This is covered in Perfect Health Diet. Here is one article for you from the blog - http://perfecthealthdiet.com/?p=2712
This was, in part, their basis for 20% carbs, 15% protein, 65% fat. For adults they essentially double the protein and take half the carbs in breast milk as one way to "justify" their optimal levels. There were other aspects of their ratio justification in the book.
I have posted this before here on PH - I think most people overestimate their protein requirements. The answer your question is one does not need nearly as protein as conventional wisdom suggests. The old 1 g/lb bodybuilder mantra seems to die a hard death like the Diet-Heart Hypothesis.
EDIT - Page 9 in PHD covers this in "Human Breast Milk and the Adult Diet" section -
"How will the optimal adult diet differ from an infant's? The brain is the body's primary consumer of carbs, and the brain accounts for 50% of calorie consumption in infants, but only 20%...in adults. Since adults require less than half the carbs that infants do, the optimal adult carb intake is likely about 20% of total calories rather than the 39% of infants. Starting from human like ratios (39%C, 54%F, 7%P), and changing half the carb calories to fat and protein, we get a ratio of 20% carb - 64% fat - 16% protein.
Other quotes from the book on Page 24
- Except for some protein deficient vegans and misguided bodybuilders, almost everyone eats a healthy amount of protein
- Only a few - very few! - protein calories can support rapid muscle growth. The protein content of muscle is 16.4%, so adding 26 pounds of muscle per year requires only 5 gms (20 calories) of protein per day [Aravind editoral - this one has me a bit vexed]
- In the normal range of protein intake, controlled trials have not been able to detect any additional muscle gains from higher protein consumption
I am NOT vehemently asserting that PHD is the gospel, just giving you a relatively popular Paleo source for your consideration.