All living things must necessarily eat other living things to survive. 

Animals eat either plants or other animals and we eat them along with certain types of plants.

Failure to do so will be our demise.

When we eat beef we are indirectly consuming the nutrients the cow had obtained from the various grasses and grains it consumed.

When our dead body decays it will return to the earth where it will be consumed by plants which will be consumed by animals which, along with certain plants (vegetables) we will consume.

Thus, in an indirect way we are eating one another.