Here's an interesting letter from New Scientist, debunking the notion that grain 
will feed all those billions, a common argument against Palaeo diets:-


From William Stanton:-

Richard Dobson says that if the grain now fed to animals were used to feed 
people instead there would be plenty for everyone on Earth(26 April, page 
20) . He omits to say that for corn and wheat to be grown in the huge 
tonnages that are currently produced requires diesel-powered machinery.

The world is now passing through “peak oil”, so petroleum production will soon 
be declining. Liquid fuels such as diesel and petrol will become steadily more 
scarce and more expensive. A farmer using liquid fuels is far more productive of 
grain than a farmer deprived of them. In addition, essential nitrogen fertilizer is 
made from natural gas, which is approaching peak production,. Taking these 
factors together leads to the possibility that in a few years’ time the world will 
have passed “peak grain” too.

   Meat from animals fed on grass will be available to a small proportion of the 
world’s population, but the diminishing supplies of corn and wheat will soon be 
inadequate to feed everyone else.”

Geoff