Lev Vygotsky was a Russian psychologist born in 1896 who focused on social contributions to cognitive development. He emphasized that social interaction and culture shape human development through mediating tools like language. Vygotsky introduced concepts like the zone of proximal development and the more knowledgeable other to explain how social learning occurs in collaboration between a child and expert. Though he died at age 37, his work formed the foundation for research on cognitive development.