Tamed
Page 6
Breeds may be relatively reproductively isolated now, but their genes tell us that there was once plenty of gene flow between breeds or proto-breeds. Breeds from separate countries share characteristics and genes which show that they must have interbred in the past. The Mexican hairless dog and the Chinese crested dog share hairlessness and missing teeth and in both breeds these traits are caused by precisely the same mutation in a single gene. The odds against this gene mutating in exactly the same way in two different dog populations are infinitesimally small. Instead, these shared traits and shared genetic signature speak of common ancestry. Dachshunds, corgis and basset hounds all have very short legs. Together with sixteen other dog breeds, they all have exactly the same genetic signature associated with this form of dwarfism – the insertion of an extra gene. It’s most likely that this insertion happened just once, in early dogs, long before any of the modern short-legged breeds appeared.
Genetic research provides us with this astonishing opportunity to understand the evolutionary history of dogs, from the pleiotropic exuberance of variety produced by selecting tameness, right through to the selection of peculiar features, suited to very particular tasks, in our modern breeds. We can see how certain mutations, and the traits associated with them, popped up amongst early dogs, and were later – much later – promoted and propagated by selective breeding to create the modern breeds we know today. With inbreeding producing problems with increased risk of disease, geneticists are also working to understand the basis of particularly prevalent diseases and it may be possible to reduce that risk by even more careful selective breeding, and judicious outcrosses, underpinned by genotyping.
Some breeds have been outcrossed beyond the bounds of domestic dogs. Such extreme outcrossing was the basis of the Saarloos wolfdog, created in 1935 by breeding together a male German shepherd with a female European wolf. The Dutch breeder, Leendert Saarloos, was hoping to create a more ferocious and formidable working dog, but he ended up with a meek and cautious beast. Saarloos wolfdogs make good family pets, and have been used as guide dogs and rescue dogs. Another breed – the Czech wolfdog – was also created by crossing a German shepherd with a wolf, this time in 1955 in Czechoslovakia. The Czech wolfdog, originally bred for military service, has also been used for search and rescue and is increasingly popular as a pet. Will Walker owns a Czech wolfdog, called Storm. ‘She’s just as friendly as any other dog. She loves every dog and every human she sees,’ he told me. She makes an excellent guard dog, too. ‘She’ll bark at anything – she’s very keen to defend me and my house.’ ‘You’re like those early hunter-gatherers with wolves protecting their camps!’ I commented.
But the growing popularity of wolfdogs – spurred on by the appearance of these impressive animals in Game of Thrones – is balanced against increasing concern about their suitability as household pets. There’s an important distinction to be made between animals bred from recent hybridisation and those established breeds such as the Saarloos and Czech wolfdog, which are genetically much more ‘dog’ than ‘wolf’. Some breeders of wolf-dog hybrids, however, offer animals which are advertised as the product of much more recent crosses, which raises concerns about the potential for wild, unpredictable behaviour.
Hybrid wolf-dogs have attacked and killed a number of children in the US, and are banned outright in some states. In others, wolf-dog hybrids are legal, as long as the hybridisation happened at least five generations ago. In the UK, a first- or second-generation wolf-dog hybrid is considered risky enough to be regulated by the Dangerous Wild Animals Act – the same law governing owning a lion or tiger. It seems odd that breeders would exaggerate the wolf content of their puppies – but wildness is part of the cachet of these animals. With buyers seeking ‘high-content’ and ‘wild looks’, and willing to part with £5,000 to feel more like Jon Snow, wolf-dog hybrids are big business. It’s difficult to know just how ‘wolfy’ the product of a cross is, several generations down the line. The first-generation animals will be 50:50 in their genes, but after that, the shuffling of DNA that happens as eggs and sperm are made introduces messiness – second-generation wolf-dogs could have up to 75 per cent wolf genes in their genome, or as little as 25 per cent. There’s also the possibility that some purported ‘wolf-dog hybrids’ are nothing of the sort, and are just cross-breeds of German shepherds, huskies and malamutes – which already look fairly wolf-like – to create animals which appear even more like wolves. The ‘wolfiness’ of a wolf-dog hybrid, a few generations after hybridisation, is impossible to pin down without genotyping. And even with that genetic measure of wolfiness, it’s difficult to know how this would relate to the potential behaviour of an individual animal.
There are also concerns about wolf-dog hybrids on the other side as dog genes make their way into the genomes of wild wolves. Genetic studies have shown that 25 per cent of Eurasian wolf genomes contain dog ancestry. This is problematic from a conservation perspective – could an injection of domestic dog genes into wild, grey wolves cause problems for Canis lupus? Wolf populations have declined in Europe, under pressure from both hunting and the fragmentation of habitats. But hybridisation could also supply beneficial genes and traits. North American wolves got their black coat colour by interbreeding with dogs centuries, if not millennia, ago. Most hybridisation appears to occur through free-ranging male dogs mating with female wolves, but one recent study showed up dog mitochondrial DNA in two Latvian wolf-dog hybrids. Mitochondrial DNA is exclusively inherited from the mother, so the only way that this DNA could have ended up in wolf genomes is by female dogs having mated with male wolves. Once dog genes have entered wolf populations, it’s very difficult to remove them. Some hybrids look a bit like dogs, but many look exactly like wild wolves. So experts have advised that the best way of reducing the impact of hybridisation is to reduce the number of free-ranging dogs. Once they mate with wild wolves, it’s too late.
Hybridisation raises all sorts of questions. There are biological questions about the integrity of species, and about just how much interbreeding occurs across our once sacrosanct species boundaries. If there’s plenty of interbreeding, with fertile offspring, does this mean our species boundaries are too narrow? These are widely debated questions right now. But in fact, taxonomists, the people who make it their business to name and circumscribe species, have never been quite as rigid as textbooks may have led us to believe. Species are simply snapshots of evolutionary lineages – diverging (and sometimes converging). They are defined by being diagnosably different from the nearest cousins on the tree of life. But sometimes they are defined for human convenience – especially when it comes to conferring separate species names on domesticates and their wild ancestors.
The potential for hybridisation also leads to ethical questions about the ‘contamination’ of wild species with genes from domesticated species. Having created domesticated species, we’re now keen to preserve any surviving, closely related wild ones. But does this invoke an idea of the purity of species that just doesn’t really exist in the real world? That’s a challenging question, and one that will only become more pressing as our own population grows, and the species we’ve become allied with burgeon alongside us. It’s such a conundrum. The species that have become our allies have secured their future, by becoming companionable, useful, even indispensable to us. But together, we represent a threat to whatever wildness remains.
It seems that the safest way for humans and wolves to co-exist on the planet is to avoid each other. Our ancestors once tolerated wild wolves – long enough to domesticate them. Wolves may be naturally much more shy around humans now than they were in the past. Wolves were changed by becoming domesticated dogs, in so many ways, but the wild wolves may have changed as well. Persecution and hunting of wild wolves probably exerted a selection pressure of its own – the most successful wolves are likely to have been the ones that stayed away from humans. Wolves that are more fearful, and that avoid us, may be products of human-mediated selection – as much
as dogs are.
The genetics of grey wolves and dogs suggests that the wolf lineage which gave rise to dogs is now extinct. Times were tough around the last glacial maximum, so that’s certainly possible. But there’s another way of looking at the family tree – that particular lineage of wolves is not extinct at all; in fact, it’s the most populous branch of the wolf family tree: dogs. Genetically speaking, dogs are grey wolves. Most researchers simply subsume them within the grey wolf species, Canis lupus – not a separate species, the previously recognised Canis familiaris, but a subspecies: Canis lupus familiaris.
So that terrier, that spaniel, that retriever that you know so well … it’s a wolf at heart. But a much friendlier one – even more tail-wagging, hand-licking, and altogether less dangerous – than its wild cousins.
2
WHEAT
Triticum
History … celebrates the battlefields whereon we meet our death, but scorns to speak of the ploughed fields whereby we thrive; it knows the names of kings’ bastards but cannot tell us the origin of wheat. That is the way of human folly.
Jean-Henri Casimir Fabre, nineteenth-century French botanist
A ghost in the ground
Eight thousand years ago, a seed fell on to the fertile ground, somewhere near the coast, in north-west Europe. It had travelled far. Not windblown, not even carried on the beak or in the guts of a bird. But on a boat. It had been part of a precious cargo, but it was so small it fell to the ground in a clearing in the forest, and no one noticed.
The seed started to germinate. It sprouted and its long leaves grew up. But the weeds around it were stronger. The interloper never managed to produce its own seeds. It died back. Nevertheless, its ghost remained in the earth. Even after the saprotrophic fungi and bacteria had done their best to pull apart every last shred of its being, a few molecules of that exotic plant survived. And with every year that passed, that layer of soil was buried deeper, as the forest floor built up. Then trees disappeared, to be replaced by sedge and reeds. They grew and died and half-rotted down. The sea level was rising, and the reedbeds were replaced with samphire and seablite. The rising tide brought in fine sediment to create a layer of mud over the peat. For a time, this new mudflat was only inundated at the highest, spring tides. Then twice a day. Then it was submerged and even the seablite couldn’t hang on any more. The sea level rose, and the waves rolled in. But the molecular ghost of the ancient, exotic plant still lingered in the deep peaty sediments – buried under metres of marine clay – at the bottom of the Solent.
A lobster makes an archaeological discovery
In 1999, a lobster living on the seabed, close to Bouldnor – just to the east of Yarmouth – on the north coast of the Isle of Wight, made an astonishing discovery. The lobster had been digging its burrow into the base of a submerged sea cliff, excavating sand and cobbles out of the bank.
Two divers spotted the lobster, and the excavated ditch that it had made, leading to its burrow. The furrow ran alongside an ancient, fallen oak tree. And in it, the divers found stones that the lobster had pushed out of its burrow. The divers were maritime archaeologists, interested in the well-preserved submerged forest at Bouldnor Cliff. They picked up the stones that the lobster had excavated – and saw that they were man-made: worked flints. These weren’t the first stone tools that the archaeologists had discovered in this area, but the others had eroded out of the sediments and had been shifted by currents. The lobster’s flints looked as though they had only moved a short distance – and the divers suspected that the original context of these artefacts may well be in the cliff itself, just where the lobster had chosen to make its home.
The submarine archaeologists set to work – diving for an hour at a time, surveying and excavating the area at the base of Bouldnor Cliff. Despite poor visibility and the strong currents, they found an astonishing wealth of archaeological material, and started to build up a picture of the environment, when it was dry land. They found relics of an ancient forest – pine, oak, elm and hazel. Alder was there too, a tree that likes to grow with its feet in the water, perhaps along the banks of an ancient river. And in the sandy sediments that must have once been the banks of that river, the archaeologists found evidence of human activity: plenty of flints, some of them burnt, together with charcoal and charred hazelnut shells, and the oldest piece of string in Britain. Radiocarbon dating revealed that the site had been occupied around 6000 BCE. Nearby, the divers found evidence of a pit containing burnt layers, and a mound of timber that might, just might, represent a raised platform that once supported a Mesolithic house. There was also plenty of worked timber, with the marks of ancient tools still clear. These timbers included a large piece of split oak – possibly part of a log-boat – and a wooden post, still standing upright in the ancient sediments. The preservation was fantastic. It became clear that, after the site had been abandoned in antiquity, peat must have grown over it quickly, sealing the archaeology in situ. And there it all lay, just waiting for that fortunate lobster to come along and discover it, 8,000 years later.
The underwater excavations at Bouldnor Cliff went on from 2000 to 2012. Analysis of all the material will take many more years. There’s just so much – from an archaeological and a palaeo-environmental perspective – for a diverse team of researchers to get their teeth stuck into. Along with all the obviously archaeological material that the divers brought up from the seabed – the chipped flints, the fragments of charcoal, and the carbonised hazelnut shells – they brought up mud. Lots of it. These samples of sediment would undoubtedly contain more tiny clues to the prehistoric environment at Bouldnor Cliff – perhaps minute rodent bones, small pieces of plant, and even pollen – which would be yielded up by sieving and microscopy. But in 2013, another team of researchers contacted the Isle of Wight archaeologists. They wanted the mud, but what they hoped to find wouldn’t be visible under even the most powerful microscope. They were after molecules. Long, stringy molecules, rich with information. They were after DNA.
The geneticists approached the Solent mud with open minds. They didn’t start with preconceptions about what they might find, and then try to find it (or not). They looked at samples from the layer that included the hazelnut shells, and applied a technique known as ‘shotgun sequencing’ – as indiscriminate as the name suggests. It sounds like the antithesis of hypothesis-driven research – the gold standard that all good scientists should be striving for: the Scientific Method. But there isn’t just one ‘Scientific Method’. Sometimes the best way of starting out to understand something better is simply just to ask – what’s out there? And then you collect data and try to make sense of it. Arguably there is still a hypothesis even in such broad approaches – which directs which data are to be collected – but there’s not an experiment as such, just good looking. Much of genomics works in this way – amassing large amounts of data and looking for patterns. In this case, the hypothesis was expansive: ‘ancient DNA from contemporary organisms will be found in the sample’. And although it may be heretical to say it, I think it’s when you keep hypotheses as broad as possible, when you break away from any preconceptions and expectations, that you can have the best chance of finding something truly novel and exciting.
The geneticists working on the Bouldnor Cliff mud pulled out all manner of DNA sequences from things that had lived there, 8,000 years ago (6000 BCE). They found genetic traces of oak, poplar, apple and beech trees, as well as grasses and herbs. Canis was there too – either from dogs or wolves, and Bos – which must have come from aurochs, the ancient ancestor of cattle. The molecular ghosts of deer, grouse and rodents were also hidden in the sediment. Piece by piece, the geneticists put together the details of the ancient ecosystem of the Solent forest, where the Mesolithic hunter-gatherers had made their camp.
But amongst the fragments of DNA from the seabed, there was something which came as a complete surprise: the unmistakable trace of Triticum. Wheat. It shouldn’t have been there. This was Britain, pre
-farming. The samples of sediment had already been checked for pollen – usually a good indicator of the plants growing at the time. But there was no wheat pollen in the samples. Was it a mistake? It was such an unusual finding, the geneticists had to be absolutely certain that they weren’t looking at something else. But the Triticum sequence seemed real enough. The team checked carefully to make sure that the signal couldn’t have come from some other wheat-like grass that was indigenous to Britain – perhaps lyme-grass, couch grass or wheatgrass. But the ancient DNA was different to all of these. Instead, the closest match was one particular species of wheat: Triticum monococcum, or einkorn – ‘single corn’. Each of the little spikelets in the ear of this wheat contains a single seed, enclosed in a tough husk. Einkorn was one of the first cereals to be domesticated and cultivated but it wasn’t thought to have arrived in Britain until 6,000 years ago (4000 BCE) – a full 2,000 years later than the unmistakable genetic traces of it at Bouldnor Cliff.
So the einkorn buried in the sediments at the bottom of the Solent had travelled far and fast to get there, so long ago. The birthplace of cultivated einkorn was two and half thousand miles away, at the eastern end of the Mediterranean. And the first person to start focusing in on the original homeland of einkorn, and other wheats, was a botanist and geneticist born in Moscow in 1887.
Vavilov’s courageous quest
In 1916, the 29-year-old Nikolai Ivanovich Vavilov left St Petersburg to embark on an expedition to Persia – modern-day Iran. He had a particular objective in mind: to track down the origins of some of the world’s most important crops.