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For this question, we'll assume that life on Earth was brought by aliens. They didn't do anything specific to our planet: they saw that it was compatible with the lifeform they wanted to seed, so they just got down and put life on Earth.

What did this form of life look like?

I want to assume that they dropped the most evolved lifeform possible, without denying any of today's science. Fossils and other proofs of the earliest stages of life on Earth must be taken into account in your question. But see that in this question, everything that we have no scientific proof was physically on earth at some point needn't have been.

Also these aliens dropped one and only one form of life, many specimens if need be. If the alternative is fun, you can include in your response a scenario where they dropped more than one lifeform, but this is not the main purpose of this question.

Bonus: how long ago was this?

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  • $\begingroup$ So, this is the very first lifeform on our planet? A single-celled organism? Not very evolved... $\endgroup$ – theonlygusti Apr 26 '15 at 18:41
  • $\begingroup$ For clarification, let me check: (1) they drop one lifeform at time X; (2) the evolutionary fossil record looks as it does today; (3) they drop the most evolved form they can; (4) they do nothing else of significance. So, (A) what lifeform, and (B) when was the drop. Right? $\endgroup$ – CAgrippa Apr 26 '15 at 18:43
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    $\begingroup$ It has to be four point something billion years ago, and non-oxygen-requiring, but beyond that there are far too many possibilities. $\endgroup$ – Gilles 'SO- stop being evil' Apr 27 '15 at 11:18
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    $\begingroup$ I don't think the question -- now that I understand it -- is too broad. It's very specific. But it looks as though it may not be answerable. $\endgroup$ – CAgrippa Apr 28 '15 at 0:44
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    $\begingroup$ @CAgrippa Well it wouldn't take half a novel to answer it like some other questions, but and answer still could be provided. I'd be satisfied with a short but well justified answer. Even if it is broad because it's impossible to give a very narrow answer. $\endgroup$ – Sheraff Apr 28 '15 at 5:28
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Since presumptive traces of biological activity can be found in the oldest (in an absolute sense) rocks, life seems to have formed as soon as the earth cooled enough, and instantly on a geological timescale.

Perhaps batches of what we woukd term proto-life were dropped as soon as Earth cooled to that point, first providing concentrated environments in isolated places, and then seeding the ocean with what could be used as food for the rapidly emerging life forms.

A more subtle idea would be to provide a bit of luck, making sure any needed conditions are met if they are lacking naturally. For example, provide a load of bio-active "fixed" nitrogen — one of the remaining weak links in our understanding of how it might have happened.

Maybe they could curate the infalling material, still massive though not so much as to boil the oceans and remelt the lot, making sure the rate doesn't get too high on occasion, and the right elements are introduced and the wrong ones kept away, with optimal timing for the emerging life's needs.

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What you describe is pretty futile.

If they drop one form of life it cannot be very advanced - as it will have nothing to feed on. No oxygen to breathe, etc.

A single celled photosynthesizing bacteria or algae is probably your best bet...

You want one and only one form of life - but the most evolved. Those factors heavily counter each other - all higher life forms are dependent on a vast ecosystem of other life forms. Plants producing oxygen and food, symbiotic bacteria, etc. To be able to exist in isolation as the only life form on the planet then your organism has to be very simple.

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  • $\begingroup$ The oldest (known) form of life on Earth didn't breathe oxygen — they breathed out oxygen, creating the oxygen-rich atmosphere we know today. $\endgroup$ – Gilles 'SO- stop being evil' Apr 27 '15 at 11:16
  • $\begingroup$ I don't understand why this is futile. Would you care explaining my "best bet"? $\endgroup$ – Sheraff Apr 27 '15 at 11:37
  • $\begingroup$ @Sheraff Added more explanation. $\endgroup$ – Tim B Apr 27 '15 at 12:23
  • $\begingroup$ @Gilles Yes, that's my point. $\endgroup$ – Tim B Apr 27 '15 at 12:24
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    $\begingroup$ Why would anaerobic organisms have to be “not very advanced”? For all we know, there could be very rich forms of anaerobic life, that died out when the atmosphere was poisoned by O₂. $\endgroup$ – Gilles 'SO- stop being evil' Apr 27 '15 at 12:30

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