Well I first suggested the idea in the early 1990s that life could have come from Mars to Earth inside rocks blasted off the Red Planet by comet and asteroid impacts, I think a lot of people felt that this was a pretty crackpot notion. But it became clear during the 1990s that not only is there a large traffic of material exchanged between Mars and Earth, but microbes, if protected by the rock, cocooned inside, could survive the harsh conditions of outer space for a long time - many millions of years.

So the evidence both theoretical and experimental has firmed up and I think many people now realize that if you get life on either Mars or Earth you’ll get it on both planets from this splashing phenomenon.  Now the case for it beginning on Mars is not very strong.  Mars is a smaller planet, so it cooled quicker so it was ready for life sooner.  Conditions there were more congenial for life to get going, but as we don’t know how life ever got going this is a bit of a leap in the dark, so we certainly can’t say that it definitely started on Mars, but it seems very plausible that it did.  On Mars seems as good a place as Earth for life to get started.  

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