The Greatest Benefit to Mankind. Roy Porter

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СКАЧАТЬ a louse-borne disease), brucellosis and haemorrhagic fevers. Other threats came from organisms co-evolving with humans, including tapeworms and such spirochaetes as Treponema, the agent of syphilis, and the similar skin infection, yaws.

      Hunter-gatherers being omnivores, they were probably not malnourished, at least not until rising populations had hunted to extinction most of the big game roaming the savannahs and prairies. Resources and population were broadly in balance. Relative freedom from disease encouraged numbers to rise, but all were prey to climate, especially during the Ice Age which set in from around 50,000 BC. Famine took its toll; lives would have been lost in hunting and skirmishing; childbirth was hazardous, fertility probably low, and infanticide may have been practised. All such factors kept numbers in check.

      For tens of thousands of years there was ample territory for dispersal, as pressure on resources drove migration ‘out of Africa’ into all corners of the Old World, initially to the warm regions of Asia and southern Europe, but then farther north into less hospitable climes. These nomadic ways continued until the end of the last Ice Age (the Pleistocene) around 12,000–10,000 years ago brought the invention of agriculture.

      Contrary to the Victorian assumption that farming arose out of mankind’s inherent progressiveness, it is now believed that tilling the soil began because population pressure and the depletion of game supplies left no alternative: it was produce more or perish. By around 50,000 BC, mankind had spilled over from the Old World to New Guinea and Australasia, and by 10,000 BC (perhaps much earlier) to the Americas as well (during the last Ice Age the lowering of the oceans made it possible to cross by land bridge from Siberia to Alaska). But when the ice caps melted around ten thousand years ago and the seas rose once more, there were no longer huge tracts of land filled with game but empty of humans and so ripe for colonization. Mankind faced its first ecological crisis – its first survival test.

      Necessity proved the mother of invention, and Stone Age stalkers, faced with famine – elk and gazelle had thinned out, leaving hogs, rabbits and rodents – were forced to grow their own food and settle in one place. Agriculture enhanced mankind’s capacity to harness natural resources, selectively breeding wild grasses into domesticated varieties of grains, and bringing dogs, cattle, sheep, goats, pigs, horses and poultry under control. This change had the rapidity of a revolution: until around 10,000 years ago, almost all human groups were hunter-gatherers, but within a few thousand years cultivators and pastoralists predominated. The ‘neolithic revolution’ was truly epochal.

      In the fertile crescent of the Middle East, wheat, barley, peas and lentils were cultivated, and sheep, pigs and goats herded; the neolithic peoples of south-east Asia exploited rice, sweet potatoes, ducks and chickens; in Mesoamerica, it was maize, beans, cassava, potatoes and guinea pigs. The land which a nomadic band would have stripped like locusts before moving on was transformed by new management techniques into a resource capable of supporting thousands, year in, year out. And once agriculture took root, with its systematic planting of grains and lentils and animal husbandry, numbers went on spiralling, since more could be fed. The labour-intensiveness of clearing woodland and scrub, weeding fields, harvesting crops and preparing food encouraged population growth and the formation of social hierarchies, towns, courts and kingdoms. But while agriculture rescued people from starvation, it unleashed a fresh danger: disease.

      The agricultural revolution ensured human domination of planet earth: the wilderness was made fertile, the forests became fields, wild beasts were tamed or kept at bay; but pressure on resources presaged the disequilibrium between production and reproduction that provoked later Malthusian crises, as well as leading to ecological deterioration. As hunters and gatherers became shepherds and farmers, the seeds of disease were sown. Prolific pathogens once exclusive to animals were transferred to swineherds and goatherds, ploughmen and horsemen, initiating the ceaseless evolutionary adaptations which have led to a current situation in which humans share no fewer than sixty-five micro-organic diseases with dogs (supposedly man’s best friend), and only slightly fewer with cattle, sheep, goats, pigs, horses and poultry.

      Many of the worst human diseases were created by proximity to animals. Cattle provided the pathogen pool with tuberculosis and viral poxes like smallpox. Pigs and ducks gave humans their influenzas, while horses brought rhinoviruses and hence the common cold. Measles, which still kills a million children a year, is the result of rinderpest (canine distemper) jumping between dogs or cattle and humans. Moreover, cats, dogs, ducks, hens, mice, rats and reptiles carry bacteria like Salmonella, leading to often fatal human infections; water polluted with animal faeces also spreads polio, cholera, typhoid and viral hepatitis.

Settlement helped disease to settle in, attracting disease-spreading insects, while worms took up residence within the human body. Parasitologists and palaeopathologists have shown how the parasitic roundworm Ascaris, a nematode growing to over a foot long, evolved in humans, probably from pig ascarids, producing diarrhoea and malnutrition. Other helminths or wormlike fellow-travellers became common in the human gut, including the Enterobius (pinworm or threadworm), the yards-long tapeworm, and the filarial worms which cause elephantiasis and African river blindness. Diseases also established themselves where agriculture depended upon irrigation – in Mesopotamia, Egypt, India and around the Yellow (Huang) River in China. Paddyfields harbour parasites able to penetrate the skin and enter the bloodstream of barefoot workers, including the forked-tailed blood fluke Schistosoma which utilizes aquatic snails as a host and causes bilharzia or schistosomiasis (graphically known as ‘big belly’), provoking mental and physical deterioration through the chronic irritation caused by the worm. Investigation of Egyptian mummies has revealed calcified eggs in liver and kidney tissues, proving the presence of schistosomiasis in ancient Egypt. (Mummies tell us much more about the diseases from which Egyptians suffered; these included gallstones, bladder and kidney stones, mastoiditis and numerous eye diseases, and many skeletons show evidence of rheumatoid arthritis.) In short, permanent settlement afforded golden opportunities for insects, vermin and parasites, while food stored in granaries became infested with insects, bacteria, fungoid toxins and rodent excrement. The scales of health tipped unfavourably, with infections worsening and human vitality declining.^*

      Moreover, though agriculture enabled more mouths to be fed, it meant undue reliance on starchy cereal monocultures like maize, high in calories but low in proteins, vitamins and minerals; reduced nutritional levels allowed deficiency diseases like pellagra, marasmus, kwashiorkor and scurvy to make their entry onto the human stage. Stunted people are more vulnerable to infections, and it is a striking comment on ‘progress’ that neolithic skeletons are typically some inches shorter than their palaeolithic precursors.

      MALARIA

      Settlement also brought malaria. ‘There is no doubt’, judged the distinguished Australian immunologist, Macfarlane Burnet (1899–1985), ‘that malaria has caused the greatest harm to the greatest number’ – not through cataclysms, as with bubonic plague, but through its continual winnowing effect. First in sub-Saharan Africa and elsewhere since, conversion of forests into farmland has created environments tailormade for mosquitoes: warm waterholes, furrows and puddles ideal for rapid breeding. Malaria is worth pausing over, since it has coexisted with humans for thousands of years and remains out of control across much of the globe.

      The symptoms of malarial fevers were familiar to the Greeks, but were not explained until the advent of tropical medicine around 1900. They are produced by the microscopic protozoan parasite Plasmodium, which lives within the body of an Anopheles mosquito, and is transmitted to humans through mosquito bites. The parasites move through the bloodstream to the liver, where they breed during an incubation stage of a couple of weeks. Returning to the blood, they attack red blood cells, which break down, leading to waves of violent chills and high fever.

      Malarial parasites have distinct periodicities. Plasmodium vivax, the organism causing benign tertian malaria, once present СКАЧАТЬ