Some life forms are speculated to possibly even now also exist on one or more big moons. It's been guessed that possibly most beyond Venus might be just subsurface life forms, if surface conditions there are too poor and too hostile, such as in many cases being too frigid cold, while having no useful atmosphere to help support life.
These could have originated far in the past from a rare few early lifeless randomly developed natural molecule forms happening to be somewhat able to sometimes use some nearby elements or chemical forms naturally existing in such places, natural usable lifeless chemicals also existing prior to any life form ever having arisen there.
Over many centuries, some natural maybe somewhat random molecular combinations with some other surrounding also lifeless natural chemicals could in very few rare randomly accidental cases, be expected to have resulted in some more advantageously complicated molecular forms. Then gradually a rare few resulting altered fresh chemical forms might have occasionally resulted in some newer chemical product having a molecular nature which in rare cases would have permitted it to have then done something similar, using other nearby natural lifeless chemical molecule materials.
And then in a few very rare cases, over maybe long slow times, this might have occasionally resulted in some eventually ever more complicated chemical forms, which might have effectively become able to catalyze having lifeless duplicates of themselves chemically produced.
Such an ability to form duplicates is a key needed characteristic of reproductive life getting developed. So when this may happen in a planet (or moon) surface, we may have here the actual beginning of natural "life" arising! And then when molecular accidents may naturally happen in such reproduced nontrivial natural chemicals, a rare few may be expected on rare occasions to result in naturally altered slightly ever more complicated molecule forms which are able to have duplicates produced more successfully, thus in fact effectively becoming the start of evolving reproducing very simple life!
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