Travel: Of the six continents, only Europe and Asia fling themselves horizontally across the globe. This vast land-mass, lying at a comfortable latitude and extending in an unbroken expanse from Damascus to Shanghai, is the key to our existence - our past and our future.
Agriculture and urban life developed first in this region, thanks to the easy passage of crops, animals and technology, and trade inexorably followed. Chinese silk has been found in burials from ancient Egypt and Iron Age Germany.
For two and a half thousand years, the Silk Road was the main artery of our civilisation. Silk, dyes and spices, compasses, printing and gunpowder travelled west. Gold and silver, Islam and Christianity went east.
Conquest went both ways, with Roman legionaries scattering their genes in the villages of northern China and the Mongol invasions reducing the central Asian cultures to ash and rubble.
With the discovery of the Americas and the sea route to the east, this traffic dwindled. The earth's centre of gravity shifted and the great cities that marked the Silk Road fell into decay. But now this region, inflamed by radical Islam and empowered by huge deposits of gas and oil, is a fuse that could ignite the future.
In 2003, the year that US and British forces invaded Iraq, Colin Thubron set off to explore what he saw as the nerve ends of the world. Starting in Xian, at the tomb of the mythical Yellow Emperor who founded China more than 4,000 years ago, he travelled the age-old route between mountain and desert, then dipped down through war-ravaged Afghanistan and bounded across Iran and Syria, to arrive at Antalya, once Antioch, the cradle of Christianity. As he journeyed, he found the Chinese world replaced by the Turkic, the Turkic by the Persian, but nowhere did these invisible boundaries of language, faith and ethnicity match the physical ones imposed by empires and nation states.
For many travel writers, the journey is a backdrop against which they play out their adventures or pursue some spurious quest. Thubron will have none of this. Writing with an almost Buddhist effacement of self, he is simply a lens through which the reader experiences other lives, the sweep of history and the stark majesty of the landscape. As a former film-maker, he has an innate understanding of form and colour, describing the body of the earth with a precision and awe that recall the photographs of Ansel Adams.
His prose, tunnelling through space and time, has an abstract, mesmeric beauty that makes this a journey of the mind.
Once in contact with people, Thubron has a quick understanding and warm sympathy that soon evoke a confidence. He also has the rare advantage of language. Speaking Mandarin and Russian, he reaches depths of experience and emotion that elude an English-only speaker. Yet language divides as well as unites: Thubron finds himself repelled by the literalism of Islam and puzzled that the Chinese have no word for passionate love.
All travellers - all proper travellers, that is, in contrast to mere tourists - seek to understand the culture and people that they encounter. For Thubron, faith lies at the centre of the human condition: whether steady, lost or gained, it is what binds us to eternity. In Kyrgyzstan, a country forged by Stalin out of nomadic clans, angels fly and serpents sing over the graves of folk heroes; in Uzbekistan, the regime has crushed a dangerously militant form of Islam and replaced it with a sanitised version. Afghanistan lives on a knife-edge between the ignorant fanaticism of the Taliban and the cautious orthodoxy of the moderates, while Iran is suffocated by the grievance culture engendered by Shiism.
Thubron is an erudite guide to the past, and an intrepid explorer of the present, but he provides no trite roadmaps to the future. Shoulder to shoulder with worshippers in the great mosque in Meshed, or alone and clambering around ruined tombs and fortresses, he finds a world in which the past pokes up through the present like ruins in the desert. Shimmering in the distance is the mirage of pornography and violence, prosperity and freedom spun by the internet.
Lucy Trench works at the Victoria and Albert Museum, which has recently opened a new gallery of Islamic art. She has travelled east, but only as far as Gilgit
Shadow of the Silk Road By Colin Thubron Chatto & Windus, 363pp. £20