IELTS Academic Reading Practice Test 33

A The treatment of teeth is widely regarded as among the oldest of medical specialisms, with archaeological evidence pushing its origins back nine thousand years. In 2006, a team of researchers excavating a Neolithic burial site at Mehrgarh in modern Pakistan reported eleven adult molars showing what appear to be deliberate, conically shaped holes drilled into their crowns. The bores, measuring between one and three millimetres across, had been largely produced using a bow-driven flint drill of the type also employed for bead-making at the same site. Crucially, several of the perforations showed clear signs of post-drilling wear, indicating that the patients had largely continued chewing on their treated teeth and that the procedure was not a post-mortem ritual but a therapeutic intervention performed during life.

B The earliest written text devoted to dentistry is said to come from Mesopotamia and to date to roughly 1800 BCE. Known as the Tooth Worm legend, it attributes dental decay to a tiny worm that gnaws within the tooth, an explanation that proved astonishingly tenacious: variants of the same theory appeared in Egyptian, Chinese, Greek, and medieval European writings well into the eighteenth century. The Edwin Smith Papyrus, an Egyptian medical document from around 1700 BCE, describes the lancing of dental abscesses and the splinting of loose teeth with gold wire, while the Ebers Papyrus from a century later prescribes herbal pastes containing frankincense and honey to soothe toothache.

C Greek and Roman writers placed dentistry within general medicine. Hippocrates and Aristotle both wrote on the eruption of teeth, the treatment of fractured jaws, and the extraction of decayed molars using forceps. The Roman physician Celsus, writing in the first century CE, gave detailed instructions for stabilising loose teeth with linen ligatures and recommended that practitioners examine the gums carefully before any extraction. Affluent Romans had cavities filled with a paste of pulverised animal bone, while wealthy Etruscan women wore partial gold bridges fitted with replacement teeth carved from ox bone, which have been recovered today from tombs across central Italy.

D After the fall of the Western Roman Empire, dentistry in Europe entered what is regarded as a long period of decline. From roughly 500 to 1500 CE, tooth extraction was performed not by physicians but by barbers and itinerant tooth-pullers, who set up stalls at fairs and used pliers, chisels, and brute force. Pain relief, when offered at all, consisted of wine, distilled spirits, or pastes made from opium and mandrake. In contrast, dental knowledge in the Islamic world advanced considerably during the same period. The tenth-century Andalusian surgeon Abulcasis described thirty different dental instruments in his medical encyclopaedia and pioneered the careful scaling of dental calculus, which he correctly identified as a cause of gum disease.

E The transformation of dentistry into a recognised profession is credited largely to the French surgeon Pierre Fauchard, whose 1728 treatise Le Chirurgien Dentiste is said to have largely established the discipline on a scientific footing. Fauchard rejected the Tooth Worm theory, identified acid produced by food residues as the true cause of decay, and described techniques for filling cavities with tin, lead, or gold foil. He also designed an early dental drill powered by a bow-and-string mechanism and devised orthodontic appliances of silver wire for straightening misaligned teeth. His insistence on the use of a dental chair, rather than the floor or a doorstep, signalled a new conception of dentistry as a careful, seated procedure rather than a roadside spectacle.

F The nineteenth century broadly brought rapid technical progress. The first reclining dental chair was patented by James Snell in 1832, and the foot-treadle drill, capable of rotation at up to two thousand revolutions per minute, was introduced by James Beall Morrison in 1871. Anaesthesia transformed the patient experience: the American dentist Horace Wells demonstrated the use of nitrous oxide in 1844, and his colleague William Morton famously used ether at Massachusetts General Hospital in 1846. Amalgam, a mixture of mercury with silver and tin, became the standard filling material from the 1830s onwards, largely supplanting more costly gold foil despite recurrent controversies about mercury exposure.

G By the twentieth century, dentistry had become a fully professionalised field with formal training, licensure, and increasingly specialised branches including orthodontics, endodontics, and oral surgery. The introduction of fluoridated public water supplies, first implemented in Grand Rapids, Michigan in 1945, produced dramatic declines in childhood cavities, while the development of high-speed air-turbine drills in the 1950s and adhesive composite resin fillings in the 1960s made restorative work faster and more aesthetically pleasing. Today, the World Health Organization estimates that some 3.5 billion people worldwide may suffer from untreated dental disease, a reminder that the global picture is still shaped less by frontier technology than by access to even the most basic preventive care.

A For most of human history, the great seasonal movements of birds, fish, and mammals were inferred only from the times of their arrival and disappearance. Aristotle famously believed that swallows hibernated in mud during winter, and the idea that small songbirds crossed entire oceans was widely dismissed as fanciful until well into the nineteenth century. The modern science of animal migration is said to have begun with the simple metal leg ring. The first systematic ringing scheme was established in 1899 by the Danish schoolteacher Hans Christian Cornelius Mortensen, who fitted aluminium rings stamped with his return address to several hundred starlings near his school in Viborg. When recovered birds began to appear in the post from across Europe, the routes and distances involved finally became measurable rather than mythical.

B Ringing remains useful for studies that depend on very large sample sizes and very small budgets, but it suffers from a fundamental limitation: data are obtained only when a marked individual is recaptured or found dead. Even in well-studied species, recovery rates are below one percent. The shift from this passive form of monitoring to active tracking is said to have begun in 1957, when American zoologist William Cochran fitted a tiny radio transmitter to a thrush near Champaign, Illinois, and followed the bird's signal by car as it migrated southward. Radio telemetry, however, was constrained by the limited range of its receivers and by the weight of early transmitters, which largely restricted the technique to animals weighing several kilograms or more.

C The launch of the Argos satellite system in 1978 transformed wildlife tracking by allowing investigators to receive position fixes from any tagged animal anywhere on Earth. Early Argos units weighed close to a kilogram and were therefore generally deployable only on large mammals and the biggest seabirds; researchers nevertheless used them to reveal that wandering albatrosses reportedly circumnavigate the entire Southern Ocean during a single non-breeding season and that adult leatherback turtles may dive to depths exceeding twelve hundred metres. The Argos system continues to operate today, although a new generation of low-power GPS tags may offer far more accurate locations and is the technology of choice for medium-sized animals.

D Smaller species presented a stubborn problem until the development of the geolocator, an extremely light device that records ambient light levels at fixed intervals and uses the timing of sunrise and sunset to estimate latitude and longitude. Geolocators weigh as little as half a gram, can be carried by birds the size of a thrush, and apparently run for over a year on a single battery. The catch, however, is that data must be downloaded from the tag itself, so the bird has to be recaptured. Despite this drawback, geolocator studies have produced some of the most dramatic recent revelations in ornithology, including the discovery that the bar-tailed godwit flies non-stop from Alaska to New Zealand on a journey of more than eleven thousand kilometres lasting around eight days.

E For very small fish and insects, even geolocators reportedly remain too heavy. The arrival of passive integrated transponder (PIT) tags, which weigh only a few hundredths of a gram and respond to a brief radio pulse from a nearby antenna, has revolutionised the study of stream-dwelling species. Antennae buried beneath fish ladders on the Columbia River system, for example, generally log every tagged salmon that passes upstream during its spawning migration, generating millions of detections each year. Researchers studying monarch butterflies have used tiny radio tags barely larger than a grain of rice, although power remains too limited for true long-distance tracking; for monarchs, citizen-science recoveries of paper tags glued to wings still apparently play a central role.

F Tracking technology has produced more than a list of routes and destinations. By coupling location data with measurements of altitude, water depth, temperature, light, and even heart rate, biologists can now apparently reconstruct the moment-to-moment decisions of migrating animals in remarkable detail. Studies of European white storks have shown that birds raised in different parts of their range generally follow distinct flyways into Africa and that climate change is gradually shifting the timing and the geography of these flyways year by year. Data from acoustic tags on Atlantic bluefin tuna have demonstrated that the species crosses the ocean far more often than was once thought, complicating the management of regional fisheries that had assumed a more sedentary stock.

G The next frontier is widely regarded as the integration of tracking with global biodiversity infrastructure. The International Cooperation for Animal Research Using Space, or ICARUS, project, hosted on the International Space Station since 2020, aims to receive signals from tags weighing only a few grams attached to tens of thousands of animals worldwide. The open-data Movebank archive operated by the Max Planck Institute reportedly already holds tracks from more than a thousand species and three billion individual locations, available to researchers free of charge. As tags become smaller, batteries more durable, and computational models more sophisticated, the gap between knowing that an animal migrates and understanding why and how it does so continues to narrow.

A A memory is rarely fixed in the brain at the moment of an experience. Instead, an initially fragile representation undergoes a prolonged process of stabilisation and reorganisation that neuroscientists broadly call consolidation. The term itself is said to have been coined by the German researchers Georg Elias Müller and Alfons Pilzecker, who reported in 1900 that lists of nonsense syllables learned by their volunteers were better remembered when followed by a quiet interval than by further learning. The interference of the second task, they argued, must somehow have disrupted a fixative process apparently still running in the background. Their observation has proved one of the most durable in psychology, and the search for the cellular and systems-level machinery behind it has occupied generations of researchers since.

B Two broad theoretical traditions appear to have developed in parallel during the twentieth century. The first, often called synaptic consolidation, locates the relevant changes at individual brain connections. According to this view, the storage of new information involves the rapid strengthening of synapses in the hippocampus, a structure deep within the temporal lobe, through a cellular process termed long-term potentiation, or LTP. Repeated stimulation of a pathway produces sustained increases in synaptic efficacy that depend on the synthesis of new proteins; if protein synthesis is blocked within a few hours of learning, the memory fails to stabilise. Eric Kandel's Nobel-winning work on the marine snail Aplysia is said to have demonstrated the same basic mechanism in a vastly simpler nervous system, providing a powerful link between molecular biology and human memory.

C The second tradition, known as systems consolidation, addresses a different question: namely, how a memory shifts from depending on the hippocampus to depending on the broader cortex. The classic evidence comes from the patient known as H.M., who in 1953 underwent bilateral removal of his medial temporal lobes to treat severe epilepsy. After the operation, H.M. could no longer form new long-term memories of facts or events, yet he largely retained childhood memories formed long before the surgery. Studies of H.M. and similar patients suggested that the hippocampus is critical for the initial storage of declarative memories, but that, over weeks, months, or years, the same information becomes stored in distributed cortical networks that can function without hippocampal support.

D The dominant account of systems consolidation, widely known as the standard model, proposes that during sleep and quiet wakefulness the hippocampus replays patterns of activity associated with recent experiences, training cortical networks to represent the same information. Direct evidence for hippocampal replay was first reported in 1994 by Matthew Wilson and Bruce McNaughton at the University of Arizona, who recorded from rats running on a track and observed that the same sequences of place-cell firing repeated, at compressed timescales, during subsequent slow-wave sleep. Subsequent work appears to have shown that replay also occurs during quiet rest within minutes of an experience, and that disrupting it impairs later memory.

E Not all theorists accept the standard model in its strong form. The multiple-trace theory, proposed by Lynn Nadel and Morris Moscovitch in 1997, argues that vivid, detail-rich episodic memories continue to depend on the hippocampus throughout life, while only more schematic, semantic forms of memory apparently become genuinely independent of it. Their case rests partly on patients with extensive hippocampal damage who lose access to detailed autobiographical recollections even when more generic factual knowledge from the same period of life is largely preserved. The trace-transformation account, developed by Moscovitch and his collaborators since the early 2000s, refines this view by proposing that memories are not so much copied from hippocampus to cortex as transformed into different representational formats as they age.

F Two phenomena have complicated all of the above accounts. The first is reconsolidation, a process discovered or rediscovered by Karim Nader and his colleagues at New York University in 2000. They showed that reactivating an largely established fear memory in rats by re-exposing the animals to its triggering cue made the memory transiently vulnerable to protein-synthesis blockers, apparently exactly as if it had just been encoded. The implication, in the view of most commentators, is that every retrieval is also a re-encoding, and that memories are continually rewritten by the act of remembering them. The second is sleep-dependent consolidation, which involves not just generic rest but specific stages: slow-wave sleep appears to favour the consolidation of declarative memories, while rapid-eye-movement sleep is said to be more important for procedural and emotional ones.

G Recent work has begun to explore engrams, defined as the physical traces left by individual memories in groups of neurons. Using optogenetic methods that permit specific neurons to be switched on or off with pulses of light, researchers at the Massachusetts Institute of Technology demonstrated in 2012 that artificially reactivating the cells that had been active during a learning episode could elicit the corresponding behaviour days later, even in the absence of the original cue. By tagging engram cells with fluorescent markers, the same group later showed that the population of neurons holding a given memory expands and migrates between brain regions over time, providing perhaps the most concrete physical correlate of systems consolidation discovered to date.

H There is broad agreement today that consolidation is not a single process but a family of overlapping mechanisms operating at different timescales, ranging from minutes for synaptic stabilisation to years for the cortical settling of complex autobiographical memory. The therapeutic implications are widely held to be considerable. Targeted disruption of reconsolidation has been explored as a treatment for post-traumatic stress disorder, on the grounds that an emotional memory recalled in a safe context and then destabilised may be re-encoded in less debilitating form. Strategic timing of sleep, exercise, and even brief naps after learning is being investigated for educational settings, and pharmacological enhancement of memory consolidation, while generally still experimental, is no longer purely speculative. Whether for ageing patients or for the design of artificial learning systems, the slow scientific story of consolidation appears to have begun to deliver practical dividends.