A new study whose abstract appears below, may be the first direct
genetic evidence of differences in dietary adaptations between
humans and chimps. [Note: there are obvious morphological differences
in human vs chimp digestive systems -- that is "well known".]

The study found evidence of positive selection pressure (vs chimps)
for the human genes in the area of amino acid catabolism. In this
regard, the study notes (p. 1962):

"A speculative suggestion is that this signal of positive selection
may arise from different dietary habits or pressures in the two
lineages. For example, branched-chain amino acid catabolism,
which involves the ALDH6A1, BCKDHA, and PCCB genes, is the primary
pathway for energy production from muscle protein under starvation
conditions (25). For all seven genes, mutations have been found that
result in human metabolic disorders, consistent with the idea that
natural selection shifted these genes in a manner that is relevant
to reproductive fitness."

I would advise that the paper is highly technical and takes some
effort to read.

Citation

Inferring Nonneutral Evolution from Human-Chimp-Mouse Orthologous
Gene Trios

Andrew G. Clark et al. [L-O-N-G author list deleted for brevity]
Science (magazine), 12 December 2003, vol. 302: pp. 1960-1963.

Abstract
Even though human and chimpanzee gene sequences are nearly 99%
identical, sequence comparisons can nevertheless be highly informative
in identifying biologically important changes that have occurred
since our [human vs chimp] ancestral lineages diverged. We analyzed
alignments of 7645 chimpanzee gene sequences to their human and
mouse orthologs. These three-species sequence alignments allowed us
to identify genes undergoing natural selection along the human and
chimp lineage by fitting models that include parameters specifying
rates of synonymous and nonsynonymous nucleotide substitution. This
evolutionary approach revealed an informative set of genes with
significantly different patterns of substitution on the human lineage
compared with the chimpanzee and mouse lineages. Partitions of genes
into inferred biological classes identified accelerated evolution in
several functional classes, including olfaction and nuclear transport.
In addition to suggesting adaptive physiological differences between
chimps and humans, human-accelerated genes are significantly more
likely to underlie major known Mendelian disorders.

Tom Billings