They lived in caves and recently started to control fire. This allowed them to break down the cells of difficult to eat foods so that they were edible. Fibrous and hard food that were not consumable in nature now could be eaten.
Another use was the killing of germs. Obviuosly as soon as a dead corpse is killed there are opportunistic life forms that take advantage of the loss of immune system to kill them off.
Microbes of all types will take up residence in a dead corpse within minutes of it's death. That is why we use refrigeration and other forms of preservation. One of the ways to reduce this was to use fire.
Fire not only killed most of these microbes but it modified the food as well. Not for the better but obviously for the worse. There were new chemicals created when the heat touched the food.
For those who have some organic chemistry background, when heat is applied to almost any organic substance in the presence of oxygen there will be new organic substances created. Also the remaining substances will become modified as bonds break and new ones form. Many of these substances are carcinogenic, meaning that they promote the genesis of cancer.
The people who eat lots of cooked and especially barbequed flesh are exposing themselves to these carcinogens on a regular basis. Also when the body is in constant ketosis, which means that the body is turning fat and protein into sugar, there is less energy to seek out and destroy cancer cells.
All of these contribute to higher than average chance of getting cancer. Here is an article documenting the real fact that the inuit in Greenland in the 1400's were developing cancer. They had no contact with the white man up until that point and they were unable to consume anything but the low carb high fat flesh from animals. Which is similar to the paleolithic diet.
A Google search of the keyword "paleo cancer" will bring up a long list of links that include admission by many paleolithic dieters who have been diagnosed with cancer after they were on the diet (some for many years).
A vegan low fat diet is used to return yourself to health and remain healthy for a long happy life (as well as a happy life for the animals who you did not kill to eat).
Eating a paleo diet is also a big environmental disaster with most of the rain forests of the planet being cut and burned down to raise animals. The amount of water that is used to raise animals versus fruits and vegetables is astronomical. Animals also produce toxic waste products which need to be dealt with, fruit and vegetables produce no toxic waste, only healthy compost to grow more fruit and vegetables.