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BenGarvey writes
"CNN.com cites a new study claiming huge meteors hit the Earth and Moon around the time that life formed, leaving two possible conclusions. That any existing life died out and had to start again, or the "panspermia" theory of life arriving on the meteors themselves."
dammit (Score:1)
Re:Or microbes survived. (Score:1)
How does pangaea fit in? (Score:1)
Re:Or microbes survived. (Score:1)
Given the high reproductive rate and thus high mutation rate, bacteria seem very unlikely to have been entirely wiped out by anything short of crashing into the sun.
Re:Time scale for life to get started? (Score:1)
RNA-only seems perfectly plausible to me, though.
While it may not be especially compelling evidence FOR panspermia, it certainly isn't an argument against it. Whether or not there might have been enough time for life to evolve (which is impossible to know for sure), the fact is that to my mind it's no more or less unreasonable to think the life came here from somewhere else than that it sprung out of nowhere here.
Re:How does pangaea fit in? (Score:1)
Before this even the sea levels on the Earth may have been too deep for the development of signifigant (sp?) self replicating chemicals. On the other hand there may have been a number of sites for the successful development of these chemicals around the edges of minor meteorite impacts.
Re:Time scale for life to get started? (Score:1)
Now, if you showed a biologist a system that, e.g., didn't have a separate phenotype and genotype, they probably wouldn't even consider it alive. But when evolving from nonlife to life, there obviously has to be a whole gray area in between.
While it may not be especially compelling evidence FOR panspermia, it certainly isn't an argument against it.
I see no real reason to introduce an extra hypothesis like panspermia when there's no evidence for it. Panspermia is testable, however. We should send an uncrewed sample-return mission to Mars sooner rather than later. If, for example, it brings back Martian bacteria that use DNA and RNA, that would be extremely strong evidence for panspermia!
re which way (Score:1)
Genetic techniques have also allowed a tree of life to be constructed for earth life, showing that the archaea (bacteria found today in extreme environments) are the most similar to the common ancestor. Presumably one could extend that technique and see whether the earliest branch on the tree of life was Mars or Earth life.
Anyhow, if we found life on Mars, it would be one of the two or three greatest scientific events ever, even if the genetic evidence showed that panspermia was wrong!
Time scale for life to get started? (Score:2)
But how much time is enough? We really know very little abou the biochemical processes that led to the first life. For example, there may have been an RNA-only world before the current RNA/DNA setup, but the original genetic code may have been even simpler than RNA or DNA. We just don't know. Given that we know essentially diddly squat about the very beginnings of life, how do we know what the time-scale would be for it to happen?
I was also left wondering if there's any proposed mechanism for creating such a plague of impacts. I would have imagined the impacts to be pretty much uncorrelated in time.
Or microbes survived. (Score:3)
Or the bacteria simply spored while the atmosphere was choked, and survived on Earth.
Or the first organisms on Earth weren't dependent on the Sun. Chemosynthesis of geothermally-synthesized chemicals is a perfectly reasonable basis for a biosphere, and some theorize that the majority of Earth organisms are actually part of an underground chemosynthetic ecology. (They even argue that petroleum may not be a fossil fuel, but a byproduct of this ecology, which explains several facts the fossil theory does not.)
Other theories may also be viable, of course, but I happen not to know what they may be...