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My time traveler is tasked with finding information on previous visits to Earth of a potentially hostile alien civ.

What sort of traces might aliens leave that could identify them? I'm thinking some kind of biological or light trail visible to a type of spectrum analyser...this could pinpoint the civ in question.

A sort of who dunnit, before dunnit gets dun...

He's going back in time to investigate suspected places, but I'm after ideas on what sort of traces (e.g. footprints (not like Armstrong's...) he COULD be looking for to ID the civ. Some kind of DNA trail is my best bet at the moment, but I was hoping for something a bit more exotic! He will be using some kind of scanner to survey terrain for traces. There is also a reason for going back in time as the traces will disappear over time.

I'm thinking something along a snail trail, but different and not readily visible. Perhaps time/space footprint that points to an entity

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closed as off-topic by Mołot, Renan, Separatrix, Ash, elemtilas Oct 30 '18 at 14:00

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    $\begingroup$ If you want to find traces of a rocket passing by you won't look for the same evidences you would look for if you were looking for a horse. If you don't tell us how this alien travel there is no way we can answer this question. $\endgroup$ – L.Dutch Oct 30 '18 at 7:15
  • $\begingroup$ This is more of finding some kind of footprint, e.g. a biological (E.G. DNA) or some other property (light, matter etc) footprint of some kind that identifies the alien civ. $\endgroup$ – IlludiumPu36 Oct 30 '18 at 7:20
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    $\begingroup$ Why doesn't the time traveller go back in time and checks out? Isn't this what time travelling is all about? $\endgroup$ – Alexander von Wernherr Oct 30 '18 at 7:26
  • $\begingroup$ Yep, he's going back in time to investigate suspected places, but I'm after ideas on what sort of traces (e.g. footprints (not like Armstrong's...) he COULD be looking for to ID the civ. Some kind of DNA trail is my best bet at the moment, but I was hoping for something a bit more exotic! He will be using some kind of scanner to survey terrain for traces. There is also a reason for going back in time as the traces will disappear over time. $\endgroup$ – IlludiumPu36 Oct 30 '18 at 7:36
  • $\begingroup$ what if the guy sets up cams all over earth and records the aliens? $\endgroup$ – Jannis Oct 30 '18 at 8:12
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Here are some ideas: The aliens interbred with a species on earth. This builds off your DNA idea.

Image by The Australian

The hero doesn't know what the aliens look like, but has a sample of their DNA. He finds a species (or even humans) that have a partial match to the alien DNA. So he goes back in time taking samples of the ancestors. As he goes further in the past, he finds versions of the evolutionary line that are increasingly disturbing and similar to the alien. Finally, in the distant past, he encounters a creature with perfectly matching DNA, showing him the startling reality of the aliens and their visit to earth.

To take it further, the hero could follow them 3 billion years into the past and discover the earliest life forms on earth were placed here by the aliens. Since the earliest life is microbial, the aliens might be sentient microbes.

The second idea is an old one. Look for radioactive materials which are traces of ancient power plants. Old traces of plastic give him hints. He goes into the past in these places, trying to nail down a century when the power plant was standing.

If the aliens merely landed on earth, there would a smaller window in time he needs to visit to witness them. After landing the ship, the aliens discarded materials that do not decay easily. These would be uranium, plastic, carbon nano tubes, or indestructible unobtainium.

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