat, smooth plane, as normal feathers do, the dense, tiny branches of this feather curved upward by 30 degrees and were edged with spikes. That bristling structure, the researchers showed, created cavities of an ideal size and shape for trapping light.

The peacock spider, by contrast, builds its superblackness convexly. Cannily placed bumps on its cuticle channel incident light toward melanin-rich patches primed to absorb it.

In both birds and spiders, the animal superblacks seem to be part of a masculine ruse. The blacks are always next to bright colors: the vivid splashes of teal, yellow, lime-green, violet and electric blue that males must flaunt in their mating displays.

By aggressively absorbing light in the areas surrounding the colorful bits, the superblacks stanch the sort of visual cues the female might use to judge the relative brightness of the ambient light. Without such comparative information, the female can only conclude the male’s colors are better than brilliant: They’re lit from within.

Little black dresses are slimming — and little black feathers scam the eye.

This article originally appeared in

(Science Times) ; (Basics)

Ultra-Black Is the New Black

Most female flies take a low-rent approach to parenthood, depositing scores of seed-sized eggs in the trash or on pet scat to hatch, leaving the larvae to fend for themselves.

Not so for the female tsetse fly. She gestates her young internally, one at a time, and gives birth to them live. When each extravagantly pampered offspring pulls free of her uterus after nine days, fly mother and child are pretty much the same size.

“It’s the equivalent of giving birth to an 18-year-old,” said Geoffrey Attardo, an entomologist who studies tsetse flies at the University of California, Davis.

The newborn tsetse fly looks like a hand grenade and moves like a Slinky, and if you squeeze it too hard the source of its plumpness becomes clear — or rather a telltale white. The larva, it seems, is just a big bag of milk.

“Rupture the gut,” Attardo said, “and the milk comes spilling out.”

And milk it truly is — a nutritional, biochemical and immunological designer fluid that the mother fly’s body has spun from her blood meals and pumped into her uterus, where her developing young greedily gulped it down.

Thus fattened on maternal largesse, a tsetse fly larva can safely burrow underground and pupate for 30 days before emerging as a full-blown adult with a nasty bite and a notorious capacity to transmit a deadly disease called sleeping sickness.

In a recent chemical and genetic analysis of tsetse fly milk, Attardo and his colleagues were startled to discover how similar it was to the product of the beloved gland that stamps us as mammals. “I was expecting something completely off the wall and different,” he said. “But there are frightening, fascinating overlaps with mammalian milk in the kinds of proteins we see.”

The new tsetse fly research is just one example of scientists widening the ranks of adherents to nature’s Milky Way. Researchers lately have found the equivalent of mother’s milk in an array of unexpected, breast-free places: in spiders, cockroaches and burying beetles; in great white sharks, male emperor penguins and flamingos of both sexes.

Other scientists have traced the evolution of mammalian lactation and concluded that it arose not as a convenient way to feed a baby, but to help our egg-laying ancestors tackle the challenge of water management on dry land.

Still other scientists are seeking to tally and understand the compositional differences in the milks from a broad sample of the world’s 5,500 or so mammals. They have unearthed a number of compelling concordances between the demands of oddball mammals and the makeup of their milk.

Assaying the milk of the nine-banded armadillo, for example, Michael Power, a lactation researcher at the Smithsonian National Zoological Park in Washington, and his colleagues were impressed by the high levels of calcium and phosphorus they detected, and by the even greater concentrations of protein.

“The protein was through the roof, way above anything else in the milk,” said Power, co-author with Jay Schulkin of “Milk: The Biology of Lactation.”

The elevated mineral counts made sense. “What does an armadillo build? A bony shell,” Power said. “So there’s going to be a lot of calcium and phosphorus going into this baby.”

The researchers soon realized it was a matter of chemistry. If you simply dropped large quantities of calcium and phosphorus into most types of mammalian milk, the minerals would glom together into insoluble phosphate compounds.

“They’d get stuck in the mammary gland and never reach the baby,” Power said. The solution? Throw in extra doses of casein proteins to bind the minerals into compact, usable nano-clusters.

“If I’m going to have a high-calcium, high-phosphorus milk, I have to have a high-protein milk,” Power said, “because a lot of that protein is a calcium-phosphorus delivery device.”

Olav Oftedal, now an emeritus researcher at the Smithsonian Institution, and his colleagues sought to estimate milk consumption rates among black bear cubs as they nursed in a den. The researchers were startled to detect in the mother bear’s blood and milk traces of the distinctively labeled water samples they’d earlier given to the cubs as part of the experiment.

There was only one way the water isotopes could have ended up in mother’s milk. “She’s lactating in a den,” Oftedal said. “She’s not eating or drinking. But she is consuming all the excreta of her young, which she then puts back in her milk.”

Small wonder, then, that the amount of milk the denning mother produced exceeded the weight she lost. “She’s recycling everything,” Oftedal said.

Expensive Lactation

Biologists warn against the wanton use of the word milk — sorry, almond “milk” really isn’t — and some mammalogists would like to restrict the term to the secretions of a dedicated mammary gland, which only their study subjects happen to possess.

But many scientists concur that if a parent synthesizes or highly modifies a substance on which its offspring’s life then depends, that parent is making a milk. By this measure, predigested food alone may not count, but if the parent first adds essential ingredients to the bolus, the regurgitate can fairly be deemed a milk.

Sandra Steiger of the University of Bayreuth in Germany and her colleagues recently reported on an American species, Nicrophorus orbicollis — a handsome, inchlong burying beetle with orange and black stripes — in which both parents care for their young. A parent beetle will eat a bit of carrion, predigest it and, on being tapped on the mouth by an offspring’s front legs, transfer the morsel into the little supplicant’s mouth.

“It’s like kiss-feeding,” Steiger said. “It looks really nice.” But as the researchers demonstrated, there is more to the osculatory exchange than pulped meat: The parent’s oral fluids are also critical to the young beetle’s survival.

Steiger’s team showed that if they presented larval beetles with liquefied mice carrion, the little insects could feed themselves just fine, but nearly all of them died before reaching pupation. Only when the scientists stirred parental oral fluids into the mix did the young beetles thrive.

The researchers have yet to analyze this chocolate-colored beetle milk, but Steiger suspects that it supplies the larvae with gut microorganisms, antibodies, digestive enzymes and other must-haves for mulching cadavers.

Wherever it appears, lactation is expensive and demands evolutionary justification. Flamingos are among the few birds that make milk for their young, and the effort drains them of all color — but at least it’s an egalitarian affair.

A male and female will jointly build a nest, incubate a single large egg and, when the egg hatches, churn out the rich crop milk on which the flamingo chick will feed for nine very long months. Begging calls from the chick stimulate in the parent’s brain the release of prolactin — the same hormone that subserves human lactation — which in turn prompts cells lining the crop, at the base of the parental throat, to swell and secrete the magic formula.

Brimming with protein and fattier than mammalian milk, flamingo milk “has the consistency of cottage cheese,” said Paul Rose, a flamingo researcher at the University of Exeter in Britain.

It is also bright pink. The parents spike the milk with the same carotenoid pigments that normally tint a flamingo’s feathers and that happen to be antioxidants — ideal for promoting a chick’s health and rapid growth.

Weeks and months pass. The parents must steadily step up milk production to meet ballooning demand. By the time the young flamingo is close to full-grown, robust in body and blushing of tone, its parents look thin and depleted, and their once-fuchsia feathers are now winter white.

“All of their energy, all their pink pigment, has gone into the crop milk,” Rose said. “Raising a flamingo is a very hard job.”

What explains the need for such full-throated lactation? Why can’t flamingos simply feed their young on beetles and flies, the way many birds do? Rose attributes the practice to the flamingo’s exceptional foraging style and the mouthparts necessary to accommodate it.

Like baleen whales, flamingos are filter feeders, and their distinctively crooked bills act as elaborate sieves. It takes time for a flamingo chick’s straight bill to thicken and bend, and longer still to master the practice of panning for a meal.

An Essential Antimicrobial

Only in the class Mammalia do all member species nurse their young, yet evolutionary biologists now believe that the roots of mammalian lactation date back more than 300 million years, a good 100 million years before the first mammals appeared.

The ancestors of modern mammals are thought to have laid the sort of porous, parchment-shelled eggs seen today among lizards, snakes and a couple of weird, monotreme mammals like the platypus. In contrast to the hard-shelled calcified eggs of birds, parchment eggs are at chronic risk of drying out, which means modern snakes and lizards often are constrained by the need to lay their eggs in a relatively damp setting.

Our ancient forebears stumbled on a liberating solution: Make yourself into a watering can, and you can lay your eggs wherever you want.

“The likely first function of milk was to hydrate parchment-shelled eggs laid on dry ground,” said Amy Skibiel of the University of Idaho, an expert in mammalian lactation. By this scenario, pre-mammals dribbled fluid onto their eggs through pores on their chest; nipples came much later.

Parchment eggs also are at risk of microbial infiltration, and genetic studies suggest that the earliest egg hydration solutions were fortified with anti-pathogen components. It’s a moisturizer — no, it’s a cleanser! And why not continue with the watering post-hatch, souping up the fluids, through natural selection, into food for the babies?

Once the mammalian lineage had settled on the secretory approach to parenthood, milks quickly diversified, the recipes dictated by a combination of need, diet and who your relatives were.

Hooded seal pups nurse on rapidly shrinking ice floes for just four days, and during that time they manage to double their weight. Not surprisingly, hooded seal milk is more than 60 percent fat — the fattiest milk among mammals.

It also smells overwhelmingly of fish, as I discovered when I sniff-tested a series of exotic milks at the National Zoo’s world-class milk bank. On the other end of the lipid scale, rhinoceros milk, at 2 percent fat, looks and smells like skim milk.

Elephant milk is less watery, and I was sure I caught notes of ice cream. Lion’s milk has no discernible odor and, like most carnivore milk, is low in sugars; meat eaters are designed to efficiently wrest their glucose from protein and fat.

Human milk, by contrast, is extremely sweet. Skibiel, who tasted her own milk while nursing her baby, said it reminded her of cantaloupe.

The number and variety of sugars in human milk outstrips that seen in any other great ape, Power said, and he proposes a surprising reason for that bounty: not to build our big brain, as some have argued, but because we needed sugar’s anti-microbial powers to help us cope with all the novel pathogens we encountered after the agricultural revolution, when we started crowding into villages and living in close quarters with other animals.

“Our ability to use animals in lots of different ways is one of the reasons we’re successful,” he said, “but it was a huge shock to the system.” Luckily, our milk rose to the challenge.

“Our brain made our milk,” Power said, “not the other way around.”

This article originally appeared in <a href="https://www.nytimes.com/2019/02/11/science/milk-animals-evolution.html" id="f0c8e344-c913-3795-a6cc-b960eeda0011">The New York Times</a>.

Everywhere in the Animal Kingdom, Followers of the Milky Way

The theory of plate tectonics is one of the great scientific advances of our age, right up there with Darwin’s theory of evolution and Einstein’s theory of relativity.
 The idea that Earth’s outer shell is broken up into giant puzzle pieces, or plates, all gliding atop a kind of conveyor belt of hot, weak rock — here rising up from the underlying mantle, there plunging back into it — explains much about the structure and behavior of our home planet: the mountains and ocean canyons, the earthquakes and volcanoes, the very composition of the air we breathe.
 Yet success is no guarantee against a midlife crisis, and so it is that half a century after the basic mechanisms of plate tectonics were first elucidated, geologists are confronting surprising gaps in their understanding of a concept that is truly the bedrock of their profession.
 They are sparring over when, exactly, the whole movable plate system began. Is it nearly as ancient as the planet itself — that is, roughly 4.5 billion years old — or a youthful 1 billion years, or somewhere in between?
 They are asking what caused the shell to crack apart in the first place, and how the industrious recycling of Earth’s crust began.
 They are comparing Earth with its sister planet, Venus. The two worlds are roughly the same size and built of similar rocky material, yet Earth has plate tectonics and Venus does not. Scientists want to know why.
 “In the 1960s and 70s, when people came up with the notion of plate tectonics, they didn’t think about what it was like in the distant past,” said Jun Korenaga, a geophysicist at Yale University.
 “People were so busy trying to prove plate tectonics by looking at the present situation, or were caught up applying the concept to problems in their own field. The origin issue is a much more recent debate.”
 Researchers also are exploring the link between plate tectonics and the evolution of complex life. Fortuitously timed continental collisions and mountain smackdowns may well have supplied crucial nutrients at key moments of biological inventiveness, like the legendary Cambrian explosion of 500 million years ago, when the ancestors of modern life-forms appeared.
 “The connection between deep Earth processes and Earth surface biology hasn’t been thought about too clearly in the past, but that’s changing fast,” said Aubrey Zerkle, a geochemist at the University of St. Andrews in Scotland.
 It is increasingly obvious that “you need plate tectonics to sustain life,” Zerkle added. “If there wasn’t a way of recycling material between mantle and crust, all these elements that are crucial to life, like carbon, nitrogen, phosphorus and oxygen, would get tied up in rocks and stay there.”

The origin and implications of plate tectonics were the subject of a recent meeting and themed issue of Philosophical Transactions of the Royal Society.
 Researchers said that pinning down when and how Earth’s vivid geological machinations arose will do more than flesh out our understanding of our home base. The answers could well guide our search for life and habitable planets beyond the solar system.

Robert Stern, a geoscientist at the University of Texas at Dallas, argues that if we are looking for another planet to colonize, we want to avoid ones with signs of plate tectonic activity. Those are the places where life is likely to have evolved beyond the “single cell or worm stage, and we don’t want to fight another technological civilization for their planet.”
 “A relatively benign way for the Earth to lose heat” The idea that continents are not fixed but rather peregrinate around the globe dates back several centuries, when mapmakers began noticing the complementarity of various land masses — for example, the way the northeast bulge of South America looks as if it could fit snugly in the cupped palm of the southwest coast of Africa.
 But it was not until the mid-twentieth century that the generic notion of “continental drift” was transformed into a full-bodied theory, complete with evidence of a subterranean engine driving these continental odysseys.
 Geologists determined that Earth’s outer layer is broken into eight or nine large segments and five or six smaller ones, a mix of relatively thin, dense oceanic plates riding low and thicker, lighter continental plates bobbing high.
 At large fissures on the ocean floor, melting rock from the underlying mantle rises up, adding to the oceanic plates. At other fracture points in the crust, oceanic plates are diving back inside, or subducting, their mass devoured in the mantle’s hot belly.
 The high-riding continental plates are likewise jostled by the magmatic activity below, skating around at an average pace of 1 or 2 inches a year, sometimes crashing together to form, say, the Himalayan mountain chain, or pulling apart at Africa’s Great Rift Valley.
 All this convective bubbling up and recycling between crust and mantle, this creative destruction and reconstruction of parts — “tectonic” comes from the Greek word for build — is Earth’s way of following the second law of thermodynamics. The movement shakes off into the frigidity of space the vast internal heat that the planet has stored since its violent formation.
 And while shifting, crumbling plates may seem inherently unreliable, a poor foundation on which to raise a family, the end result is a surprising degree of stability. “Plate tectonics is a relatively benign way for Earth to lose heat,” said Peter Cawood, an Earth scientist at Monash University in Australia.
 “You get what are catastrophic events in localized areas, in earthquakes and tsunamis,” he added. “But the mechanism allows Earth to maintain a stabler and more benign environment overall.”

Unfortunately for geologists, the very nature of plate tectonics obscures its biography. Oceanic crust, where the telltale mantle exchange zones are located, is recycled through the upwelling and subducting pipeline every 200 million years or so, which means hard evidence of tectonic origins was destroyed long ago.
 Continental crust is older, and rocks dating back more than 4 billion years have been identified in places like Jack Hills, Australia. But continental plates float above the subductive fray, revealing little of the system’s origins.
 Nevertheless, geoscientists are doing their best with extant rocks, models and laboratory experiments to sketch out possible tectonic timelines. Korenaga and his colleagues have proposed that plate tectonics began very early, right after Earth’s crust solidified from its initial magmatic state.
 “That is when the conditions would have been easiest for plate tectonics to get started,” he said. At that point, he said, most of the water on Earth — delivered by comets — would still be on the surface, with little of it having found its way into the mantle. The heat convecting up through the mantle would exert a stronger force on dry rocks than on rocks that were lubricated.
 At the same time, the surface water would make it easier for the hot, twisting rocks beneath to crack the surface lid apart, rather as a sprinkling of water from the faucet eases the task of popping ice cubes from a tray. The cracking open of the surface lid, Korenaga said, is key to getting the all-mighty subduction engine started. With subduction established, water, like oceanic crust, would cycle between Earth’s surface and mantle.

Water is constantly recycled between the mantle and crust On the opposite end of the origins debate is Stern, who argues that plate tectonics is a mere billion years old or less, and that Earth spent its first 3.5 billion years with a simple “single lid” as its outer shell: a crust riddled with volcanoes and other means of heat ventilation, but no moving plates, no subduction, no recycling between inside and out.

As evidence of the youthfulness of the plate regimen, Stern points to two classes of rocks: ophiolites and blueschist.
 Ophiolites are pieces of oceanic crust atop bits of underlying mantle that have made their way onto land and thus have escaped the relentless recycling of oceanic crust. Recent research has shown that ophiolites are not just any slice of oceanic crust, Stern said, but rather were formed by the forces of subduction.
 Similarly, blueschists are rocks that are fashioned under very high pressure but low temperatures, and “the only place you can do that is in a subduction zone,” Stern said.
 Nearly all ophiolites are less than 1 billion years old, he added, while the most ancient blueschists, found in China, are just 800 million years old. No ophiolites, no blueschists, no evidence of subduction or plate tectonics.

Most geologists opt for a middle ground. “Science is a democratic process,” said Michael Brown, a geologist at the University of Maryland and an editor of the themed issue, “and the prevailing view is that Earth started to exhibit behaviors that look like plate tectonics 2.5 to 3 billion years ago.”
 Significantly, that chronology decouples plate tectonics from the origin of life on Earth: Evidence of the earliest single-celled organisms dates back more than 3.6 billion years. Nevertheless, scientists view plate tectonics as vital to the sustained evolution of that primordial life.
 Plate tectonic activity did not just help to stabilize Earth’s heat management system. The movement kept a steady supply of water shuttling between mantle and crust, rather than gradually evaporating from the surface.
 It blocked the dangerous buildup of greenhouse gases in the atmosphere by sucking excess carbon from the ocean and subducting it underground. It shook up mountains and pulverized rocks, freeing up essential minerals and nutrients like phosphorus, oxygen and nitrogen for use in the growing carnival of life.

Zerkle discerns a link between geological and biological high drama: “It’s been suggested that time periods of supercontinental cycles — when small continents smash together to make large supercontinents, and those supercontinents then rip apart into smaller continents again — could have put large pulses of nutrients into the biosphere and allowed organisms to really take off.”
 Plate tectonics also built the right playing fields for Darwinian games. “Think about what drives evolution,” Stern said. “It’s isolation and competition. You need to break continents and continental shelves apart, and separate one ocean from another, for speciation to occur.”
 Life is always falling apart, on the rocks — and a good thing, too.

This article originally appeared in <a href="https://www.nytimes.com/2018/12/18/science/plate-tectonics-continents-earth.html" id="3dff30f0-a467-3adf-abbe-48179998c77b">The New York Times</a>.

The Earth's Shell Has Cracked, and We're Drifting on the Pieces

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Natalie Angier

Articles written by the author

Ultra-Black Is the New Black

Everywhere in the Animal Kingdom, Followers of the Milky Way

The Earth's Shell Has Cracked, and We're Drifting on the Pieces