Most of us know enough anthropology to know that the natural diet of our species in not herbivorous. The evidence for meat consumption goes back millions of years and our closest great ape cousins consume at least small amounts of meat. We also have digestive and other biochemical adaptations to meat consumption.
Unfortunately there are vegans who subscribe to what is really a form of creationism, a fictional evolutionary narrative in which humans are a peaceful herbivorous species. Proponents of this often cite "evidence" that is very poor and completely irrelevant, such as our lack of fangs (plenty of herbivores have fangs and many omnivores do not).
Other vegans recognize their diet as a modern diet for their modern philosophy. These people are willing to follow science and supplement to make up for the animal foods they are not consuming.
These folks need to be very critical of the woo vegans, because frankly woo veganism kills people and makes, This baby that died in France of b12 deficiency died of woo. I would never argue that veganism can ever provide optimal nutrients for childhood development and there is ample and growing body research to support this contention (DHA, Taurine, Carnitine, are known issues, what is unknown is more interesting…also the massive amounts of human variation which is why some children seem to do OK, but who knows how many IQ points have been lost?), but there is no reason to die from veganism. b12 supplements are widely available, though some people don't absorb them very well and may need injections. Vegans might also want to consider that the environmental impact of having children is definitely greater than that of eating a steak every day.
Paleo and real foodists also have to be aware of woo. There are plenty of people parroting ideas that have not only no evidence, but no plausible scientific basis in our side of the internet. I guess the main advantage we have is that we are usually not ideologically attached to our diets the way some vegans are. My doctor tells me my kid needs some corn? OK, we're having grits! My diet is about humans living well, not humans sacrificing themselves.
Remember the movie Independence Day? An awesome piece of campy b-movie sci-fi, but animal rights folks really remind me of the people on the roof of that building in New York with the "Welcome Aliens!" signs that subsequently get blown to smithereens.
In conclusion: I think veganism is suboptimal and anti-humanistic, but there is no reason to also be antiscientific and die for your cause, however foolish it might be. Just admit you are not a cud-chewing herbivore and take some freakin b12 and DHA supplements and stop making yourselves look stupid. Or how about eating just a few brainless oysters a week? Oh, sorry, that's against your religion even though oysters possess no thoughts, feelings, or nerves.
A few other woo victims (strict vegetarianism =vegan):