Lifextension argues it as well as anyone I’ve read. Some quotes:
Chimpanzees produce salivary amylase to digest fruit; similarly, carnivores also possess amylase in order to process the glycogen residing in muscle meat. Moreover, animals fed alternatives to their natural diet will produce amylase in amounts corresponding to the quantity of carbohydrates consumed. Humans too have their own primordial amylase gene copy; we have possessed it ever since we were primates. The second copy mutation occurred somewhere between 100 – 200,000 years ago, however this may have resulted even more recently, as single nucleotide polymorphisms and copy number mutations can result in just thousands of years. The additional – and currently incomplete – copies occurred at the very most, around 25,000 years ago, but most plausibly they came about around 10,000 years ago, concurrent with the onset of agriculture, and confirming that high starch consumption was a historically late phenomenon. Many present day human populations from low-starch consuming ancestries still only have two copies, indicating that adaptation to high-starch consumption was not globally widespread.
Moreover, the current evidence engendered from nitrogen stable isotope analysis of hominin bone data – being studied by Professor Michael Richards and the Max Planck Institute for Evolutionary Anthropology – has confirmed that our human ancestors truly were high-level carnivores. In fact, one-hundred percent of the early hominin bones studied from Upper Palaeolithic Europe reveal an even more carnivorous stable isotope footprint than that of foxes and wolves; while, comparatively, the data from omnivores such as pigs or the Brown Bear validates that these species truly did have an omnivorous diet.
Read the whole enchilada. The debate continues.
A lot of these arguments depend on the presumably slow rate of adaptation of a species to a
novel diet. So here is an idea to test out. Take some fruit flies, who presumably eat fruit,
and make them think the fruit is running out and all that is left is meat and kale, and see
how many generations it takes for most of them to die out and the rest to adapt!