IELTS Academic Reading Practice Test 11

A The shift from hunting and gathering to systematic agriculture is the most consequential change in the human past. For approximately 200,000 years of anatomically modern human existence, Homo sapiens lived in small mobile groups that subsisted on wild foods. Beginning around 12,000 years ago, in a handful of widely separated regions, this pattern broke down: some populations began to actively cultivate plants and confine animals, eventually settling in permanent villages, building public structures, and accumulating the surpluses on which urban civilisation depends. The transition is widely said to have unfolded in different ways in different places, but its long-term effects have, in many archaeologists' view, reshaped almost every aspect of how human societies are organised.

B The geography of agricultural origins is now well established, although the chronology has been steadily pushed back as new evidence accumulates. The Fertile Crescent of southwest Asia produced agriculture earliest, with wheat and barley cultivation evident by approximately 10,500 years ago. Independent agricultural transitions occurred in northern China with millet, southern China with rice, central Mexico with maize and squash, the Andean highlands with potatoes and quinoa, and the Eastern Woodlands of North America with chenopod and sumpweed. Sub-Saharan Africa and New Guinea developed their own crop suites slightly later. The independence of these centres has profound implications: agriculture was not a singular invention diffusing outwards but a response that multiple societies independently developed when comparable conditions occurred. The pattern invites the conclusion that the agricultural transition, far from being a unique cultural innovation, was a response that human populations were capable of making whenever local circumstances made it advantageous.

C The question of why agriculture began at all has produced extensive debate. Hunter-gatherer populations typically work fewer hours per week, eat more varied diets, and suffer less disease than early farmers. Skeletal evidence from the earliest farming populations shows increases in dental cavities, growth stunting, and infectious disease. Why, then, did people make the change? The most widely accepted modern view is that agriculture emerged when climatic and demographic conditions combined to make wild-food strategies unsustainable, with population pressure forcing intensification of food production despite the immediate costs. The explanation has, in many archaeologists' view, broadly come to replace earlier accounts that attributed the transition to a single moment of cultural insight or to the diffusion of practices from a single source.

D Domestication itself involved gradual genetic change in both plants and animals over many generations. Wild cereals shatter their seeds at maturity to disperse them, but rare mutant individuals retain seed on the plant; selection by harvesters with primitive sickles favoured these non-shattering forms, producing within a few centuries the harvestable cereals familiar today. Animal domestication followed parallel patterns: docile, tolerant variants of wild populations were selected over generations until they became reproductively dependent on humans. The cumulative effect of such selection across millennia has produced crops and livestock whose phenotypes typically bear only limited resemblance to their wild ancestors, and which can no longer survive without human cultivation or husbandry. Modern genetic studies have, in many archaeologists' view, confirmed the multi-generational picture, showing measurable selective sweeps on specific loci over the timescales the archaeological record had already suggested.

E The consequences for human society were far-reaching. Sedentism allowed accumulation of material goods. Agricultural surplus enabled specialisation: craftspeople, priests, soldiers, and rulers could be supported by farmers who produced more than they themselves consumed. Property and inheritance became major social categories. Population densities rose by orders of magnitude. The social structures we now describe as civilisation — cities, writing, taxation, hierarchical government — all emerged in the wake of agricultural intensification. The constellation of developments that followed has, in many historians' view, meant that nearly every feature of modern social organisation can be traced, in some form, to the changes set in motion by the agricultural transition twelve thousand years ago. Whether one regards the transition as a generally welcome development or, with some recent commentators, as a long detour from a more healthful earlier mode of life, the basic point that contemporary civilisation depends on it is widely held to be uncontroversial.

A The human gut hosts a remarkably diverse community of microorganisms — including bacteria, archaea, fungi, and viruses — whose collective number is widely thought to rival that of the body's own cells and whose total genetic content exceeds the human genome more than a hundredfold. Until relatively recently, this community was studied principally as a source of disease, with individual pathogens cultured in isolation. Over the past two decades, advances in DNA sequencing have allowed researchers to characterise the microbiome as an integrated ecosystem, transforming understanding of its role in human health. The reframing is widely said to have moved gut microbes from an afterthought of human physiology to a central organising principle of contemporary biomedicine.

B The composition of an individual's gut microbiome is shaped by a complex set of influences. Birth mode appears to be the first major determinant: babies delivered vaginally typically acquire a microbiome dominated by their mother's vaginal and faecal bacteria, while babies delivered by Caesarean section reportedly acquire a community more similar to the skin bacteria they first contact. Breastfeeding, weaning, antibiotics, diet, geographic region, and even cohabitation with pets are widely understood to continue shaping the microbiome through childhood and adulthood. Studies of identical twins separated by long distances have shown that diet and environment outweigh genetic background in determining adult microbiome composition. The implication, in many researchers' view, is that the gut microbiome is a dynamic feature of an individual rather than a fixed inheritance.

C The functions of the gut microbiome are, according to researchers, remarkably varied. The most obvious is digestion: bacterial fermentation of dietary fibre yields short-chain fatty acids that the human gut absorbs as an energy source, and bacterial synthesis is credited with providing vitamins K and several B vitamins that humans cannot produce themselves. Less obviously, the microbiome interacts continuously with the immune system, training it to distinguish pathogens from harmless commensals and influencing the calibration of inflammatory responses. The microbiome also produces or modifies an extraordinary range of small molecules that circulate through the bloodstream and reach essentially every organ. The breadth of these functions has, in many writers' view, led researchers to describe the microbiome as a kind of additional organ system whose chemistry the rest of the body has been routinely co-evolving with.

D Studies linking the microbiome to disease have proliferated in recent years. Inflammatory bowel disease, obesity, type 2 diabetes, colorectal cancer, autism spectrum disorder, depression, and Parkinson's disease have all reportedly been associated with distinct microbiome characteristics in human populations. Many of these associations remain correlational and the question of causation is, in the view of many researchers, largely unresolved, but mouse experiments — in which the microbiome of a sick human is transferred to germ-free mice and observed to induce some features of the original disease — appear to provide increasingly strong evidence that gut microbes can act causally on distant body systems. The mouse models have, in many researchers' view, elevated microbiome-disease links from intriguing correlations to provisional causal hypotheses that warrant clinical investigation.

E Therapeutic interventions targeting the microbiome have, in many researchers' view, advanced from the speculative to the clinical. Faecal microbiota transplantation, in which stool from a healthy donor is administered to a patient, has emerged as a highly effective treatment for recurrent Clostridioides difficile infection, with cure rates approaching 90 percent. Approved drugs based on this principle entered the US market in 2023. Probiotic and prebiotic products are sold widely, although the regulatory framework around their efficacy claims is widely considered contested. Personalised dietary interventions based on individual microbiome composition — promoted by several commercial startups — reportedly show promising preliminary results in metabolic studies but have not been validated by large independent trials. The current commercial landscape contains a wide range of products with very different levels of scientific support, and consumer judgement is widely said to require greater discrimination than the marketing encourages. The implication, in many writers' view, is that the microbiome has joined the small number of biological systems whose direct clinical manipulation has moved from speculation to standard practice within the span of a single generation.

A The carbon cycle is described as the continuous movement of carbon atoms through Earth's atmosphere, oceans, soils, rocks, and living organisms. Over geological timescales, the cycle is, according to most earth scientists, approximately balanced: carbon released from rocks by weathering and volcanism is matched by carbon removed from the atmosphere into ocean sediments and biological tissues. Over shorter timescales, however, the various reservoirs can fall in or out of balance, and the response of the cycle to perturbations has been one of the most consequential topics of modern earth science. The framing of the carbon cycle as a connected system, rather than as separate atmospheric, oceanic, and biological problems, has, in many writers' view, defined the modern climate-science agenda.

B The atmosphere holds approximately 870 billion tonnes of carbon, mainly as carbon dioxide and methane. The terrestrial biosphere — plants, animals, and especially the carbon-rich soils beneath them — is estimated to hold approximately 2,300 billion tonnes. The ocean is widely understood to hold far more: an estimated 38,000 billion tonnes, mostly as dissolved inorganic carbon in the form of bicarbonate ions. Vastly more carbon — perhaps as much as 75,000,000 billion tonnes — is said to be locked in sedimentary rocks, principally as limestones formed from the remains of marine organisms over hundreds of millions of years. The size of the rock reservoir underscores how small a fraction of the planet's total carbon is in active circulation at any given time.

C The fluxes between these reservoirs are mediated by specific physical and biological processes. Photosynthesis is reported to remove approximately 120 billion tonnes of atmospheric carbon per year into plant biomass, with most of it returning via respiration and decomposition. Ocean uptake adds another estimated 90 billion tonnes per year, partially balanced by ocean release. Geological fluxes are much smaller in any given year — volcanism releases about 0.1 billion tonnes — but their long-term integration is widely considered to control atmospheric carbon over geological time. The comparison of annual fluxes with reservoir sizes is, in many researchers' view, broadly the simplest way to grasp why the carbon cycle is so sensitive to apparently small additional inputs from human activity.

D Human activities, in the view of many earth scientists, have substantially perturbed the cycle. Fossil-fuel combustion currently releases approximately 10 billion tonnes of carbon per year into the atmosphere — a flow that, while smaller than annual photosynthetic uptake, represents the addition of carbon that has been locked away from the atmosphere for hundreds of millions of years. The atmosphere absorbs roughly half of this anthropogenic emission, with the ocean taking up about a quarter and terrestrial ecosystems absorbing the remainder, although the proportions are widely understood to vary year by year. The partitioning of the additional carbon among reservoirs has emerged as one of the most important quantitative parameters for projecting the climatic effects of continued emissions.

E The ocean's response to elevated carbon dioxide is widely regarded as complex. Increased dissolution of carbon dioxide produces carbonic acid, which lowers ocean pH — a phenomenon called ocean acidification. Surface ocean pH has fallen by about 0.1 unit since pre-industrial times, representing what researchers describe as a 30 percent increase in hydrogen ion concentration. This appears to affect organisms that build shells or skeletons from calcium carbonate, including corals, molluscs, and certain plankton, with implications said to cascade through marine food webs. The biological consequences of acidification have been described by some researchers as the climate-change problem's silent twin, in that they unfold without the visible signals of weather extremes.

F Terrestrial ecosystems also respond to elevated atmospheric carbon dioxide, but in ways that vary considerably between systems. The "carbon fertilisation effect," in which higher carbon dioxide accelerates plant growth, has been measured in temperate and tropical forests, although the magnitude of the effect appears to saturate at concentrations not far above current levels. Other terrestrial responses are regarded as less benign: thawing of Arctic permafrost is releasing previously locked organic carbon, and increased fires in boreal forests appear to be accelerating return of biomass carbon to the atmosphere. The net effect of these competing terrestrial responses is, in many ecologists' view, broadly the subject of intensive research, because the direction of the terrestrial carbon flux is regarded as one of the largest unresolved uncertainties in climate projection.

G Reducing atmospheric carbon dioxide is widely agreed to require either reducing emissions or actively removing carbon from the atmosphere. Natural climate solutions — reforestation, soil-carbon enhancement, wetland restoration — can sequester carbon at relatively modest cost but cannot, even at full deployment, fully offset current emissions. Engineered solutions including direct air capture and bioenergy with carbon capture and storage are at various stages of development, with limited operational deployment and costs that remain far above any current emissions-trading price. The scientific consensus is described as unequivocal: achieving climate stability will probably require both rapid emissions reduction and substantial deployment of removal technologies.