>> Does anyone know what part of the world legumes come from?  Did they
>> evolve somewhere in Africa- in which case humans would have evolved
>> along side them- or somewhere else?

Did a little study on this a couple of years ago. Legumes include hundreds
of plants found all over the world, and come in all shapes and sizes.
Legumes range from tiny (clover) to huge trees like locust and catclaw, and
pretty much anything in between. Legumes are classified as such because of
their flowers, bean-like seeds, and their ability to add nitrogen to the
soil (via bacteria). It's likely that your flower garden or landscape
includes many different legumes.

Yes, it is true that most legumes are poisonous to us, but some legumes are
edible in their raw states. The tendency is to lump all legumes into the
bean or pea families even though those families are but a minority.

>> Also, commercially prepared peanuts often have aflatoxins.

 As do many wild (natural) fruits and berries. It's caused by a mold.