People often mention how prestoric men who lived on a diet of mostly meat had healthy bodies yet usually fail to mention that they all die before they turn 40. The comparisson is pretty pointless in my opinion though I don't believe that it totally dispproves the low-carb diet. It's just a half-sighted banel boast.

This is true, but as Beth pointed out, 40 was an average age which can be highly influenced by a high infant mortality rate. Something else you have to realize is that without medicines and medical care, if you got sick chances were far higher that you would die from whatever you got than they are now with antibiotics and advanced medical care...even life support. Something like an infected cut which would be treated easily with an antibiotic today could be life-threatening then. Same thing for if you happened to get injured which was also a much more likely occurance since hunting for prey was a dangerous task and also put you at risk for becoming prey or having your prey turn on you and injure or kill you.
As for the health of their bodies, it's pretty unlikely that those prehistoric men and women suffered from obesity, which is really the whole point of eating more protein and less carbs. More likely than not they lived from meal to meal, walked (or ran depending on what was chasing you) wherever they went, and had to hunt or gather every bit of food that went into their mouths. Man survived prolonged periods of drought and ice ages when vegetable matter was hard to find or non-existent. What did they survive and even thrive on? Meat and animal fat. Sugar? Nope. If you wanted something sweet the question that you had to ask yourself was how many bee stings you could tolerate to get it. If fruit happened to be native to your area, you only got it when it was in season...a few weeks out of every year or a few months if there were different varieties.
It's interesting that a recent show on the Discovery channel, Walking with Cavemen, showed our prehistoric ancestors (at least those who survived) eating mostly meat and very little vegetable matter and credited the addition of large amounts of protein with man's development of a large brain.