Cooking is an old and universal practice. It certainly is much older than germ theory (which was developed in the 1870s), so for most of the history of cooking, the rationale could not have been to kill germs. Fresh meat, particularly wild game, will not have much of a bacterial load, and cooking it would have little microbiological benefit. The need to be concerned about bacterial contamination of meat really only began with the invention of feedlots and industrial food processing.
The most prominent scientific rationale given for cooking meat (and other foods) is that it increases digestibility. Richard Wrangham argues that the invention of cooking was a major driver of hominid evolution – the increased calories made available by cooking allowed our guts to get smaller and our brains to get bigger.
This sounds like a good theory, but the actual evidence for it is slim. The oldest archeological evidence for cooking is about 20,000 years old. Wrangham thinks that cooking goes back much further – to 1.9 million years[1] . But his argument is indirect, if not downright circular: this is about the time that our hominin ancestor Homo erectus began sporting smaller jaws and larger skulls. Cooking would have made food easier to chew and more nutritious, therefore this is when cooking began. A plausible story but not a compelling one.
The evidence that cooking makes food in general and meat in particular more nutritious is not much stronger (I think we can all agree that it makes meat easier to eat. I can attest that fresh raw liver from a deer takes a long time to chew). Some nutritionists claim the opposite, that raw foods are more nutritious. They have a point: many vitamins are degraded by heating. But this degradation affects nutrition (and health, and thus natural selection) only if the heating leads to an overall deficit of a particular nutrient. Even if cooking degrades 90% of a given nutrient, that’s not a problem if 10% is all you need or if you get it from other foods. This is one of the advantages of being an omnivore.
Cooking certainly makes meat easier to break down into amino acids that can be absorbed by the gut – this has been known since the 1930s[2] . But digestion and nutrient absorption are complex processes that can’t really be modeled in a calorimeter or test tube.
A few experiments support the notion that cooking meat results in a net gain in nutrition. Pythons (!) fed cooked meat reduced their metabolic costs by 12% as compared to uncooked meat[3] . Wrangham’s group published a study showing that mice (who of course are not carnivores) lost weight more slowly on a cooked-meat vs raw meat diet[4]
But that’s about it for scientific evidence that cooked meat is more nutritious. Pretty thin gruel in my opinion (sorry).
The best evidence that cooking meat is adaptive is sociological: cooking meat is a universal practice. If cooking did not provide a net benefit, then non-cooking hominin species or non-cooking human societies would have outcompeted their cooking cousins. Since we are not ruled by tartare-loving overlords, it’s pretty evident that cooking meat is good for us.
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Footnotes
[1] Phylogenetic rate shifts in feeding time during the evolution of Homo
[2] The effect of cooking on the digestibility of meat
[3] Cooking and grinding reduces the cost of meat digestion.
[4] Energetic consequences of thermal and nonthermal food processing