Some of our ancient ancestors did indeed practice cannibalism, often for purposes of ritual and ceremony, but almost certainly not for good nutrition. A new study from the University of Brighton comparing the nutritional value of human flesh compared to other Paleolithic prey animals shows that, calories per pound, humans just aren’t very nourishing.
Boars and beavers, for example, pack about 1,800 calories per pound of muscle. As for humans? 650 calories. One mammoth could feed 25 neanderthals for a month, but one human would provide the same group only 1/3 of their needed calories for one day.
So, why does cannibalism come up so often when we explore the remains of our ancient ancestors? Evidence of cannibalism in ancient humans goes back as far as 800,000 years – long before homo sapiens came about – and while cannibalism sometimes appears to have been practiced out of necessity, more often than not, it appears it was part of a cultural ritual, or a warning to other early humans to stay away. Essentially what it may boil down to, in many cases, is that our ancestors survived because they were very opportunistic. Sometimes cannibalism was just easier than going out and hunting a boar.
NatGeo has a great article on this topic that you should totally read here.