Most exoplanets making the news are larger than Earth. I wonder about the smaller worlds, how small is too small? Reword to keep the question narrow enough to be answerable.
When the Earth was created in it's orbit around our sun, if Earth had been smaller, how small could it have been and still allowed human life to evolve and survive?
I imagine that the ability to maintain an atmosphere is the key. There are several things impacting atmosphere retention. If we make the Earth smaller and/or less dense, at what point does life as we know it today became impossible to have evolved and survived to present day?
I recognize the Moon is important in the process also, but for simplicity answers may choose to ignore lunar influences.