Since humans have been eating solids since before there were books about child-rearing... nature ought to provide a pretty good guide to how to do it.  It wouldn’t be good for the species to screw this up.  The biggest hint is that babies develop an interest in picking up food and bringing it to their mouths around 6 months give or take a couple months.  Often their first teeth come in during this interval as well, as another cue to the parent and a driver for the child to want to chew on things as they're teething.  

Now cooking is a recent invention, so the easiest foods a child could start with are foods that are soft in their natural, uncooked state, and appealing to them... this pretty much means ripe fruit!  The child will progress slowly onto other solid foods as there isn't that much you can eat in its raw state without molars.  Seeds, nuts, grains and meats, and most vegetables will be hard to eat until the appearance of molars.  Breastfeeding supplies most of the nutrition until the molars are present, at the earliest.  The iron stores in the baby are also wearing off around this time and they need to start getting iron from dietary intake.  But if you offer them what they are capable of feeding themselves, it should all work out naturally.

Modern caveats ...

-We have an epidemic of food allergy and allergic conditions... so we have to be more careful with food introduction

.-We have a penchant for cooking food and spoonfeeding it, which will affect the child's natural appetite and food preferences - allowing them to completely self-feed raw foods can seem really tedious and prolonged of a process compared to the schedules you read about in books and hear about others trying, but breastmilk is quite adequate for their nutritional needs while they spend a few months learning how to work this food thing.

-We introduce non-breastmilk foods to babies before gut closure.  Usually cow milk formula, sometimes others, sometimes rice solids in thickened formulas.  Some babies are started on "solids" before gut closure, since it happens sometimes between 4-6 months, but we don't know when for any particular baby.

-We tend to clamp a newborn's umbilical cords right after birth, which keeps some of their iron stores from getting to them, and may require introduction of iron-containing foods before the time of molar eruption.