With changes in our solar system, such as planets moving away from the sun and the reduction in solar radiation, which other planets in our solar system could have been, in the past, in a suitable zone for life to originate?
This answer is also speculative, but according to the planetary habitable zone models proposed by "Habitable zone for Earth-like planets in the solar system" (Franck et al. 2000), a key finding was that:
an Earth-like planet at Martian distance would have been habitable up to about 500 Ma ago while the position of Venus was always outside the habitable zone.
So, Mars could have been habitable, but as it is much smaller than Earth, it has a smaller gravitational field presumably a major factor in how it lost much of its atmosphere.