Editing Fermi paradox (section)

=== Rarity of intelligent life===
==== Extraterrestrial life is rare or non-existent ====
{{main|Rare Earth hypothesis|Firstborn hypothesis}}
Those who think that intelligent [[extraterrestrial life]] is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the [[evolution of biological complexity]]—are rare or even unique to Earth. Under this assumption, called the [[rare Earth hypothesis]], a rejection of the [[mediocrity principle]], complex multicellular life is regarded as exceedingly unusual.<ref name="rare-earth">{{cite book |title=Rare Earth: Why Complex Life is Uncommon in the Universe |first1=Peter D. |last1=Ward |author-link1=Peter Ward (paleontologist) |first2=Donald |last2=Brownlee |author-link2=Donald Brownlee |year= 2000 |page=368 |publisher=Springer |edition=1st |isbn=978-0-387-98701-9}}</ref>

The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a [[galactic habitable zone]], a star and planet(s) having the requisite conditions, such as enough of a [[Circumstellar habitable zone|continuous habitable zone]], the advantage of a giant guardian like Jupiter and a large [[natural satellite|moon]], conditions needed to ensure the planet has a [[magnetosphere]] and [[plate tectonics]],<ref>{{cite journal |last1=Stern |first1=Robert J. |last2=Gerya |first2=Taras V. |title=The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations |journal=Scientific Reports |date=12 April 2024 |volume=14 |issue=1 |page=8552 |doi=10.1038/s41598-024-54700-x |pmid=38609425 |pmc=11015018 |bibcode=2024NatSR..14.8552S |language=en |issn=2045-2322}}</ref> the chemistry of the [[lithosphere]], [[atmosphere]], and oceans, the role of "evolutionary pumps" such as massive [[glaciation]] and rare [[Meteoroid#Bolide|bolide]] impacts. Perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) [[prokaryote|prokaryotic cells]] to [[eukaryote|eukaryotic cells]], [[sexual reproduction]] and the [[Cambrian explosion]].

In his book ''[[Wonderful Life (book)|Wonderful Life]]'' (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.<ref>''The Nature of Nature: Examining the Role of Naturalism in Science'', editors Bruce Gordon and William Dembski,
[https://books.google.com/books?id=pe5nAwAAQBAJ&pg=PT491 Ch. 20 "The Chain of Accidents and the Rule of Law: The Role of Contingency and Necessity in Evolution"] by Michael Shemer, published by Intercollegiate Studies Institute, 2010.</ref>

==== Extraterrestrial intelligence is rare or non-existent ====
It is possible that even if complex life is common, intelligence (and consequently civilizations) is not.<ref name="Lineweaver, 2009"/> While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology,<ref>{{cite journal|title=Detecting Life-bearing Extrasolar Planets with Space Telescopes|bibcode=2008ApJ...684.1404B|author1=Steven V. W. Beckwith|doi=10.1086/590466|issue=2|journal=The Astrophysical Journal|volume=684|pages=1404–1415|date=2008|arxiv = 0710.1444|s2cid=15148438}}</ref><ref>{{cite journal|title=Detection of circular polarization in light scattered from photosynthetic microbes|author=Sparks, W.B. |author2=Hough, J. |author3=Germer, T.A. |author4=Chen, F. |author5=DasSarma, S. |author6=DasSarma, P. |author7=Robb, F.T. |author8=Manset, N. |author9=Kolokolova, L. |author10=Reid, N. |display-authors=etal|journal=Proceedings of the National Academy of Sciences|volume=106|issue=14–16|pages=1771–1779|date=2009|url=http://www.pnas.org/content/early/2009/04/28/0810215106.full.pdf |archive-url=https://web.archive.org/web/20150924175633/http://www.pnas.org/content/early/2009/04/28/0810215106.full.pdf |archive-date=2015-09-24 |url-status=live|doi=10.1016/j.jqsrt.2009.02.028|hdl=2299/5925 |hdl-access=free }}</ref> none of them have the ability to determine if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.<ref name="Tarter-NYAS" />

Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as [[aardvark]]s and elephants. As he points out, "dolphins have had ~20{{nbsp}}million years to build a radio telescope and have not done so".<ref name="Lineweaver, 2009">[https://arxiv.org/abs/0711.1751 Paleontological Tests: Human Intelligence is Not a Convergent Feature of Evolution.] {{Webarchive|url=https://web.archive.org/web/20191220142600/https://arxiv.org/abs/0711.1751 |date=December 20, 2019 }}, Charles Lineweaver, Australian National University, Canberra, published in ''From Fossils to Astrobiology'', edited by J. Seckbach and M. Walsh, Springer, 2009.</ref> In addition, Rebecca Boyle points out that of all the species that have evolved in the history of life on the planet Earth, only one—human beings and only in the beginning stages—has ever become space-faring.<ref name="Rebecca Boyle, Quanta Magazine">[https://www.quantamagazine.org/galaxy-simulations-offer-a-new-solution-to-the-fermi-paradox-20190307/ "Galaxy Simulations Offer a New Solution to the Fermi Paradox"], Quanta Magazine "Abstraction Blog," Rebecca Boyle, March 7, 2019. "The sun has been around the center of the Milky Way 50 times," said Jonathan Carroll-Nellenback, astronomer at the University of Rochester.</ref>

==== Periodic extinction by natural events ====
{{see also|Global catastrophic risk|Neocatastrophism}}
[[File:Coast Impact.jpg|thumb|An [[asteroid impact]] may trigger an [[extinction event]].]]
New life might commonly die out due to runaway heating or cooling on their fledgling planets.<ref>{{cite news |url=http://astrobiology.com/2016/01/the-aliens-are-silent-because-they-are-extinct.html |title=The Aliens Are Silent Because They Are Extinct |work=Australian National University |date=January 21, 2016 |access-date=2016-01-22 |archive-url=http://arquivo.pt/wayback/20160523051724/http://astrobiology.com/2016/01/the%2Daliens%2Dare%2Dsilent%2Dbecause%2Dthey%2Dare%2Dextinct.html |archive-date=May 23, 2016 |url-status=live }}</ref> On Earth, there have been numerous major [[extinction event]]s that destroyed the majority of complex species alive at the time; the [[K-T extinction|extinction of the non-avian dinosaurs]] is the best known example. These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as [[gamma-ray burst]]s.<ref>{{cite journal |doi=10.1017/S1473550404001910 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |vauthors=Melott AL, Lieberman BS, Laird CM, Martin LD, Medvedev MV, Thomas BC, Cannizzo JK, Gehrels N, Jackman CH |journal=International Journal of Astrobiology |volume=3 |issue=1 |pages=55–61 |date=2004 |url=http://acdb-ext.gsfc.nasa.gov/People/Jackman/Melott_2004.pdf |arxiv=astro-ph/0309415 |bibcode=2004IJAsB...3...55M |hdl=1808/9204 |s2cid=13124815 |access-date=August 20, 2010 |archive-url=https://web.archive.org/web/20110725132522/http://acdb-ext.gsfc.nasa.gov/People/Jackman/Melott_2004.pdf |archive-date=July 25, 2011 |url-status=live }}</ref> It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species.<ref>{{cite book |title=Global catastrophic risks |author1=Nick Bostrom |author2=Milan M. Ćirković |chapter=12.5: The Fermi Paradox and Mass Extinctions}}</ref>

However, the chances of extinction by natural events may be very low on the scale of a civilization's lifetime. Based on an analysis of impact craters on Earth and the Moon, the average interval between impacts large enough to cause global consequences (like the [[Chicxulub crater|Chicxulub impact]]) is estimated to be around 100 million years.<ref>{{Cite journal |last1=Mazrouei |first1=Sara |last2=Ghent |first2=Rebecca R. |last3=Bottke |first3=William F. |last4=Parker |first4=Alex H. |last5=Gernon |first5=Thomas M. |date=2019-01-18 |title=Earth and Moon impact flux increased at the end of the Paleozoic |url=http://dx.doi.org/10.1126/science.aar4058 |journal=Science |volume=363 |issue=6424 |pages=253–257 |doi=10.1126/science.aar4058 |pmid=30655437 |bibcode=2019Sci...363..253M |issn=0036-8075}}</ref>