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NigelGlitter wrote:
Abiogenesis cuts both ways. If evidence comes up that some advanced civilization planted the seeds of life throughout the universe, we've simply rolled back the clock, and have to answer how they got here, and we're back to arguing turtles. So, we get right to the meat of the whole ID debate, it either has to be some supernatural creator that created life, the universe, and canned tuna fish, or it's nature at work. Since there is no evidence of any supernatural entities, we are left to ponder the profoundly obvious avenue that is left to us.

To give this argument a bit more immediacy, it's possibly worth considering just how many "turtles" we can go back...

Let's say (rounding off the numbers a bit): That the universe is 14Bn years old, that the Earth is 4.5Bn years old (though started to form about 4.6Bn years ago), and that life arose around 4Bn years ago (though this last figure seems open to a fair amount of revision).

What this means is that life on Earth began when the universe was about 10Bn years old.

If intelligent aliens deliberately seeded the Earth, it's quite reasonable to suggest that they came from an Earth-like planet around a Sun-like star (too few heavy elements and Earth-like planets can't form) that formed 4.5Bn years previously - ie when the universe was 5.5Bn years old, with life arising there when the universe was 6Bn years old.

If we repeat the cycle again and we the require the formation of an Earth-like planet around a Sun-like star when the universe was just 1.5Bn years old. That's 3 generations (as it were) and we've pretty much run out of time - it doesn't seem likely that Sun-like stars could have formed until the universe was at least 1Bn years old. Even if we assume fairly wide error bars, we're not going to get orders of magnitude higher than 3 generations.

And of course... if life could arise naturally back then, it's even more likely to arise naturally now, as there would be more planets that are in just the right conditions.


PS For actual figures and some references, see here:
http://en.wikipedia.org/wiki/Age_of_the_universe
http://en.wikipedia.org/wiki/History_of_Earth
http://en.wikipedia.org/wiki/Panspermia
http://arxiv.org/abs/astro-ph/0012399
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