I'm afraid mention of this article below makes it clear that cancer was indeed a standard occurrence in palaeolithic-era tribes, and therefore must also have been quite prevalent  among hunter-gatherer tribes as well, later on:-

"
History of Cancer
							The
August 2006 issue of "Bulletins in Cancer" found that paleopathology is
useful to expand our knowledge of the natural history of cancer.
However, the author points out that the interpretation of neoplasms, or
cancerous growths, is one of the more interesting but extremely
difficult aspects of this science. The article states that the
occurrence and frequency of neoplasms in ancient populations, which may
seem lower than they are now, is still a subject of debate for both
paleopathologists and medical historians. The author states that all
forms of benign and malignant tumors of the bones have been documented
in ancient human specimens from the Paleolithic period." taken from:-
http://www.livestrong.com/article/381304-paleo-diet-cancer/

The main problem is, of course, that palaeo corpses and those of hunter-gatherers are usually not in plentiful supply and not in good shape, so that it is difficult to ascertain the exact extent of cancer. Howeever, absence of evidence is not evidence of absence, of course.

There is a whole science concerning cancer in antiquity, namely paleooncology. Here is a report showing cancer as being standard in homo erectus skeletons:-

http://www.traveldoctoronline.net/paleopathology-of-human-tumors-MTE2NDAxNjI=.htm

The clincher is that wild animals also get cancer even when eating natural foods, that is, unless one assumes that humans are somehow uniquely protected compared to other species. Cancer doesn't seem all that rare either among wild animals given reports re the Tasmanian Devils and their facial tumour cancer etc.

Geoff