The cannibal habits of ancient humans probably caused epidemics of diseases similar to CJD, claim researchers.
They also possibly helped promote the genes which now appears to protect the vast majority of us from such illnesses, they say.
Early humans may have been cannibals
The question of whether our ancestors ever used to feast on each other, however, remains controversial.
The research team, from University College London, UK, normally study the genetics of prion diseases such as CJD hoping to find ways to protect modern humans from them.
However, their genetic testing of more than 2,000 people from a wide variety of races and cultures worldwide threw up an interesting clue to the cannibalism question.
vCJD and other prion diseases are caused when the introduction of "rogue proteins" into the brain cause other normal prion proteins to misfold, clump together and kill cells.
However, people who have two particular variations of a gene associated with the prion protein appear to be protected from the disease.
The UCL team, led by Professor John Collinge, found that these variant genes were present in humans across the world.
Normally, if one variant gene gives a bigger advantage than another, the other will disappear, but the continuing presence of both suggests the coupling offers a distinct evolutionary advantage.
This suggests that, at some point in prehistory, people who carried them were able to thrive and spread throughout the world when perhaps people who lacked the gene pairing were dying out.
Precisely what this advantage was is unclear, but Professor Collinge believes that the likely answer can be found in more recent history in Papua New Guinea.
There, a tribe which had a tradition of eating its dead were struck down by an epidemic of prion disease called kuru.
He believes that similar epidemics, but on a far wider scale, caused by widespread cannibalism among ancient humans, are likely to be the cause.
He told the BBC: "We have been trying to understand the genetic basis of prion disease, and why some people get the disease later than others.
"We found evidence that was strong balancing selection at the prion protein gene.
Pressure to change
"At some stage of human evolution there was a selection pressure which results in the pattern of genetics we see at the prion gene today.
"The question is - what has caused that selection pressure?
"The logical explanation - because we know that cannibalism can lead to these diseases - we know that there is evidence that cannibalism occurred in our ancestral past, that this is a plausible explanation of what caused this strong selection."
Although some anthropologists believe that the argument for human cannibalism is not yet proven, evidence that it did happen includes human bones with human teeth marks on them, and fossilised human faeces which include human proteins.
The study is published in the journal Science.