There were tribes and peoples that started solids early. (I specifically remember certain African tribes that started supplementing with raw liver and blood at three months). Babies do need that iron supplementation at a young age as they will run out of iron by six months. The iron is available from raw liver, raw egg yolk, or blood (uggh.)
My personal experience is this-- with my first son, during that pregnancy I had a decent diet with lots of supplementation but still not ideal. I was coming off of years of not eating well at all which makes a big difference. So, my first son was a skinny (though tall), fussy, colicy baby. He did not take any interest in any supplementation of anything whatsoever until he was like 10 or 11 months old when he started barely playing with it. This did cause me concern at the time being a 1st time mom!
After a few years of healthy eating and a near-perfect diet during pregnancy, my second son was born pleasantly fat and roly poly. He has a broader face too, btw. He is solidly built and strong and consistently ahead in physical milestones, and shows mental acuity as well. Although his rolls of fatness are beginning to lessen, he is still quite large, being 5 months old now he is wearing size 18 months. He is the opposite of the fussy colicy constitution my first son had-- he is so laid back and happy, he just will sit there and laugh and laugh for almost no reason at all. To me, the stark difference between the two boys is the perfect testament to the NT way of eating during pregnancy. (BTW my older son later filled out and became more laid-back and happy as we adjusted his toddler diet.)
Now, my second son (5 mo) has been VERY interested in food for the past 4-6 weeks and I give him tastes of things on my finger (barely any) let him play with my bowl or spoon, and feed him his raw egg yolk about once a week. We just started the raw egg yolk about three weeks ago. He LOVES his egg yolk but only actually eats a couple of little bitty mouthfuls; mostly he just mouths it and spits it out. But he loves it, he likes being just like mama and his big brother.
My conclusion, based mainly on my own experience, is that when a baby is recieving all of the nutrition they need from the breastmilk, and has received all the nutrition they need in utero, they can spare calories to other foods for more experimentation and fun, and they feel playful and adventurous enough to do so. In my first son's case, I don't think his body felt it could spare any calories to anything but breastmilk, because all of his nutritional needs weren't being met. Also his fussiness/colic made him unwilling to try new things. So that's what I think anyway.