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WHEN the Romans invaded Britain two thousand years ago, they probably took one bite of the local apple and immediately sent home for scions of juicy-fruited Italian trees to graft onto the rootstocks of the bitter native crabs. And that鈥檚 how eating apples got to England, suspects Barrie Juniper, now an emeritus fellow at Oxford University. But where, he wondered, did the Romans get their large and luscious fruits? DNA analysis might just provide an answer.

Historians had already provided clues. The Romans knew about grafting, a clever way of propagating a particular variety that sidesteps the genetic lottery of seeds. They had learned it from the Greeks, who had probably pinched it from the Persians some 2300 years ago. The technique itself seems to have originated in Mesopotamia about 3800 years ago. The dating fits perfectly with Juniper鈥檚 hunch that apples began to come west just about this time.

But where exactly did they come from? Somewhere in Central Asia was everyone鈥檚 guess. Yet tradition has it that the modern domesticated apple is a hybrid, the result of chance crosses among wild species growing along the east-west trading route known to us as the Silk Road. Travellers took the most appealing fruits westwards, and the Persians propagated the choicest varieties. Then the Romans carried them to the furthest reaches of their empire. The varieties underwent further hybridisation with other cultivars and probably with wild species too.

That鈥檚 the conventional story, but is it true? Juniper had his doubts, and in 1998 he set off to find wild apples first in Uzbekistan and then around Almaty, the capital of Kazakhstan and an ancient staging post along the trade route. He found a virtually treeless landscape, stripped bare by generations of nomads鈥� goats. So in a borrowed Russian jeep and with heavily armed local guides, Juniper and his colleagues travelled hundreds of kilometres east to the Heavenly Mountains of Tien Shan. And there, beyond the reaches of the goats, they found apples galore, growing wild.

Taking DNA samples from more than 100 trees proved easy: Juniper and his colleagues simply plucked three leaves from each tree, tore them up and popped them in a plastic bag with a spoonful of silica gel, ready for DNA extraction back at the lab. The following year, Juniper launched an expedition further through the Tien Shan into the Ili Valley of southern China, and then to Kyrgyzstan.

The results from all the expeditions were startlingly consistent: modern apples, wherever they come from, all share genetic sequences with only one Central Asian species, Malus sieversii. The implication, says Juniper, is that all the apples we eat today are descended from this one species from the forests of Tien Shan.

The true history of English apples now begins to unfold. Since the Romans arrived, deliberate breeding has created many new varieties鈥攈ybrids of existing cultivars. Other varieties have arisen as 鈥渨ildings鈥�, grown from pips shed by a discarded apple core. But all this genetic mixing seems to have happened with apples that were already domesticated. It鈥檚 apparently the same story throughout most of the world: local wild species have made no contribution to the domesticated apple鈥檚 gene pool.

Molecular biologists Stephen Harris and Julian Robinson of Oxford University caution that further sampling of Central Asian apple trees is needed to clinch the argument. But Juniper is pretty confident. No one, he says, would consume the small, bitter fruits of the now rare native British crab apple Malus sylvestris by choice. Plant breeders found out long ago that hybrids of cultivated varieties of apple and other species such as the native crab were always a waste of time, producing tiny fruits. For some reason, the species that evolved in Tien Shan is so well-endowed genetically that it has generated a huge number of varieties without any input from other species.

So just how did this remarkable fruit evolve? Juniper thinks it probably started off much like the other 20 or so wild species still growing in central and southern China, with small fruits bearing hard but edible seeds that were spread by fruit-eating birds.

Meanwhile, the Tien Shan mountains began to rise out of the ancient Tethys Ocean. Suddenly isolated in mountainous hills and valleys, our proto-apple found a Shangri-La, says Juniper. There was plenty of time to evolve, as these high, spiky peaks have never been glaciated.

Over some 10 million years, forests grew up and our ancestral apple tree found itself surrounded by deer, wild pig and bears, all with an inordinate fondness for fruit. As these beasts selected the largest and juiciest apples, the small, cherry-like delicacy favoured by birds gave way to a large, mammal-friendly fruit. Even the seeds changed into hard, poisonous ones that would pass unscathed through a mammal鈥檚 gut. And emerging in a nutrient-rich bear dropping gave the pips an excellent start in life.

By the time people turned up in the region, between 5000 and 8000 years ago, the apple as we know it had nearly evolved. It was poised to head west鈥攈elped by the now domesticated horse, which probably carried it in both saddlebag and gut.

Not everyone is yet sold on Juniper鈥檚 story. Devotees of cider, made from bitter apple varieties, argue that there could be a bit of crab in some old cultivars. Juniper is about to test the idea, but says he wouldn鈥檛 put money on it. He鈥檚 convinced that early in their history, the wild crabs of Europe and North America were shunted onto evolutionary branch lines鈥攁nd now they鈥檙e not going anywhere that would ever make Homo sapiens鈥檚 mouth water. If we鈥檇 never had anything but these bitter little fruits, we鈥檇 probably be in Eden still. Adam and Eve simply wouldn鈥檛 have been tempted.