'Complementary feeding' is indeed breastmilk and other foods. I am not sure of whateme's point - there is evidence from current round-the-world practices that many, many babies are given other foods from an early age (birth, even, and the first few months, though usually in tiny amounts), even in cultures in developing countries which routinely do longer-term breastfeeding. It's a public health issue there, because of the risks of non-exclusive breastfeeding - paper from Bangladesh here: www.ncbi.nlm.nih.gov/pmc/articles/PMC2753993/
It's very difficult for anthropologists, archeologists and others to be sure of what happened thousands of years ago, but there is good inferential evidence: some of the inferring is done by looking at what pre-industrial cultures still do today; some of it is done by looking at the teeth and the bones in the remains of children.
As I understand it, the teeth/bone evidence will indicate when breastmilk stops being a major part of nutrition, and solid foods take over. You can tell, apparently, if a child has eaten meat habitually. The teeth/bone evidence can't tell us when bf actually stops for good. Yes, solid food does/did take over from about 2 years, but it's likely that in many societies breastfeeding carried on, ceasing gradually over a period of several years - we know that from seeing what goes on today.
The 'normal' way to feed for our species, historically, is to breastfeed long-term, with solid/non-milk foods part of the infant diet from early on ('complementary food'). The 'exact' age for these other foods varies, but in a lot of societies, probably began/begins earlier than is good for health.