Tamed

by Alice Roberts

  Which came first?

  We can speculate about where the first commercial, directly genetically modified chickens will emerge. But we’ve been indirectly editing the chicken genome for centuries – where and when did that all start? The answer provides us with the solution to an enduring question that has left our best philosophers completely and utterly stumped. A riddle that addles the mind and draws one into a downward spiral with insanity seemingly the only end point.

  Which came first? The chicken or the egg.

  And here’s the thing. Evolutionary biologists have an answer for this question. Because, before the chicken, there was the junglefowl – and that laid eggs too. And so did its ancestors, all the way back to the dinosaurs, and still further back in time. Eggs, clearly, came first.

  With that epic question answered, we still need to pin down the actual origin of chickens. In the 1990s, researchers seemed pretty sure that all chickens came from a single origin, that the ancestral species was the red junglefowl (as Darwin had, once again, rightly predicted), and that domestication had occurred in a discrete area of south or south-east Asia. The genetic diversity of modern chickens is highest across that region, and much lower in China, Europe and Africa. Some researchers have suggested that the chicken homeland was, very specifically, the Indus Valley, 4,000 to 4,500 years ago (2000 to 2500 BCE), during the Bronze Age. References to ‘the bird of Meluhha’ in cuneiform tablets from Mesopotamia, dating to 2000 BCE, could relate to chickens – Meluhha itself is thought to be an ancient name for the Indus Valley. Others, though, have favoured an origin further east. Today, several distinct subspecies of red junglefowl scratch around in forests stretching throughout south and south-east Asia, from India, Sri Lanka and Bangladesh, to Thailand, Myanmar, Vietnam, Indonesia and southern China.

  Doesn’t this sound like a familiar story? We know what comes next. As more information came in from wider genetic studies, the theory of chicken origins was rewritten – as one which involved diverse geographic centres of origin, across south and south-east Asia. But that impression of multiple origins is also compatible with a single origin – perhaps across a relatively wide area – followed by dispersal and extensive interbreeding with wild species along the way. Modern chicken genomes contain strands of ancestry woven in through interbreeding with closely related birds including other subspecies of red junglefowl, as well as different species such as the grey junglefowl and Ceylon junglefowl.

  In 2014, a piece of research published by Chinese geneticists really put the cat among the pigeons, or the fox among the chickens, for want of a better metaphor. They articulated an astonishing claim – that chickens had been domesticated by 8,000 years ago on the North China Plain. It was so out of kilter with the rest of chicken science – and feathers flew. Most researchers remained sceptical, however, for several reasons. Firstly, the climate of the North China Plain 10,000 years ago was decidedly unsuitable for tropical junglefowl – the accepted wild progenitors of chickens. Secondly, the identification of bones from archaeological sites on this plain seemed rather suspect. Some bones thought to be those of chickens were probably pheasants. And other bones appeared to have been entirely misidentified – not belonging to any bird, even, but to dogs. It seems that this was an extraordinary claim – which turned out to lack the requisite robust, extraordinary evidence. South and south-east Asia remained the most likely homelands of chicken-kind. And from that starting point, chickens set off to conquer the world.

  Pacific chickens

  Thousands of miles away from the homeland of chickens, these birds have been drawn into an ongoing debate about the human colonisation of the Americas. The premise is this: if chicken history is so tightly bound up with human history, then elucidating events in the deep past of chickens will shed light on what humans were up to as well. Reconstructing the population movements that led to the colonisation of the Pacific has been a tricky challenge. The peopling of Pacific Islands happened relatively recently, within just the last 3,500 years – but waves upon waves of colonisers have left confused trails behind them. The challenge is like looking for footprints in the sand, to recover the evidence of these ancient journeys. Imagine standing on a popular beach in Britain at the end of a summer’s day, just as the last families are packing up their windbreaks, their towels, and their buckets and spades, and heading off. If you mapped all the footprints on the beach, could you reconstruct all the events of the day? Could you work out how many people had been there, from which direction they’d walked on to the beach, and roughly what time they’d arrived? It would be a huge challenge.

  Reconstructing ancient migrations is an even more daunting task. And yet, with archaeological and genetic evidence combined, it is just about possible. Humans didn’t arrive on the remote islands of Oceania alone; they took various other species with them – some deliberately, others less so, but all of them have a story to tell. Geneticists have attempted to track the human expansion into the Pacific by studying the molecular secrets hidden away in species as diverse as bottle gourds, sweet potatoes, pigs, dogs, rats – and chickens.

  The islands of Near Oceania, in the south-western Pacific, were colonised way back in the Pleistocene, over 30,000 years ago. But Remote Oceania – including the groups of islands also known as Micronesia and Polynesia – wasn’t peopled until much later, in the Neolithic. It was the last major migration of humans to entirely unpopulated lands. Archaeologists and linguists have suggested that this colonisation had happened in two waves, with an earlier migration of farmers, bearing characteristic Lapita pottery, starting around 3,500 years ago, and a later one, around 2,000 years ago. But chickens don’t seem to bear witness to this two-wave model. Studying mitochondrial DNA from both modern and ancient chickens, geneticists found a distinct signature stemming from a single prehistoric introduction of chickens into Polynesia. The picture was extraordinarily clear – there was a founding lineage, from which all later Pacific Island lineages had evolved. The mitochondrial lineages of chickens, from the Solomon and Santa Cruz Islands in the west to Vanuatu and the Marquesas Islands further east, hark back to the original prehistoric arrival of farmers – and their fowl – in the Pacific Islands. For a while, genetic studies of humans also suggested that the colonisation had happened in one wave, but recent analysis of ancient human genomes has lent new support to that two-wave model suggested by the spread of material culture and languages amongst the islands of Polynesia. The chickens, it seems, were leading us up the garden path – and not for the first time.

  For a while, it seemed as though that eastwards spread of farmers – and their chickens – might even have kept going right across the Pacific. The identification of a particular mitochondrial DNA type in chickens both from Rapa Nui (Easter Island) and from South America suggested just such a link. This was exciting, and controversial, as it indicated pre-Columbian contact between Pacific Islanders and the Americas. But the latest research on chickens, with exquisitely careful checks to rule out contamination, have revealed no such link. The DNA of Rapa Nui chickens and South American chickens is, in fact, quite distinct. South American chickens are essentially an offshoot of European chickens, which fits with the – much less controversial – idea of a post-Columbian introduction from Europe. This isn’t to say that there was no early contact between Pacific Islanders and South America, though – sweet potatoes made their way from South America to Polynesian islands long before the arrival of Europeans in the New World. And the genomes of modern-day Easter Islanders show traces of admixture with Native Americans, dating to between 1280 and 1495 – whereas Europeans only reached the island in 1722. But this is still only circumstantial evidence: what’s really needed to clinch this one is DNA evidence of admixture from pre-Columbian bones, either from the Americas, or from Polynesia. And that, for now, is proving elusive.

  Genetics adds an important line of evidence, enriching and complementing the story that we can obtain from archaeological, linguistic and historical data – but not
displacing those sources. Each provides us with a separate perspective on ancient reality. But when we contemplate prehistory in this way – staring at the past through such a wide lens – it’s easy to forget that these were people, and animals and plants, just as we encounter them today. Not species, but individuals. Science is powerful – it can answer our questions – but sometimes I feel the chill of the abstract quite acutely. We build knowledge in this way, certainly – but perhaps, at times, we lose sight of the personal, the intimate, the moment.

  Considering the people, then – we can imagine early farmers making voyages into the Pacific and settling on islands alongside hunter-gatherers. And there would undoubtedly have been a two-way flow of information. The hunter-gatherers shared their local knowledge of plants and animals – where to find them, and what was good to eat. The farmers shared their knowledge too – and their domesticates. It may not always have been this friendly, but by and by, the hunter-gatherers adopted more of the farmers’ ways, and started to grow crops and breed animals. Gradually, and probably without any definite decision to do so, they became part of the Neolithic Revolution.

  Early birds in the west

  Chickens were not part of the original spread of people and ideas and livestock that marked the beginning of the Neolithic in Europe – they were domesticated too late for that. By the time chickens made their way into Europe, the Bronze Age had dawned. By 2000 BCE, chickens had spread from the Indus Valley into Iran. From the Middle East, chickens could have spread via a coastal route, to Greece and across the Aegean into Italy. Maritime trade had really taken off by the Bronze Age – this was the era of the Mycenaeans, the Minoans and the Phoenicians. The Mediterranean was crammed with merchant ships plying their trade. An alternative route may have seen chickens spreading north from the Middle East, through Scythia, and then west into central Europe. But it’s also possible that some chickens could have spread from much further east – from China, and then via a northern route, through southern Russia, into Europe.

  Several researchers have suggested that differences between the chickens of northern and southern Europe reflect these two distinct routes of introduction. But once again, the history of this domestic species is horrendously complicated – and knottily tangled up with human history. It’s hard to trace those first migrations of chickens into Europe. Since those first feathery pioneers arrived, chickens have been subject to natural and artifical selection; flocks have been lost to diseases and replaced; birds have been brought in from further afield. The chicken breeders of the late nineteenth century picked and chose, breeding for particular traits, creating hybrids and generally mixing up the genetic history of European chickens, until they got what they wanted. And yet it is possible to disentangle the threads – the history is still there, embedded in the DNA of living birds.

  A huge survey of chickens from the Netherlands – including sixteen ‘fancy breeds’ as well as commercial varieties – produced fascinating results. Most of these chickens had mitochondrial DNA which formed a neat cluster with that of chickens from the Middle East and India. The Indian subcontinent is the likely geographic origin for this cluster of maternal lineages. But a handful of breeds had mitochondrial DNA which was characteristic of chickens from the Far East – from China and Japan. These included three Dutch fancy breeds – the Lakenvelder, Booted Bantam and Breda Fowl – as well as some commercial egg-laying breeds from the US. It’s tempting to imagine that these breeds, with their Far Eastern mitochondrial genes, provide some support for the idea of a pioneering northern route into Europe. But in fact this handful of eastern lineages, which are not even that closely related, is much more likely to reflect more recent genealogy. It’s likely that these erratic traces of East Asian ancestry have a very short history, deriving not from that first wave of chickens arriving into Europe during the Bronze Age – but from exotic birds imported much later, by nineteenth-century breeders. So far, then, genetic studies of chickens fail to provide any support for that northerly route from the Far East. Instead, the main flow of these familiar fowls into Europe was via the Mediterranean.

  The first evidence of chickens in Britain dates to the late first millennium BCE, in the Iron Age, but it was the Romans who really made chickens popular in this corner of north-west Europe. Chickens are by far the most well represented of any bird species in Romano-British archaeological sites. And yet the evidence is still rather thin on the ground, especially compared with mammal bones – like those of pigs, sheep and cattle. Bird bones are relatively fragile, and easily crunched to bits by scavengers, so it’s somewhat surprising to find any surviving at all. In rural settlements, away from centres of power and Roman influence, there’s not much evidence of chickens. But we glimpse them in more Romanised sites – in towns, villas and forts. Given the slim chances of chicken bones surviving, this evidence suggests that chickens – and their eggs, of course – may have been an important food, at least for the elite, in Roman Britain. It seems that chickens became equally popular further north, beyond the reach of the Romans. In South Uist, in the Outer Hebrides, there’s a little evidence of chicken bones dating back to the Iron Age, though it’s not until the subsequent Norse period that there’s more widespread evidence of domesticated chickens, braving the chill of the Hebridean Islands.

  Although it’s tempting to assume that evidence of domestic chickens is evidence of people eating them, and their eggs, we shouldn’t rush to conclusions. It’s been suggested instead that the initial spread of domestic chickens across the Middle East, and then into Europe, was in fact less about meat and eggs – and more about blood sport. Images of cockerels fighting appear on seals and pottery from Egypt, Palestine and Israel, dating to the seventh century BCE. The sport was popular in Ancient Greece, and also seems to have been exported across the Roman Empire. Archaeological collections of chicken bones from Velsen in the Netherlands, and from York, Dorchester and Silchester in Britain, contain surprisingly high proportions of cockerels. Artificial cock-spurs have been found at Silchester and Baldock, but it seems that Ancient Britons may have been indulging in cockfighting even before the Romans arrived. Julius Caesar, in his Gallic Wars, wrote that the Britons ‘regard it as unlawful to eat the … cock … but they breed them for amusement and pleasure’.

  The idea that the spread of chickens across Europe may have been about something other than meat is supported by a couple of lines of evidence. Firstly, during the Middle Ages, chickens were relatively small – suggesting that meat may not have been the foremost concern of breeders. Perhaps keeping hens for eggs and cocks for fighting was more important. And there’s written evidence too: geese and pheasants were much more frequently to be found on medieval menus than their now more popular cousins.

  Domesticated chickens were transformed during the twentieth century, through the systematic approach to selective breeding largely unleashed by the Chicken-of-Tomorrow competition. But even before this, chickens had started to plump up, diverging away from their red-junglefowl ancestors. In just the last few years, geneticists have been able to identify particular regions of the genome that appear to have changed over time, and which seem to be linked to a size increase. They’re also able to estimate when these changes to the genome took place. A study of modern chickens from around the world revealed that they all carried two copies of a particular variant of a gene associated with metabolism. The gene in question made a protein which was a receptor for thyroid stimulating hormone (TSH). The particular version of the gene which had become ubiquitous in modern chickens made them nice and plump. It seemed like a gene variant that surely must have been associated with the initial domestication of chickens – as essential as large seeds in domesticated wheat or corn. And yet the DNA of chickens from before a thousand years ago was almost completely devoid of this variant. It was only during the Middle Ages that the gene suddenly became much more common, sweeping through chicken populations.

  This sudden spread of a gene for plumpness coincides with an
equally sudden and significant increase of chicken bones at European archaeological sites in the tenth century, from 5 per cent to almost 15 per cent of animal bones. This seems to tie in with a religious and cultural change – the Benedictine Reform – which prohibited the consumption of four-legged animals during fasts (which could take up a third of the year) but permitted two-legged creatures, as well as eggs and fish, to be eaten. Suddenly fatter chickens became incredibly desirable, and human-mediated natural selection then worked its wonders, promoting the spread of that metabolic gene variant through chicken populations. Urbanisation probably played a role as well: although city-dwellers would have relied heavily on rural-agriculture produce, they could also keep some animals – such as goats, pigs and chickens – in their own backyards.

  Hormones can also affect the way animals behave, as well as their metabolism, and are implicated in an extremely important element of domesticated-chicken behaviour: a complete failure of maternal instinct. This sounds as though it should be bad for survival – and in the wild, it certainly would be. A hen who walked off from her eggs after laying them wouldn’t stand much of a chance of passing her genes on to the next generation, but in domesticated hens, that’s exactly what we want them to do. A hen that goes broody, that sits on her eggs and stops laying, is never going to win any prizes for egg production. The original red junglefowl lays fewer than ten eggs a year, whereas the most productive of our modern, domesticated egg-layers manages 300. That’s only feasible because the instinct to incubate the eggs has somehow been bred out of our chickens. That possibility only arose when chicken farmers discovered the trick of artificial incubation. The earliest egg incubators are very ancient – going right back to Ancient Egypt. But the genetic changes associated with the most profound loss of maternal behaviour in chickens appear to have happened much more recently. The loss of broodiness in chickens is the equivalent of the non-shattering rachis in wheat and maize – incompatible with successful reproduction in the wild, but advantageous under domestication.