Is human civilization Earth’s first?

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(NASA Goddard and Steve Byrne)

A paper recently published in International Journal of Astrobiology asks a fascinating question: “Would it be possible to detect an industrial civilization in the geological record?” Put another way, “How do we really know our civilization is the only one that’s ever been on earth?” The truth is, we don’t. Think about it: The earliest evidence we have of humans is from 2.6 million years ago, the Quarternary period. Earth is 4.54 billion years old. That leaves 4,537,400,000 years unaccounted for, plenty of time for evidence of an earlier industrial civilization to disappear into dust.

The paper grew out of a conversation between co-authors Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, and astrophysics professor Adam Frank. (Frank recalls the exchange in an excellent piece in The Atlantic.) Considering the possible inevitability of any planets’ civilization destroying the environment on which it depends, Schmidt suddenly asked, “Wait a second. How do you know we’re the only time there’s been a civilization on our own planet?”

Schmidt and Frank recognize the whole question is a bit trippy, writing, “While much idle speculation and late night chatter has been devoted to this question, we are unaware of previous serious treatments of the problem of detectability of prior terrestrial industrial civilizations in the geologic past.”

There’s a thought-provoking paradox to consider here, too, which is that the longest-surviving civilizations might be expected to be the most sustainable, and thus leave less of a footprint than shorter-lived ones. So the most successful past civilizations would leave the least evidence for us to discover now. Hm.

Earlier humans, or…something else?

One of the astounding implications of the authors’ question is that it would mean — at least as far as we can tell from the available geologic record — that an earlier industrial civilization could not be human, or at least not homo sapiens or our cousins. We appeared only about 300,000 years back. So anyone else would have to have been some other intelligent species for which no evidence remains, and that we thus know nothing about. Schmidt is calling the notion of some previous non-human civilization the “Silurian hypothesis,” named for brainy reptiles featured in a 1970 episode of Dr. Who.


Dr. Who’s Silurians evolved from rubber suits to prosthetics (BBC)

Wouldn’t there be fossils?

Well, no. “The fraction of life that gets fossilized is always extremely small and varies widely as a function of time, habitat and degree of soft tissue versus hard shells or bones,’ says the paper, noting further that, even for dinosaurs, there are only a few thousand nearly complete specimens. Chillingly, “species as short-lived as Homo Sapiens (so far) might not be represented in the existing fossil record at all.”

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