Warmflash and Weiss (2205): Did Life Come from Another World? in Scientific American 24-Oct-05

Warmflash and Weiss put the case (again) for panspermia (more properly, exogenesis). They're gentle on themselves:

In modern times, several leading scientists--including British physicist Lord Kelvin, Swedish chemist Svante Arrhenius and Francis Crick, co-discoverer of the structure of DNA--have advocated various conceptions of panspermia. To be sure, the idea has also had less reputable proponents, but they should not detract from the fact that panspermia is a serious hypothesis.

"Leading scientists" support panspermia? True, but are they being scientists when they do? Kelvin had a few off-days, being at least as famous for his dogged insistence on an incorrect age of the Earth as he is for his thermodynamics. Aarhenius was a smart guy who died nearly 80 years ago - give him a break. Crick talked in public about panspermia but was far from an uncritical advocate of it. Odd that Hoyle isn't mentioned.

Anyway, so what? Ever heard of the first of Clarke's laws? Ever counted how many learned scientists don't agree with panspermia? Arguments from authority carry little weight since the Rennaissance. And extraordinary claims require at least ordinary evidence.

"Panspermia is a serious hypothesis"? Hmm. Actually it occupies a peculiar niche in science, being for a long time almost but not quite falsifiable. There's no evidence for it but also nothing fully compelling against it. As hypotheses go, it doesn't impress. It makes for good copy though.

Plausible deniability or deniable plausibility?

While life-from-space might be possible (they say "plausible" but that's a connotation too far for me), why might Mars be a more congenial cradle for life than Earth? Earth is, at least currently, the poster boy for a planet on which life could begin: it provides tectonics, vulcanism, liquid water, and fantastically complex geochemistry, all of which are likely to help abiogenesis along. At it's worst it has been at least survivable (fossil evidence goes back to the Hadean).

Any good hypothesis has to be able to survive a slash attack from a fourteenth century razor-wielding monk. Invoking Occam, it seems more plausible that life began on Earth and stayed there than that it began on Mars, sent emissaries to Earth, then died out back "home". Why invoke a perilous and improbable crossing of the gulf of space at all? What additional explanatory power is provided? What missing features does areogenesis confer over geogenesis? On current knowledge: nothing. If we found DNA-based life on Mars, it would still be more likely that it originated on Earth.

I personally find polygenesis a more likely, yet still unproven, hypothesis, one which, if successful, would boost Occam further at the expense of panspermia. If you can make it at home, why send out for it? Undoubtedly we came from outer space (we are starstuff, said Sagan). But, while our sample size is poor, there is so far no reason to suppose that we arrived on Earth alive and replicating.