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Numerous workers have recognized that microscopic wear on the incisors and
molars of primates reflects tooth use and diet. For example, those primates that
often use their front teeth in ingestion have high densities of microwear
striations on their incisors. Furthermore, folivores have a high incidences of
long narrow scratches on their molars, whereas frugivores have more pits on
those surfaces. Among frugivores, hard-object feeders have even higher pit
incidences than soft-fruit eaters. These and other relationships between
microwear and feeding behaviors in living primates have been used to infer diet
in fossil forms. Miocene apes have a remarkable range of microwear patterning,
greatly exceeding that of living hominoids. For example, relatively high scratch
densities suggest that Micropithecus, Rangwapithecus, and especially
Oreopithecus (66) included more leaves in their diets. In
contrast, high pit percentages suggest that Griphopithecus and
Ouranopithecus (66) were hard-object specialists. Finally,
intermediate microwear patterns suggest that most other species studied, such as
Gigantopithecus, Dendropithecus, Proconsul,
Dryopithecus, and, perhaps, Sivapithecus (66–68), had diets dominated by soft fruits. These
data give us a glimpse of the extraordinary variation from which the last common
ancestor of apes and hominids evidently arose.

Continual mutation also means that potentially useful features can get lost.
Many primates cannot make vitamin C, an ability that wasn't missed in animals
that get lots of vitamin C in their diet. However, such losses can be limiting
if the environment changes, as one primate discovered on long sea voyages

The history of evolution could be written in an animal's excrement.

a census of the microbes

has found that closely related animals and those with similar diets tend to have
the same species of gut microbes.

By reading the chemical letters of a molecule called 16s ribosomal RNA – a
cousin to DNA –

researchers can catalogue bacterial species and conduct a rough census of their
numbers.

Gordon's team says the findings could help explain the evolutionary success of
mammals, particularly plant eaters such as cattle and giraffes that flourished
over the last 2 million years after a large increase in grassland habitats.

"If animals from an omnivorous background have moved into a more herbivorous
lifestyle they have absolutely needed bacterial partners and microbes to allow
that to happen,"

It's possible that our microbial makeup has guided and played a major role in
defining who we are and what we do, but it could also be the flip side," he
says. Gut microbe populations could passively respond to changes in host
physiology and diet.