RealClimate logo

The Silurian Hypothesis

Filed under: — gavin @ 17 April 2018

One of the benefits of working for NASA is that the enormous range of science the agency covers – from satellite records for the present day, to exoplanet climates, from early Mars and deep time on Earth to the far future – and the opportunity to think ‘big’. This week sees the publication of a paper I wrote with Adam Frank that we hope might provoke some ‘big’ thinking.

The Silurian Hypothesis (preprint) is the idea if industrial civilization had arisen on Earth prior to the existence of hominids, what traces would be left that could be detectable now? As a starting point, we explore what the traces of the Anthropocene will be in millions of years – carbon isotope changes, global warming, increased sedimentation, spikes in heavy metal concentrations, plastics and more – and then look at previous examples of similar events in the geological record. What is unique about our presence on Earth and what might be common to any industrial civilization? Can we rule out similar causes?

(Dino Street (University of Rochester illustration/Michael Osadciw)

Adam had a nice piece in the Atlantic and there is also a good write up on Motherboard.

The naming of this idea comes from a 1970 Dr. Who episode where an ancient race of reptilians (“Silurians”) who had put themselves in hibernation to avoid a global catastrophe were awakened by experimental nuclear physics experiments. (I tried to find ‘prior art’ on pre-human terrestrial civilization that wasn’t based on notions of panspermia or ancient astronauts, but I haven’t yet been successful – anyone?). Needless(?) to say, we aren’t proposing any such occurrence (not least because the Silurian period is too early for the development of complex life on land).

The ideas in the paper lead naturally to many lines of speculation, some of which are relevant to us today, and some of which are just interesting (to us at least). For instance, given that the more sustainable a civilization is, the smaller its geophysical footprint might be, what does that imply for the detectability of long-term civilizations? Does the onset of ocean anoxia at the end of many of these events suggest a possibility of cycle where the collapse of one civilization provides the seeds (fossil fuels) for the next?

The whole idea is so intriguing that I wanted to do more with it than is possible in a journal article. Other scientists have occasionally dabbled in science-fiction (notably Carl Sagan and Fred Hoyle) and so, following their lead, I wrote a short story “Under the Sun” about the consequences for finding such a signal.

Literary as well as scientific criticism welcomed!


  1. G.A. Schmidt, and A. Frank, "The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?", International Journal of Astrobiology, pp. 1-9, 2018.

110 Responses to “The Silurian Hypothesis”

  1. 101
    James Cross says:


    There might have been other differences between the Eemian and Holocene but I don’t time has much to do with it. The Eemian lasted about 15,000 years and the early civilizations of the Holocene began in about 5-6,000 years after it began.

    There might be something to do with needing a population critical mass.

    But I think more likely is that humans were still lacking certain cognitive capabilities or perhaps they had no capability of living in large groups. I have speculated that humans at that point may have been more like super-intelligent wolves living in relative small packs. But there is also some evidence that modern language may not have arisen until 70-90,000 years ago. Cognitive abilities (language among them) may be related to ability to have extended families and kinship and to live in larger groups.

    Whatever the explanation the point of the question is that Earth had a species seemingly on the verge of being capable of creating a technological civilization yet still didn’t manage it for another 100,000 years. If the species had wiped out (Toba?), Earth might still not have a technological civilization.

  2. 102

    #98, 100–

    OT, but an interesting side question, as stated.

    My guess, FWIW, is that there is a lot of cultural (and perhaps even neurological) development going on between the first ‘anatomically modern’ humans 200,000 years back (and, according to recent finds, now further back than that) and the ‘Upper Paleolithic,’ which is the period ~50,000 years ago when we start to see differentiation and development of stone tool technologies in the archeological record:
    My guess is essentially the second attribution hypothesis in the Wiki article, the first being that it was the stimulus of the Last Glacial Maximum that forced technological advancement upon ancient humans. Of course, these ideas aren’t mutually exclusive; it may be that it’s as fortuitous that the environmental challenge arrived only after H. Sap had developed enough to survive it, as it is that it arrived and helped push us out of a long cultural stasis.

  3. 103
    James Cross says:


    I would look to Southern Africa around 70-100,000 years ago for origins of modern behavior.

    We have some fairly consistent genetic, linguistic, and archaeological evidence that points to Southern Africa as the origin of modern human capabilities about 100,000 years ago.

    A Tishkoff, et al. 2009 study concludes that “the people who today hold the most diverse genetic structure in the world are the San bushmen of South Africa. Thus, the San are likely to be the descendants of the first group who stayed behind, the first group of humans who colonized Africa and then the rest of the world.” (3)

    We have archeological evidence pointing to the same general region. At the Howiesons Poort and Stillbay sites in South Africa are found “lithic stone industries that are comparable to European Upper Paleolithic in their sophistication, yet they date fully 20,000 to 30,000 years earlier.” (5) The dates for this are 70-77,000 years before present and predate significant human migration from Africa. There is also evidence of artistic workshops from Blombos Cave in South Africa from as long as 100,000 years ago.

    Linguistic evidence, while more controversial and subject to criticism, also supports a South African origin of language. Analysis of the number of phonemes used by language shows a clear pattern of less phonemes in use in languages the greater the distance from Africa. San bushmen use over 200 phonemes. English uses 45. Hawaiian has only 13. Quentin Atkinson states : “An origin of modern languages predating the African exodus 50,000 to 70,000 years ago puts complex language alongside the earliest archaeological evidence of symbolic culture in Africa 80,000 to 160,000 years ago. Truly modern language, akin to languages spoken today, may thus have been the key cultural innovation that allowed the emergence of these and other hallmarks of behavioral modernity and ultimately led to our colonization of the globe.”

    Still the key point is there were several varieties of human species – Neanderthals, Denisovans among us – in the world 100,000 years ago and it isn’t at all clear that any of the other species would have made the advance to a technological civilization. So just having a large brain might not be enough. There are certainly other more subtle factors at work.

  4. 104
    nigelj says:

    Regarding the question of why humans didn’t produce an advanced culture 300,000 years ago. I think I recall early humans developed in the middle of Africa and may not have migrated into the fertile crescent area to the north until more recently. This area was favourable for grain farming, and farming is the beginning of advanced culture.

    I think farming would also require a certain level of population.

  5. 105
    Russell says:

    Recent discoveries in the archaeology of the Mediterranean have pushed the start of navigation back several eons. Underwater archeology there could shed yield rather more light o npalaeoclimate than the heat of the MWP debate.

  6. 106
    Mal Adapted says:

    James Cross:

    But I think more likely is that humans were still lacking certain cognitive capabilities or perhaps they had no capability of living in large groups. I have speculated that humans at that point may have been more like super-intelligent wolves living in relative small packs.

    In a punctuationist model of speciation in Homo, the anatomical and cognitive adaptations unique H. sapiens would have evolved together in a small, isolated population of its parent species (currently thought to be H. heidelbergensis). That is, the earliest anatomically modern humans had the cognitive capacity for behavioral modernity when their fossils first appeared in the record. I lean toward the “small, mobile bands skulking warily around the bad neighborhoods of Africa” theory for the delayed appearance of culture. Human populations may have been small simply because there hadn’t been very much culture accumulated; and culture wasn’t accumulating very rapidly because there weren’t enough individuals generating new cultural adaptations, and because transmission between mutually-hostile bands was limited.

    Mr. Cross’s analogy to ‘super-intelligent wolves in small packs’ seems apt to me. I made the following comment on another blog:

    We didn’t have much culture at the outset, even if we had the individual mental wattage. Cultural evolution is a process of accumulating adaptive behavior in individual brains and laboriously passing it on to each new generation by speech (remember, durable information storage only became available in the last 5ky). Again, our early populations were probably low, and there’s evidence the rate of cultural innovation is partly a function of population size. And if early nomadic bands seldom met other bands, innovations would diffuse more slowly through the continental meta-population.

    Another commenter responded with an analogy to smart-phones:

    I find this one of the most credible explanations. We were wandering around with super-smart phones, but no network to link too until population density increased. Then its synergistic, the increased population evolves better social cooperation, (more apps!) which increases food production, that increases population…

    That one’s good too.

  7. 107

    R 105: Recent discoveries in the archaeology of the Mediterranean have pushed the start of navigation back several eons.

    BPL: An eon means 1 billion years.

  8. 108
    JRClark says:

    107 BPL: An eon means 1 billion years

    Well, although the term aeon may be used in reference to a period of a billion years (especially in geology, cosmology or astronomy), its more common usage is for any long, indefinite, period.

    Given Russell’s comment was in the context of ‘human navigation’ and ‘archaeology of the Mediterranean’ it is self-evident his use of the word eon/aeon meant a long, indefinite, and of an indeterminate period of time and nothing more than that.

    Which makes BPL’s 107 comment more than irreverently moot but bordering on embarrassing.

  9. 109
  10. 110
    jgnfld says:


    We are not talking “common” usage here but rather scientific usage here. For example, if your criticism is correct, you should be able to supply examples of archaeology papers which speak about human history as existing for “eons” of time.

    Try for example using to find papers which include the terms “eons human history”.

Leave a Reply

Comment policy. Please note that if your comment repeats a point you have already made, or is abusive, or is the nth comment you have posted in a very short amount of time, please reflect on the whether you are using your time online to maximum efficiency. Thanks.