First: the period between now and when Earth will first become uninhabitable, which is 1.1 billion years.
I’m not too clear on why this would be related to hard steps; it’s just a fact about changes in the brightness of the Sun that on the face of it seems unrelated to evolution. If I was asked about a planet orbiting a red dwarf star at a distance that gives an Earth-like temperature, then I’d expect the time between the formation of the planet and life emerging to still be about the same 0.4 billion years, but the time until the planet becomes uninhabitable would be many times longer.
I’m not too clear on why this would be related to hard steps; it’s just a fact about changes in the brightness of the Sun that on the face of it seems unrelated to evolution. If I was asked about a planet orbiting a red dwarf star at a distance that gives an Earth-like temperature, then I’d expect the time between the formation of the planet and life emerging to still be about the same 0.4 billion years, but the time until the planet becomes uninhabitable would be many times longer.