Chapter 4 Chapter 4 The Prehistoric World

1.Interglacial Over a period of more than a billion years, the ice sheets have slid back and forth between the Earth's poles, sometimes meeting at the equator.There are many reasons, including continental plate drift, the earth's elliptical orbit, a skewed axis, and fluctuations in atmospheric carbon dioxide levels.For several million years, the structure of the continental plate at that time was basically the same as ours today, and glacial periods recurred quite regularly. This process lasted for as long as 100,000 years, and the average length of the interglacial period was between 12,000 and 28,000 years. between.

The last ice age left New York 11,000 years ago.Under normal circumstances, the next Manhattan-leveled ice age could strike any day now, although we are now increasingly skeptical that it will happen.Many scientists speculate that the interglacial period before the next cold snap will last longer because we pump more carbon dioxide into the atmosphere, delaying the ice age that would surely come.By comparing ancient gas bubbles in Antarctic ice cores, we find that there is now more carbon dioxide in the atmosphere than at any time in the past 650,000 years.If humans ceased to exist tomorrow, we would no longer be able to send carbon-containing molecules into the sky, and the troubles we caused would be over.

Even though our standards are changing, by our standards, such a thing would not happen immediately, because modern humans did not have to wait for the day to become fossils and enter geological time.As a veritable force of nature, we've done just that.After we are gone, the longest surviving of human masterpieces will be our reshaped atmosphere.So Tyler Falk finds it no surprise that she is a designer who teaches environmental physics and ocean chemistry at NYU's biology department.He thinks he has to bring all of those disciplines together to describe how humans have altered the atmosphere, biosphere, and oceans—a feat so far accomplished only by volcanoes and colliding continental plates.

Falk is lanky, with dark hair slightly curly, and his eyes narrowed when he thinks.Leaning back in his chair, he carefully studied a poster that almost covered the bulletin board in the office.This poster depicts the atmosphere and oceans as layers of fluid with increasing density.As recently as 200 years ago, atmospheric carbon dioxide was still dissolving into seawater at a steady rate, keeping the world's carbon dioxide in balance.Now, the level of carbon dioxide in the atmosphere is so high that the oceans have to readjust themselves.Because the ocean is so big, this adjustment process will take time, he said.

"Suppose there were no human beings in the world, and all fuels were no longer used. At first, the surface of the ocean would absorb carbon dioxide rapidly. As the ocean is saturated with carbon dioxide, this rate will slow down. Some carbon dioxide will be able to be converted. Assimilated by cooperative organisms. Gradually, as seawater merges, old, unsaturated seawater emerges from the depths to replace the saturated seawater." It will take a thousand years for the seawater to completely turn over, but this will not bring the earth back to the purity of the pre-industrial era.The ocean and atmosphere are in balance with each other, but both absorb too much carbon dioxide.The same goes for the earth, where excess carbon will cycle between the soil and the life forms that absorb it, but will eventually be released.So where can it go? "Normally," says Fokker, "the biosphere is like an upside-down glass bottle: the top layer is basically cut off from other matter, except for the occasional meteor. At the bottom, the cap faces the volcano. The direction is slightly opened."

The problem is that we've dug up Carboniferous coal and dumped it into the atmosphere—we've turned into a volcano that's been erupting since the eighteenth century. As volcanoes toss excess carbon into ecosystems, the next step for Earth is business as usual. "Rock layers die, but it takes a lot longer." Silicates such as feldspar and quartz are the main components of the Earth's crust, and carbon dioxide from rain and carbon dioxide gradually weathers them into carbonates.The carbonic acid breaks down into soil and minerals, which release calcium into the groundwater.The river flows into the sea, where calcium is deposited and becomes the seabed.This is a long process, which is accelerated slightly by excess atmospheric carbon dioxide.

"In the end," Falk concluded, "geological cycles will restore the carbon dioxide levels to prehistoric levels. This process will take about 100,000 years or so." The process may take longer: One of our concerns is that the warmer the oceans are, the more carbon dioxide they emit (rather than absorb).Another problem is that although small marine organisms can lock carbon into their "armor," increased levels of carbon dioxide in the upper ocean may dissolve their shells.The good news, though, is that up to 90% of the excess CO2 would have been absorbed by the ocean overturning in the first millennium, leaving the atmosphere with just 280 parts per million more than the pre-industrial level of CO2 Give ten to twenty points.

We have 380 parts per million of CO2 today and scientists who have spent a decade studying the Antarctic ice swear to us that the difference in CO2 means there is no chance of an invasion for the next 15,000 years glacier.But when the excess carbon is slowly absorbed, palm trees and magnolias may reproduce faster than oaks and beeches in New York.Perhaps the moose had to forage for gooseberries and elderberries in Labrador 6, while Manhattan became home to armadillos and squirrels from the south... After a long-term investigation of Antarctica, some other equally prestigious scientists gave the following answer: Unless the ice and snow melted in Greenland freezes the Mexican warm current and cuts off this huge "transmission device" that supplies warm water around the world, Such things can happen.If so, Europe would enter an ice age, and the east coast of North America would also be doomed.Probably not so severe as to form a thick layer of glaciers, but temperate forests might turn into barren tundra or permafrost.Berry bushes might also degenerate into stunted, speckled groundcover next to moss that attracts reindeer south.

The third is our preferred hypothesis, where the two extreme forces might cancel each other out, so that the temperature stays in between.Either way, hot, cold, or somewhere in between, as long as humans exist, atmospheric CO2 levels will rise to 500 to 600 parts per million, or by our estimates, by 2100 It reached 900 parts per million years ago.If we don't change the way we do business now, much of what was once frozen over Greenland will melt and pour into the Atlantic Ocean.Depending on the precise volume of melted ice, what Manhattan will become will be just two rocky islands—one of the mountains that once towered over Central Park and the other of Washington Heights' outcropping.For a while, groups of buildings a few miles to the south would scan the surrounding waters like periscopes, but in vain, the rough waves would eventually knock them down one by one.

2.Frozen Eden What would the Earth be like if humans never evolved?Is our evolution inevitable? If we disappear, will we—or equally complex creatures—reappear? * Lake Tanganyika in East Africa sits within a rift valley that split Africa in two 15 million years ago.The Great Rift Valley is a continuation of an earlier tectonic valley in what is today the Bekaa Valley in Lebanon, which developed southward to form the Jordan River and the Dead Sea.It then gradually widened to form the Red Sea.Today, it splits into two parallel rift valleys in Africa.Lake Tanganyika, located on the Western Fork of the Great Rift Valley, stretches for 420 miles and is the longest and narrowest lake in the world.

Lake Tanganyika is one mile deep and 10 million years old. It is also the second deepest and oldest lake in the world, behind Lake Baikal in Siberia.Thus, Lake Tanganyika is quite an interesting lake for scientific researchers who extract ore samples from lake bed sediments.Annual snowfall seals the history of climate change in glaciers, and pollen from surrounding vegetation sneaks into deep freshwater bodies.The water bodies are neatly and clearly layered: the dark edges are vegetation carried away by runoff during the wet season, and the light edges are algal blooms during the dry season.In ancient Lake Tanganyika, mineral samples reveal more secrets than plants.They reveal how a tropical jungle was transformed into the fire-resistant Tanzanian deciduous woodlands that now cover much of Africa.The Tanzanian woodland is another human masterpiece: the Paleolithic humans obtained pastures by burning trees, and developed the woodland to attract and raise antelope, and the Tanzanian woodland has been developed since then. The pollen was mixed with thick layers of charcoal, suggesting that the onset of the Iron Age was accompanied by more severe deforestation, as people learned to smelt ore and, later, how to make hoes for plowing.They grow crops such as palm millet, which also leave traces in the pollen.Subsequent crops, such as soybeans and corn, either produced too little pollen or had grains too large to be carried very far by rainwater.However, the pollen of exotic ferns increased, evidence of agricultural development. We tied ten-meter steel pipes to the cables.Aided by a humming generator, it descends under its own gravity to the lake bed, deep into layers of pollen deposited over hundreds of thousands of years.Andy Cohen, a paleolnologist at the University of Arizona who directs a research project on the eastern shore of Lake Tanganyika in Tanzania's Kigoma region, said the next step is the piercer, which needs to be able to pierce five million Years or even 10 million years old sediments were sampled. Such machines are quite expensive, and they resemble small drill ships.The lake is too deep for the drilling machine to be anchored, so it relies on several thrusters linked to a GPS to constantly adjust its position above the cave.But Cohen says it's worth it because it's the oldest and richest archive on Earth. "We've long thought that advancing and retreating polar ice caused climate change. But there's reason to believe that material cycles in the tropics play a role too. We know a lot about climate change in the poles, but we don't know much about the sources of Earth's heat There are very few, but this is where people live." Sampling the strata gives you "ten times as much climate history as you can get in the glacier layers, and with a lot more precision. There's maybe a hundred different things to look at," Cohen said. We analyze." They retain the history of human evolution, because the mine samples record the leap of ages - the first step of walking upright in primates, and the australopithecus to hominids, able man, Homo erectus, Eventually to the transcendence of Homo sapiens.The pollen is identical to the ones our ancestors inhaled, and even originated from the same crops they touched and ate, as they also occur in this rift valley. The other branch of the Great Rift Valley, east of Lake Tanganyika, is a shallower saline lake that has evaporated and reappeared several times over the past two million years.Today it is a meadow where the Maasai7 graze their cattle and sheep, sprinkled with sandstone, clay, tuff and ash, and topped by a layer of volcanic basalt.A river flowing eastward through the Tanzanian highlands gradually cut a 100-meter-deep canyon in these formations. In the 20th century, archaeologists Louis Leakey and Mary Leakey discovered 1.75 million years ago here. Anterior hominid skulls.The gray Olduvai scree gorge, now a sisal-grown semi-desert, is where hundreds of sheet-shaped tools and axes made of basalt were eventually unearthed.Some tools date back two million years. In 1978, 25 miles southwest of Olduvai Gorge, Mary Leakey's group found footprints frozen in tuff.They are the tracks of an Australopithecus family, most likely parents and children, who walked through the muddy ash that erupted from nearby Sadiman Volcano in heavy rain.Their findings push back the existence of hominids who walked upright to 3.5 million years ago.Here, and other sites in Kenya and Ethiopia, paint a vivid picture of human origins.We now know that humans actually walked upright for millions of years before they thought of striking one stone against another to make sharp tools.From the remains of hominid teeth and other nearby fossils, we deduce that humans were once omnivorous, using our molars to crush nuts—but as we went from finding stones that looked like axes, we later learned to make Axes, we also have weapons for effectively hunting and eating animals. Olduvai Gorge and other hominid fossil sites, shaped like a crescent moon stretching south from Ethiopia parallel to the continent's east coast, prove beyond doubt that we are all descended from Africans.The dust we breathe is blown up by the wind.Zephyr dusts Olduvai's sisal and black locusts with a gray tuff powder that contains the calcified fragments of DNA we carry with us.From here, human beings march towards every continent and every corner of the world.Finally, after a full circle, we come back—we are so different from our ancestors that we enslave our backward blood relatives in order to assert our right to exist. The animal bones at these sites include hippopotamus, rhinoceros, horses and elephants, which were extinct by human reproduction; many of the bones were sharpened by our ancestors What the world was like before mammals came to the fore.However, they fail to show how we stand out.But there are clues in Lake Tanganyika.These clues point to ice. Lake Tanganyika is fed by many rivers that pour down from the cliffs of the Rift Valley several miles high.There was a time when these rivers came from the Promenade Rainforest.Then came the Tanzanian woodlands.Today, most cliffs are treeless.The slopes were burned to grow cassava; their fields were so steep that farmers were said to have tumbled off the slopes. The Gombe River is an exception.It is located on the coast of Tanzania, east of Lake Tanganyika, and has been studying gorillas since 1960 by primatologist Jane Goodall, who was an assistant to the Rikge Olduvai Canyon Project.Her field study of how a species behaves in the wild is the longest in human history.Their center was in a tent, accessible only by boat.The surrounding national park is one of the smallest in Tanzania, covering only 52 square miles.When Jane Goodall first came here, the surrounding hills were overgrown with jungle.Where the jungle borders woodlands and grasslands, there live African lions and black buffaloes.Today, three sides of this national park are surrounded by cassava fields, oil palm fields, residential houses on the hills, and several villages with more than 5,000 people living on the shore of the lake.The famous gorillas number in the ninety or so. Although gorillas are the most studied primate in Gombe, the rainforest is also home to many green baboons and several species of monkeys: vervet, red-bearded, red-tailed and blue. In 2005, a Ph.D. student at the Center for the Study of Human Origins at New York University, Kate Dettvila, spent several months there investigating the strange phenomenon of red-tailed and blue monkeys. The face of the red-tailed monkey is narrow and black, with white spots on the nose, white cheeks, and a very flexible chestnut-yellow tail.The blue monkey's fur is slightly blue and triangular in shape. Its face is almost hairless and its prominent browbones are impressive.They have different colors, different shapes, and different names. No one will be confused between the red-tailed monkeys and blue monkeys here.In the Gombe area, however, it is now apparently impossible to tell them apart because they have started interbreeding.So far, Dettvila has demonstrated that, despite the different numbers of chromosomes in the two species, at least some are fertile.She scraped their feces from the forest floor—residues from their guts that suggested the mixing of DNA gave rise to new species. Only she thinks more.In the history of genetics, sometime between 3 and 5 million years ago, two monkey species with a common ancestor diverged.In order to adapt to the environment, the two gradually separated.Charles Darwin was the first to deduce the process of evolution through a situation similar to this—the phenomenon in which chaffinches on the Galapagos islands became isolated and isolated from each other.In this case, thirteen different species of chaffinch have emerged to adapt to the local diet, and their pecks have different functions: pecking open seeds, eating insects, sucking sap from cacti, and even sucking the blood of seabirds. In Gombe, the exact opposite happened.At some point in history, barriers that once restricted the free movement of blue and red-tailed monkeys were replaced by new forests, and the two species began to share the environment.But as the forests around the Gombe River National Park gave way to cassava fields, they took to the fugitive life together. "As mates within their own population become increasingly difficult to find," says Dettwila, "these animals are forced to resort to desperate, or creative, means of survival." Her thesis was that the interbreeding of two species could be a force of evolution, just as natural selection is a force of evolution. "Maybe initially the offspring of hybrids weren't as adaptable as their parents," she said. It will go on until one day there will be hybrids that are as fertile as their parents and may have a survival advantage over their parents because the habitat has changed." And so the future descendants of these monkeys were human masterpieces again: the farming moderns scattered across East Africa expelled their parents, and species like monkeys, shrikes or flycatchers had to interbreed, interbreed, go extinct—or just do. Make some other pioneering work, such as evolution. Something similar may have happened here.Once upon a time, when the Great Rift Valley was just beginning to form, tropical rainforests stretched across central Africa from the Indian Ocean to the Atlantic Ocean.Gorillas have appeared on the continent, and one species resembles chimpanzees in many ways.But we've never found any remains of this species, and chimpanzee remains are rare, for the same reason: In tropical rainforests, torrential rains wash away minerals from the ground, making it hard to fossilize, and bones decay quickly.Scientists know it did, though, because genetics proves that we descend directly from the same ancestors as chimpanzees.American physical anthropologist Richard Jean Ham gave this never-before-discovered gorilla a strange name: Pamplier (meaning "before the chimpanzee"). This species predated today's chimpanzees and predated a major drought that hit Africa seven million years ago.The swamps shrunk, the soil dried up, the lakes disappeared, and the forests shrank. Surrounded by savannahs, there were few places to hide.Ice age activity at the poles contributed to the accident.Because most of the world's water is locked up in glaciers in Greenland, Scandinavia, Russia, and much of North America, Africa is incredibly hot.Although volcanoes like Kilimanjaro and Mount Kenya are covered in snow and ice, no ice reaches Africa.The climate change that has thinned Africa's forests (more than twice the size of today's Amazon basin) is precisely because a distant but terrifying white force is destroying the taiga that stands in its way. Distant ice activity poses a dilemma for forest-dwelling mammals and birds on the African continent.Over the next few million years, they evolved in different ways in different forests.We know that at least one of them was forced to make a daring attempt to relocate to the savannah. If humans disappeared, if a species eventually replaced us, would they evolve like us?In southwestern Uganda, we can see human history recreated in microcosm.Chabula Canyon is a narrow, ten-mile-long gash cut through the dark-brown volcanic ash that has accumulated on the floor of the Great Rift Valley.In stark contrast to the surrounding yellow plains, a green ribbon of tropical hardwoods and emblical trees covers the gorge along the Chabra River.For the chimpanzees, this oasis is both a sanctuary and an ordeal.The verdant canyon is only 500 yards wide, and the limited fruit here does not meet the food needs of all the orangutans.Therefore, some brave orangutans always take risks, climb the canopy of trees, cross the canyon, and lead to another field of hope. There were no branches for them to use as ladders to look beyond oats and citronella, so they had to stand on their hind legs—for a little while, but at least biped.They watch lions and coyotes through the sparse fig trees of the savannah.They choose a tree that they presumably can reach, so they don't make themselves prey to predators.Then, they ran away. A remote glacier has driven some brave but hungry "Panpril" out of forests that aren't big enough to support us—some of whom have really used their imagination and creativity to survive.About three million years have passed, and the world has warmed again.The glaciers retreated.The trees regained their lost ground, and some even reached Iceland.The trees on the African continent were united again, across the coast from the Atlantic to the Indian Ocean, but at this time, the Pamperel had evolved into a new species: the first great apes to choose to live in the grasslands and woodlands on the edge of the forest.For the next hundred years they were bipedal, so their legs became longer and their big toes shortened.They are gradually losing their ability to perch in trees, but their skills at surviving on the ground have taught them more. Now, we are primitive humans.Almost in the process of evolving from australopithecines to humans, we not only learned to live on the savannah that had been burned by fire, but also learned how to make fire and burn grass by ourselves.For the next three million years, the distant ice sheets did not drive out the grasslands and forests for us, and our numbers were not large enough to do so.At that time, although the descendants of the so-called modern man were far from Pamplil, we must have had enough of them to make some innovative moves again. Were the primitive humans who came out of the African continent fearless adventurers looking forward to vast areas beyond the prairie? Or are they nothing more than losers driven out of their cradles by their more powerful blood-kin? Or were they just breeding and advancing along the grasslands all the way to Asia, like any beast that saw an abundance of resources?Darwin thought it didn't matter: when isolated communities of the same species evolve in different ways, the real winners are those who learn to thrive in new environments.Whether uprooted or daringly adventurous, the survivors gave birth and multiplied in Asia Minor and India.In Europe, they learned a skill already familiar to temperate creatures like squirrels, but entirely new for primates: planning reserves.A good memory and forethought are essential in order to store them for winter when food is plentiful.They crossed land bridges to most of Indonesia, but to reach New Guinea and Australia they had to learn to steer ships.This was about 50,000 years ago.Then, some 11,000 years ago, keen Homo sapiens living in the Middle East discovered a secret known only to certain kinds of insects: how to grow vegetation to gain food sources, rather than destroy them. Since we know that the Middle Eastern wheat and barley they planted soon went south along the Nile, we can surmise—as shrewd Jacob lured his mighty twin brother Esau on his return (see allusions) )—from there some one who knew the knowledge of agriculture returned home to Africa with the seeds.Well done to him, as food sources became strained as another ice age (the last) stole water from lands that glaciers couldn't reach.So much seawater was frozen into glaciers that sea levels were 300 feet lower than they are today. It was at this stage that other humans who spread out across the Asian continent arrived as far away as Siberia.Parts of the Bering Sea dried up, and a thousand-mile land bridge led all the way to Alaska.It spent 10,000 years under a layer of ice more than half a mile thick.By then, though, much of the ice had receded, and it became a channel up to 30 miles wide in localized areas.Humans bypassed the lake formed by melting ice and snow and crossed this land bridge. Chabula Canyon and the Gombe River are now archipelagic atolls with remnants of the forests that once gave us life.This time, the shattering of African ecosystems is not because of glaciers, but because of us - in the last evolution, we have become the masters of nature, with the same power as volcanoes and ice sheets.The forest is surrounded by agriculture and dwellings, like a lonely island, the other descendants of Pamperel still abide by the living habits when we left, and we have moved from the woodland to the grassland and finally settled in the city.To the north of the Congo River, our brethren are the gorillas and chimpanzees; to the south, the bonobos.We are most genetically similar to the latter two; Louis Leakey sent Jane Goodall to Gombe because bones and skulls he and his wife found showed that our common ancestor looked and behaved like Chimpanzees have huge similarities. Whatever the reason for our ancestors to leave the land, their decision sparked an unprecedented evolution—sometimes described as the greatest success, and sometimes as the world's greatest disaster.Still, assuming we stayed here, or assuming we stayed on the grasslands, the ancestors of today's lions and coyotes must have killed us.If such a species existed, what would have evolved to where we are today? If we had stayed in the forest then, we would now see the world with the same eyes as wild chimpanzees.Their thinking may not be clear, but there is no doubt that they have intelligence.A chimpanzee in its natural environment will watch you calmly from a branch without feeling inferior to a higher primate.The image of orangutans in Hollywood is misleading, because those trained orangutans are very young and cute like children.They keep growing, however, sometimes weighing as much as 120 pounds.For a person of the same weight, about 30 pounds will be fat.For chimpanzees who climb up and down all day, they may only have 3-4 pounds of fat, and the rest is muscle. Curly-haired Dr Michael Wilson is the young leader of the Gombe River Field Research Project.He demonstrated the strength of chimpanzees.He witnessed the whole process of them tearing up and devouring the red bearded monkeys.They are excellent hunters, 80% of their attacks are successful kills. "Lions only have a 10-20 percent kill rate. Chimpanzees are pretty smart species." But he also found them sneaking into the territory of other chimpanzees nearby, ambushing individual unsuspecting males and killing them.He observed that they patiently eliminated males from other tribes around them until they claimed the entire territory and all the females.He had also seen fierce fighting and bloody battles within the clan for the top spot.Comparing these phenomena with human aggressive wars and power struggles has become his research direction. "I hate thinking about it. It's kind of depressing." But why are bonobos, who are smaller and thinner than chimpanzees, but closely related to us, not aggressive at all?This is a mystery.Although they also defend the territory, we have never seen the killing of other people.They are peaceful by nature, love to play and flirt with multiple partners, maintain a matriarchal social structure, and all members take responsibility for raising the next generation... In the eyes of those who insist that the weak can also claim a place on earth, this almost becomes a reality. There is a myth. However, in a world without humans, if they came into conflict with chimpanzees, their numbers would be greatly reduced: 10,000 bonobos would survive, or even fewer, while the chimpanzee population would balloon to 150,000.A century ago, the sum of the two was about 20 times higher than it is now, and as time went by, the two branches were further and further apart on the path of evolution. Michael Wilson, who was walking in the rainforest, heard the sound of drumming, and he knew it was chimpanzees tapping the plank roots of plants to pass signals to each other.He followed them and ran all the way, crossing the valley of thirteen rivers in Gombe, jumping over the morning glory vines connected with the tracks of baboons, and following the chimpanzee's shouts, he ran for two hours at a stretch, finally at the bottom of the Rift Valley. The top catches up to them.Five chimpanzees climbed into a tree at the edge of the woodland to nibble on their favorite mangoes, which come all the way from the Arabian Peninsula along with wheat. A mile below, Lake Tanganyika sparkled in the afternoon sun.This behemoth stores 20% of the world's fresh water and raises many regional fish species. Aquatic biologists call this place the Galapagos Lakes.To the west of the lake lie the smoky hills of the Congo River, where chimpanzees are still used for food.In the opposite direction, across the border of the Gombe region, are farmers who still use rifles and resent the chimpanzees who rob them of their oil palm fruit. Apart from their own kin, humans and chimpanzees have few natural enemies here.These five chimpanzees climbed a tree in the middle of the meadow, which just proved that they inherited highly adaptive genes-their adaptability is much stronger than that of gorillas who only eat food in the forest. They live on a variety of foods and are able to adapt to a variety of environments.Without humans, they may not need to adapt to the environment.Because the forest will soon recover, Wilson said. "The Tanzanian woodlands will move in here to re-occupy the cassava fields. Maybe the baboons will get close enough to multiply, and the seeds carried in their droppings will be planted in various areas. Before long, the trees will take root wherever they are suitable for living Sprout. Eventually, the chimpanzees will follow." Lions are back as prey numbers pick up, followed by the big game: black buffalo and elephants in game reserves in Tanzania and Uganda. "In the end," Wilson said with a sigh, "I think the chimpanzee population will continue to grow, and Malawi, Burundi, and Congo will all have their footprints." The forest is back, with fruit that chimpanzees love and plenty of red bearded monkeys to hunt.The narrow Gombe is a small piece of dusty African past and a window to the future "post-human era".Here, nothing can tempt a primate to leave this verdant landscape to follow our senseless footsteps. Of course, all this can only last until the day when the Ice Age returns.