One autumn day, 170 years ago, a very relieved young man stepped ashore at Falmouth dock. He had sailed away nearly five years before and despite never overcoming terrible seasickness, he had circumnavigated the globe.

On landing, he hurried home to his family and for the remainder of his life he travelled very little. But a seed of an idea had formed during that journey and over the next 23 years it would grow and finally bear fruit. Charles Darwin would write the book that changed the way the human race thought about itself forever.

Darwin was a latecomer to the crew of the HMS Beagle. The ship had sailed before, but the previous captain a man named Pringle Stokes had committed suicide following a long bout of depression. Captain FitzRoy had replaced Stokes and had sailed the Beagle home. FitzRoy was well aware of the loneliness of command, so rather than dine alone every night, he decided to take a companion on the next trip and an enthusiastic young naturalist was recommended to him.

Charles Darwin was a respectable man with no idea what he wanted to do with his life. He had tried to study medicine, but surgery made him queasy. He tried religious studies, but it bored him. His real passion was his beetle collection, an interest he shared with the Reverend Frederick Hope, a family friend back in Shropshire. Darwin imagined that the trip would allow him time to reassess a kind of 19th-century gap year.

The invention of reliable, portable clocks, called chronometers, had enabled Britain to start mapping the world very accurately. So, 24 new chronometers were installed in FitzRoy's cabin and the Beagle's job was to fix the precise longitude of various remote islands. FitzRoy also intended to fill in some of the blanks on existing maps of South America.

Darwin himself had no purpose in mind other than to expand his knowledge of the world. He was, however, a very meticulous record keeper and collector. At the end of his journey, his personal diary totalled almost 800 pages and his scientific notes more than 2,000 pages. He also collected biological specimens to send home for analysis. In all, he dried 3,907 samples and preserved a further 1,529 in alcohol. Darwin sent some of his insects back home, including some to his old friend Frederick Hope. A few of Darwin's specimens are still preserved today in Oxford University's Hope Entomological Collection.

Darwin took with him the first volume of a new book called Principles of Geology by Charles Lyell and more than half the notes he made were geological observations. Lyell's book suggested that the landscape of the world had changed over millennia, which was radical thinking at that time. Darwin soon found physical evidence to support Lyell's theory and a few months later, Darwin received the second volume. In this book, Lyell went further and proposed that animal species had become extinct and new ones arisen as the environment changed, but he stopped just short of suggesting that animal species themselves had evolved.

While he was away, Darwin's scientific reputation steadily rose back in Britain. In particular, he sent home the fossilised remains of eight huge extinct mammals, including that of an elephant-sized sloth called Megatherium. On Darwin's return, Lyell himself was quick to use the fossils to support his own theories about a changing world.

Towards the end of his time away, Darwin wrote about the ups and downs of the journey. He was honest about the hardships of travelling and concluded that it would be a worthwhile experience only if the intended traveller had 'a decided taste for some branch of knowledge' and could 'look forward to a harvest, however distant that may be, when some fruit will be reaped'.

Darwin was a careful and methodical man, and one who struggled with religious belief all his life. Over the next 20 years he though deeply about publishing his theories. In the meantime, he made detailed studies of domestic animals and also published books on geology. He married, fathered ten children and worried obsessively about his health.

When Darwin finally did publish the fruit of his life's work, called On the Origin of Species, it suffered a barrage of criticism. Darwin was attacked both by the church and by many scientists. Even FitzRoy, who had gained a scientific reputation himself as a meteorologist, turned against him at one famously stormy meeting in Oxford.

The debate about evolution continues today, with groups in America recently renewing their efforts to restrict Darwin's ideas in schools. Despite this controversy, there is no doubt that the young Darwin's wide-eyed wonder at the world during one incredible journey spawned an idea that changed both science and the world.