The latest issue of "Nature" magazine has a commentary on
the recent "Science" article that gave insight into the differences
between chimp and human diets.

Citation:

Nature 427, 208 (15 January 2004); doi:10.1038/427208a
Evolutionary biology: Our relative genetics
DAVID PENNY

Abstract:
Data on the chimpanzee genome help in detecting differential
selection on individual genes, and in judging whether normal
microevolutionary processes are sufficient to account for human
origins.

A quote from the article:
"One of Clark and colleagues' findings is that human enzymes for
amino-acid breakdown (catabolism) have been under positive selection.
This is concordant with the generally high proportion of meat
(and thus protein) in the human diet, at least in comparison with
the more herbivorous chimpanzee and gorilla. The increased capacity
to break down amino acids is not surprising in another respect. For
example, failure to catabolize phenylalanine has several adverse
effects, including brain damage. Overall, the finding lends support
to theories4 that an increased proportion of meat in the diet of
early humans was important for an increase in brain size. Regardless
of that, there could also be ethical implications. If early humans
ate meat 'naturally', then for example being vegetarian could be
considered a personal choice rather than a universal ethical
decision. But all that can be claimed here is that scientific
knowledge will be necessary, even if not sufficient, for solving
such ethical questions."

Tom Billings