Elephants != chimpanzees
Some animals, humans and chimps included, can't synthesize certain things that other animals can, and must get them from diet, which mostly means eating animals that can synthesise these things..
I can't find the link now, but there was a controlled study where two Swedish guys ate nothing but animal products for two years and they exited the experiment in a better shape than they entered it.
*edit* Found a different study, only twelve months though:
linkSUMMARY AND CONCLUSIONS.
1. Two men lived on an exclusive meat diet for 1 year and a
third man for 10 days. The relative amounts of lean and fat,
meat ingested were left to the instinctive choice of the individuals.
2. The protein content varied from 100 to 140 gm., the fat from
200 to 300 gm., the carbohydrate, derived entirely from the meat,
from 7 to 12 gm., and the fuel value from 2000 to 3100 calories.
3. At the end of the year, the subjects were mentally alert,
physically active, and showed no specific physical changes in any
system of the body.
4. During the 1st week, all three men lost weight, due to a
shift in the water content of the body while adjusting itself to the
low carbohydrate diet. Thereafter, their weights remained
practically constant.
5. In the prolonged test, the blood pressure of one man remained
constant; the systolic pressure of the other decreased 20
mm. and the diastolic pressure remained uniform.
6. The control of the bowels was not disturbed while the subjects
were on prescribed meat diet. In one instance, when the
proportion of protein calories in the diet exceeded 40 per cent,
a diarrhea developed.
7. Vitamin deficiencies did not appear.
8. The total acidity of the urine during
the meat diet was increased
to 2 or 3 times that of the acidity on mixed diets and acetonuria
was present throughout the periods of exclusive meat.
9. Urine examinations, determinations of the nitrogenous
constituents of the blood, and kidney function tests revealed no
evidence of kidney damage.
10. While on the meat diet, the men metabolized foodstuffs with
FA: G ratios between 1.9 and 3.0 and excreted from 0.4 to 7.2 gm.
of acetone bodies per day.
11. In these trained subjects, the clinical observations and
laboratory studies gave no evidence that any ill effects had
occurred from the prolonged use of the exclusive meat diet.
And you can't really consider post-farming diet, as that is very recent and we couldn't have obtained all essential nutrients from plants alone before it.