Ever since the first animals evolved, there was a need to absorb nutrients from food. They would have had primitive mouthparts and rudimentary structures for ingesting food. On a microscopic scale, organisms may have used intracellular digestion to digest and absorb food. This is similar to how an amoeba would absorb food today.
When more complex, multicellular life forms evolved, there was a need to ingest a wide variety of matter. From plant matter to meat, all these materials had to be broken down into their constituents and dispersed throughout the body. Today, the human digestive system is a very robust system, able to break down food into its constituents. Even our teeth are an evolutionary piece of art due to its structure and function. We even have specialized cells that can secrete special substances, which help with the digestion and absorption of food.
Other animals have even more advanced digestive systems – birds and crocodiles cannot chew their food; hence, they possess an additional organ called the gizzard. This organ essentially acts as a grinder, grinding the food down. For the gizzard to function, birds have to swallow stones, which will eventually be regurgitated or passed on into the rectum once the function is done. Surprisingly, even earthworms have gizzards which require stones to function.
Many ruminants such as cows have an interesting digestive system. In fact, they have four stomachs, each designed to perform a specific function. Cows rely on fermentation to digest their food; hence, they have a rich microbiota in their gut. On the other hand, a carnivores’ diet is primarily meat; hence, they have a much shorter digestive tract. This is because meat is rich in nutrients, and a short digestive tract can easily extract the same with ease. Moreover, carnivores such as lions and tigers try to shred the meat with their teeth to help with digestion.
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