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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.There's also the problem of metallicity.If memory serves me, our Sun has a higher metalicity than the average for the Milky-Way galaxy. In fact our Sun was born in the Sagitarus (sp?) dwarf galaxy.If the level of metalicity found in our Sun is a requirement for life, there may not have been any stars around with that level of metalicity 10 Gigayears ago - let alone 13.5!Jim
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