In Powered by Plants, I discussed the human stomach acid level according to data I could find at the time of writing. I wrote:
"Ancient human species lived in Africa and like all other species lacked any type of rapid refrigeration. Animals biologically adapted to eating flesh of large game animals often obtain more meat from a kill than they can eat in a single day, despite having very large stomachs that we lack. They will guard the carcass for several days while continuing to feed on the rotting flesh, in order to maximize return on the energy that they invested in the hunt.
"To succeed at this lifestyle, animals biologically adapted to eating flesh produce sufficient stomach acid to keep the stomach at a pH of approximately 1.0-2.0 during food digestion, a necessity for killing microbes and parasites present on raw and decaying meat and for denaturing the large amount of protein that meat supplies.
"Most studies have shown that killing of bacteria requires a pH of less than 2.0, which in humans is 'rarely maintained for any length of time, especially during food intake' [Zhu et al, 2006]. The inability of the human stomach to maintain a pH below 2.0 for very long during food digestion makes it possible for microbes to survive and even colonize the human stomach, as illustrated by the fact that humans frequently contract infections of H. pylori and zoonotic microbes such as E. coli, salmonella, and campylobacter."
In a 2015 study entitled "The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome" Beasley et al searched gastrointestinal biology, animal physiology and avian physiology textbooks, published and unpublished papers for measured stomach pHs of 68 species (25 birds and 43 mammals) from 7 trophic levels.
"Our hypothesis was that foregut-fermenting herbivores and animals that feed on prey more phylogenetically–distant from them would have the least acidic stomachs. Tukey-Kramer comparisons indicated that scavengers (both obligate and facultative) had significantly higher stomach acidities compared to herbivores (both foregut and hindgut) and specialist carnivores feeding on phylogenetically distant prey. Specifically, foregut-fermenting herbivores had the least acidic stomachs of all trophic groups while omnivores and generalist carnivores, with more intermediate pH levels, were not distinguishable from any other group (Fig 1)."
Most interesting was their finding that "humans, uniquely among the primates so far considered, appear to have stomach pH values more akin to those of carrion feeders than to those of most carnivores and omnivores." They found human stomach pH to be an "extremely low" 1.5, contradicting the data I used in Powered by Plants. The authors searched for an explanation:
"Baboons (Papio spp) have been argued to exhibit the most human–like of feeding and foraging strategies in terms of eclectic omnivory, but their stomachs–while considered generally acidic (pH = 3.7)–do not exhibit the extremely low pH seen in modern humans (pH = 1.5) [38]. One explanation for such acidity may be that carrion feeding was more important in humans (and more generally hominin) evolution than currently considered to be the case (although see [39]). Alternatively, in light of the number of fecal-oral pathogens that infect and kill humans, selection may have favored high stomach acidity, independent of diet, because of its role in pathogen prevention."
Humans have stomach pH lower than many other carnivores and more similar to carrion-feeding birds. In light of humans' extreme aversion to oral contact with feces, it seems most likely that this stomach acidity evolved as an adaptation to eating meat, whether carrion or fresh meat.
This finding thus indicates that human ancestors could not survive and reproduce in their natural habitat without regularly eating microbe-laden animal flesh, and that their need to eat meat exerted a natural selection for survival and reproduction of individuals who had stomach acidity greater than many other omnivores and carnivores and similar to scavengers.