Cannibalism, eating your young, rape, etc.
I’m not sure why killing for food is the one place we should choose to define our values and ethics based on what animals in the wild do.
Even if you could, you would also need to explain all of the evolutionary problems that could come from some humans going vegetarian while others don't.
What if being vegetarian makes you smaller and weaker physically (perhaps the case in some vegetarian countries now). If you had the answer, and it was clear a diet consisting of vegetables causes reduction in physical size, then I have to ask:
Would you want your kids to be shorter and physically weaker than you are?
As someone who eats meat, that's probably one of the worse arguments against vegetarianism/veganism I've heard. If eating animals is immoral, sure why not? If pillaging your neighbors makes your society better off, do you think a good objection to "maybe we shouldn't pillage our neighbors" is "Would you want your kids to be shorter and physically weaker than you are"?
Do you also have a problem with red meat?
> you can't prove that it's actually healthier, because the current state of dietary science is pretty poor.
Almost every decision in life must be made without proof, but with evidence and judgment. We know a lot about nutrition, and a lot of evidence points toward health benefits in eating more vegetables and less meat. We can also see lots of vegetarians in our communities and they don't seem sickly or shorter, etc. - we also see elite athletes in public who are vegetarians.
> a diet consisting of vegetables
Vegetarianim is much more than vegetables; it's everything but meat - legumes (generally beans), vegetables, fruits, grains, nuts - plus eggs and cheese. Vegans cut out the latter two items.
> What if being vegetarian makes you smaller and weaker physically (perhaps the case in some vegetarian countries now).
Where?
> evolutionary problems that could come from some humans going vegetarian while others don't.
What problems? How does diet affect evolution? We'll lose our hunting muscles over the next 500,000 years? Remember humans haven't changed much biologically in 200,000+ years.
Taiwan — 12-13% vegetarian — 174 cm avg male height
Mexico — 10-19% vegetarian — 170 cm avg male height
Italy — ~10% vegetarian — 174 cm avg male height
Brazil — 8-14% vegetarian — 176 cm avg male height
UK — ~7% vegetarian — 178 cm avg male height
Australia — 5-6% vegetarian — 179 cm avg male height
Switzerland — 5-9% vegetarian — 179 cm avg male height
Austria — 5-9% vegetarian — 179 cm avg male height
Germany — 4-8% vegetarian — 180 cm avg male height
I mean, if you think height doesn't matter for men, I think you may want to think about it.
Also, height is determined early in life. Many people become vegetarian in adulthood. Becoming vegetarian at 30 won't affect your height, I'm pretty sure.