Charles Darwin was a British scientist who transformed our understanding of the evolution of the natural world.
What is less well known is that Darwin had help.
In 1831, during his expedition to the Galapagos islands, Darwin noticed variation between species on different islands.
He wondered if these changes occurred over time, from generation to generation, with many different descendants evolving from one common ancestor.
But in 19th century Britain it was unanimously believed that God had created every species of life, fixed and unchanging.
Adrian Desmond, University College London - "Darwin was radical. His dynamic view was extraordinary, nothing had ever been seen like it."
Rebecca Stott, Author 'Darwin and the Barnacle' - "The notion that species had evolved, that humans had evolved, would be deeply upsetting, because of this presumption that humans are at the top of the ladder, of the hierarchy... to suggest that we had evolved from apes must have seemed deeply heretical."
Terrified of public humiliation, and hopeful of proving this hypothesis beyond doubt, Darwin worked secretly on his theory for two decades.
But it would be another man - Alfred Russell Wallace - who would prompt Darwin to reveal his theories to the world...
Wallace was a self-trained naturalist and explorer.
In early 1858, Wallace was in Malaysia when he contracted malaria.
When the fever subsided - inspiration struck.
Fittest variations - those best equipped for survival against factors such as disease or predation - will survive longest and eventually evolve into new species, he realized.
Wallace wrote down his theory, named Survival of the Fittest, and posted it to Darwin.
To Darwin's shock, Wallace's thoughts replicated his own.
Amazed and horrified that 20 years of work might go uncredited, Darwin turned to his friends, the botanist Joseph Hooker and the geologist Charles Lyell.
They suggested that Darwin and Wallace present their papers together at the Linnean Society in London.
Despite religious outcry, a year later in 1859, Darwin published "The Origin of Species" now regarded as the foundation of evolutionary biology.
Darwin remains one of the most celebrated scientists of all time. While Wallace has been forgotten.
Yet, in the spirit of discovery, Wallace was thankful for his role in spurring Darwin on, describing himself as "More Darwinian than Darwin."