Book 2 Listening (1108796), страница 16
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They probably becomeextinct well before that. Dr Habib proposes that what actually did for Microraptor was what hecalls a drag tax. His model suggests the manoeuvrability that its hind wings granted theanimal came at the expense of increased drag. Modern birds do not pay the drag taxbecause their manoeuvrability results from a single pair of wings that are better able to copewith stress than Microraptor's forewings were.
That stress-resistance is provided by muscleswhich are attached to a keel-like extension of the sternum. Microraptor lacked this keel.According to Dr Habib, birds adapted for forest flight are able to make the same turnsthat Microraptor made, but without losing nearly as much energy to drag. This, he argues,made bird flight more efficientand that would have made it impossible for Microraptor tocompete. (From The Economist, November 10, 2012)61Script 28. Marine ecologySea wolvesSharks, it seems, are necessary for the ecological health of coral reefsFOR decades, rangers in Yellowstone National Park, in the American West, had to cullthe area's red deer (known locally as elk, though they bear no resemblance to European elk,known locally as moose) because the animals' numbers were grazing the place to death andthus threatening the livelihoods of other species.
Many ecologists argued that the deer hadonce been kept under control by wolves, which had been hunted to extinction by people.When wolves were reintroduced to Yellowstone, in 1995, these arguments proved correct.The deer population fell to manageable levels, and culling stopped.Wolves, it turned out, played a crucial role in keeping the wider ecosystem intact. Nowcomes evidence that the same is true for another top predator: sharks.
Jonathan Ruppert ofthe University of Toronto, in Canada, and his colleagues reached this conclusion by studyingdata on shark activity around two groups of coral reefs between Indonesia and Australia.They gleaned their information from baited underwater video stations and also fromrecords collected from those reefs between 1994 and 2008 by the Australian In statute ofMarine Science. They have just published their results in the Public Library of Science.The reef groups in question, known as the Scott Reefs and the Rowley Shoals, areclose to each other. They experience similar sea temperatures. And both were hit bypowerful cyclones and affected by coral bleaching in the mid-1990s.
One, however, turnedout to be rich in fish species whereas the other is impoverished, and the one with morespecies seemed to recover faster from the storms and the bleaching. Dr Ruppert believesthese differences are the result of the presence in one, and the absence in the other, ofsharks.The impoverished group, the Scott Reefs, has been fished for centuries by peopleinterested in catching sea cucumbers, certain snails and sharks - all of which areinternationally traded and fetch high prices. In recent decades the demand for sharks, inparticular, has boomed, as China has grown richer and its citizens have been supping moreshark-fin soup. The Rowley Shoals, by contrast, are a protected area where all fishing isprohibited.It is hardly a surprise that fishing reduces a reef's species diversity.
But what didsurprise Dr Ruppert and his colleagues was exactly how that diversity was diminished. Forbesides the species that fishermen are hunting (silvertip and grey reef sharks were threetimes as common around the Rowley Shoals as around the Scott Reefs), many other sorts ofanimal had suffered.This was not because they were being caught accidentally. Sea cucumbers and snailsare hand·picked by divers, and sharks are caught on lines, rather than in nets, with bait thatattracts only carnivores. All this means there is little bycatch. Yet the Scott Reefs also lackedherbivorous species such as parrotfish, though midsized predators, like snappers, were moreabundant than in the Rowley Shoals.
Dr Ruppert thinks that eliminating sharks meanspopulations of the midsized predators they feed on rise and those predators' prey then sufferthe consequences. Hence the reduced numbers of parrotfish.Their absence, however, has knock-on effects. Seaweed grows more thickly withoutparrotfish constantly gnawing at it. That growth smothers young coral and probably, thoughDr Ruppert cannot prove it in this particular case, makes it harder for reefs to recover fromcyclones and bleaching. Healthy reefs, then, seem to need sharks in the way that healthyforests need wolves. It's not much fun if you are a snapper or a deer. But Mother Natureprefers it thatway. (From The Economist, September 28, 2013)62Unit 11.
Human evolutionScript 29. Human evolution and palaeobotanyGrassed upA cherished theory about why people walk upright has just bitten the dustAfrica’s great grasslands are one of that continent's most famous features. They arealso reckoned by many to have been crucial to human evolution. This school of thought holdsthat people walk upright because their ancestors could thus see farther on an open plain.Forest primates do not need to be bipedal, the argument continues, because the trees limittheir vision anyway.As "Just So" stories go, it is perfectly plausible. But some go further and argue that thetransition took place when the savannahs themselves came into existence, replacing the preexisting forest and forcing human ancestors to adapt or die out.
Fossil evidence suggestshumanity's upright stance began to evolve between 6m and 4m years ago. So the questionis, did that coincide with the formation of the savannah?A paper in Geology, by Sarah Feakins, of the University of Southern California,suggests not. Dr Feakins studied sediment cores from the Gulf of Aden, a place whereoffshore winds deposit detritus from a goodly part of the east of the African continent. Inthese, she discovered plant molecules that date back between 12m and 1m years. Suchmolecules contain carbon, and carbon atoms come in various isotopes, whose ratios giveaway their history.
In particular, the ratio 12C to 13C can tell you what sort of plant made themolecule in question.Plants in rainforests tend to discriminate against 13C. Those in modern Africangrasslands are less selective and 13C is thus more abundant in their molecules. Dr Feakinswas therefore able to ask when these grasslands came about. To her surprise, they seem tohave been there even 12m years ago. Close examination of the cores shows that the natureof the grass changed over the millennia, as species that were adapted to dry condilions tookover from those that prefer wetter weather, but savannah of some form there always was.The climatic change she observed was already known about.
It was the reason peoplesuspected forests had given way to savannah. But, contrary to that suspicion, Dr Feakins hasshown that early humanity's east African homeland was never heavily forested, so the ideathat people were constrained to walk upright by the disappearance of the forests is wrong.Perhaps it was more pull than push - a preexisting, but empty ecological niche cryingout to be filled by an enterprising species that could make the transition. But perhaps thosewho seek an ecological explanation of this sort are, as it were, barking up the wrong tree.(From The Economist February 16, 2013)Script 30. Human evolutionYou look familiarAnother piece of humanity's family tree is fitted into placeThe opening scene of Mel Brooks's film "History of the World: Part One" dispenses withhuman origins in one line: "And the ape stood, and became man." Would that it were thateasy for palaeontologists to sort out.
The transition to humanity is generally agreed to haveoccurred between Australopithecus, a genus of small-brained, bipedal primates whose mostfamous member is a fossil nicknamed "Lucy", and the big-brained species Homo erectus. Butpinning down when precisely this took place, and which of the various australopithecinespecies were involved, has been challenging. Now the most human-like australopithecinefound to date is clarifying things- and staking a claim to be the species from which earlyhumans evolved.Fossils of the new species, Australopithecus sediba, were discovered in 2008 in a cavein South Africa. Initial research, led by Lee Berger of the University of the Witwatersrand, inJohannesburg, concluded that the species came too late in the fossil record to be the63ancestor of the Homo lineage.This week, however, a range of new research into sediba, again led by Dr Berger, hasbeen published in Science.
These studies conclude that sediba did in fact predate Homoerectus and, moreover, that parts of its anatomy are surprisingly similar to modern man.The fossils examined in the Science papers are of an adolescent boy and an adultwoman. They are well preserved, and encased in sediments that allow uncommonly precisedating. They lived 1.977m years ago, predating the appearance of Homo erectus by 77000years. The period is an especially muddled one for palaeontology, being full of fragmentaryfossils that are difficult to assign either to Homo or to Australopithecus.
The sediba fossils, bycontrast, have some of the most complete features in the early human record.The new studies centre on the most telling bits of anatomy in the story of humanevolution: the brain, pelvis, hands and feet. The brain itself does not fossilise, but the insideof the cranium retains an impression of its contours. The researchers mapped these withhigh-powered x-ray beams to create a three-dimensional model of the surface of sediba'sbrain.
They found that its size was on a par with other australopithecines, but its shape wasmore like that of a human brain. Specifically, the frontal lobes, which are the seat in modernhumans of higher cognitive functions such as abstract reasoning, looked more humanlike insediba's brain than they do in the brains of other australopithecines. That suggests theneurological changes which gave rise to humanity may have predated the brain's expansionan event that had, hitherto, been regarded as crucial to the emergence of humans.The hands, feet and pelvis of sediba indicate that it both climbed trees and walkedupright, though with a different gait from that of humans or chimpanzees.
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