Passage
Around 13,000 years ago North America had a more diverse mammal community than modern-day Africa. There were multiple horse species, camels, llamas and a now-extinct animal called Glyptodon, which looked something like a Volkswagen bug–size armadillo. Smilodon, a sabertoothed cat around the size of today’s African lion, skulked across the grasslands in search of ground sloths and mammoths. Seven-foot-long giant otters chowed down on massive trees. And such massive creatures were not just found in North America. On every continent mammals on average were a lot larger in the late Pleistocene, the geologic epoch spanning from around 2.5 million until about 11,700 years ago.
Scientists have long debated what caused all these large-bodied critters to go extinct while many of their smaller counterparts survived. A team of researchers led by University of New Mexico biologist Felisa Smith analyzed evidence from millions of years’ worth of mammalian extinctions and found that on each continent large mammals started to die out around the same time humans first showed up.
If the extinction trend continues apace, modern elephants, rhinos, giraffes, hippos, bison, tigers and many more large mammals will soon disappear as well, as the primary threats from humans have expanded from overhunting, poaching or other types of killing to include indirect processes such as habitat loss and fragmentation. The largest terrestrial mammal 200 years from now could well be the domestic cow, Smith’s research suggests.
Some scientists lay the blame squarely on humanity’s shoulders, arguing overhunting doomed the planet's megafauna. After our hominid relative Homo erectus fanned out from Africa into Eurasia starting some two million years ago, Homo sapiens followed around 60,000 to 80,000 years ago and became widespread in Eurasia, joining our close cousins, the Neandertals and Denisovans. It is thought Homo sapiens later reached Australia between 50,000 and 60,000 years ago and finally settled the Americas between 13,000 and 15,000 years ago. In the time line of mammalian extinctions, large animals started to disappear only after humans or their hominid cousins showed up. But could that be a coincidence? Others have argued the main culprit behind these die-offs was the changing climate.
In North America the average mammal weighed around 98 kilograms before the ancestors of humans showed up. Today the average size is closer to eight kilograms.“We’ve lopped a couple orders of magnitude off the distribution of mammals’ [body sizes],” Smith says. For most of mammalian evolutionary history, an animal’s size was not predictive of its extinction risk. That link only appeared once hominids began to live alongside large mammals.
This finding does not mean climate-related changes could not have stressed some wildlife populations, enabling humans to more easily bring about their eventual downfall. Rather it suggests the greater likelihood of large-bodied mammals going extinct is tied to human activities. A suite of animals that evolved in Eurasia, Australia and the Americas without the risk of predation from tool-using, fire-making, group-living hominids were suddenly faced with a new threat. They simply could not adapt fast enough to survive the incursion of these omnivorous bipedal apes.
In addition, Smith’s analysis looked at the size distribution of African mammals prior to the hominid migration into Eurasia. She found African mammals were also smaller on average once hominids began appearing on the landscape there—and they evolved right alongside one another. “They have evidence that hominids in Africa had already been impacting the size distribution of mammals on that continent before Homo sapiens evolved,” says paleoecologist Emily Lindsey, assistant curator and excavation site director of the La Brea Tar Pits Museum in Los Angeles, who was not involved in the study. What that means, she says, is “these groups of hominid species were having impacts on a continental scale before the evolution of modern humans.” And it does not take all that many hominids to have such broad effects. Driving a large species to extinction does not mean killing every last one of its members. “You just have to kill slightly more than are being produced each year,” Lindsey says. If a population's reproduction rate cannot compensate for its losses each year, within a few hundred to a couple of thousand years the species will simply die out.
Large-bodied mammals are especially vulnerable because they reproduce slowly. Mammoths and mastodons, for example, likely had a two-year gestation period, akin to modern elephants, and would have typically produced just one offspring at a time. It is therefore a lot easier to decimate a population of 100,000 mammoths than a population of 100,000 rabbits, which reproduce twice a year and birth by litter.
Question for 100 RCs for Practice Questions- 53
Try yourself:The central idea of the passage is that …
Explanation
The passage starts by claiming that“On every continent mammals on average were a lot larger in the late Pleistocene …” The second paragraph ties the cause of extinction of these large mammals to human evolution by stating that “large mammals started to die out around the same time as humans first showed up.” The next paragraph states that if this extinction trend continues, then modern large mammals will also disappear, and the domestic cow may be the largest mammal alive 200 years from now. The author then explores in detail the link between human activities and the extinction of large species. Later, the passage states that “Large-bodied mammals are especially vulnerable [to extinction] because they reproduce slowly.” Thus, the main idea of the passage is that among other reasons human activities accelerate the extinction of large mammals even today.
Option 1 is incorrect. The passage does not suggest whether we must start thinking about minimizing our impact on Earth. Eliminate option 1.
Option 2 is incorrect. This option is an exaggeration, we cannot conclude from the passage whether humans or their ancestors deliberately killed huge number of large mammals. Eliminate option 2.
Option 4 is incorrect. This option correctly covers one of the themes of the passage that human activities led to the extinction of large mammals in the late Pleistocene. But it fails to mention the other major point of the passage that human activities are driving modern large mammals to extinct as well. Eliminate option 4.
Option 3 is correct. This option covers the idea that human and other hominids were primarily responsible for extinction of large species in the past, and that humans may be driving modern large mammals to extinction through their activities.
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Question for 100 RCs for Practice Questions- 53
Try yourself:Which one of the following statements can be inferred from the passage?
Explanation
Option 1 is incorrect. The information in paragraph 4 is inadequate to infer that the arrival of Homo sapiens in Australia between 50000 to 60000 years ago was the“first” significant impact that Homo sapiens had. Eliminate option 1.
Option 3 is incorrect. Paragraph 5 states that the extinction risk of an animal does relate to its size after the hominids began to live alongside large mammals. Eliminate option 3.
Option 4 is incorrect. The author states in paragraphs 5 and 6 that generally the size of the animal is not linked to its extinction risk, but the link appeared after hominids began to live alongside large mammals. And, though there could have been climate related reasons for their extinction, there is “the greater likelihood of large-bodied mammals going extinct (because of) human activities.” Thus, we can infer that human activities were the primary reason for extinction while climate change only aided that process. Eliminate option 4.
Option 2 is correct. Paragraph 4 states that “It is thought Homo sapiens later reached Australia between 50,000 and 60,000 years ago and finally settled the Americas between 13,000 and 15,000 years ago.” Also, paragraph 1 states that Pleistocene was the geological epoch spanning from around 2.5 million until 11,700 years ago. Thus, we can conclude that modern humans settle in the Americas in the late Pleistocene.
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Question for 100 RCs for Practice Questions- 53
Try yourself:According to the passage, which of the following is NOT a risk to the existence of modern large mammals?
Explanation
Option 1 is incorrect. Paragraph 2 states “If the extinction trend continues apace, modern elephants, rhinos, giraffes, hippos, bison, tigers and many more large mammals will soon disappear as well, as the primary threats from humans have expanded from overhunting, poaching or other types of killing to include indirect processes such as habitat loss and fragmentation.” The discontinuities in the environment of large mammals mean habitat fragmentation of large mammals. It is an extinction risk for modern large mammals according to the passage. Eliminate option 1.
Option 2 is incorrect. The destruction of the environment inhabited by large mammals amounts to habitat loss mentioned in paragraph 2. Thus, it is an extinction risk and we can eliminate the option.
Option 3 is incorrect. The last paragraph states that “Large-bodied mammals are especially vulnerable because they reproduce slowly.” Thus, we can conclude that large mammals are at a greater risk of extinction due to human activities since they have a slower reproductive rate and longer gestation period. Eliminate option 3.
Option 4 is correct. The passage contains no reference to the growing demand for meat and overpopulation of humans; also, large mammals are generally killed not for meat but for other commercial purposes. Also, even if they are killed for meat we have no data from the passage to conclude if they are being killed more than are being produced each year. So, option 4 is not advanced as an extinction risk to modern mammals.
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Question for 100 RCs for Practice Questions- 53
Try yourself:All of the following statements about hominids are true EXCEPT:
Explanation
Option 2 is incorrect. The third last paragraph describes hominids as“tool-using, fire-making, group living”. Thus, group-living implies that hominids tended to live in social groups and cooperated with each other. This option is not an exception. Eliminate option 2.
Option 3 is incorrect. “fire-making” suggests that hominids could start and maintain fire. Eliminate option 3.
Option 4 is incorrect. The 6th paragraph describes hominids as “omnivorous bipedal apes”. The word ‘bipedal’ means “two-footed”. So, eliminate option 4.
Option 1 is correct. The word omnivorous means: feeding on both animal and vegetable substances. “Omnivorous” is used to describe hominids; hence, option 1 is contrary to the passage.
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Question for 100 RCs for Practice Questions- 53
Try yourself:Which of the following best sums up the findings of the study led by biologist Felisa Smith?
Explanation
Option 1 is incorrect. The 6th paragraph states that human activities were primarily responsible for the extinction of large mammals and climate-change may have aided this extinction, not the other way round. Eliminate option 1.
Option 2 is incorrect. We see in the6th paragraph that the author does not deny the role of climate change in the extinction of large mammals. He states climate change could have stressed the large mammalian populations. Hence, eliminate option 2.
Option 3 is incorrect. This option is irrelevant in the context of the question asked. The findings were not that hominids were extremely proficient predators but that the link between extinction-risk of animals and their size appeared only after hominids began living alongside them. Eliminate option 3.
Option 4 is correct. Paragraph 5 states that“For most of mammalian evolutionary history, an animal’s size was not predictive of its extinction risk. That link only appeared once hominids began to live alongside large mammals.”
Thus, option 4 correctly sums up the findings of the team of researchers led by biologist Felisa Smith and is the correct answer.
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