To answer the “why is Homo sapiens so prone to religious belief?” you first have to distinguish between traits that are adaptations, that is, products of Darwinian natural selection, and traits that are byproducts of adaptations, also called spandrels or exaptations. An example: Why is our blood red? Is there some adaptive advantage to having red blood, maybe as camouflage against autumn leaves? Well, that’s unlikely, and we don't need any other adaptive explanation, either. The explanation for why our blood is red is that it is adaptive to have a molecule that can carry oxygen, mainly hemoglobin. Hemoglobin happens to be red when it's oxygenated, so the redness of our blood is a byproduct of the chemistry of carrying oxygen. The color per se was not selected for. Another non-adaptive explanation for a biological trait is genetic drift. Random stuff happens in evolution. Certain traits can become fixed through sheer luck of the draw.
To distinguish an adaptation from a byproduct, first of all you have to establish that the trait is in some sense innate, for example, that it develops reliably across a range of environments and is universal across the species. That helps rule out reading, for example, as a biological adaptation. Kids don't spontaneously read unless they are taught, as opposed to spoken language, which is a plausible adaptation, because it does emerge spontaneously in all normal children in all societies.
The second criterion is the causal effects of the trait would, on average, have improved the survival or reproduction of the bearer of that trait in an ancestral environment -- the one in which our species spent most of its evolutionary history, mainly the foraging or hunter-gatherer lifestyle that predated the relatively recent invention of agriculture and civilization.
Crucially, the advantage must be demonstrable by some independently motivated causal consequences of the putative adaptation. That is, the laws of physics or chemistry or engineering have to be sufficient to establish that the trait would be useful. The usefulness of the trait can't be invented ad hoc; if it is, you have not a legitimate evolutionary explanation but a “just-so story” or fairy tale. The way to tell them apart is to independently motivate the usefulness of the trait. An example: Via projective geometry, one can show that by combining images from two cameras or optical devices, it is possible to calculate the depth of an object from the disparity of the projections. If you write out the specs for what you need in order to compute stereoscopic depth, you find that humans and other primates seem to have exactly those specs in our sense of stereoscopic depth perception. It's exactly what engineers would design if they were building a robot that had to see in depth. That similarity is a good reason to believe that human stereoscopic depth perception is an adaptation.
Likewise for fear of snakes. In all societies people have a wariness of snakes; one sees it even in laboratory-raised monkeys who had never seen a snake. We know from herpetology that snakes were prevalent in Africa during the time of our evolution, and that getting bitten by a snake is not good for you because of the chemistry of snake venom. Crucially, that itself is not a fact of psychology, but it helps to establish that what is a fact of psychology, namely the fear of snakes, is a plausible adaptation.
Our sweet tooth is yet another example. ItÂ’s not terribly adaptive now, but biochemistry has established that sugar is packed with calories, and therefore could have prevented starvation in an era which food sources were unpredictable. That makes a sweet tooth a plausible adaptation.
