If you lined up the digestive systems of mammals and dialed into their stomach chemistry, one detail about humans might raise an eyebrow: our gastric acid is fierce. Not just mildly corrosive — it’s on par with the pH found in scavengers like vultures and hyenas. While typical omnivores hover between a stomach pH of 2.5 to 3.5, and herbivores digest comfortably at pH 4 or higher, the human baseline — under fasting conditions — averages a sharp 1.5. That’s stomach acid strong enough to burn through more than just a salad.
Which leads to a strange question: why does an animal that now eats soft fruits, steamed greens, and grilled grains need a digestive system equipped for decaying meat?
Acid: Evolution’s Disinfectant
The best guess science can offer is simple, though hardly flattering — acid is our first line of microbial defense. Fresh plant matter isn’t especially dangerous in microbial terms; it rots slowly, and with relatively little bacterial load. But meat — especially the kind that’s been dead for a while — is a microbial playground. And for species that rely on such food, the ability to neutralize pathogens before they reach the gut is a life-or-death matter.
From this perspective, our stomach acidity makes less sense as the legacy of a peaceful forager than as the battle-hardened inheritance of a creature that spent hundreds of thousands of years scavenging and hunting — and often eating under less-than-ideal conditions. Cooked food is a recent luxury. Refrigeration, even more so.
Anatomy Tells Its Own Story
When you start mapping human digestive traits alongside those of other mammals, a clearer evolutionary fingerprint emerges — and it doesn’t look particularly plant-based.
| Trait | Carnivores | Herbivores | Humans |
|---|---|---|---|
| Stomach acidity (pH) | 1–2 | 4–7 | 1.5–2.0 |
| Intestinal length (× body) | 3–6× | 10–15× | ~8× |
| Dominant enzymes | Proteases, lipases | Cellulases (via microbes) | Proteases + amylase |
| Vitamin B12 source | Meat | Microbial synthesis | Animal foods / fermentation |
| Gallbladder bile | High concentration | Low concentration | High concentration |
In short: we are neither obligate carnivores nor gentle herbivores. Our digestive setup is a mismatched toolkit — part butcher, part baker — but heavily tilted toward animal consumption. Especially when it comes to what the body is most desperate to protect: its microbial entry points.
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