Darwin's work on emotional expression, from notes in his Beagle diary and observations of his own children, to questionnaires, and experiments with photographs, was an integral part of his broad research on human evolution. It provided one of the main bodies of evidence for the descent of humans from animals.

Thanks to all who took part in our online emotions experiment - over 18,000 of you! The formal stage of the experiment is now over, but it will be staying online as an activity, so if you don't want to know the results, look away now. If you'd like to find out more about the experiment, or have a go yourself, click here. We promised to give some feedback and so here is what we made of the results. We've summarised the top 20 responses to each photograph in the pie charts below.

The year 1871 was an extremely busy and productive one for Darwin, with the publication in February of his long-awaited book on human evolution, Descent of man. The other main preoccupation of the year was the preparation of his manuscript on expression. Darwin continued to investigate the mechanisms of various emotions, such as grief, shame, and astonishment, drawing on the expertise of physicians and physiologists, as well as zoo-keepers, pet owners, parents of young children, and professional photographers. The year also brought a significant milestone for the family, as Darwin's eldest daughter Henrietta was married in August.