Humanity First is an anti-steppers political movement led by Brian Cowley.
The Long Earth
The Humanity-Firsters think "Stepping" is unnatural and an abomination and see natural steppers as mutants who will replace them like the Homo Sapiens replaced the Neanderthals.
This movement gathers all kind of people who suffered from the discovery of the Long Earth like Rod Green (phobic, left behind by his family who founded a colony in the Long Earth) or Jim Russo (who has lost fortunes because of the economical consequences of Step Day).
The Firsters are ones responsible for the Madison bombing.