Maybe I am being a little hopeful here, but I think it future civilisations will have much more knowledge of human society in the twentieth and twenty-first centuries than we have of the ancient world. Admittedly, our steel and concrete structures may not be as enduring as the stone buildings of the ancients, but the sheer number of objects in existence now will provide future archaeologists with a wealth of information.
Imagine yourself living in the sparsely populated world of two thousand years ago. How many objects would you own? Unless you were nobility, then maybe a few pieces of fabric, a pot or two, a poorly constructed house or shelter, and some animals if you were lucky. And yet we know a remarkle amount about these people. In comparison, how many objects do you, one of our six billion modern people, actually own? A few hundred? A few thousand? Your home, which may crumble, will no doubt leave a footprint for those willing to dig. A car comprised of thousands of components of various materials? Dozens of items of clothing, CDs, tapes, disks, books, crockery, cutlery, appliances, lighting, clocks, computers, furniture? Much of this stuff is non-biodegradable. Even our nuclear and atomic technologies alone would leave incredible amounts of clues behind. There are countless millions of square metres of our planet’s surface under tarmac. Railway cuttings and tunnels blasted through solid rock have such gentle gradients and curves that it will be obvious they were some sort of transport path for relatively high speed vehicles. Airport runways are solidly built on very strong and thick concrete to cope with the weight of modern aeroplanes, and these too will surely survive.
And all of the above is based on the premise of a worst case scenario in which war, disease, or other catastrophe has destroyed our civilisation, and has left our descendants to piece together the clues from scratch. In a better outcome, our society may indeed survive such that the future one is merely an evolved version of our own. In this case, the knowledge of our time would have been preserved by conscious effort. I’d wager that much of the digital data of our time would be readable in the future, albeit by specialised hardware and software used by historians. I find it hard to believe that some incedibly advanced future computer would not be able to read a floppy disk (even if this means it has to think of a way to do it, and come up with a design for a disk drive). Needless to say, any surviving phonograph records will be fairly straightforward to those discovering them - this was the idea behind Carl Sagan’s “gold record” project on the Voyager spacecraft of the Seventies.
I think we’re going to leave our mark.