地球上的生命每2700万年就会灭绝一次(但我们还有大约1600万年才能到下一个)

上周也看到了关于宇宙的类似解释。由于地球的年龄约为45.4亿年，这意味着大约有168个不同的灭绝周期。据我所知，地球会在极端温度下变热或结冰，然后变冷或变暖，直到水和生命再次成为可能。< / p >

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?

我们有数十亿年前的大量化石和生命记录吗?< / p >

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.

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.

Furthermore based on the estimates for various population sizes it's quite obvious far less then 1% of animals are ever fossilized. So it's not a stretch to assume any number of species ( intelligent or otherwise) could have easily evolved and died out and never left a recognizable trace. And finally... They have found human like species dating back like 4 million years. And 4 million years is a long long time.

In conclusion... I sincerely appreciate the question, but the only correct answer is nobody knows. No one has ever found proof there was ancient civilizations so to some mainstream researchers that means there never was. However since no one alive today was around to witness the past then any answer you get is speculation and opinion. It's quite possible humans have evolved repeatedly, even fairly probably if you look at repeats in the fossil record. Unfortunately there is so much political tension over this subject hardly any one will actually read consider and respond accordingly with an answer opinion or idea. It's possible. That's a fact.. is it probable... In my opinion yes. Any one who tries to definitively tell you it never happened isn't worth their weight in salt.

The most factual correct answer is we haven't found recognizable proof of multiple cycles...nor has any one ever or will any one ever find proof there were not. You can't prove a negative