The evolutionary origin of religions theorizes about the emergence of religious behavior during the course of human evolution.

Humanitys closest living relatives are common chimpanzees and bonobos. These primates share a common ancestor with humans who lived between four and six million years ago. It is for this reason that chimpanzees and bonobos are viewed as the best available surrogate for this common ancestor. Barbara King argues that while non-human primates are not religious, they do exhibit some traits that would have been necessary for the evolution of religion. These traits include high intelligence, a capacity for symbolic communication, a sense of social norms, realization of "self" and a concept of continuity.[1][2][3] There is inconclusive evidence that Homo neanderthalensis may have buried their dead which is evidence of the use of ritual. The use of burial rituals is evidence of religious activity, but there is no other evidence that religion existed in human culture before humans reached behavioral modernity.[4]

Marc Bekoff, Professor Emeritus of Ecology and Evolutionary Biology at the University of Colorado, Boulder, argues that many species grieve death and loss.[5]

In this set of theories, the religious mind is one consequence of a brain that is large enough to formulate religious and philosophical ideas.[6] During human evolution, the hominid brain tripled in size, peaking 500,000 years ago. Much of the brain's expansion took place in the neocortex. This part of the brain is involved in processing higher order cognitive functions that are connected with human religiosity. The neocortex is associated with self-consciousness, language and emotion[citation needed]. According to Dunbar's theory, the relative neocortex size of any species correlates with the level of social complexity of the particular species. The neocortex size correlates with a number of social variables that include social group size and complexity of mating behaviors. In chimpanzees the neocortex occupies 50% of the brain, whereas in modern humans it occupies 80% of the brain.

Robin Dunbar argues that the critical event in the evolution of the neocortex took place at the speciation of archaic homo sapiens about 500,000 years ago. His study indicates that only after the speciation event is the neocortex large enough to process complex social phenomena such as language and religion. The study is based on a regression analysis of neocortex size plotted against a number of social behaviors of living and extinct hominids.[7]

Stephen Jay Gould suggests that religion may have grown out of evolutionary changes which favored larger brains as a means of cementing group coherence among savannah hunters, after that larger brain enabled reflection on the inevitability of personal mortality.[8]

Lewis Wolpert argues that causal beliefs that emerged from tool use played a major role in the evolution of belief. The manufacture of complex tools requires creating a mental image of an object which does not exist naturally before actually making the artifact. Furthermore, one must understand how the tool would be used, that requires an understanding of causality.[9] Accordingly, the level of sophistication of stone tools is a useful indicator of causal beliefs.[10] Wolpert contends use of tools composed of more than one component, such as hand axes, represents an ability to understand cause and effect. However, recent studies of other primates indicate that causality may not be a uniquely human trait. For example, chimpanzees have been known to escape from pens closed with multiple latches, which was previously thought could only have been figured out by humans who understood causality. Chimpanzees are also known to mourn the dead, and notice things that have only aesthetic value, like sunsets, both of which may be considered to be components of religion or spirituality.[11] The difference between the comprehension of causality by humans and chimpanzees is one of degree. The degree of comprehension in an animal depends upon the size of the prefrontal cortex: the greater the size of the prefrontal cortex the deeper the comprehension.[12]

Religion requires a system of symbolic communication, such as language, to be transmitted from one individual to another. Philip Lieberman states "human religious thought and moral sense clearly rest on a cognitive-linguistic base".[13] From this premise science writer Nicholas Wade states:

Another view distinguishes individual religious belief from collective religious belief. While the former does not require prior development of language, the latter does. The individual human brain has to explain a phenomenon in order to comprehend and relate to it. This activity predates by far the emergence of language and may have caused it. The theory is, belief in the supernatural emerges from hypotheses arbitrarily assumed by individuals to explain natural phenomena that cannot be explained otherwise. The resulting need to share individual hypotheses with others leads eventually to collective religious belief. A socially accepted hypothesis becomes dogmatic backed by social sanction.

Frans de Waal and Barbara King both view human morality as having grown out of primate sociality. Though morality awareness may be a unique human trait, many social animals, such as primates, dolphins and whales, have been known to exhibit pre-moral sentiments. According to Michael Shermer, the following characteristics are shared by humans and other social animals, particularly the great apes:

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