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This is a popular argument among some Paleo diet enthusiasts and vegans. They state that if other animals do not drink milk from other species, therefore, it must be unnatural and humans past infancy shouldn’t drink milk either.
There are two factors which make humans different from animals when it comes to milk consumption: our ability to extract milk from animals and the genetic changes that have occurred in some populations over thousands of years, as a result of that ability to extract milk.
Ability to extract milk from other mammals
Humans have a superior intelligence and the physical ability to extract milk from milkable domesticated animals. That’s a great advantage compared to other mammals. It started about 80,000-10,000 years ago, when our ancestors learned how to domesticate animals and then to extract milk without harming them. (1, 2, 3)
It is technically difficult and impractical for other adult animals to feed on milk from either their mother or animals of other species.
Genetic changes in humans but not in other mammals
After farming replaced hunting and gathering and the domestication of animals became widespread in some areas of the world, farmers started to extract and use milk. Initially they produced tolerable, low in lactose fermented dairy products such as yogurt or cheese.
People, who still were not able to produce the lactase enzyme, adapted slowly to this new source of nutrients. This change in the diet created a need for the breakdown of lactose, triggering a genetic mutation which spread over thousands of years to the surrounding areas. (1, 2)
The spread of these genetic mutations could only occur after many generations of consistent consumption of milk and milk products. Lactase persistence is currently found in approximately 32% of adults worldwide.
In contrast, other mammals did not go through the same evolutionary path and so could not experience similar genetic mutations. The ability of mammals to produce lactase reduces during the weaning period, making them gradually lactose intolerant.
Logical aspect of the myth
It is also simply not logical to draw such a conclusion as in the myth question. Even if all the other mammals are unable to drink milk for any reason, we cannot assume that humans can’t. Moreover, there are many things that humans can do and eat that other mammals just can’t.