Oxford English dictionary defines facultative as: occurring optionally in response to circumstances rather than by nature.
If we understand that as humans we evolved as facultative carnivores then this implies that our dependence on meat is imperative for our health and survival. Vegetables become an alternate under circumstances of scarcity of our main food — -animals, usually ruminants.
Different ways of eating have evolved in recent times as agrarian cultures replaced, by degree, the hunter facultative process for humans. The reality that our genetics have not altered much during this time places the burden on vegans and vegetarians to disprove that humans are healthier eating predominantly a meat based diet.
The issue is deeply complicated by cultural rites and traditions that have perhaps at times skewed and deviated humans from their most natural mode as facultative carnivores. Just because a human can get away with eating ocassional vegetables, fruit, pulses, grains and tubers does not mean it is optimal. Far from it. The obvious alteration in microbiome, neurohormonal regulation and organ health have suffered immeasurably since we veered in this direction of replacing meats with plants as our mainstay.
The return trend toward eating meat in recent years among scientists, clinicians and lay persons is demonstrating and countering the so-called conventional wisdom of modern medicine that has villinized fat for the last 40 years.
People are getting healthier eating as zero carb and low carb facultative carnivores. Autoimmune disease is receding and cancers are becoming more amenable to orthodox treatment as a result. People are happier eating the way evolution guided them. Their brains function optimally, and ironically, they are calmer than their vegan counterparts.