I believe the original ones were mutated humans brought to you by Adam Fenix
I've always assumed the Locust queen was Marcus's mom who Adam tried to save from cancer (or something similar) by turning her into one.
Elaine Fenix didnt die of cancer. She was found dead in the Hollow under Jacinto while she was working, she was a biologist specializing in mutation. Its possible this is the first encounter with the Queen and Adam Fenix.
From what I gathered throughout the three games is that the Locust have always been underground, juding by their structures. Plus theres a childrens story about boogey men taking kids away underground, urban legen type stuff.
The Sires were early stages of eveolution Locusts. Like a neanderthal is to a human.
The Locusts became aware of the lambent infection, and needed to flee the underground, to do so they had to invade and kill humanity. Or another theroy is that the Queen wanted Adam to find a way to kill the Lambet problem, but he couldn't so they decide to commit genocide on humanity. Which would make sence when she says 'humanity brought this war upon itself'...for not being able to help them.