Darwin’s theory of evolution emphasizes on natural selection occurring among the offspring of a species. According to Darwin, humans, like animals, are the product of evolution, and humans and the great apes share a common distant primate ancestor. In his work The Expression of the Emotions, Darwin directly compared humans with other animals, and concluded that "humans differ from other animals only in degree" (Hergenhahn, 2009, p. 301). Darwin's work initialized modern comparative and animal psychology, which supported the idea that "much could be learned about humans by studying nonhuman animals" (Hergenhahn, 2009, p. 301).
For example, a research question that can only be answered by doing experiments with nonhuman animals could be, can manipulation of connectome cause and cure mental disorders such as schizophrenia? Such research can shed light on human psychology but it cannot be done through experiments on human brain. However scientists can conduct experiments on animals. According Seung (2012), brains of small animals such as rats have similar neurons and connectomes as that in human brains but much less, that could simplify the research.
Hergenhahn, B. R. (2009). An introduction to the history of psychology (6th ed.). Belmont, CA: Wadsworth.
Seung, S. (2012). Connectome: How the brain's wiring makes us who we are. New York, NY: Houghton Mifflin Harcourt.