Considering what we know today about cancer, it's likely that cancer was already a thing before we even became the species that we are now. I've been reading The Emperor of All Maladies recently and regarding oldest known occurences of cancer, it says pretty much what this ACS document does:

"Human beings and other animals have had cancer throughout recorded history. So it’s no surprise that from the dawn of history people have written about cancer. Some of the earliest evidence of cancer is found among fossilized bone tumors, human mummies in ancient Egypt, and ancient manuscripts. Growths suggestive of the bone cancer called osteosarcoma have been seen in mummies. Bony skull destruction as seen in cancer of the head and neck has been found, too. Our oldest description of cancer (although the word cancer was not used) was discovered in Egypt and dates back to about 3000 BC. It’s called the Edwin Smith Papyrus and is a copy of part of an ancient Egyptian textbook on trauma surgery. It describes 8 cases of tumors or ulcers of the breast that were removed by cauterization with a tool called the fire drill. The writing says about the disease, “There is no treatment.” "

Perhaps r/askscience could be helpful on this as well.