Emergence of religious behavior discussed in terms of natural evolution
The evolutionary origin of religion and religious behavior is a field of study related to evolutionary psychology, the origin of language and mythology, and cross-cultural comparison of the anthropology of religion. Some subjects of interest include Neolithic religion, evidence for spirituality or cultic behavior in the Upper Paleolithic, and similarities in great ape behavior.
Nonhuman religious behavior.
Humanity's closest living relatives are common chimpanzees and bonobos. These primates share a common ancestor with humans who lived between six and eight 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, and realization of "self" continuity. 
Elephants perform rituals for their dead. They demonstrate long periods of silence and mourning at the point of death; later, elephants return to grave sites and caress the remains. Some evidence suggests that many species grieve death and loss.
Relevant prerequisites for human religion.
Increased brain size.
In this set of theories, the religious mind is one consequence of a brain that is large enough to formulate religious and philosophical ideas. 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. The cerebral neocortex is presumed to be responsible for the neural computations underlying complex phenomena such as perception, thought, language, attention, episodic memory and voluntary movement. 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.
Stephen Jay Gould suggests that religion may have grown out of evolutionary changes that favored larger brains as a means of cementing group coherence among savanna hunters, after that larger brain enabled reflection on the inevitability of personal mortality.
Tool use.
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 that 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. Accordingly, the level of sophistication of stone tools is a useful indicator of causal beliefs. 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.
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.
Development of language.
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". From this premise science writer Nicholas Wade states: 
"Like most behaviors that are found in societies throughout the world, religion must have been present in the ancestral human population before the dispersal from Africa 50,000 years ago. Although religious rituals usually involve dance and music, they are also very verbal, since the sacred truths have to be stated. If so, religion, at least in its modern form, cannot pre-date the emergence of language. It has been argued earlier that language attained its modern state shortly before the exodus from Africa. If religion had to await the evolution of modern, articulate language, then it too would have emerged shortly before 50,000 years ago."
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.
Language consists of digital contrasts whose cost is essentially zero. As pure social conventions, signals of this kind cannot evolve in a Darwinian social world—they are a theoretical impossibility. Being intrinsically unreliable, language works only if one can build up a reputation for trustworthiness within a certain kind of society—namely, one where symbolic cultural facts (sometimes called 'institutional facts') can be established and maintained through collective social endorsement. In any hunter-gatherer society, the basic mechanism for establishing trust in symbolic cultural facts is collective "ritual".
Transcending the continuity-versus-discontinuity divide, some scholars view the emergence of language as the consequence of some kind of social transformation that, by generating unprecedented levels of public trust, liberated a genetic potential for linguistic creativity that had previously lain dormant. "Ritual/speech coevolution theory" exemplifies this approach. Scholars in this intellectual camp point to the fact that even chimpanzees and bonobos have latent symbolic capacities that they rarely—if ever—use in the wild. Objecting to the sudden mutation idea, these authors argue that even if a chance mutation were to install a language organ in an evolving bipedal primate, it would be adaptively useless under all known primate social conditions. A very specific social structure—one capable of upholding unusually high levels of public accountability and trust—must have evolved before or concurrently with language to make reliance on "cheap signals" (words) an evolutionarily stable strategy. The animistic nature of early human language could serve as the handicap-like cost that helped to ensure the reliability of communication. The attribution of spiritual essence to everything surrounding early humans served as a built-in mechanism that provided instant verification and ensured
the inviolability of one's speech.
Animal vocal signals are, for the most part, intrinsically reliable. When a cat purrs, the signal constitutes direct evidence of the animal's contented state. The signal is trusted, not because the cat is inclined to be honest, but because it just cannot fake that sound. Primate vocal calls may be slightly more manipulable, but they remain reliable for the same reason—because they are hard to fake. Primate social intelligence is "Machiavellian"—self-serving and unconstrained by moral scruples. Monkeys and apes often attempt to deceive each other, while at the same time remaining constantly on guard against falling victim to deception themselves. Paradoxically, it is theorized that primates' resistance to deception is what blocks the evolution of their signalling systems along language-like lines. Language is ruled out because the best way to guard against being deceived is to ignore all signals except those that are instantly verifiable. Words automatically fail this test.
Morality and group living.
Frans de Waal and Barbara King both view human morality as having grown out of primate sociality. Although 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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De Waal contends that all social animals have had to restrain or alter their behavior for group living to be worthwhile. Pre-moral sentiments evolved in primate societies as a method of restraining individual selfishness and building more cooperative groups. For any social species, the benefits of being part of an altruistic group should outweigh the benefits of individualism. For example, a lack of group cohesion could make individuals more vulnerable to attack from outsiders. Being part of a group may also improve the chances of finding food. This is evident among animals that hunt in packs to take down large or dangerous prey.
All social animals have hierarchical societies in which each member knows its own place. Social order is maintained by certain rules of expected behavior and dominant group members enforce order through punishment. Additionally, higher order primates also have a sense of fairness.
Chimpanzees live in fission-fusion groups that average 50 individuals. It is likely that early ancestors of humans lived in groups of similar size. Based on the size of extant hunter-gatherer societies, recent Paleolithic hominids lived in bands of a few hundred individuals. As community size increased over the course of human evolution, greater enforcement to achieve group cohesion would have been required. Morality may have evolved in these bands of 100 to 200 people as a means of social control, conflict resolution and group solidarity. According to Dr. de Waal, human morality has two extra levels of sophistication that are not found in primate societies.
Psychologist Matt J. Rossano argues that religion emerged after morality and built upon morality by expanding the social scrutiny of individual behavior to include supernatural agents. By including ever-watchful ancestors, spirits and gods in the social realm, humans discovered an effective strategy for restraining selfishness and building more cooperative groups. The adaptive value of religion would have enhanced group survival.
Rossano is referring here to collective religious belief and the social sanction that institutionalized morality. According to Rossano's teaching, individual religious belief is thus initially epistemological, not ethical, in nature.
Evolutionary psychology of religion.
Cognitive scientists underlined that religions may be explained as a result of the brain architecture that developed early in the genus "Homo" in the course of the evolutionary history of life. Nonetheless, there is disagreement on the exact mechanisms that drove the evolution of the religious mind. The two main schools of thought hold:
Stephen Jay Gould, for example, saw religion as an exaptation or a spandrel, in other words: religion evolved as byproduct of psychological mechanisms that evolved for other reasons.
Such mechanisms may include the ability to infer the presence of organisms that might do harm (agent detection), the ability to come up with causal narratives for natural events (etiology), and the ability to recognize that other people have minds of their own with their own beliefs, desires and intentions (theory of mind). These three adaptations (among others) allow human beings to imagine purposeful agents behind many observations that could not readily be explained otherwise, e.g. thunder, lightning, movement of planets, complexity of life. The emergence of collective religious belief identified such agents as deities that standardized the explanation.
Some scholars have suggested that religion is genetically "hardwired" into the human condition. One controversial proposal, the God gene hypothesis, states that some variants of a specific gene, the VMAT2 gene, predispose to spirituality.
Another view builds on the concept of the triune brain: the reptilian brain, the limbic system, and the neocortex, proposed by Paul D. MacLean. Collective religious belief draws upon the emotions of love, fear, and gregariousness and is deeply embedded in the limbic system through socio-biological conditioning and social sanction. Individual religious belief utilizes reason based in the neocortex and often varies from collective religion. The limbic system is much older in evolutionary terms than the neocortex and is, therefore, stronger than it – much in the same way as the reptilian is stronger than both the limbic system and the neocortex.
Yet another view is that the behavior of