I don’t know how most people get IBS, but my IBS started in my early teens. I always had a shy bowel and this probably led to a habit of incomplete or rushed evacuations. Over time the colon learns an ingrained, routine behaviour, just as we do. The brain and colon share a history and decades of gut / brain experiences can’t easily be undone. The gut’s “brain”, the enteric nervous system, is extremely complex.
I also was an alcoholic for a number of years from late teens to mid thirties, and that could have damaged my gut lining as well. I noticed IBS symptoms before then though. I was also a compulsive lolly eater when young, so an addictive personality doesn’t help.
FODMAPs aren’t necessarily the cause of IBS, because most people eat these without any problems at all. Grains and other antinutrients could be a factor in causing IBS, because, as the Paleo argument goes, our guts weren’t designed to digest grass seeds. But most people don’t have problems with antinutrients either, at least not for quite a while.
IBS could be due to a complex number of reasons. It could be that a shy bowel just naturally develops as our genetics didn’t endow us with the ability to digest certain foods all that well, so the evacuation process naturally takes longer. The longer you need in order to go, the more conscious you are of time slipping by, and perhaps getting increasingly nervous about this as well.
Were we born with colons with defective motor coordination? This doesn’t really make sense to me. If you eat foods that naturally propel along in the colon, vegetables, it’s hard to see how constipation would develop or why it would develop. But if you eat things that don’t propel in the colon – a lot of animal protein, grains etc or if you drink alcohol you can see how colon would be in two minds: tugging forwards to expel the fibre but holding back at the same time due to the constipating effect of the antinutrients or toxins.
If you typically have a prolonged and difficult evacuation, try the diet on my blog which excludes all grains, legumes, nuts, white potato, high FODMAPs and allows only a minimum of sugar.