In a moment of idle curiosity, I wondered what would happen if for some reason, human beings suddenly became extinct. Not a cheery thought but I was curious about how long it would be before all traces of our existence disappeared so that a new future intelligent species or maybe extra-terrestrial visitors to Earth would see no trace that intelligent life had ever existed because all the artifacts we created had simply decomposed away. It seems hard to imagine since humans have covered the Earth with our creations but surely all things must pass at some time?
I came across this list put out by the US National Park Service of the times things take to decompose in nature. What struck me was that these times, long by our scale, were not so much when compared to geological times. Glass bottles were the clear winners, taking about a million years to disappear. I thought that plastic would take longer than 450 years and I was surprised that aluminum cans disappear after just 200 years.
This made me curious about how fossils of dinosaurs and even older objects survive for so much longer. It is because the bones have been transformed into rock.
But a buried bone isn’t the same thing as a fossil — to become a fossil, the bone has to become rock. The organic parts of the bone, like blood cells, collagen (a protein), and fat, eventually break down. But the inorganic parts of the bone, or the parts made from minerals like calcium, have more staying power. They remain after the organic materials have disappeared, creating a fragile, porous mineral in the shape of the original bone.
Other minerals reinforce this bone, burning into a fossil. Water gradually makes its way into the bone, carrying minerals like iron and calcium carbonate picked up from the surrounding sediment. As the water penetrates the dinosaur’s bones, some of these minerals precipitate into their microscopic pores. As this process continues, the bone becomes more and more rocklike. It’s like filling a sponge with glue — the sponge’s physical structure stays the same, and the pores and pockets within it fill up. The glue makes the sponge sturdier and more resistant to damage. Large, thick bones, which have more room for mineral glue, make better fossils than small, flat bones.
Over the course of millions of years, the sediment around these reinforced bones becomes sedimentary rock. Erosion, tides and other natural processes continue to deposit more sediment, and this sediment becomes rock, too. As long as they can withstand the pressure from the surrounding rock, the bones remain safely hidden and preserved. After millions of years, some natural process, like the gradual shifting of the planet’s surface, can reveal these layers of rock and the fossils they contain.
So it appears that the best (and perhaps the only) way to identify extremely ancient artifacts is if they have been transformed into rocks that have some shape that is not a random formation.
This aricle by Mayank Vahia of the Tata Institute of Fundamental Research suggests that ‘purified metals’ (which I assume are metals that have been reduced down to the basic elements) and alloys may last for hundreds of millions of years and in some cases re-homogenization may never occur. But how would we know that they were created by a species and not naturally occurring ores?
Of course the conditions matter. Objects that are exposed to the elements are likely to decompose faster than those that are buried deep, and the nature of the surrounding environment, whether wet or dry, likely matters too. But I was curious if there was anything that would survive for hundreds of millions or even billions of years. Will the fossils of human bones be the only things that survive hundreds of millions of years after we are gone or could human-made artifacts also become rocks and survive almost indefinitely?
Vahia thinks some things may survive.
However, the most long lasting impact of human existence, in the form of isolation of different materials and metals is likely to endure over millions of years until the normal geological activity absorbs these materials in the Earth’s crust and sends it down to the magma level where it can be melted and re-mixed. Some of these manmade differentiations may well be permanent, visible to alien visitors even a few hundreds of millions of years later.
The humble staple pin in the Tibetan Plateau may well become a clinching evidence of the one-time existence of humans on this planet several hundred million years after they disappear.
There is of course a great deal of uncertainty about estimates of how long things take to decompose so it may be impossible to get a definitive answer.