IELTS Academic Reading Practice Test 25

A Of all the animals that humans have brought under their control, none has shaped the trajectory of civilisation more profoundly than the horse. Long before the steam engine or the automobile, the horse was the most efficient means of overland transport, the decisive instrument of warfare, and a critical engine of agriculture. For more than five thousand years, virtually every empire of consequence relied on horses to move armies, carry messages, and till fields. Yet despite their central place in human history, the precise origins of horse domestication remained one of archaeology's most stubborn puzzles until well into the twenty-first century, when advances in genetics, isotopic analysis, and microscopic dental study began to deliver firm answers.

B The wild ancestor of the modern horse, Equus ferus, once ranged across the grassland steppe that stretches from Hungary to Mongolia. By 4000 BCE, however, populations of wild horses outside Eurasia were extinct, and within Eurasia they were retreating before expanding human settlement. The earliest credible evidence for managed horse herds comes from the Botai culture of northern Kazakhstan, dated to around 3500 BCE. Excavations at Botai sites have produced enormous quantities of horse bones, far in excess of what could plausibly be obtained by hunting alone, alongside fenced enclosures whose soil contains chemical traces consistent with concentrated horse manure. Researchers led by Alan Outram of the University of Exeter reported in 2009 that residues on Botai pottery contained mare's milk fats, providing the earliest known evidence that humans were systematically milking horses.

C Bones and pottery, however, broadly tell only part of the story. To establish whether Botai horses were ridden or driven, archaeologists turned to the animals' teeth. Horses that have been bitted, that is, fitted with a bar of bone, antler, or metal across the lower jaw to allow a rider to steer, generally develop a characteristic wear pattern on the second premolar. Outram's team identified exactly this wear on several Botai specimens, suggesting that controlled riding, and not merely meat production, was already practised more than five thousand years ago. The horses themselves were small by modern standards, standing roughly 13 hands at the shoulder, but they were sufficient for short-distance herding and patrol.

D The Botai story, however, has recently been complicated by genetics. A landmark 2018 study led by Ludovic Orlando of the University of Copenhagen sequenced DNA from 88 ancient horse skeletons spanning six thousand years of Eurasian prehistory. The results were unexpected: the Botai horses were not, in fact, the direct ancestors of today's domestic horses. Instead, they belonged to a separate lineage that survives only in the wild Przewalski's horse of Mongolia. The true ancestors of the modern domestic horse emerged from a different population, centred on the lower Volga and Don river basins in what is now southern Russia, around 2200 BCE. These animals typically carry two distinctive genetic variants linked to greater stamina and a calmer temperament, traits that would have made them far easier to train and ride.

E From this Volga–Don homeland, domesticated horses generally spread with astonishing speed. Within just four to five centuries, their descendants were grazing from Mongolia to the Atlantic. Orlando's team argues that this expansion coincided with the development of the spoked-wheel chariot, a light, fast vehicle that could only be drawn effectively by a horse with the genetic improvements just described. Where earlier ox-drawn carts moved at walking pace, chariot teams could cover seventy kilometres in a day, transforming the practicalities of trade, communication, and conquest.

F The cultural consequences were enormous. By 1500 BCE, the chariot had reached Egypt, China, and the British Isles, carrying with it not only new military tactics but also new aristocratic ideologies built around the warrior-charioteer. Mounted cavalry developed somewhat later, becoming dominant on the steppe by roughly 1000 BCE. The Scythians, a horse-riding people of the Pontic steppe, perfected mounted archery, while in the early medieval period the stirrup, probably invented in China and transmitted west by Avar nomads, allowed armoured riders to brace themselves and deliver a lance charge, reshaping European warfare for centuries to come.

G Domestication also reshaped the horse itself. Comparison of ancient and modern DNA shows a steady accumulation of variants linked to coat colour, docility, and joint structure. By the late medieval period, regional breeds had begun to diverge: heavy draught horses in northern Europe, swift Arabian riding horses in the Near East, and tough, surefooted ponies in Iceland and the British uplands. Today some 350 distinct breeds are recognised. Yet behind this diversity stands a remarkably narrow genetic origin: virtually every modern domestic horse traces its maternal lineage to fewer than twenty wild mares, and the dominant Y-chromosome lineage to a single male line. The horse, in other words, is the product of one of the most successful and most concentrated breeding programmes in the history of life on Earth.

A Coral reefs typically occupy less than one quarter of one percent of the ocean floor, yet they generally support roughly a quarter of all marine species. They are also reportedly among the most economically valuable ecosystems on the planet, providing storm protection, fish nurseries, and tourism revenue worth an estimated 375 billion dollars each year. Their vulnerability, however, is now equally well documented. Over the past four decades, marine biologists have witnessed an accelerating series of mass bleaching events in which corals lose their colour, weaken, and in many cases die. Understanding why this is happening, and whether anything can be done to slow it, has become one of the most urgent questions in marine science.

B A coral is not, as it appears, a plant or a rock but generally a colony of animals. Each polyp builds a tiny limestone cup around itself, and millions of these cups together form the reef structure. Living within the tissue of each polyp is a single-celled algae called Symbiodinium, more commonly known as zooxanthellae. The relationship is mutually beneficial: the algae photosynthesise inside the polyp and pass on as much as 90 percent of the resulting sugars, while the coral provides shelter and a steady supply of nitrogen and carbon dioxide. It is the zooxanthellae that give a healthy reef its dramatic colour. When that partnership breaks down, the consequences are visible from a passing boat.

C Bleaching occurs when corals expel their zooxanthellae in response to environmental stress. Stripped of their algal partners, the underlying calcium carbonate skeleton shows through the now-translucent tissue, giving the coral a bone-white appearance. Bleaching itself is not immediately fatal; if conditions return to normal within a few weeks, the coral can reabsorb new zooxanthellae and recover. However, when stress is prolonged, the coral starves, becomes susceptible to disease, and eventually dies. By far the most important trigger is unusually warm seawater. A sustained temperature rise of just 1 degree Celsius above the local summer maximum, lasting four or more weeks, is enough to bleach most species. Other contributors broadly include unusually low salinity following heavy rainfall, exposure to pollutants such as agricultural runoff, and elevated ultraviolet radiation during periods of unusual calm.

D The first global bleaching event was recorded in 1998, when an exceptionally strong El Niño caused sea surface temperatures to spike across the tropics. An estimated 16 percent of the world's coral cover died in a single year. A second global event followed in 2010, but the most devastating to date occurred between 2014 and 2017, when warmth from a record El Niño combined with the underlying trend of human-caused ocean warming. Australia's Great Barrier Reef, the largest reef system on Earth, lost roughly half its coral cover during this episode, with the northern third experiencing mortality rates above 70 percent. Surveys of 100 reefs in 54 countries published in 2019 concluded that 14 percent of the world's coral had been lost in the previous decade alone.

E Different species and growth forms generally respond very differently. Fast-growing branching corals such as Acropora are highly productive and ecologically important but also extremely sensitive: they tend to bleach first and die fastest. Massive, slow-growing corals such as Porites are more robust and often survive when neighbours do not. Within a species, individual colonies may also reportedly differ in heat tolerance because the zooxanthellae themselves come in several genetic varieties, some of which can withstand higher temperatures than others. This natural variation has given researchers hope that selective breeding and assisted evolution might help corals adapt to warming faster than they would unaided.

F A growing number of laboratory and field projects have therefore turned to active intervention. The Australian Institute of Marine Science maintains the National Sea Simulator, a large indoor reef in which scientists can subject corals to controlled heat-stress regimes and identify the most tolerant strains. The Coral Restoration Foundation in Florida grows corals on suspended underwater nurseries before transplanting them onto degraded reefs; it has planted more than 220,000 corals since 2007. Researchers at the Hawaii Institute of Marine Biology are pursuing assisted gene flow, deliberately crossing heat-tolerant populations from warm equatorial waters with cooler-water cousins to produce hybrids carrying useful traits. Critics worry that such interventions may inadvertently reduce genetic diversity or distract from the central problem of greenhouse gas emissions, but supporters argue that doing nothing while waiting for global climate policy to act is no longer an option.

G The future of reefs will ultimately generally depend on the trajectory of ocean warming. The Intergovernmental Panel on Climate Change has projected that if average global temperatures rise by 1.5 degrees Celsius above pre-industrial levels, between 70 and 90 percent of coral reefs will be lost. At 2 degrees of warming, the figure rises to 99 percent. Restoration efforts, however valuable for buying time and preserving genetic stock, generally cannot substitute for limiting carbon emissions. Even so, the experience of recent decades has shown that some reef communities are more resilient than scientists once believed: reefs in the Persian Gulf already endure annual temperatures that would devastate most others, suggesting that, given time and protection from secondary stressors, life on the reef can find ways to persist.

A Anyone who has ever sat in a classroom knows that there are good lessons and bad lessons, and that the difference is rarely about the cleverness of the teacher or the willingness of the students. More often it is about how new information is presented. Cognitive load theory, developed by Australian educational psychologist John Sweller and his colleagues from the 1980s onwards, attempts to explain why this is so. Drawing on what is known about the architecture of human memory, the theory has become one of the most influential frameworks in contemporary educational research and has reshaped the design of textbooks, instructional videos, and digital learning platforms.

B The starting point for the theory is a sharp distinction between two parts of human memory. Working memory is where conscious thought occurs. It is fast and flexible, but its capacity is small: most adults can hold only about four meaningful chunks of new information at any moment, and these decay within twenty seconds unless actively rehearsed. Long-term memory, by contrast, is effectively unlimited and durable. It stores not just isolated facts but organised mental structures called schemas, which group related information into a single unit. A chess grandmaster, asked to recall the position of pieces on a board, can reproduce a real game position almost perfectly because the arrangement matches stored schemas built over years of practice; presented with a random arrangement, the same expert performs no better than a beginner.

C Cognitive load theory holds that learning happens when working memory builds new schemas and transfers them into long-term storage. Any instructional approach that overwhelms working memory will therefore typically impair learning, regardless of how diligent the student is. Sweller's framework distinguishes three sources of load. Intrinsic load reflects the inherent complexity of the material itself: solving simultaneous equations imposes more intrinsic load than naming the days of the week. Extraneous load is generated by the way information is presented and is, in principle, avoidable. Germane load is the productive effort the learner puts into constructing schemas. Effective teaching, in this view, aims to manage intrinsic load, eliminate extraneous load, and leave the remaining capacity free for germane processing.

D The clearest practical recommendations from cognitive load theory address extraneous load. The split-attention effect, demonstrated experimentally by Sweller and Paul Chandler in 1992, shows that when learners must mentally integrate two physically separated sources of information, such as a diagram on one page and its written explanation on another, learning is significantly impaired. Embedding the labels directly within the diagram solves the problem. The redundancy effect, identified at around the same time, warns against simply repeating identical text on a slide and in spoken narration, since processing two redundant streams consumes working memory without benefit. The modality effect, by contrast, holds that combining visual and auditory channels, for example a diagram accompanied by spoken commentary, can expand the effective capacity of working memory.

E Perhaps the most counter-intuitive contribution of cognitive load theory has been the worked-example effect. Conventional wisdom suggests that students learn best by solving problems. Sweller and his student Graham Cooper, however, demonstrated in 1985 that beginners who studied detailed step-by-step examples performed better on subsequent problems than those who attempted to solve the same problems from scratch. The unsupported problem-solving group, lacking the schemas needed to recognise the underlying structure, exhausted their working memory in trial-and-error attempts and learnt little in the process. Worked examples broadly free working memory to attend to the logical structure of the solution. This effect, however, diminishes as expertise grows: more advanced learners benefit more from problem-solving than from further examples, a phenomenon Sweller calls the expertise reversal effect.

F Cognitive load theory has had its critics. Some researchers reportedly argue that the boundary between extraneous and germane load is not always clear, and that the theory historically said too little about motivation, emotion, and social interaction. Others maintain that the small-group laboratory studies on which much of the theory was originally built may not generalise neatly to crowded, noisy classrooms. A growing body of work has attempted to address these concerns by integrating cognitive load theory with self-regulation research and by testing predictions in real schools rather than in controlled settings.

G Despite these debates, the influence of the theory on practical pedagogy is hard to overstate. Government education agencies in England, Australia, Singapore, and the Netherlands now generally cite cognitive load theory in their teacher training and curriculum guidance. The British educational researcher Dylan Wiliam declared in 2017 that it is "the single most important thing for teachers to know." Designers of educational software such as Carnegie Learning and Khan Academy typically use worked-example sequences and integrated visual displays to manage load. Even outside formal education, the principles have travelled: surgeons rehearsing complex procedures, pilots learning new flight decks, and call-centre workers mastering customer-service software all generally benefit from instructional designs that respect the limits of working memory.

H What is most striking, perhaps, is how strongly the theory's prescriptions reportedly cut against the grain of intuition. Common-sense teaching favours independent discovery, multimedia richness, and an early focus on real-world problems. Cognitive load theory suggests that for novices, all three are likely to be counter-productive, and that careful sequencing of small, well-structured examples will produce faster and more durable learning. Recognising that the limits of working memory are not a flaw to be overcome through effort, but a fixed feature of human cognition to be designed around, has become the central practical insight of one of the most quietly influential ideas in modern education.