I didn't pay much (zero) attention to the grass-fed vs. grain-fed beef thing for a long time. A couple of years ago I got into Mark Hyman and his 'Eat Fat, Get Thin' routine (book title also) and he sang the praise of grass fed beef and butter. Makes excellent sense. One of the best guides to healthy eating methinks is to examine the diet our species evolved on for many thousands of years. That was what our digestive organs evolved on, just the way cows evolved on grass. Word is that grass fed beef has higher omega threes - meat and milk/butter - than grain fed.
I used to think that wheat was primal human food. Not true it turns out, entered human diet around 9,000 years ago. A long time but compared to a half a million years a short time. Not to mention our digestive systems are virtually identical to those of the great apes, and them critters evolved for a long, long time w/o wheat in their diet. Or cows milk. It's a bit odd that butter receives praise but apparently the science indicates that it's far better than cows milk for human consumption. I should study it more.
I've read varying accounts of how long ago we branched off from the other apes on the family tree. Some accounts have it around 10 million years ago - this link has it at 30 million years.
http://theconversation.com/when-humans-split-from-the-apes-55104
Amazing to me though that our digestive systems would be that similar after 30 million years.
Edited to add: Upon finishing the piece they make it clear that the 30m figure is by no means certain. Some have it as low as 6m years ago that we split off from chimps. Pretty large amount of time nonetheless.