Aliens visited Earth once, during the Holocene Epoch of the Quaternary Period. Along with the official samples collected for scientific analysis, one of them smuggled some souvenirs back home as toys for its 5-year-old squirmling.
All quite harmless, it thought, until a beetle crawled out of a piece of deadwood and infected the squirmling with a deadly virus that wiped out every living creature on the planet.
In a couple of centuries from now, human astronauts will visit the star Wolf 1061 and discover exoplanet 1061c is dead. Then they will excavate and find a 2016 copper-plated coin from Earth.