Molecular biology has recently provided genetic evidence indicating that some interbreeding between Homo sapiens and Homo neanderthalensis likely occurred. Erect posture and large brain are the two most conspicuous human anatomical traits. The advent of culture has brought with it cultural evolution, a superorganic mode of evolution superimposed on the organic mode, which has, in the last few millennia, become the dominant mode of human evolution. Ethics and ethical behavior may serve as a model case of how we may seek the evolutionary explanation of a distinctively human trait. Humans evaluate their behavior as either right or wrong, moral or immoral, as a consequence of their eminent intellectual capacities, which include self-awareness and abstract thinking. Ethical behavior is an attribute of the biological make-up of humans and is, in that sense, a product of biological evolution. One major contribution of sociobiology to evolutionary theory is the notion of inclusive fitness.