THE SILURIAN HYPOTHESIS
The search for life elsewhere in the universe is a central occupation of astrobiology and scientists have often looked to Earth analogues for extremophile bacteria, life under varying climate states and the genesis of life itself. A subset of this search is the prospect for intelligent life, and then a further subset is the search for civilizations that have the potential to communicate with us.
A common assumption is that any such civilization must have developed industry of some sort. In particular, the ability to harness those industrial processes to develop radio technologies capable of sending or receiving messages. In what follows, however, we will define industrial civilizations here as the ability to harness external energy sources at global scales.
One of the key questions in assessing the likelihood of finding such a civilization is an understanding of how often, given that life has arisen and that some species are intelligent, does an industrial civilization develop? Humans are the only example we know of, and our industrial civilization has lasted (so far) roughly 300 years (since, for example, the beginning of mass production methods). This is a small fraction of the time we have existed as a species, and a tiny fraction of the time that complex life has existed on the Earth's land surface (~400 million years ago, Ma). This short time period raises the obvious question as to whether this could have happened before. We term this the ‘Silurian Hypothesis’
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. Given the vast increase in work surrounding exoplanets and questions related to detection of life, it is worth addressing the question more formally and in its astrobiological setting. We note also the recent work of Wright which addressed aspects of the problem and previous attempts to assess the likelihood of solar system non-terrestrial civilization such as Haqq-Misra & Kopparapu .
This paper is an attempt to remedy the gap in a way that also puts our current impact on the planet into a broader perspective. We first note the importance of this question to the well-known Drake equation. Then we address the likely geologic consequences of human industrial civilization and then compare that fingerprint to potentially similar events in the geologic record. Finally, we address some possible research directions that might improve the constraints on this question.
Relevance to the Drake equation
The Drake equation is the well-known framework for estimating of the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy . The number of such civilizations, N, is assumed to be equal to the product of; the average rate of star formation, R*, in our Galaxy; the fraction of formed stars, f p, that have planets; the average number of planets per star, n e, that can potentially support life; the fraction of those planets, f l, that actually develop life; the fraction of planets bearing life on which intelligent, civilized life, f i, has developed; the fraction of these civilizations that have developed communications, f c, i.e., technologies that release detectable signs into space, and the length of time, L, over which such civilizations release detectable signals.
If over the course of a planet's existence, multiple industrial civilizations can arise over the span of time that life exists at all, the value of f c may in fact be >1.
This is a particularly cogent issue in light of recent developments in astrobiology in which the first three terms, which all involve purely astronomical observations, have now been fully determined. It is now apparent that most stars harbor families of planets. Indeed, many of those planets will be in the star's habitable zones . These results allow the next three terms to be bracketed in a way that uses the exoplanet data to establish a constraint on exo-civilization pessimism. In Frank & Sullivan such a ‘pessimism line’ was defined as the maximum ‘biotechnological’ probability (per habitable zone planets) f bt for humans to be the only time a technological civilization has evolved in cosmic history. Frank & Sullivan found f bt in the range ~10−24–10−22.
Determination of the ‘pessimism line’ emphasizes the importance of three Drake equation terms f l, f i and f c. Earth's history often serves as a template for discussions of possible values for these probabilities. For example, there has been considerable discussion of how many times life began on Earth during the early Archean given the ease of abiogenisis including the possibility of a ‘shadow biosphere’ composed of descendants of a different origin event from the one which led to our Last Universal Common Ancestor (LUCA) . In addition, there is a long-standing debate concerning the number of times intelligence has evolved in terms of dolphins and other species . Thus, only the term f c has been commonly accepted to have a value on Earth of strictly 1.
THINGS YOU MAY NOT KNOW: That is the condition of the human race: we have woken to life with no idea how we got here, where that is or what happened before. Nor do we think much about it. Not because we are incurious, but because we do not know how much we don’t know. Warp speed ahead or behind in which the time traveler visits an ancient species of advanced, long-extinct lizard people who’d achieved technological mastery 450 million years before modern man. The lizards were called Silurians, hence the Silurian Hypotheses.
THINGS YOU MAY WANT TO SAVE: Reptile aliens & bulldog aquatic lizards. They are way cool even the way they breathe reminds me of Zen Training.
ZENTRAVELER SAYS: If you feel fishy you may not be alone in this world? Wearing lizard skin cowboy boots might give you a clue who we were.
From here to Infinity is a relatively short ride! The next leg takes eons and eons as you fly through the Barycentric Dynamical Time Zone! …and on and on and on. Follow the Zentraveler Newsletter often for Travel, Health and Zen-like stories and such. Where else can you get a THREE IN ONE NEWSLETTER FOR THE PRICE OF FREE.
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