Sixteen hundred years ago Europe descended into the shadows. Waves of migration from Asia had brought the Roman Empire to its knees by the start of the 5th century AD. Large numbers of refugees had fled from the terror of the Huns – “an exceedingly savage” people, who dressed in the skins of field mice and bound the heads of their children so tightly that the frontal and occipital bones were flattened, distorting the shape of the skull. As waves of tribes flooded across the Danube, into what is now Germany, France and Spain, the empire collapsed. Rome itself, once capital of a mighty empire, was sacked, as Europe was delivered a death knell.

The period that followed used to be called the Dark Ages, a reference to the scale of the profound collapse that followed, though scholars these days prefer to talk about continuities between the period before the fall of Rome and the centuries that followed. Nevertheless, the decline was dramatic. Literacy levels plummeted, while building in stone all but disappeared, a clear sign of collapse of wealth and ambition; long-distance trade that once took pottery from factories in Tunisia as far as Iona in Scotland collapsed, replaced by local markets dealing only with exchange of petty goods; there was a major contraction too in smelting work, as measured from pollution in polar ice-caps in Greenland, with levels falling back to those of prehistoric times.

In fact, gloom did not envelope all of Europe, for much of the eastern half of the Roman Empire managed to escape intact – first, hanging on for dear life, before stabilizing and then flourishing. Centered on the great city of Constantinople, modern Istanbul, the Eastern Roman Empire – that nowadays we call the Byzantine Empire – included territories stretching from the Balkans through Asia Minor and the Middle East, as well as Egypt. These were rich lands that were home to major cities whose tax revenues paid for the construction of astonishing buildings like the great church of Haghia Sophia on the banks of the Bosphoros, where Europe meets Asia.

As bastions of Christianity, the Byzantine world took a keen interest in its eastern frontier, regularly fighting against the Persian Empire that dominated the heart of the world in the region linking East with West. It also controlled enviable trade routes that brought spices, silks and textiles from India, China and South Asia to customers spread out across the Persian Gulf and the Eastern Mediterranean. By the early 7th century, rivalries flared up and led to a war that lasted two decades, with both sides fighting each other to a standstill – and to a state of exhaustion.

As both teetered, armies inspired by the messages of the Prophet Muhammad swarmed north through the Arabian peninsula and delivered knockout blows that felled Persia and brought the Byzantines to a point of catastrophe almost as great as that experienced by Rome two hundred years earlier. Although the Byzantines hung on, regrouped and rebuilt an imperial state that survived for another eight centuries, a new world was born.

The Arab armies took control of a region that spanned from Spain and the Pyrenees across North Africa through Egypt, Palestine and Syria deep into Central Asia as far as the border with China. Cities began to spring up, paid for by vast incomes brought from all corners of this new empire to the center. Damascus boomed, as did cities like Mosul and Samarra in what is now Iraq, and Merv, in modern Turkmenistan. A new capital was founded that was bigger and brighter than all. Known at the time to all as the City of Peace, it is today more famous by another name: Baghdad.

The wealth that flowed into the heart of the Islamic world was breathtaking. As new elites found themselves with vast new fortunes, they set about spending them on the best that money could buy. Texts were written that set out where the best place was to buy tasty figs or the finest saddles – exactly the sort of thing that today’s hedge fund managers might turn to when it comes to finding out where to buy the best cigars and the best speedboats. Guide books were written about how to entertain dinner party guests: hosts and hostesses of the time were no less eager than Martha Stewart to make sure the right sort of drinks were served on arrival (water, served with a conserve of rose was bound to go down well, one writer observed), and that portion sizes should be generous – no one likes to leave a feast still hungry, the same author advised.

Those in Europe were left behind by this dazzling world that was not just about the fine things in life. The wealthy were great patrons of the arts and of scholarship, with the result that the best minds in the world were drawn to cities like Bukhara, Balkh, Samarkand and Baghdad. Nor was religion a barrier or an advantage to success: many of the great scholars of the time prospered despite the fact that they were Christian, Buddhist, Jewish or Hindu. This was a Muslim empire that was self-confident, tolerant and generous.

Europe had little to offer in return, and continued to founder while philosophers in the east debated about Aristotle, mathematicians pushed forward the frontiers of algebra, and astronomers calculated the phases of the moon – and the circumference of the earth – ever more accurately. With no mineral resources to speak of, traders from Western Europe had little more to sell than amber, falcons and fine swords.

There was, however, one commodity that Europe had in abundance that proved to be greatly in demand in the center of Asia: men – but above all, women and children.

When we think about slavery today, we think of the horrific treatment of Africans brought across the Atlantic to work in the Americas and in the Caribbean in the most terrible conditions. But the European experience with slavery was much more extensive than the centuries that followed Columbus’s crossing west across the ocean.

As I explain in my new book, The Silk Roads: A New History of the World, it is not just the numbers of those who were trafficked from all parts of Europe that are shocking; so too was the impact that this had on the emergence of the western half of the continent from the darkness. Silver began to flow back into the Mediterranean and above all through Scandinavia in quantities that are scarcely believable – but evidenced by finds of silver coins that are mind-boggling in scale, suggesting that not millions or tens of millions of coins flowed out of the Islamic world, but hundreds of millions.

  It was this wall of cash that prompted the rise of some of the great cities in Europe – including Prague, Utrecht, Mainz and the jewel of the Adriatic: Venice. The legacy of the slave trade will shock and surprise. But the fact that the rise of the West into the medieval period owed much to the willingness of Europeans to sell their fellow men (and women) into bondage is significant; so too is the fact that Western Europe owed its rise from the shadows to world that it was soon to come into bitter conflict with.