My bet is the thing that last the longest will be the Pioneer Spacecraft. I have seen some mindbogglingly long estimates as to how long something like the pioneer would last asuming no catastrophic accidents. something in the order of 10^(10 - 30) years.
Good point, Mangetout. Hope you don’t mind I’m trying to find an answer, if such exists, by offering hypothesis?
It looks by the various replies that some evidence of civilization will endure very long. I don’t really think radio signals or the Pioneer count. The radio signals will quickly become too weak to be much use, and Pioneer is nearly impossible to find. For all we know, there might be lots of evidence of an ancient civilization one kilometer below a major city, but we are not going to find it. I think we should consider only evidence with a reasonable chance of detection. The moon-landers do count, I’d completely forgotten about it. That might be a very good bet for it. Still I wonder if aliens would surmise that these were sent from the earth, or that they would develop a theory that the moon contained an ancient civilization that died when the atmosphere left.
Furthermore, while I don’t dispute there will be for a very long time evidence that there was something intelligent around, a lot of what was suggested will not give an indication of what exactly was around. In the same way we find evidence of primitive humans or dinosaurs but have a really hard time to find out what they actually were doing. There will certainly be enough evidence of some kind of civilization from 1000-2000 A.D. but most of these years people lived in a manner quite remote from our current life-style. If we look at the life-style from the last thirty years, how much will remain for future explorers to make reasonable assumptions? Internet is mostly a virtual set-up; the data connections themselves do not give a clue about HTML, TCP/IP or DNS.
Landfills might indeed provide a lot of evidence. So there would for example be television sets to be found. How hard would it be to deduce the existence of television from a television set? For older ones, especially with tubes, it might be easier. I wonder whether aliens would find it easy to deduce the workings of transistors or I.C.’s (chips) if they had no other clues.
Cars could be good evidence, since these are fairly self-contained.
About paper in land-fills, I don’t know how long that will exist. Some might get fossilized, but I imagine it would be hard to extract the paper from the surrounding mass. Furthermore, a lot of current ‘paper’ output is done with toner (copying and laser printing), which can be erased from the paper. Printed documents in landfills would give an interesting subsection of civilization: probably little ‘high’ culture, a lot of porn, commercial mail, comics. Newspapers might help.
Just some thoughts on a Sunday morning… Well, human civilization still exists, at least that’s how it appears from where I’m sitting.
Let’s not forget the vaults of Scientology. Cute little time capsules that would entertain our ET scientists to no end I’m sure.
Perhaps so much of it that they will use the photos to construct what our religious life must have been like. 
Well, you know, I’m thinking about all those chthonic ‘earth-goddess’ statuettes archeologists are poring over… Do we have a new theory?
I’m not sure if we’re talking about the same statuettes, but in his book Goddess Unmasked, Davis discusses how if a future archaeologist found all our Barbie dolls, they might conclude that we were using them for Goddess worship. Similarly, all those statuettes that look like dolls for little girls (and boys?) might be just that.
Yeah, that doesn’t do much to help the porn=>weird religion joke you were making, but I had nothing to add that could be amusing. Sorry about that.
All evidence that humanity ever existed? Until the sun goes nova, assuming we’re still confined to this system by then.
My father and I were discussing a similar idea a few days ago. He suggested that, given the age of the Earth and the time Homo Sapiens have been around it might be possible that there were advanced cultures that died out in the past. (Not that he’s convinced of this, just speculating)
I countered that, among other things, the estimates of early populations were small and an agrarian society was sufficient, for a long time, to provide for peoples’ needs, so there was less need for technological development, which is why we never see relics of Olmec pickup trucks.
Since we can currently discover things about the world from 4.5 billion years ago, I’d say that’s the lower limit.
Well, for one thing, we’ve been emitting unnaturally high amounts of RF radiation for the past seventy years. That’s a tipoff right there, even if they don’t detect the subtler modulations.
Secondly, we’ve polluted nearly every planet with some form of technological debris. True, Venera is long gone, but the Pathfinder will be around for a while, bearing odd insignia and being composed of odd materials and generally being extremely artificial.
Our planet’s near orbit is a junk heap. There are cameras, bolts, screws, and other unnatural metal parts that have been machined to a suspiciosly high degree of precision to be meteorites, and why would such a small world attract a cloud that big, anyway?
Under the oceans, metal hulks and plastic cabling and jars of fine wine will moulder and sit for a long time to come. Subduction and quakes may claim some of it, but there’s plenty down in our polluted seas.
Speaking of pollution, we’ve created and expelled massive amounts of chemicals that just don’t show up in nature. When was the last time you saw a beetle with a bakelite shell, or a flower made of cellophane? How many plants odour the air with formeldahyde? Radioisotopes are another dead giveaway that nature didn’t intend everything that is found on Sol-3.
(Hell, the oceans probably contain massively high amounts of oil and pesticides. Pesticides might be written off as venom, but petroleum?)
Finally, we have concrete. Manmade rock composed of silica crystals cured though the action of water has proven just as durable as real rock. Even if all of the structures collapse, it would look odd to see so goddamned much of the same kind of silica compounds on America’s coasts and Japan and Europe and so forth.
So, would the Altairians be able to deduce the existence of sentient apes? Probably not, especially if mammals in general have fallen from dominence. But would they be able to deduce the existence of a toolmaking, barely spacefaring civilization with a love of nuclear energy and odd carbon compounds? Sure.
For a humourous take on what future archeologists might make of modern civilization after it collapses I recommend Motel of the Mysteries by David Macaulay, it gives a rather hysterical account of the unearthing of a motel after the Earth has been buried under a blizzard of junk mail.
Worked flints and mine workings in continental shield areas are my candidates for very long lasting relics of intelligence… concrete could last a long time too, altough it could be subject to water and carbonic acid degradation after a while.
Metals and plastics would mostly degrade, unless golden artifacts were preserved- but being soft gold would easily deform under pressure.
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
Don’t forget the few spacecraft that are drifting somewhere out there shouting “here we are, look at the prettiful capsule”…
Won’t our radio transmissions be drifting off into the far reaches of space even after the sun goes nova?
“prettiful”?
Yes, they would. Would they be recognizably artificial after that time? Maybe.
Anything made of glass could be preserved for literally billions of years. Middens of beer bottles have the potential to last until the crust they are on is subducted into the mantle. Concrete will last ages and ages too. Metals will eventually rust away, even if sealed in an anaerobic environment I wouldn’t expect anything metal to last more than a few tens of millions of years.
Perhaps what the OP meant to ask was, if there was a previous civilization on earth, how long ago must it have been since we don’t find any artifacts of it? Given human penetration all over the globe, I would say that any civilization as vast as ours could never have existed on earth.