Many early twin studies where looking to see if intelligence (whatever that is ) is determined by what we are born with, our genotype, or as a result of interactions with the envirionment (experience), our phenotype. Twins are a special case as they are monozygotic (essentially a single egg divided) or dizygotic (two eggs fertilised at the same time). Monozygotic twins share the same genes and possibly the same environmnent, dizygotic possibly share the same envirionment, but not genes.
However, there does not seem to be any conclusive outcome from twin studies on many issues. This could be because, the genes of identical twins are not identical, genetic difference can cause phenotypic variation with will in turn have differential interactions with environments.
Regarding humanity, if you mean humankind, then it will not explain anything more than we have to work with what we have got. The devlopment of humankind is complex, for example non genetic transgenerational effects can also be inherited. Many studies have shown that deprivation in one generation can have effects on subsequent generations even if they do not suffer deprivation.