If aliens existed we could readily see copious surface engineering in galaxies: "The germs of existence [on] earth if they could freely develop themselves would fill millions of worlds in the course of a few thousand years"(Malthus) https://waitbutwhy.com/2014/05/fermi-paradox.html … https://www.centauri-dreams.org/2012/01/23/rethinking-setis-targets/comment-page-1/ …
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Circa 1990 Andrew Cutler of Energy Science Laboratories in La Jolla claimed he found hydrothermal ore depletions corresponding to the Cambrian explosion and that the asteroid belt could be explained by a planetoid being dismantled for its nickel iron core around the same time.
That's one interesting fertilization hypothesis. Another one is that, before lichens and later plants had colonized land, oceans had been very depleted in phosphates. So life's colonization of land fertilized oceans, leading to the Cambrian explosion in the oceans.
