Final answer:
Cows can eat grass because their specialized digestive systems, including a complex stomach with a rumen, allow them to ferment and digest cellulose. Human digestive systems lack this capability, so we cannot directly obtain nutrients from grass. Instead, humans have adapted to different dietary sources with mixed health effects.
Step-by-step explanation:
Why Cows Can Eat Grass and Humans Cannot
Cows and other grazing animals can eat grass and benefit from its nutritive value, but humans cannot due to the specialized digestive systems of grazers. Grazers like cows have a complex stomach, particularly the rumen, which hosts a rich microbial environment capable of breaking down cellulose from grass into usable nutrients. This process is called fermentation. Moreover, grasses are often found on pasturelands unsuitable for other agricultural purposes, thus utilizing this land more efficiently while transforming grass into protein sources that humans can consume.
While humans cannot digest grass in the same way due to the lack of a rumen and the necessary gut flora, they have adapted over time to other dietary sources. Despite the agricultural revolution introducing grains and dairy into the human diet, this shift has led to adaptations like lactose tolerance in certain populations but also to health issues such as tooth decay and dietary intolerances.