Oral grooves are the mouths of single-celled microorganisms. Unlike complex organisms such as humans, cells can only ingest their food whole since they don’t have hard body parts like a jaw and teeth to help them chew. Another use for the oral groove is during conjugation or genetic exchange (microorganism sex, basically). You can often see paramecia engaged in this activity, stuck together like armless lovers.
What happens to the food inside the paramecium? Digestive enzymes in sacs called vesicles attack the food and break it down into smaller, useable molecules. Proteins become amino acids, foreign DNA become nucleotides, and carbohydrates become simple sugars. The paramecium then takes these simple molecules and rebuilds them into paramecium proteins and other paramecium cell structures. No matter how complex an organism is, this is what happens to food in all of us. Some organisms have fancy systems to break down larger food items while others survive by drinking juices and sucking in tiny animals.
The same can be said for reproduction. There are many ways living organisms can spread their genetic material. Some ocean dwellers spew their sperm and eggs into the environment and hope for the best. Some land dwellers have special organs designed specifically for sperm delivery and egg production. For simple single-celled organisms like the paramecium, oral grooves play a role in sexual reproduction.
Paramecia can engage in both asexual and sexual reproduction. Asexual reproduction happens when the environment has plenty of resources and the paramecium make copies of itself. Population rises quickly and life is great. When conditions sour, paramecia will engage in sexual reproduction, connecting their oral grooves together and exchanging bits of DNA. The hope is to create a more diverse gene pool so that at least one or two of the paramecia cells survive.
A simple oral groove on a paramecium tells the story of consumption and survival. These two acts are what drives all life on earth. So no matter how complicated society is, how difficult our human lives are to comprehend, or how murky our purpose may seem, remember that the meal you ate and the partner you love/are searching for are all the accomplishments that evolution is looking for from you.