A human zoo was an exhibition that showed people from other cultures. Very often, this was done in similar ways than animals are shown in zoological gardens today. Most such exhibitions were done between 1870 and 1940. Human zoos attracted many people in Europe. They were often based on the idea of unilinealism, scientific racism and social Darwinism. Unilinealism or unilineal evolution is the idea that evolution only happens in one direction; some animal will be "at the peak" of evolution. As a consequence of these ideas, some human zoos placed indigenous people (particularly Africans) in a continuum somewhere between the great apes and humans of European descent. Ethnographic zoos have since been criticized as highly degrading and racist.