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I've seen in several places that there seems to be an extinction cycle on Earth of approximately 27 million years. I've read this here:

http://www.dailymail.co.uk/sciencetech/article-1294372/Life-Earth-wiped-27-million-years.html

And also saw a similar explanation on Cosmos last week. Since the Earth is about 4.54 billion years old, that means there have been around 168 different extinction cycles. From what I understand, the Earth will either heat or freeze at extreme temperatures and then either cool or warm until water and life is a possibility again.

Given that this is correct (and definitely correct if I'm wrong, I'm eager to learn!), what are the chances there was a full civilization like ours that got completely wiped out in a different extinction cycle than ours? Is it possible or are there other factors in the history of our planet that would have prevented life from flourishing to the point that we are in now? I do understand that the factors of creating humans exactly like us are pretty drastically low but I suppose I'm referring to 'civilization' as intelligent life.

And a second part to the question, I know there are fossil records in the different layers of sedimentary rock but would there ever have possibly been an extinction event that ruined the Earth so drastically that it destroyed fossils and rocks (i.e. records of the past)? I've read that the oldest fossil found was 3.5 billion years old but it's not conclusive. Do we have a vast record of fossils and life that is billions of years old?

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    $\begingroup$ I'm not sure that this is a good question for this site. It's really two questions in one, periodic nature of extinctions and a prehuman civilization. The latter question is highly suspect for a site that purports to be scientific. The skeptics sister site might be a better home for this second question. $\endgroup$ – David Hammen May 31 '14 at 19:53
  • $\begingroup$ Extinction of life on Earth every 27 M years? The limules, the nautiles and the sphenodons did not get the message. $\endgroup$ – Nicolas Barbulesco Apr 7 '15 at 23:56
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You need to be very wary of anything written in the non-scientific media about science. The media loves woo and controversy because those are the things that garners readers, and that in turn garners advertising revenue.

By way of analogy, suppose as near-adult in gym class someone said "My gym shoes smell bad. Bad! Awfully bad!" Someone else would inevitably take a sniff. If it was bad, really, really bad, he would say so, while gagging. Someone else would say "That bad? Are you sure? Pass it over." That shoe would have quickly been passed around the locker room if it truly did smell worse than death. (Apologies if you are female; females don't do stupid stuff like this.)

The non-scientific media love scientific articles that smell, particularly those articles that smell extremely bad. The media falls under the compulsion of passing these articles around. Good, solid journalism is sorely lacking nowadays. It's much easier to copy someone else's writing than it is to investigate. It's not plagiarism if the original source receives acknowledgement and a fee. You can typically trace those dubious reports that are all over the internet to a single source.

So what's the source of this 27 million year recurring extinction event figure? It's two men who keep peddling this conjecture every other year or so. Here's the article that caused this imbroglio:

A. Melott and R. Bambach, "Nemesis reconsidered," Monthly Notices of the Royal Astronomical Society: Letters, 407:1 99,102 (2010).


Rather than looking to the non-scientific media, it is much better to look to the scientific community and see how much credence they have given to this conjecture. The answer is essentially none. That article has received a grand total of seven (CrossRef), maybe nine (Web of Science) citations in the four years it has been in print. The vast majority of those are either self citations or citations by other fringe scientists who are pushing their own brand of woo. This hypothesis is viewed rather lightly by the scientific community.

Moral of the story: Don't believe everything you read on the internet.


Let's suppose your conjecture is true. What signs would we see of a prior civilization? If the civilization was sufficiently advanced we would see a depletion of resources. If that prior civilization had advanced to the stage of sending stuff into space, we wouldn't see iron where we see it now. We see iron in very, very old banded iron deposits. We don't see any anomalous iron deposits in what would have been cities millions and millions of years ago.

The same goes for coal and oil. Our first discoveries of oil and coil was stuff easily accessible from the surface. Easy pickings. Those easy pickings would not exist if some prior civilization had advanced to the Elizabethan stage, perhaps even Roman level.

What if some calamity had wiped out humanity 2000 years ago? 10,000 years ago? What would a future civilization see millions of years from now? That's a better question. The answer is not much. That future civilization might well not see anything at all. Archaeologists have found fossil records of our ancestors, but that's because we humans are driven to look for those ancestors. A future civilization might not find any fossil humanoids that exhibited a tripling in brain size in just a few million years.

That of course is assuming that intelligent life arose before humanity. That too is dubious. Evolution shows a more or less steady progression in brain size. Dinosaurs might not have been as dumb as people thought 50 years ago, but they were still pretty dumb. Their brains were reptilian. Science fiction writers love the idea of intelligent dinosaurs. That's a rather non-standard view amongst scientists.

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    $\begingroup$ "You need to be very wary of anything written in the non-scientific media about science." -- and more particularly, you need to be very wary of anything written in the Daily Mail, about anything (but especially science). $\endgroup$ – Pont May 31 '14 at 8:47
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    $\begingroup$ While the Nemesis theory for mass extinctions is certainly not consensus, the original 1984 paper outlining the evidence for mass extinction periodicity has been cited 794 times. Just something to bear in mind. $\endgroup$ – AlexLipp May 31 '14 at 9:07
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    $\begingroup$ @AlexLipp - That 794 figure is the count from scholar.google.com. Their count is notoriously dubious. Web of Science reports only 485 citing articles. A good number of those citing articles refute Raup's and Sepkoski's claim of periodicity in extinction events. $\endgroup$ – David Hammen May 31 '14 at 15:29
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    $\begingroup$ Note also that after 20+ years, only one of those mass extinctions has been associated with a major bolide impact. A number of others have been associated with flood basalts though. That is a bit of a problem. $\endgroup$ – winwaed Jun 2 '14 at 13:44
  • $\begingroup$ Instead of this long development that not everything printed is true, it would be more interesting to say what this conjecture is false — if it is. $\endgroup$ – Nicolas Barbulesco Apr 8 '15 at 0:04
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Firstly, I'd like to state that so far there is no evidence that a civilization similar to ours has appeared on Earth before us. It might be apparent that we as a species have left quite a significant geological footprint, (if we didn't Archeologists would be out of a job!), and as of yet we've discovered no evidence to indicate a civilization that cannot be explained by either our species or our close relatives, such as Neanderthals. Also this evidence is completely contained within the last few hundred thousand years (very recent in Geological time).

In addition for 'eukaryote' life forms to exist (such as humans) oxygen is required so that respiration can occur. If we look at evidence for oxygen in Earth's atmosphere for the majority of it, oxygen is purely a trace element, and simply not enough to sustain complex life forms (as we know it I might add!). This would suggest that not only is there no evidence for complex life forms to exist, there is evidence that complex life forms are simply not possible for most of Earth's history.

Do we have a vast record of fossils and life that is billions of years old?

Essentially, life on Earth before about 540 million years ago (The so called Cambrian 'Explosion') was a little boring, as in it was primary simple multi-cellular organisms that didn't have many hard parts (the bits that get recorded in the fossil record). Think like a sponge today. These kind of life forms have a pretty poor preservation potential as its called, so before about 550 million years ago the fossil record (in terms of actual fossils) is absolutely terrible.

However we do have existence of life before this time, despite this. For example the Stromatolites which go back 3.5GA. And while fossils as we think of them today are rare in this period, we have other evidence of life. For example, life has a habit of 'preferentially' uptaking certain isotopes of carbon, meaning that life leaves a so called isotopic signature in the geological record. These 'signatures' are not direct evidence and are known as 'proxies'.

So essentially we have no evidence of a civilization other than ours existing anywhere in Earth's history, and in fact life itself is pretty boring before about 540 million years ago.

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    $\begingroup$ Lack of evidence is not evidence of lack. You have to look beyond that lack of evidence to show that the lack of evidence truly is evidence of lack. $\endgroup$ – David Hammen May 31 '14 at 19:51
  • $\begingroup$ @DavidHammen This is true! For eukaryote life forms to exist (such as humans) oxygen is required so that respiration can occur. If we look at evidence for oxygen in Earth's atmosphere for the majority of it, oxygen is purely a trace element, and simply not enough to sustain complex life forms (as we know it I might add!). This would suggest that not only is there no evidence for complex life forms to exist, there is evidence that complex life forms are simply not possible for most of Earth's history. $\endgroup$ – AlexLipp May 31 '14 at 21:10
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    $\begingroup$ You missed my point. Suppose there was a just-budding dinosaur civilization that just happened to have been wiped out by the KT extinction event. Would we see signs of that civilization in fossils? Maybe, maybe not. Suppose humanity had been wiped out 10,000 years ago, when civilization first started arising. Would some future intelligence species see any signs of that early human civilization? Would they find any fossils of humanity at all? Probably not. They would be focusing on the intelligent birds out of South America theory rather than the intelligent apes out of Africa theory. $\endgroup$ – David Hammen Jun 1 '14 at 23:38
  • $\begingroup$ Note that there are previous (and still existing) human civilisations that have preceded us that appear quite similar to ours in many aspects... Only for the last 10-15 thousand years though. $\endgroup$ – naught101 Jul 2 '14 at 9:36
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    $\begingroup$ ...Or so we think, until we discover a different life form. For instance, anaerobic eukaryote life does exist. The first eukaryotes probably did not need oxygen (Cavalier-Smith hypothesis). $\endgroup$ – Nicolas Barbulesco Apr 7 '15 at 23:39

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