Читать книгу Invasive Aliens - Dan Eatherley - Страница 9

‘But while men slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also.’

King James Bible, Matthew 13:25–26

For a million years a windswept peninsula in a corner of northwest Europe had seen various species of humans coming and going. The arrivals and departures were synchronised to the advance and retreat of continental glaciers, a dance choreographed by climatic change. They barely registered. A cluster of footprints here, a tidy pile of knapped flints there. Overwintering in caves, the people would emerge to gather shellfish from grass-fringed estuaries, pad through woodland in search of berries and nuts or pick at the carcasses left by lions and giant hyenas. The more ambitious, coveting the freshest meat, bone and fur, would rally family and friends in adrenaline-fuelled pursuits of deer, horse or mammoth.

Make no mistake, even the earliest people were unusual. Britain had never welcomed visitors quite like them, and over the aeons these experiments in humanity forged in the evolutionary crucible of an African valley generated ever more sophisticated results: the grunts of the most obtuse of cavemen took on deeper meanings; people fashioned better weapons and perfected their hunting techniques; they got the hang of butchery and learned to tame fire. Humans would turn their new-found skills on each other from time to time. Yet, for a great sweep of history, these pioneers – Homo antecessor, Homo heidelbergensis and maybe others – were but minor players on a stage dominated by rhinoceros and sabre-toothed cat, bison and bear. A low profile was often the best strategy given the monsters with which the land was shared. People were no more masters of their destiny than were grains of pollen in the air. And, every time the cold rushed back in and the fragrance of the dwindling forest was lost once more to the bitterness of endless tundra, so would humans again retire to more hospitable refuges in southern and southeastern Europe, abandoning the briefly colonised outpost to musk ox, wolverine and ice.

In the milder periods, when permafrost meltwaters inundated what would be known as the English Channel, the peninsula became an island. On one such occasion, some 125,000 years ago, humans found themselves shut out of the party altogether: things were warming up once again and a wealth of plant and animal species had spread back into Britain. But by the time people were on the scene, the land bridge from the continent had been claimed by the rising seas. Elephant, hyena, lion, deer, hippopotamus, elk and other animals had the place to themselves, enjoying a halcyon human-free interlude lasting 65,000 years.

Even Homo sapiens, the most successful hominid – in population terms, at least – to arrive in Britain was no great shakes at first. Originating perhaps more than 200,000 years ago, modern humans took their time getting here. Not for millennia would the most substantial exodus from Africa occur, with one wave of migrants moving along the Indian Ocean coastline towards southeast Asia, and eventually Australasia; another meandering north and west across the Middle East and Europe. When, from around 40,000 years ago, small bands of nomads, each with its own distinctive material culture, started to reach our shores, perhaps in seasonal visits, they found they’d been beaten to it.

Homo neanderthalensis had been eking out a living on Britain’s cold treeless steppes for at least the previous 20,000 years hunting, with flint-tipped wooden spears, woolly mammoth, woolly rhinoceros and probably quite woolly bison. The Neanderthals, with their heavy eyebrow ridges, flared nostrils and stocky physiques, were well suited to the hostile conditions. Yet their days were numbered, the British contingent vanishing within a thousand years of the arrival of modern humans. No one knows why.

It’s tempting to equate correlation with causation and accuse Homo sapiens of behaving as the archetypal invasive species, outcompeting and eradicating a vulnerable native with cunning and violence, perhaps passing on some disease for good measure. But the story appears more complicated: the significant amount of Neanderthal DNA in the modern human genome suggests a peaceful, even romantic, coexistence between the two hominids across continental Europe dating back 100,000 years. Could it be that the two varieties of early human preferred to make love not war? In Britain, at least, it seems they had little direct contact, and in any case the argument is academic since dropping temperatures led to the most recent glacial maximum about 22,000 years ago. Any prospect of human existence was snuffed out for another ten millennia. The ice crept forward, smothering everything, wiping the sheet clean. Another fresh start.

The people who returned to a warming Britain from around 15,000 years ago could still be classed as hunter-gatherers, but there was a greater sophistication about them, judging by the plethora of artefacts and art left behind. They were dog-lovers too, grey wolves having been domesticated to mutual advantage, possibly more than once, during or even before this most recent Ice Age. Moving along the Atlantic coast, the humans tracked herds of reindeer, horse, deer and elk north from their southern European refugia. Some perhaps crossed the English Channel in boats, while others may have sauntered through Doggerland, an expanse of terrain today submerged beneath the North Sea.

These people exploited natural resources with unprecedented intelligence: flint-tipped arrows of hazel, fired from bows of elm, felled aurochs (wild ox), red deer and wild boar with accuracy; the slipperiest of fish were trapped in river weirs purpose-built from willow; birds and smaller mammals were noosed and snared; a wider range of plants was collected, stored and cooked than ever before. People were thinking ahead. Fire was used to manage woodland. Freshly burnt clearings, the ash festooned with appetising plant regrowth, could be used to lure hungry game, which was much easier than tracking a deer or boar through dense forest. Nevertheless, impacts on the landscape were minimal. As in previous migrations people travelled light and, save for the plant seeds brought as food or stuck to clothing and bedding, few in the way of new species were conveyed to Britain during this period. Things though were about to change.

Danger. Tree felling in progress. A yellow warning sign greeted us as we approached the kissing gate. I had expected this: Hembury Hillfort’s website requested visitors to ‘observe cordoned off areas with red and white tapes’, and please to ‘not climb on timber stacks’. Thankfully, given my five-year-old daughter’s enthusiasm for outdoors rampaging, neither woodpiles nor tape were in evidence today. The works programme, aimed at reducing root damage to the site’s archaeology, was finished for the season. The tree clearance had a secondary function, to open up the view: that’s what partly drew us here. Hembury didn’t disappoint.

Twenty minutes later saw us picnicking amid bluebells at its southernmost tip. From the 240-metre-high bluff we were offered stupendous views across the Otter river valley towards the coast at Budleigh (the sea itself was lost in haze). The landscape was a hodgepodge of greens, interrupted here and there with the dull copper of a newly ploughed field, a yellow patch of oilseed rape, and the occasional pale minaret of wood smoke. Just visible to the west was Exeter, and beyond the grey eastern tors of Dartmoor from whose direction a brisk wind blew. Birds sang and robber flies buzzed. There was the faint drone of distant air traffic. Above us circled a pair of buzzards.

‘What can you see?’ I asked my daughter.

‘Cows,’ she replied, mouth stuffed with cheese-and-onion crisps.

Today’s miscellany of embankments, trenches, mounds and other vestiges of Hembury’s convoluted history confounds those wishing to understand it. The modern visitor is further disorientated by colossal beech trees which have erupted from the earthworks, clinging on with tentacular moss-covered roots. Yet its secrets are yielding to the archaeologist’s trowel.

Hembury’s strategic location and defensive qualities have long been recognised by those keen to defend themselves and command the region. It’s a real Russian doll of a place: ostentatious double-ditched ramparts dug in the Iron Age, some 3,000 years ago, surround the entire three-hectare monument, which is perched at the edge of the Blackdown Hills in East Devon. Easy access to nearby iron ores and smelting works perhaps justified the investment in time and effort to shift the countless tonnes of earth by hand. Members of the Belgae tribe, from northern France and the Low Countries, subsequently laid claim to Hembury, making their own mark in about 50 BCE with additional defensive ditches and ridges across the centre of the fort. Then, in the middle of the first century CE, the Roman military too added Hembury to its network of forts – apparently taking it without a fight.

More fascinating still was Hembury’s much earlier, Neolithic, incarnation, dating to around 6,000 years ago. This period was the focus of a pioneering series of digs in the early 1930s undertaken by the Devon Archaeological Exploration Society. The work was led by Dorothy M Liddell, a formidable and inspirational personality, and one of an emerging breed of female archaeologists. (A 17-year-old illustrator called Mary Nicol was one of Liddell’s protégés at Hembury. Later, as Mary Douglas Leakey, she would make her own name with palaeontological discoveries in Africa.) Through meticulous excavations, Liddell detected signs of earlier inhabitation at Hembury, including a causewayed (or interrupted) enclosure; post-holes denoting a once-grand timber gateway; the remnants of daub huts; shallow cooking pits, a metre and a half in diameter; and traces of a circular wooden building, possibly a guard house. Her team also recovered flint arrowheads and axes, and other stone implements, along with jet and greyish steatite beads and some of the earliest pieces of southern English pottery. Known as ‘Hembury ware’, the latter included simple round-bottomed bowls with lug handles, made using gabbroic clay, an orange-coloured mineral naturally occurring around the Lizard in Cornwall, 200 kilometres to the west. The finds hinted at a connection to an ancient and extensive commercial network stretching across the region and beyond.

But, for me, Liddell’s most important discovery at Hembury were some charred grains of spelt, an ancient form of wheat. Carbon dated at roughly 5,000 years old, these represent some of the earliest archaeological evidence for the cereal anywhere in Britain. Liddell also turned up stone querns for grinding the crop into flour. Evidence of the importance of cereals in the diet of Hembury’s Neolithic occupants was bolstered by the later discovery of 13 impressions of wheat grains embedded within some of the Neolithic ceramics.

How and why did a food plant native to the Middle East – 3,500 kilometres distant – come to be eaten atop a windy promontory in southwest England? The answer lies much further back in time.

Some 23,000 years ago, while Britain and the rest of northern Europe was gripped in an endless winter, people basking in the more benign climate of the eastern Mediterranean were gathering, grinding and cooking the grains of wild wheat, barley, oats and other grasses. It’s possible that the most far-sighted and patient among them may have planted out some of their seeds and waited to harvest a crop. The evidence for such an innovation back then is patchy, but certainly by around 12,500 years ago farming communities had materialised across the region.

The specifics of the transition from restless nomadism to a sedentary way of life based on cereal cultivation are still to be understood, but the shift is remarkably well documented in the Natufians, a people whose settlements are scattered across what is today Israel, Palestine, Jordan, northern Syria and southeastern Turkey. From about 14,500 years ago they started exploiting wild grasses such as emmer wheat and barley to make flatbread, beer and, later, animal feed. The transition from hunter-gatherer to settled farmer was by no means simple and direct. For some reason, the Natufians, having earlier taken up agriculture based on the intensive harvesting of wild grains, decided to resume a more mobile existence around 12,800 years ago. This about-turn has been linked to a colder period known as the Younger Dryas that reduced the natural availability of wild cereals in the Mediterranean region, forcing people to keep moving to fill their bellies.

Eventually, the Natufians and others returned to the cultivation of cereals. By selecting varieties with the greatest yields, or those which thrived in diverse conditions, crops were gradually domesticated. Early agriculturalists benefited from a common mutation in wild wheat and barley that causes the grain-carrying spikelets to be more tightly gripped to the plant after ripening – just when they should be releasing them. In wild conditions, these ‘non-shattering’ mutants are at a competitive disadvantage compared to normal grasses which can spread their seed far and wide, but they lend themselves to being harvested and cultivated by humans. People learned to exploit other plants too, including flax, pea, chickpea, lentil and bitter vetch, intentionally planting, tending and harvesting them.

Scientists wonder whether the timing of this shift to crop domestication, which probably occurred independently in different places across the Fertile Crescent – as well as in parts of eastern Asia where wild varieties of millet and rice were the grains of choice – might not be a coincidence. One suggestion is that as the climate warmed at the end of the last Ice Age, and sea levels rose, so people were forced to higher ground where they would have encountered wild wheat and barley growing naturally. Levels of carbon dioxide were also increasing in the atmosphere – possibly due to its release from warming oceans – boosting worldwide plant production, including grasses such as cereals, and kick-starting what is often called the Neolithic revolution.

With the right kind of seeds, well-prepared soil and a favourable climate, the pioneer farmers soon found themselves amassing more food than they needed. This calorie boost, combined with a reduction in energy spent moving around, is thought to have ramped up human reproductive rates. A population boom led to civilisations across the Fertile Crescent, an 800-kilometre arc of territory encompassing the floodplains of the Nile, Tigris and Euphrates. But the discovery of farming may have set off a vicious cycle: the more people bred, the more food was needed and the harder everyone had to work. If they didn’t want to, or couldn’t, they might cheat or steal, requiring strong laws and even stronger rulers to keep the peace. Of course, there was nothing stopping rulers themselves from hoarding food and growing their own power in the process. At the same time, more and more of the landscape was turned over to crops which meant an acceleration in deforestation, erosion and other varieties of environmental degradation.

Unsurprisingly, given its peripheral location and challenging climate, Britain wasn’t an early adopter of agriculture. By the time wheat and barley made their appearance here some 6,000 years ago – and those precious spelt grains were being hoarded in a primitive hut on Hembury hill – the world’s first city of Uruk was already rising from the Mesopotamian floodplain. The farming of livestock also appeared in Britain, and the rest of northern Europe, around this time, again having been pioneered long before in the Middle East.

Goats and sheep are believed to have been domesticated from their wild ancestors – bezoar and mouflon, respectively – across southwest Asia from about 11,000 years ago. These low-maintenance creatures, compatible with a semi-nomadic lifestyle, were probably first kept for their flesh alone, and only later used for milk, wool and other secondary products. The fertilising properties of livestock manure was also noticed and exploited. Despite their benefits, sheep and goats would go on to become among the world’s most destructive invaders, especially on islands where their relentless chomping wipes out rare plants and degrades ecosystems. Indeed, their unfussy diet, their rapid reproductive rate, their tolerance of a breadth of environmental conditions – the very traits which first drew us to them and of course to so many other problematic species – go a long way to explaining their world domination. At the last count, two billion sheep and goats roamed the planet.

Around the time that people first domesticated sheep and goats, cattle also joined the ranks of tamed ruminants. Cows were descended from the extinct wild ox, or aurochs. This was a spectacular beast, particularly the bull which stood nearly two metres high at the shoulder and sported fearsomely curved horns. Unlike the bezoar and mouflon, aurochs were already present in post-glacial Britain – indeed, they roamed the entire Eurasian landmass; however, domestication probably occurred in the Middle East. That’s because early cattle were much smaller than our native aurochs, and DNA studies show that modern cows, including British ones, are genetically closer to Syrian aurochs than home-grown ones. In fact, today’s entire global cattle herd – numbering some 1.5 billion cows – is believed to be descended from a founding stock of just 80 animals, likely to have originated in the Middle East. There’s a good chance, however, that hybridisation would have occurred between local British aurochs and the smaller incoming cattle. Neolithic farmers may not have been thrilled about this: their petite cows, bred for milking, may have risked serious injury when attempting to birth an outsized hybrid calf.

Other modern domesticates with native British versions also seem to have derived from imported stock. These include the pig, whose ancestor, the wild boar, was widespread here before the advent of agriculture. Porkers are thought to have been first farmed in the eastern Anatolian region of modern-day Turkey about 10,000 years ago – along with a later independent domestication event in central China – and descendants of these Anatolian versions were subsequently brought to Britain. As with cattle, the amount of wild boar DNA in the genome of domestic pigs suggests frequent hybridisation between the two. To an extent, this may have benefited pig farmers, as crossbred versions may have been better suited to the more bracing local conditions in Britain, although too much of the ‘wild’ in a pig could make it a handful. A balance had to be struck.

Many of our supposedly native crops may also have come from elsewhere too. For instance, Britain’s blackberries, raspberries, carrots and parsnips, as well as the perennial ryegrass, red clover and common vetch traditionally used as animal fodder, all probably derive from southern European strains.

Whether there’s the whiff of the exotic about other domesticated species is less certain. For instance, the honeybee is thought to have originated in Asia, or maybe Africa, around 300,000 years ago, later spreading naturally across Europe, so the likely presence of this woodland insect in Britain before the most recent Ice Age would qualify it as native. Yet, the earliest known archaeological evidence for honeybee exploitation by humans in this country – as suggested by beeswax residues on seven pieces of Neolithic pottery found in southern England – dates to as recently as 4,000 years ago. That’s several millennia after sweet-toothed pioneer farmers in Turkey, and later in central Europe, began gathering honey and wax from the insects, and possibly even domesticating them. So, we’re left to wonder if Britain’s first apiarists collected honey from wild bees or perhaps were using a tamer, introduced, variety that had been bred on the continent. In a sense, this discussion is somewhat academic, since pretty much all of our honeybees are today derived from southern European stock after parasitic mites devastated Britain’s existing honeybee population in the early twentieth century.

So, how did the ‘Neolithic package’ – although this term for an apparent commonality of elements, including domesticated crops and livestock, along with other characteristic artefacts, is increasingly criticised as over-simplistic – reach our shores? Did Fertile Crescent farmers themselves migrate north and west, or was it just their agricultural practices that travelled, along with the wheat, barley, sheep, goats and other domesticated species upon which they were reliant? The question has been debated for well over a century, although recent research is beginning to support the former hypothesis. For instance, a genetic study published in 2018 found strong affinities between Mesolithic British and western European hunter-gatherers over a period spanning Britain’s separation from the continent. The authors of this paper believe that British Neolithic people derived much of their ancestry from Anatolian farmers who followed the Mediterranean route of dispersal and entered Britain from northwestern mainland Europe. One thing is certain: when times were good, farming guaranteed a steady food supply and supported a burgeoning human population. In Britain, its practitioners rubbed along with nomadic hunter-gatherers for hundreds if not thousands of years, but the agricultural way of life, and the settled civilisation it supported, proved irresistible. So too, would the invasive species that profited from both.

The omens were there from the start. For millions of years, a spectrum of fast-growing, fast-spreading pioneer plants, both annuals and perennials, evolved to benefit from landscape impacts very similar to those that humans would one day cause. Many were adept at exploiting forest clearings opened up by fallen trees or recolonising habitats scraped clean by fires, glaciers, floods, landslips, volcanic activity and other natural disturbances. So, when the first farmers razed woodland and stripped soil bare in readiness for crops, they were teeing things up for a plethora of undesirable species. Commonly known as ‘weeds’, they have plagued us ever since.

Most troublesome of all were the weeds that resembled crops. These included darnel, a toxic grass which happened to be a dead ringer for wheat, and which infested the Middle East’s earliest agricultural sites. Pastoralism only worsened the situation, as grazing and browsing livestock suppressed tree regrowth, maintaining the sort of open conditions favoured by weeds. What’s more, just like crops, many weeds were adapted to thrive on the elevated levels of soil fertility resulting from all that extra animal dung.

British farmers, like their continental antecedents, set about annihilating the wildwood with their crops and livestock. Shifting agriculture was probably practised at first, with the felling of a few trees and controlled burning of understorey, followed by successive plantings of cereals. After a few seasons, the plot’s soil nutrients were exhausted, forcing people to move on and repeat the destructive pattern. Anthropogenic deforestation was hardly a new thing – as we’ve seen, hunter-gatherers were keen on woodland openings – but its scale from the Neolithic onwards was unparalleled.

Trees were removed for reasons beyond the need for cropland: their timber was a source of both fuel and building material, while the clearances themselves may have held a symbolic value. Britain’s vanishing woodland is reflected in changes in the incidence of particular pollen species in the archaeological record. As the representation of oak, elm, lime and ash dwindled, grasses, shrubs and wildflowers came to the fore. Invertebrate communities also changed, with a decline in specialist forest insects, including those associated with old or decaying timber, their place taken by varieties adapted to open and disturbed ground; dung beetles flourished thanks to livestock. Every so often a prolonged spell of climatic deterioration – as occurred between 5,000 and 3,500 years ago – would lead to a temporary abandonment of arable farming in Britain. Forests then had a chance to recover, although pastoral farming would still have been practised.

Of course, Britain’s Neolithic farmers had their work cut out dealing with the weeds that prospered in the denuded landscape. Many unwanted plants already lurked as seeds in our soil, just waiting for their moment in the sun; others were conveyed from further afield as contaminants of grain imports. The field, or corn, poppy, well-known to early Middle Eastern civilisations, is among the more familiar of the non-natives to have debuted in Britain around this time. The ancient Egyptians were taken by the striking blood-red blooms which infested their wheat and barley fields at harvest. The poppy’s reappearance each year was a metaphor for rebirth and regeneration. The flower was woven into funerary bouquets and depicted on tombs.

Another arrival in Britain was charlock, or wild mustard, which was once described as the most troublesome annual weed of arable land. Indeed, an assortment of familiar crops including artichokes, flax, garden peas, leeks, lentils, lettuces and radishes may have started out as invaders of arable fields. Given that these are all fast-growing, short-lived species thriving on bare soil, their weedy heritage seems to fit. Even einkorn – one of the first types of wheat to be cultivated on a large scale – may have started life as a contaminant of emmer wheat crops. Furthermore, bread wheat, today’s single most important variety, thanks to its easier threshing and greater grain yield, arose in the Fertile Crescent at least 8,500 years ago as a result of hybridisation between emmer and another weed, wild goat grass.

From a British perspective, some of the most important of the arable weeds were rye and wild-oats. Although originating in the Middle East, both seemed better adapted to our miserable climate and harsh soils, and often outperformed wheat and barley. So tenacious were these grassy invaders that by the Early Bronze Age, about 4,000 years ago, central and northern European farmers stopped bothering to weed them out and instead harvested them as crops in their own right. Domesticated varieties of both rye and oats were soon cultivated for bread-making, for flavouring alcoholic drinks, and as animal feed. Wilder versions of the oat stuck around and remain intractable arable weeds to this day, in large part due to the similarities in appearance and lifecycle with those of crops. Selective herbicides are available but hand-weeding, or ‘rogueing’, of wild-oats is still practised on a small scale.

When Brits took to agriculture 6,000 years ago, the door wasn’t just opened to invasive plants. Also waved through was an assortment of animal species adapted to living among people and exploiting their way of life. The house sparrow is a case in point. Remains of this small, gregarious bird have been identified in 10,000-year-old Natufian sites, suggesting sparrows long ago learned to nest in or close to buildings, purloining stored cereals and picking through the rubbish piles. By the Late Bronze Age, about 800 BCE, sparrows are known to have been present in central Sweden, so had probably reached Britain by then too. Today, they’re one of the world’s most cosmopolitan birds, outcompeting indigenous avians and proving a serious agricultural pest. In Russia alone, they’ve been accused of consuming a third of the annual grain production. During the 1950s the Chinese leader, Mao Zedong, even declared war on the sparrow, his scientists reckoning that, for every million birds killed, 60,000 extra people could be fed for a year. Chairman Mao’s scheme backfired: the removal of sparrows resulted in plagues of locusts and other insect pests, whose populations the birds had helped suppress, which in turn led to famine. The Chinese government ended up reintroducing sparrows from the Soviet Union.

It seems therefore that house sparrows have a value in agricultural systems and in Britain, at least, we’re fond of them. The sparrow population has been falling of late: during the 1970s there were up to 12 million of them in the UK, but the population is now half that, with the worst declines in England. No one is really sure what’s killing off sparrows. Possible factors include a reduced availability of invertebrate prey, a shortage of nesting sites and increased predation by squirrels, magpies and cats. In cities, high levels of nitrogen dioxide in the air, mainly from car exhausts, also seems to be a factor, with London alone seeing a 60 per cent decline between 1994 and 2004. All this has triggered urgent conservation efforts to save the sparrow.

Such measures won’t be contemplated any time soon for the house mouse, another accomplished non-native invader, which originated up to a million years ago somewhere between the Middle East and northern India. The rodent was first drawn to the organic waste tips of hunter-gatherer settlements in the southern Levant at least 15,000 years ago and its population was primed to explode with the invention of agriculture. Recent evidence shows the house mouse sometimes shared the more mobile of the Natufian sites with a second species, the short-tailed mouse; however when people settled down for any length of time, the house mouse soon elbowed out its wilder cousin. By the Bronze Age, the rodent had scurried into western Europe but took a while to make its mark in Britain: the earliest records date from pre-Roman Iron Age settlements at Gussage All Saints in Dorset and Danebury Hillfort in Hampshire. The mouse seems to have got established after repeated introductions as a ship stowaway; by then Britain was well connected to the continent by the maritime trade and replete with granaries. Danebury alone boasted some 4,500 pits for storing crops, making it a house mouse heaven.

Along with rabbits, rats and grey squirrels, the house mouse shares the accolade of being among the few vertebrates to inflict both economic and social costs on a national scale. In addition to eating and fouling food stores, the rodent harbours a catalogue of unpalatable (and unpronounceable) diseases from tularaemia and typhus to leptospirosis and lymphocytic choriomeningitis. Humans have long waged a losing war against the species. These days baited traps and poisons tend to be used, but in times past barley cakes, spiked with black hellebore (a toxic variety of buttercup), would be placed at the entrance to their holes. Mice were also said to flee a censer of haematite stone and burning green tamarisk. But nature also provided a more elegant solution to the rodent problem.

The African wildcat’s mouse-destroying prowess, along with its skill as a bird and fish catcher, may have been what recommended the species as the perfect household pet to the Egyptians more than 4,000 years ago. If true, that would make its tame version, the domestic cat, an early agent of biological control (the use of one organism to reduce populations of another). The sacred importance of cats in ancient Egypt is the stuff of legend with the feline deity Bastet worshipped as a goddess of fertility and the moon. The Greek historian Herodotus famously – but perhaps not altogether reliably – reported that the death of a cat prompted all those in the household to shave their eyebrows. The pet would then be embalmed. One cemetery unearthed at Beni Hasan in 1888 was said to contain the remains of 80,000 cats. A 20-tonne consignment of the corpses was later exported to Liverpool as fertiliser. One or two of the mummified moggies were saved for posterity by the city’s museum. The human relationship with cats may predate ancient Egypt, with the suggestion that the felines began domesticating themselves during the Early Neolithic period; as sparrows and mice were drawn to Natufian grain stores and spoil heaps over 10,000 years ago, so cats were drawn to the sparrows and mice. A rise in the feline population may have been further sustained on proffered titbits from people, as well as rummaging through our mounting piles of rubbish.

Like the house mouse, the domestic cat first appeared in Britain towards the end of the Neolithic, with signs of the species at Gussage All Saints and Danebury Hillfort – just like those of its famous rodent quarry. Could it be that the cat’s pest control qualities were appreciated in Iron Age Britain? Cats were, however, rare until medieval times. The earliest written record dates to the reign of the Welsh king Howell the Good (880–950 CE), who issued the edict that anyone slaying or stealing a cat was liable for a financial penalty calculated in terms of the equivalent cost in grain: ‘The worth of a cat that is killed or stolen; its head is to be put downwards upon a clean even floor, with its tail lifted upwards, and thus suspended, whilst wheat is poured about it, until the tip of its tail be covered.’ Today, an estimated nine million cats prowl Britain’s towns and countryside, each year snaffling some 100 million prey items, including mammals, birds, reptiles and amphibians. One 1987 study from the village of Felmersham in Bedfordshire implicated cats in almost a third of house sparrow deaths. It seems old habits die hard.

Perhaps the greatest feline felony is a crime of passion. As with cows and pigs, keeping apart wild and domesticated versions often proves futile. The same seems true of pet pussies and Britain’s own native wildcat, an endangered beast confined to the forested margins of Scottish moorland. The two versions have interbred so often that hybrids now dominate the wildcat population. Conservationists worry that too much domestic cat in the genome of the wildcat weakens it and leaves an animal which is already threatened by habitat loss and persecution close to extinction.

The arrival in Britain of a tabby of a different sort is also linked with the advent of Neolithic agriculture. Also known as the grease moth, the large tabby gets its name from the uncanny resemblance that its forewings bear to cat fur. With an appetite for dried dung, dead skin, old feathers, bits of straw and other unmentionable detritus, tabby larvae probably first hitched a ride here ensconced in livestock bedding. Suggesting that its natural habitat might once have been caves, the insect lurks in the gloomy recesses of stables and outhouses, where the larvae spin protective silken tubes about themselves then munch away undisturbed on their rarefied diet for up to two years before turning into adults.

A similar niche is exploited by dermestid beetles, many of whose 1,000 species and subspecies are spread by human migrations and globalised trade. Some are specialist scavengers on desiccated animal remains including hides, furs, feathers, tendons and bone, and a few are associated with Egyptian mummies, as well as with human remains from Middle Bronze Age sites in the southern Levant where the larvae drilled tunnels into the bone. Museum taxidermists still use these insects to nibble flesh from animal skeletons prior to display. Some dermestids could have reached Britain as early as the Neolithic period in the same way as the large tabby moth.

Among a number of non-native insect pests arriving in crop shipments is the grain weevil, a flightless species measuring around four millimetres when full-grown. Mated females each produce 150 eggs or more, which are deposited individually into grain kernels. The developing larvae feed there for up to six months before pupation, after which the adults chew their way out of the now-empty seed hulls. There’s a theory that before agriculture came along the grain weevil’s Asian ancestors lived on food scraps in bird or rodents’ nests, before dispensing with wings altogether and becoming wholly dependent on human food stores. If true, this was a good move, as today the weevil plagues food stores worldwide, gorging on wheat, barley, rye, oats, corn, rice and millet, as well as a range of processed goodies from chocolate to pasta. The earliest western European record is from Early Neolithic Germany up to 7,000 years ago, and the insect is confirmed in Britain from the first century CE. Today, the UK alone spends an estimated £6.5 million annually on pesticides to control these and other non-native invertebrate pests of stored grains and fodder crops, including the saw-toothed grain beetle, foreign grain beetle and the red flour beetle, as well as mites and moths.

The unparalleled growth in human population and radical change in lifestyle unleashed by the Neolithic revolution benefited a different class of invading organisms; organisms that made their livelihoods not just among us, but on and even inside us. Harmful bacteria, viruses, protozoa, fungi, intestinal worms, ticks, lice and fleas, and myriad other nasties had always been present in the environment. For example, the bacteria responsible for tuberculosis, which still kills around three million people annually, was probably infecting the very earliest hominids in East Africa millions of years ago. The guts of hunter-gatherers are thought to have been crawling with roundworm, hookworm and other helminth worms, and their wounds quickly got infested with staphylococcal bacteria. In addition, a miscellany of animal-borne diseases may have infected humans before the Neolithic, from sleeping sickness and schistosomiasis to monkey malaria. But as soon as we started to form dense, semi-permanent, settlements, living side by side with livestock, and inadvertently drinking water contaminated by our own waste (never a good idea), harmful parasites and pathogens of all shapes and sizes were allowed to reach epidemic proportions for the first time.

For instance, the measles virus, in order to persist and spread, requires a sedentary population of up to half a million people with a continually replenishing supply of previously uninfected children. Malaria, yellow fever, diphtheria, leprosy, smallpox, influenza and the common cold are among a wide range of other ‘civilisation diseases’ thought to have benefited from our change of habits, many hopping from domesticated animal to human during, or after, the Neolithic. (The species-jumping may have gone both ways, with evidence that humans could have passed on harmful worms as well as certain other parasites and pathogens to their livestock, rather than vice-versa.) Furthermore, as we have seen, agriculture boosted populations of rodents, birds, invertebrates and other agents of disease. Even without close-living humans, grain stores, and herds of livestock, disturbance to the environment wrought by farming itself probably facilitated the spread of parasites and pathogens. For example, the deforested habitat resulting from slash-and-burn agriculture continues to favour malaria-carrying mosquitoes.

Britain’s remote location, temperate conditions and relatively late adoption of modern farming may have helped its people avoid early epidemics. However, disease outbreaks probably became a fact of life by the Bronze Age with the increase in trade with the continent. Indeed, a catastrophic epidemic could explain the extraordinary results of a recent study on ancient human DNA across Europe which indicates that at least 90 per cent of the ancestry of Britons can be traced to the Beaker people. Named for their characteristic bell-shaped pots, this group originated in central and eastern Europe and arrived in Britain some 4,500 years ago, seemingly replacing almost the entire indigenous population. One suggestion is that the pre-Beaker Brits might have succumbed to a disease to which the Beakers were resistant.

Not everything that arrived towards the end of the Bronze Age and into the Iron Age was quite so unwelcome. By around 2,500 years ago, trade routes were beginning to extend to the Far East, courtesy of new imperial roads built by the Persians, facilitating a westward spread of previously unknown plants and animals. During this period, Brits may have got their first taste of a domestic apple, a species originating in the mountains of Central Asia, or ridden their first donkey, derived from wild asses in Egypt.

The woad plant, a member of the cabbage family prized as a source of indigo dye, was another Asian native appearing in Britain around this time. (Extracting the pigment was a complex process, involving huge quantities of leaves, a fair amount of an alkaline substance, such as lime – made by heating up chalk or limestone in a kiln – or stale urine, and a prolonged fermentation phase.) In De Bello Gallico, Julius Caesar’s account of his seven-year campaign in the first century BCE to subdue the Gauls (another name for the Celts), he records that British warriors dyed themselves with woad to terrify their enemies. This was the inspiration for a blue-faced Mel Gibson in Braveheart. Like many of the best stories it has its doubters: the term Caesar used for ‘woad’ was vitrum, which also translates as ‘glass’, prompting some to suggest that Celts were in fact scarring or tattooing themselves. Whatever the truth, pod fragments and seeds of woad have been discovered in the Late Iron Age site of Dragonby, near Scunthorpe in Lincolnshire, and it’s believed the species was brought by Celts, via western and southern Europe.

The Romans may not have had a hand in bringing this particular plant to Britain, but that’s more than can be said for a whole new wave of non-natives about to make their presence felt. Once again, momentous changes were afoot in this corner of northwestern Europe.