Darwin was accepted as a companion to the captain of the HMS Beagle, a young aristocrat called Robert FitzRoy. The 27m-long ship, with its crew of 73 men, and state-of-the-art equipment that included 22 chronometers (instruments for accurately measuring longitude), left Plymouth harbour on December 27, 1831.

The trip was meant to be completed within two years, but actually lasted nearly five! It took the Beagle from Britain to Brazil, round the coastline of South America to Peru, then onto the Galapagos Islands, heading home on a long route, designed to verify chronometer readings at known points, that took in Tahiti, New Zealand, Australia, Tasmania, the Cocos Islands and Cape Town, and, after a detour back to Brazil to check previous readings, finally brought the ship back to England.

This diverse itinerary gave Darwin access to a huge variety of geological features, fossils and living organisms. On his many trips and journeys inland he methodically made and wrote up observations and collected specimens, many of which were new to science. The conclusions that Darwin came to during and after the trip were controversial, and - to England's religious majority - blasphemous. They suggested that the world was not only extremely old but constantly changing, challenging most people's view that the Earth hadn't altered since God created it in seven days, around 6,000 years earlier.

Natural selection

Some of Darwin's findings contributed to the development of his most famous idea: natural selection. He realised that, over time, the animals that survived in any given environment were those that inherited – randomly, rather than by design – the specific beak size, wing span or leg length that gave them an advantage in finding food or a mate. The proof was that certain fossils of extinct animals Darwin found related anatomically to similar types of animals still alive in the local area, and that the ground from which they were excavated showed no signs of a catastrophic event.

These animals, such as the Megatherium, the giant ground sloth whose remains Darwin found in Argentina, were unsuccessful versions of species that carried on evolving. This idea was backed up by living specimens: most famously, the beaks of Galapagos finches differed from island to island, proving the species had adapted differently according to its local environment.

Home at last

When he arrived back in England in October 1836, Darwin was already a scientific celebrity. His reputation had been secured by his friend John Henslow, who had published some of Darwin's letters and shown leading naturalists the strange fossils he had sent back from the boat. This recognition helped him secure the help of experts in unlocking the full significance of his findings, and the financial backing he needed from his father to start work on ideas that were only fully revealed more than 20 years later in On The Origin Of Species.

And what happened to the Beagle? Having being used as a watch vessel to help combat smuggling around Southend, it was then sold and stripped for scrap in 1870. In 2004, the remains of the ship's hull were discovered under five metres of mud near Potton Island in Essex.