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To the question, What part of a star's habitable zone has the best chance of developing life?, I made the following comment:

It's always intrigued me as to why some people have thought that life in our solar system might have developed on Mars before transferring to Earth, via meteor impacts. Mars is farther from the Sun & is colder. Also, in the early days of the solar system, the Sun's energy output was less than what it is now, implying Mars would have been even colder than it is now.

To which @ProfRob made the comment:

Mars had an atmosphere and was warmer and wetter in the past, & a thicker atmosphere I should say

I accept all of this, but my question now is, what where conditions like on Earth when Mars was warmer, wetter and had a thicker atmosphere? Could life have evolved here first or was Mars a better candidate for life to develop on first?

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    $\begingroup$ Are you essentially asking about the conditions on hadean Earth? Probably the wiki article is a good start for that, if asking such a broad question.. $\endgroup$
    – AtmosphericPrisonEscape
    Apr 24 at 3:57
  • $\begingroup$ @AtmosphericPrisonEscape: Basically the question is this: the claim has been made that Mars was once wetter, warmer & had a thicker atmosphere. How long ago was that & during that time period, of the solar systems evolution, what were conditions like on Earth? If that corresponds to Hadean Earth, the fine, or does it correspond with another time period such as the Cambrian or Jurassic, or something else? $\endgroup$
    – Fred
    Apr 26 at 11:36

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Carr & Head 2010 summarize the evidence for past martian flowing water (see Figure below), and contrast it with the corresponding timescale on Earth (timescale on far left).

Fig. 1 from Carr&Head 2010

As is shown in the figure, flowing water mostly disappeared from Mars up until around ~3Gyrs ago, when on Earth photosynthesis was already invented by bacteria (see here), but life was still single-cellular procaryotes. The dashed lines after 3Gyrs indicate spotty or unclear evidence for the formation of the corresponding geological features.

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