1/30  So to summarize.  The biochemical path taken by developing life was
      swept clean by later, more sophisticated life such that no empirical
      evidence exists for any biochemical path whatsoever.  Moreover,
      scientists don't have a complete, compelling theory, even though they
      are unburdened by ANY possibly falsifying evidence.  Well, I will say
      that support for 'life started on earth' is about as strong as support
      for any other hypothesis, including 'life started on mars', 'life started
      on the moon', 'life was brought here by aliens,' etc.  Keeping in mind
      that mars and moon may have a better preserved record than earth, and
      that aliens may have evolved like scientists think: from chemistry.
        -- ilyas
        \_ the problem with all this stuff is that you can always go one step
           back and say "ok so how did that happen?  what predates that?" all
           the way to the 'beginning' of the universe (if there is such a
           thing) and then you get into, "ok, so how did the universe come
           about?" which leads to various religious concepts or the equally
           oddball stuff coming from physics.
              \_ what oddball stuff are you talking about?
           \_ Yes but you only have to go back a little way before you start
              running into contradictions with religions like Christianity.
              (And no, I'm not talking about stuff like "virgin birth"; aside
              from the biological debate, biblical details such as whether
              someone was a virgin are impossible to verify.)
              \_ A lot of it can be written off as errors in translation and
                 misunderstandings over time.  Even so, if all religions could
                 somehow be proven to be untrue, you still have the same
                 problem with going back and back and back to "how did the
                 universe come into being, from what, and what was 'here'
                 before (wherever 'here' might be in that context?".
      \_ Humans like to think things are magic. Historically they thought things
         from weather to illness were supernaturally controlled. All through
         history, the more we learn about the universe the more we see
         how things result from natural processes. At the limits of observation
         we don't have all the answers, but we can make reasonable conjectures
         that don't involve magical beings. We know a lot of organisms for which
         we have almost no fossil record (e.g. except in amber, which obviously
         only existed in an advanced age). We have some chemical evidence for
         self-replicating processes that could occur.
         Humans also think things are magical when they are improbable. For
         example, when a someone comes out unscathed from a huge disaster.
         But these improbable things too have natural explanations. And it's
         not improbable that the lottery will have a winner.
         But these improbable things too have natural explanations.
         otherwise.  [moved because it made the other replies to op look like
         replies to this which they weren't]
         \_ Argument from history is not enough.  I can also use such an
            argument another way: 'Historically, we could always find some
            intermediate form organism or "living fossil" living in the
            backwoods of the amazon, or some deep sea trench.  Why can't
            we find the lifeform intermediate between a chemical and a
            bacterium?'  Personally, I find the existence of life truly
            puzzling.  Right now we have no answer.  Nor did my alternative
            hypotheses postulate magical beings (or magic of any kind),
            although I don't reject magic either.  I also have problems with
            some feats attributed to evolution, but that's another story.
            I don't see _any_ support for 'life started on earth' at this
            moment.  It's cheaper to assume spores traveled from mars, and
            hope mars has something illuminating in the rocks, than to assume
            carbon and hydrogen combined to form a replicating machine
            involving three separate components (each a giant molecule), none
            of which can function without the other two.
              -- ilyas
            \_ How so? How is it cheaper to assume spores traveling from Mars
               than a primordial soup of amino acids getting a spark from
               the nascent Earth atmosphere? In the first case you still need
               to explain where the spores came from, and then explain how
               it is they happen to be capable of surviving the journey from
               Mars, not to mention what force caused them to suddenly
               migrate to Earth at just the right moment to get sucked into
               the Earth's orbit rather than the much more likely scenario of
               the spores missing Earth completely and ending up in the Sun.
               If you're willing to assume that many coincidences, why not
               believe that the elements on Earth just happened to fuse in the
               right way to form the essential amino acids that form the basis
               for life?
               \_ Because there are lots of spores, and they can survive the
                  trip (spores from bacteria on earth routinely go off into
                  outer space, they are found at all layers of the atmosphere).
                  Also, because it's more likely life arose on Mars, given a
                  good fossil record on Mars (which may or may not exist --
                  we don't know, but we can't rule it out), and traveled to
                  earth than it arose on earth, given no record before bacteria
                  at all.  The missing variable here is whether there is
                  some location 'close by' where a good record exists.  In my
                  mind the probability for something like that shoots up, if
                  there is nothing here on our rock.  I don't buy the argument
                  that every single trace of pre-bacterial life simply
                  disappeared.  It just never happened at any other stage of
                  life.  -- ilyas
                  \_ 1) So because it never happened at any other stage of
                     life, _so far as we know_, it's unlikely to have
                     happened?  2) We have possible traces of pre-bacterial
                     life in the structure of our own cells and in our DNA.
                     3) If we find that fossil record on Mars or the Moon,
                     then I will certainly give your theory more credence;
                     until then, the idea that multi-cellular life is the
                     result of random molecules bonding and that subsequent
                     iterations of such life wiped out the traces or our
                     equipment is still too insensitive to detect the evidence
                     sounds much more likely. I don't expect to convince you,
                     but I do want you to understand why some of us disagree
                     with you.
         \_ humans like to think probable things are improbable, due to
            inappropriate assumption of (combinatorial) conditions that
            are really independent...
         Bottom line, there is more support for "life started on earth" than
         otherwise.
        \_ ok...so?
        \_ the problem with all this stuff is that you can always go one step
           back and say "ok so how did that happen?  what predates that?" all
           the way to the 'beginning' of the universe (if there is such a
           thing) and then you get into, "ok, so how did the universe come
           about?" which leads to various religious concepts or the equally
           oddball stuff coming from physics.
           \_ Yes but you only have to go back a little way before you start
              running into contradictions with religions like Christianity.