Editing Panspermia (section)

== History ==

Panspermia has a long history, dating back to the 5th century BCE and the natural philosopher [[Anaxagoras]].<ref>{{Cite journal |last=Hollinger |first=Maik |date=2016 |title=Life from Elsewhere – Early History of the Maverick Theory of Panspermia |url=https://www.biblioscout.net/article/10.25162/sudhoff-2016-0009 |journal=Sudhoffs Archiv |language=de |volume=100 |issue=2 |pages=188–205 |doi=10.25162/sudhoff-2016-0009 |pmid=29668166 |s2cid=4942706 |issn=0039-4564}}</ref> Classicists came to agree that Anaxagoras maintained the Universe (or Cosmos) was full of life, and that life on Earth started from the fall of these extra-terrestrial seeds.<ref name=":44">{{Cite journal |last=Mitton |first=Simon |date=2022-12-01 |title=A Short History of Panspermia from Antiquity Through the Mid-1970s |url=http://dx.doi.org/10.1089/ast.2022.0032 |journal=Astrobiology |volume=22 |issue=12 |pages=1379–1391 |doi=10.1089/ast.2022.0032 |pmid=36475958 |bibcode=2022AsBio..22.1379M |s2cid=254444999 |issn=1531-1074}}</ref> Panspermia as it is known today, however, is not identical to this original theory. The name, as applied to this theory, was only first coined in 1908 by [[Svante Arrhenius]], a Swedish scientist.<ref name=":04"/><ref name=":103">{{Cite journal |last1=Arrhenius |first1=Svante |last2=Borns |first2=H. |date=1909 |title=Worlds in the Making. The Evolution of the Universe |url=http://dx.doi.org/10.2307/200804 |journal=Bulletin of the American Geographical Society |volume=41 |issue=2 |pages=123 |doi=10.2307/200804 |jstor=200804 |hdl=2027/hvd.hnu57r |issn=0190-5929|hdl-access=free }}</ref> Prior to this, since around the 1860s, many prominent scientists were becoming interested in the theory. More recent advocates include [[Sir Fred Hoyle]], and [[Chandra Wickramasinghe]].<ref name=":9">{{Cite journal |last=Napier |first=W.M. |date=2007-04-16 |title=Pollination of exoplanets by nebulae |url=http://dx.doi.org/10.1017/s1473550407003710 |journal=International Journal of Astrobiology |volume=6 |issue=3 |pages=223–228 |doi=10.1017/s1473550407003710 |bibcode=2007IJAsB...6..223N |s2cid=122742509 |issn=1473-5504}}</ref><ref>{{Cite journal |last=Line |first=M.A. |date=July 2007 |title=Panspermia in the context of the timing of the origin of life and microbial phylogeny |url=http://dx.doi.org/10.1017/s1473550407003813 |journal=International Journal of Astrobiology |volume=6 |issue=3 |pages=249–254 |doi=10.1017/s1473550407003813 |bibcode=2007IJAsB...6..249L |s2cid=86569201 |issn=1473-5504}}</ref>

In the 1860s, there were three scientific developments that began to bring the focus of the scientific community to the problem of the origin of life.<ref name=":04"/> Firstly, the Kant-Laplace [[Nebular hypothesis|Nebular theory]] of solar system and planetary formation was gaining favor, and implied that when the Earth first formed, the surface conditions would have been inhospitable to life as we know it. This meant that life could not have evolved parallel with the Earth, and must have evolved at a later date, without biological precursors. Secondly, [[Charles Darwin]]'s famous theory of evolution implied some elusive origin, because in order for something to evolve, it must start somewhere. In his ''Origin of Species'', Darwin was unable or unwilling to touch on this issue.<ref>{{Cite book |last=Darwin |first=Charles |url=http://dx.doi.org/10.5962/bhl.title.87899 |title=The variation of animals and plants under domestication / |date=1883 |publisher=D. Appleton and company |location=New York|doi=10.5962/bhl.title.87899 }}</ref> Third and finally, [[Louis Pasteur]] and [[John Tyndall]] experimentally disproved the (now superseded) theory of [[spontaneous generation]], which suggested that life was ''constantly'' evolving from non-living matter and did not have a common ancestor, as suggested by Darwin's theory of evolution.

Altogether, these three developments in science presented the wider scientific community with a seemingly paradoxical situation regarding the origin of life: life must have evolved from non-biological precursors after the Earth was formed, and yet spontaneous generation as a theory had been experimentally disproved. From here, is where the study of the origin of life branched. Those who accepted Pasteur's rejection of spontaneous generation began to develop the theory that under (unknown) conditions on a primitive Earth, life must have gradually evolved from organic material. This theory became known as [[abiogenesis]], and is the currently accepted one. On the other side of this are those scientists of the time who rejected Pasteur's results and instead supported the idea that life on Earth came from existing life. This necessarily requires that life has always existed somewhere on some planet, and that it has a mechanism of transferring between planets. Thus, the modern treatment of panspermia began in earnest.

[[Lord Kelvin]], in a presentation to The British Association for the Advancement of Science in 1871, proposed the idea that similarly to how seeds can be transferred through the air by winds, so can life be brought to Earth by the infall of a life-bearing meteorite.<ref name=":04"/> He further proposed the idea that life can only come from life, and that this principle is invariant under philosophical [[uniformitarianism]], similar to how matter can [[Conservation of mass|neither be created nor destroyed]].<ref>{{Cite journal |last=Thompson |first=W. |date=1871-10-01 |title=Inaugural address before the British Association at Edinburgh, August 2d. |url=http://dx.doi.org/10.2475/ajs.s3-2.10.269 |journal=American Journal of Science |volume=s3-2 |issue=10 |pages=269–294 |doi=10.2475/ajs.s3-2.10.269 |s2cid=131738509 |issn=0002-9599}}</ref> This argument was heavily criticized because of its boldness, and additionally due to technical objections from the wider community. In particular, Johann Zollner from Germany argued against Kelvin by saying that organisms carried in meteorites to Earth would not survive the descent through the atmosphere due to friction heating.<ref name=":04"/><ref>{{Cite journal |last=Hollinger |first=Maik |date=2016 |title=Life from Elsewhere – Early History of the Maverick Theory of Panspermia |url=http://dx.doi.org/10.25162/sudhoff-2016-0009 |journal=Sudhoffs Archiv |volume=100 |issue=2 |pages=188–205 |doi=10.25162/sudhoff-2016-0009 |pmid=29668166 |s2cid=4942706 |issn=0039-4564}}</ref>

The arguments went back and forth until Svante Arrhenius gave the theory its modern treatment and designation. Arrhenius argued against abiogenesis on the basis that it had no experimental foundation at the time, and believed that life had always existed somewhere in the Universe.<ref name=":103"/> He focused his efforts of developing the mechanism(s) by which this pervasive life may be transferred through the Universe. At this time, it was recently discovered that solar radiation can exert pressure, and thus force, on matter. Arrhenius thus concluded that it is possible that very small organisms such as bacterial spores could be moved around due to this [[radiation pressure]].<ref name=":103"/>

At this point, panspermia as a theory now had a potentially viable transport mechanism, as well as a vehicle for carrying life from planet to planet. The theory still faced criticism mostly due to doubts about how long spores would actually survive under the conditions of their transport from one planet, through space, to another.<ref name=":112">{{Cite journal |last=Sagan |first=Carl |date=August 1961 |title=On the Origin and Planetary Distribution of Life |url=http://dx.doi.org/10.2307/3571249 |journal=Radiation Research |volume=15 |issue=2 |pages=174–192 |doi=10.2307/3571249 |jstor=3571249 |pmid=13745360 |bibcode=1961RadR...15..174S |issn=0033-7587}}</ref> Despite all the emphasis placed on trying to establish the scientific legitimacy of this theory, it still lacked testability; that was and still is a serious problem the theory has yet to overcome.

Support for the theory persisted, however, with [[Fred Hoyle]] and [[Chandra Wickramasinghe]] using two reasons for why an extra-terrestrial origin of life might be preferred. First is that required conditions for the origin of life may have been more favorable somewhere other than Earth, and second that life on Earth exhibits properties that are not accounted for by assuming an [[Endogeny (biology)|endogenic]] origin.<ref name=":04"/><ref name=":9" /> Hoyle studied spectra of interstellar dust, and came to the conclusion that space contained large amounts of organics, which he suggested were the building blocks of the more complex chemical structures.<ref>{{Citation |last1=Hoyle |first1=Fred |title=Comets - A Vehicle for Panspermia |date=1981 |url=http://dx.doi.org/10.1007/978-94-009-8528-5_15 |work=Comets and the Origin of Life |pages=227–239 |access-date=2023-12-08 |place=Dordrecht |publisher=Springer Netherlands |isbn=978-94-009-8530-8 |last2=Wickramasinghe |first2=Chandra|doi=10.1007/978-94-009-8528-5_15 }}</ref> Critically, Hoyle argued that this chemical evolution was unlikely to have taken place on a prebiotic Earth, and instead the most likely candidate is a comet.<ref name=":04"/> Furthermore, Hoyle and Wickramasinghe concluded that the evolution of life requires a large increase in genetic information and diversity, which might have resulted from the influx of viral material from space via comets.<ref name=":9" /> Hoyle reported (in a lecture at Oxford on January 16, 1978) a pattern of coincidence between the arrival of major epidemics and the occasions of close encounters with comets, which lead Hoyle to suggest<ref>{{cite journal
| last       = Hoyle
| first      = [Sir] Fred  
| title      = COMETS—A MATTER OF LIFE AND DEATH
| url        = https://www.sciencedirect.com/science/article/abs/pii/0083665680900276
| journal    = Vistas in Astronomy
| date       = 1980
| volume     = 24
| issue = 2 
| pages      = 123–139
| doi = 10.1016/0083-6656(80)90027-6 
| bibcode = 1980VA.....24..123H 
| quote      = [...] a lecture in the spirit of those wild old sessions at the R.A.S., a lecture with which most of you in this audience will quite likely disagree.
}}</ref>  that the epidemics were a direct result of material raining down from these comets.<ref name=":04"/> This claim in particular garnered criticism from biologists.

Since the 1970s, a new era of planetary exploration meant that data could be used to test panspermia and potentially transform it from conjecture to a testable theory. Though it has yet to be tested, panspermia is still explored today in some mathematical treatments,<ref name=":62">{{Cite journal |last1=Ginsburg |first1=Idan |last2=Lingam |first2=Manasvi |last3=Loeb |first3=Abraham |date=2018-11-19 |title=Galactic Panspermia |journal=The Astrophysical Journal |volume=868 |issue=1 |pages=L12 |doi=10.3847/2041-8213/aaef2d |issn=2041-8213 |doi-access=free |arxiv=1810.04307 |bibcode=2018ApJ...868L..12G }}</ref><ref name=":33"/><ref name=":24"/> and as its long history suggests, the appeal of the theory has stood the test of time.