Panspermia

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Star formation in a molecular cloud (here the Orion Nebula )

The hypothesis of panspermia / ⁠ panspɛrmiː ⁠ / ( ancient Greek πανσπερμία Panspermia , of πᾶν Pan "everything" and σπέρμα sperm "seeds."; Dt something like "All-seed") says is that simple life forms by long distance moving the universe and bringing the beginnings of life to earth . In doing so, their representatives try to resolve what they believe to be a contradiction between the high level of complexityof life on the one hand and the comparatively short time for its creation on the other. Most scientists, however, have regarded panspermia as pure speculation, as so far only life has been proven on earth.

history

Precursors of the theory of panspermia can already be seen in the ideas of the Greek philosopher Anaxagoras , who spoke of the "seeds of life". But these considerations were forgotten again by Aristotle 's theory of the spontaneous origin of life. For the medieval world of thought based on this in Europe, the question did not arise, especially since the Christian doctrine of creation contradicted it. It was not until the 19th century that Charles Darwin's justification of the biological evolution theory (1859) and Louis Pasteur's experiments on the question of spontaneous generation (generatio aequivoca) in 1884 clearly revealed the problem of the origin of the first living things on earth for many scientists.

Different hypotheses were developed about the beginning of biological evolution : In 1866 Ernst Haeckel first formulated the view of the primeval origin of life through primary spontaneous generation, archigony or autogeny . On the other hand, ideas comparable to the panspermia hypothesis were represented by Jöns Jakob Berzelius (1834), Louis Pasteur (1864), Hermann Richter (1865), Lord Kelvin (1871) and Hermann von Helmholtz (1871). Often, but not always, it was associated with the postulate of the eternity of life ; precisely this also became a point of criticism.

Early 20th century formulated Svante Arrhenius with the radio-panspermia the first theoretical description of panspermia (1903/1908). According to this theory, spores can escape from the outer layers of the earth's atmosphere and be transported into interstellar space by the pressure of sunlight (the solar wind was still unknown at the beginning of the 20th century).

The panspermia hypotheses were taken up again in 1963 by Donald Barber and in the 1970s by Francis Crick and Leslie Orgel (directed panspermia).

The British astronomer Fred Hoyle was also a great proponent of the panspermia hypothesis. He linked it with his steady-state theory of the universe, which assumes an infinite age of the cosmos and thus circumvents the question of the origin of life. At the latest when a broad scientific consensus raised the competing Big Bang model to the dominant cosmological theory of the dynamics of the universe, its ideas lost their attractiveness. Even the fact that Hoyle appeared as the author of various fantastic stories did not exactly increase the scientific reputation of his ideas, which were increasingly viewed as science fiction . Hoyle's student and former colleague Chandra Wickramasinghe , however, still actively represents panspermist ideas.

In 1996, Brig Klyce finally proposed the Cosmic Ancestry version, a combination of Hoyle's Panspermia hypothesis with the holistic Gaia conceptions of James Lovelock .

Arguments

A theory of panspermia basically encounters five problems:

  • Life has to come from somewhere else.
  • Life has to get into interstellar space.
  • Life has to survive there.
  • Life must come into the capture cross-section of a habitable planet.
  • Life there must enter the biosphere intact.

How does life get into space?

Svante Arrhenius suggested in 1908 that microbes that were transported to the outer layers by atmospheric processes could leave the gravitational field of their planet through the pressure of light from the sun or through close encounters with meteoroids . An alternative would be for material with embedded microbes to be thrown into space when a meteorite impacts. Electric fields could carry bacteria and other microbes from Earth into space and transport them to planets like Mars . This is what the electrical engineer Tom Dehel of the American aviation authority FAA suspects after calculations of electromagnetic fields in the earth's atmosphere. Until now, scientists believed that such an exchange of life would only be possible through the impact of a meteorite, in which dust containing microbes is thrown into space. More recent findings on the options mentioned:

  • Indian studies found bacteria in the stratosphere at an altitude of 40 kilometers and thus significantly higher than previously assumed.
  • Simulations at the German Aerospace Center in Cologne showed that organisms can survive the impact that is necessary to separate rock from the original body.

It is also speculated that life does not thrive on planets alone: ​​After all, in the late 20th century, various basic building blocks of life, such as amino acids, were found on comets or in their coma . However, so far there is no evidence of life forms.

Survivability in space

One argument against panspermia is that no life forms can survive under the conditions of space, i.e. above all in a vacuum , at temperatures close to absolute zero and under the high levels of UV and cosmic radiation . Even inside larger bodies, where cosmic rays are largely shielded, DNA should be destroyed over long periods of time by radiation from radioactive elements, which are present in small quantities in every naturally occurring rock .

However, there is evidence that bacteria can survive for longer periods of time under these conditions:

Surveyor 3 spacecraft under investigation by astronauts on the Apollo 12 mission
  • The US lunar mission Surveyor 3 accidentally brought bacteria of the species Streptococcus mitis to the moon. After their return to Earth 31 months later, most of the spores were able to continue their normal life cycle.
  • The BIOPAN experiments of the Institute for Aerospace Medicine of the German Aerospace Center in Cologne-Porz examine the resistance under defined conditions. On Russian Foton - Satellite containers were with the bacterium Bacillus subtilis into orbit and opened there for two weeks. After returning to Earth, several per thousand of the original population had survived the time in orbit without any cover or protective film. Further experiments showed that living organisms that are covered by UV radiation, for example by a layer of dust, can survive in space for a few years. But they could possibly also last for several million years, provided they are shielded from cosmic rays in a rock body several meters in size.
  • There is a special group of organisms that are able to survive even in very hostile places: These are cyanobacteria of the genus Chroococcidiopsis and in particular the extremophilic bacterium Deinococcus radiodurans , which is only slightly sensitive to ionizing radiation ; it was found in plants that are supposed to preserve food through irradiation.
  • The bacterium Desulforudis audaxviator lives alone in the groundwater a few kilometers deep in the rock and can only get its energy from hydrogen peroxide and hydrogen , which at this depth are only formed by natural radioactivity in the rock. Under these conditions, cell division would take on the order of 100 to 1000 years.
  • Some multicellular organisms, such as tardigrade , are capable of cryptobiosis . That allows some of them to survive space conditions.

Got into the capture cross section of a habitable planet

If life has got into interstellar space, in the best case scenario it will be several hundred thousand years to the next inhabitable planet.

Impact survivability

After their cosmic passage, the life forms have to survive the way through the earth's atmosphere to the planet's surface, which is connected with stresses due to strong deceleration forces and great heat development. Meteoroids , which penetrate the earth's atmosphere and reach the earth's surface as meteorites, are only heated and melted on the surface. From a depth of just one centimeter, the material is hardly heated, so that the survival of microorganisms seems possible. In the atmosphere, meteorites are slowed down to free-fall speed below a certain size , so that the impact energy is not sufficient to rule out survival. The amount of Martian matter that has not been heated above 100 ° C on its way to Earth in the past four billion years is around four billion tons.

Extreme habitats on earth

At the end of the 20th century, life forms were found on earth under very "hostile" conditions, under which life would not previously have been thought possible. Many strains of bacteria are now known that do not depend on the sun as a source of energy, but use other chemical processes, for example in volcanoes , the chimneys of hot springs in the deep sea ( black smokers ) and underground lakes. In the meantime, life has been found at temperatures of several hundred ° C, in strongly acidic environments or in drill cores more than 1,000 meters deep in the Antarctic ice (see Vostok Sea ). These findings confirm the assumption that life is far more resilient than was thought decades ago. In 2018, researchers at the Deep Carbon Observatory estimated that around 70 percent of the total number of bacteria and archaea on earth live in the earth's crust.

Cosmic clues

Electron microscope image of a detail of the ALH 84001 meteorite

After the discovery of increasingly complex molecules in interstellar clouds, the simplest amino acid glycine was also detected in 2002 . In the very primitive meteorite Murchison , which fell in 1969 , amino acids, diamino acids and other organic compounds were found.

Features of the found in Antarctica Mars - meteorite ALH 84001 are even interpreted by some researchers as traces of fossil bacteria. However, this interpretation is highly controversial.

Variants of panspermia

Directed panspermia

Another prominent protagonist of panspermia, Nobel Prize winner Francis Crick , formulated the theory of directed panspermia together with Leslie Orgel in 1973 . According to this theory, the spurs of life did not end up in space by accident, but were deliberately released by an extraterrestrial civilization . Sending small grains of bacteria is, according to Crick, the cheapest and most effective way of transporting life to a potentially viable planet . The reason is, for example, that civilization was facing an inevitable catastrophe, or hoped that other planets would terraform for later colonization . There are initial reflections on how directed panspermia could promote life on exoplanets.

Transpermia

In the late 1990s and early 21st century, some considerations were made that examine transport not between planetary systems but only between neighboring planets. This process is Transspermie (Engl. Transpermia ) called. This form of panspermia is also considered to be speculative, but is being considered as a possibility much more strongly than the above-mentioned panspermia in the broader sense.

According to the work of the Mileikowsky researchers mentioned above, more than four billion tons of Martian material came to earth in the history of the earth, which was not heated above 100 ° C during this process. The reverse route from Earth to Mars also took a smaller but considerable amount of material. If life is found on Mars, it could therefore be possible that there is a close relationship with earthly life. The question then, however, would be where life originated, on earth or on Mars.

Strong Panspermia and Cosmic Ancestry

The version proposed by Fred Hoyle, also referred to as strong panspermia, assumes, in contrast to "weak" panspermia, that not only the simplest life from space came to earth, from which, according to the theory of evolution, the biological diversity and especially the genetic structure of the modern organisms, but assumes that this diversity was already laid out in “genetic programs” of the germs of life coming from space. In particular, strong panspermia rejects macroevolution and only accepts microevolution as a fine adjustment to the environment. According to this, life would always have been part of an infinitely old universe.

The version propagated as Cosmic Ancestry extends the strong panspermia by including hypotheses from the Gaia environment , according to which the biosphere actively controls the environmental conditions of a planet in order to create the most favorable conditions for life.

Classification of the theory of panspermia

At the beginning of the 21st century, only a few people are systematically working on the theory of panspermia, even if it is not fundamentally rejected by many scientists and institutions such as the US space organization NASA and the German Aerospace Center (DLR) . In particular, the transspermia hypothesis in relation to Earth and Mars is at least considered as a possibility.

Origin of life

The main motivation for panspermia is the fact that life on earth left verifiable traces very early on.

The oldest known minerals in the earth's crust are zircons , around 4.4 billion years old , which suggest that the young earth was initially cooling. Presumably through the impact of many asteroids and comets and other geological processes, however, the crust that existed at that time was completely destroyed. The oldest rocks on earth, which have been dated almost four billion years ago, were only able to form a solid crust that has been partially preserved until today around 3.8 billion years ago, after the frequency of meteorite impacts had significantly decreased around 3.9 billion years ago, as confirmed by studies on lunar craters . Before this point in time, chemical evolution from the simplest molecules to more complex biomolecules to complete organisms is usually considered unlikely.

The oldest fossils are possibly 3.54 to 3.56 billion year old stromatolites found in Australia and South Africa; Geochemical isotope analyzes even show anomalies in the oldest rocks, which also indicate life. These dates are currently being discussed anew, as there are indications that organisms from later geological epochs could have penetrated the older rock, or that the geochemical anomalies could also have purely inorganic causes. If the original dates are confirmed, life on earth seems to have existed almost immediately with the presence of the first liquid water or the first oceans , 4 billion years ago, both continental crust and oceans could have existed on the earth's surface).

One possible explanation for this almost "earliest possible" evidence of life is that its formation is an almost natural process in the universe that takes place almost suddenly as soon as environmental conditions permit.

Supporters of the panspermia hypothesis, on the other hand, point out that spontaneous generation can necessarily only contain chemical-physical processes. However, it is precisely the formation of the indispensable long-chain molecules and the pronounced predominance of chirality among living beings on earth that cannot be explained by a currently known chemical or physical interaction within the time scale allowed. Although there are mechanisms that can separate living beings with different chirality, it remains unclear how environmental conditions can favor one chirality.

In addition, there is no evidence that the formation of living beings takes place automatically under favorable conditions; on the contrary, it has not yet been possible to produce structures similar to life under freely selectable circumstances (laboratory experiments). Representatives of the panspermia hypothesis also point to the discrepancy that from unicellular to multicellular the temporal steps to the development of more complex forms of life continue to decrease; While the development of cyanobacteria into other unicellular organisms took about a billion years, spontaneous generation is said to have taken place within a hundred million years.

With panspermia there would be considerably more time and, through a transfer through space, a myriad of planets available for the formation of life. A simple calculation shows that every point of the Milky Way can be reached within 20 to 50 million years even at relatively low cosmic speeds. The proof of life on planets outside the solar system with spectral analysis is currently not possible because of the far too weak reflection of the planets. An alleged special status of the earth is therefore without any basis.

Panspermia deliberately does not explain the origin of life itself, which is transferred to other places and times or even, according to some of its representatives, never took place. In the latter case, it is assumed that the universe does not have a finite age and that life is one of the fundamental components of the cosmos alongside space, time and matter. However, this notion is in contrast to the now generally accepted Big Bang model, according to which the universe is around 13.7 billion years old. The supporters of this panspermia variant are thus forced to switch to alternative models of the cosmos such as the steady-state model .

Less extreme is the idea that life originated in another place in the universe, from where it spread in the universe and finally came to earth. Compared to the above-mentioned variant, it has the advantage of not contradicting established cosmological theories, but in the opinion of its followers still offers clear advantages over the prevailing view that the first forms of life formed on earth, as at least potentially significantly more Time is available for the emergence of life.

Critique of the panspermia hypothesis and ideological aspects

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The advocates of panspermia come from diverse backgrounds; The spectrum of representatives ranges from serious scientists who consider panspermia to be very speculative, but can be treated scientifically, to interested laypeople to more pseudo-scientific or religiously influenced representatives. From science, especially from evolutionary biologists, the objection often arises that many versions of panspermia are unscientific because they are difficult or impossible to test scientifically. In particular, the radical idea that life has always existed is largely disproved by the big bang model of cosmology that is prevalent today with the resulting finite age of the universe.

But the less radical ideas are also criticized, since the only possibilities of verification would be to send probes to foreign planets and to find unambiguous and extraterrestrial traces of life in meteorites or comets. This is either extremely complex or to be expected with a very low probability. In contrast, panspermia is difficult to falsify as there is little information about the conditions on earth at the time when life arises. Creationism in particular shows that proponents set up further hurdles in overcoming apparently difficult hurdles in the case of scientific success. This always keeps a back door open.

Even the discovery of organic material in meteorites, comets or anywhere else in space does not change the dubious position of panspermia, since the formation of organic material on earth with its diverse chemistry should have been even more possible. This has been proven by various experiments, including the now famous Urey Miller experiment . Therefore, the so-called pseudo-panspermia, that is, the view that not simple life, but only the basic building blocks of life (organic compounds) comes from space, is not necessary for the explanation of the origin of earthly life.

In an organized form, panspermia is currently represented, for example, by the Interstellar Panspermia Society. This organization has officially set itself the task of promoting scientific research specifically on targeted panspermia and enabling its implementation. At the same time, this organization disseminates a catalog of ethics known as “Astrobioethic”. The latter is often interpreted to mean that it is more than just an association that wants to promote extraordinary projects, but also an ideological association - Panspermia opponents criticize this organization because of its religious features. In addition, the influence of the Gaia hypothesis within cosmic ancestry panspermia indicates that panspermia worldviews can take on religious traits due to their fascination , for example comparable to Scientology or Raelism .

Although Lord Kelvin, one of the first exponents of panspermia, took an anti-Darwinist standpoint, which also had religious motives, creationists are currently generally rejecting panspermia as another naturalistic worldview in addition to the theory of evolution. Proponents of panspermia usually regard panspermia as the third path between evolutionary theory and creationism. Nevertheless, there are also points of contact, such as the rejection of the modern theory of evolution , especially macroevolution, using pseudo-scientific arguments. For example, Brig Klyce, a representative of the form of panspermia known as “Cosmic Ancestry”, uses the concept of “irreducible complexity”, originally derived from creationism, to argue against the theory of evolution. It is also conceivable that some versions of panspermia, such as the “Cosmic Ancestry” version, will be adapted by creationists in a modified form.

With the exception of some prominent scientists such as Francis Crick and Fred Hoyle, panspermia in its general form currently only plays a very marginal role in science. Only in the form of transspermia is it seriously considered in planetology and astrobiology . Should life forms be found outside the earth on one of the planets or moons within the solar system that show similarities to earthly life that cannot be explained by chance or convergent evolution alone , transspermia would, however, gain new topicality.

Science fiction

The idea of ​​panspermia was picked up in several science fiction novels. Particular mention should be made of Jack Finney's novel Die Körperfresserommen, the Dragonrider books by Anne McCaffrey, as well as the books Quest and The Hair Carpet Knotters by Andreas Eschbach .

Often the panspermia is also used as a fictitious explanation for the mostly humanoid aliens and their mostly earth-like home planets. For example in the Star Trek series, where extraterrestrials known as primordial humanoids consciously provoke the evolution of intelligent species based on their own genetic make-up.

The PC game Spore , which is mainly about "creating" your own biosystems through evolution, always begins after you have selected a planet with the invasion of microorganisms from outside.

See also

literature

  • Aleksandar Janjic: Extremophile Organisms and Transspermia. In: Astrobiology - the search for extraterrestrial life. Springer Berlin Heidelberg, ISBN 978-3-662-59491-9 .
  • Svante Arrhenius: Worlds in the Making. Harper, London 1908.
  • Francis Crick, Leslie Organ: Directed Panspermia. In: Icarus. International journal of solar system studies. Elsevier, San Diego Ca 19.1973, 341. ISSN  0019-1035
  • Francis Crick: Life Itself, its Origin and Nature. Simon and Schuster, London 1981 (English). ISBN 0-7088-2235-5 .
  • Fred Hoyle: The Intelligent Universe. Michael Joseph Limited, London 1983 (English). ISBN 0-7181-2298-4 .
  • Gerda Horneck: Life, a cosmic phenomenon? DLR simulations on the survival of microorganisms in space. in: DLR-Nachrichten . Cologne 94.1999 (Sept.), 16–25 (PDF download). ISSN  0937-0420
  • Fred Hoyle, Chandra Wickramasinghe: Life from Space. Zweiausendeins, Frankfurt 2000, ISBN 3-86150-373-5 .
  • C. Mileikowsky et al: Natural transfer of viable microbes in space. In: Icarus. International journal of solar system studies. Elsevier, San Diego Ca 145.2000, 391-427. ISSN  0019-1035
  • Paul Davies: The fifth miracle, the search for the origin and meaning of life . Simon and Schuster, London 1999 (PDF). ISBN 0-684-83799-4 (dar: Kap 10, "A Bio-Friendly Universe?")
  • Paul Davies: How bio-friendly is the universe. In: International Journal of Astrobiology , vol. 2, Issue 02, pp.115-120. 04/2003 ( preprint (English) )
  • Paul Davies: The search for life in the universe . (PDF) Macquarie University, New South Wales 2004. in: Astrobiology and Planetary Missions. Edited by Richard B. Hoover, Gilbert V. Levin, Alexei Y. Rozanov, G. Randall Gladstone. Proc. of SPIE., Bellingham Wa 5906.2005, 59060I (PDF). ISBN 0-8194-5911-9 , ISSN  0277-786X

Web links

Individual evidence

  1. ↑ Jennifer Rieger: Panspermia Theory - What the universe has to do with the origin of life. In: Deutschlandfunkkultur.de. November 21, 2019, accessed on December 11, 2019 (German).
  2. ^ Abraham Loeb: Did Life from Earth Escape the Solar System Eons Ago? In: Scientific American. November 4, 2019, accessed November 12, 2019 .
  3. ↑ Tardigrade defy cosmic radiation orf.at; Tardigrade survive space walk Wissenschaft-online.de, September 2008 (accessed on September 2, 2010)
  4. ^ Viable Transfer of Microorganisms in the Solar System and Beyond . (PDF) dlr.de; accessed on February 20, 2015.
  5. ↑ Harvard study suggests asteroids might play key role in spreading life. In: Harvard Gazette. July 8, 2019, Retrieved August 12, 2019 (American English).
  6. ↑ Curt Mileikowsky et al .: Natural Transfer of Viable Microbes in Space: 1. From Mars to Earth and Earth to Mars . In: Icarus . tape 145 , no. 2 , 2000, pp. 391-427 , doi : 10.1006 / icar.1999.6317 .
  7. ↑ Dirk Schulze-Makuch: Turn Up the Heat: Bacterial Spores Can Take Temperatures in the Hundreds of Degrees. Retrieved June 4, 2019 .
  8. ↑ Life Deep Underground Is Twice the Volume of the Oceans: Study. Retrieved April 2, 2019 .
  9. ↑ Claudius Gros: Developing ecospheres on transiently habitable planets: the genesis project . In: Astrophysics and Space Science . tape 361 , no. 10 , September 5, 2016, ISSN  0004-640X , doi : 10.1007 / s10509-016-2911-0 .
  10. ↑ de.memory-alpha.org
This article was added to the list of excellent articles on November 9, 2004 in this version .