To interpret what humans ate during Paleolithic times it is necessary to
understand the Pleistocene:

As Loren Cordain has pointed out, the primary game species of the
Pleistocene contined much more fat than even those domestic animals who have
been bred to express their Pleistocene DNA origins and store extra fat.
That the majority of large Pleistocene animals are extinct ( 60% of all
large mammal and bird species) makes it difficult to imagine the fat
available to those who inhabited their range.

This huge grassland covered a much larger portion of the earth than do the
dry grasslands of today.  Studies of pollen sediments indicate that trees
and woody stemed vegetables were much rarer than they are today and grew
much slower due to the lower concentrations of CO2 in the atmosphere
 according to glacier gas studies) .  Lush grasslands covered most of the
Earth now dominated by forrests.

Even in the dry grasslands of today the traditional people such as Plains
Indians, Tutsis and Mogols consume a diet that is far higher in fat than
even I consume.  In such places edible vegetables are rare and fruit trees
don't grow at all.  In such conditions one must consume at least 60% of
calories as fat to survive.  Stefansson also found these conditions in the
Arctic even though the Tundra is of too high a latitude to support abundant
grasses.  At lower latitude and CO2 levels even the berries that constitute
the entire Inuit vegatable food would be even more scarce and replaced by
grasses.  These lush grasslands would support far more high fat animals than
all the domestic animals that we produce today and we were evolved to
utalize this bounty.

Ray Audette
Author "NeanderThin"
http://www.neanderthin.com