Panspermia is a Greek word that translates literally as "seeds everywhere". The panspermia hypothesis states that the "seeds" of life exist all over the Universe and can be propagated through space from one location to another. Some believe that life on Earth may have originated through these "seeds".
Mechanisms for panspermia include the deflection of interstellar dust by solar radiation pressure and extremophile microorganisms traveling through space within an asteroid, meteorite or comet.
Three popular variations of the panspermia hypothesis are:
- Lithopanspermia (interstellar panspermia) - impact-expelled rocks from a planet's surface serve as transfer vehicles for spreading biological material from one solar system to another.
- Ballistic panspermia (interplanetary panspermia) - impact-expelled rocks from a planet's surface serve as transfer vehicles for spreading biological material from one planet to another within the same solar system
- Directed panspermia - the intentional spreading of the seeds of life to other planets by an advanced extraterrestrial civilization, or the intentional spreading of the seeds of life from Earth to other planets by humans
Panspermia does not provide an explanation for evolution or attempt pinpoint the origin of life in the Universe, but it does attempt to solve the mysteries of the origin of life on Earth and the transfer of life throughout the Universe.
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The first known mention of the concept of panspermia was in the writings of the Greek philosopher Anaxagoras (500 BC – 428 BC), although his concept differs from the modern theory:
All things have existed from the beginning. But originally they existed in infinitesimally small fragments of themselves, endless in number and inextricably combined. All things existed in this mass, but in a confused and indistinguishable form. There were the seeds (spermata) or miniatures of wheat and flesh and gold in the primitive mixture; but these parts, of like nature with their wholes, had to be eliminated from the complex mass before they could receive a definite name and character.
In 1743 the theory of panspermia appeared in the writings of French nobleman, diplomat and natural historian Benoît de Maillet, who believed that that life on Earth was "seeded" by germs from space falling into the oceans, rather than life arising through abiogenesis.
The panspermia theory was rekindled in the nineteenth century by the scientists Jöns Jacob Berzelius (1779–1848), Lord Kelvin (William Thomson) (1824–1907) and Hermann von Helmholtz (1821–1894). Lord Kelvin declared in 1871, "[W]e must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instance no life existed upon this Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation."
In 1973 the late Nobel prize winning British molecular biologist, physicist and neuroscientist Professor Francis Crick, along with British chemist Leslie Orgel, proposed the theory of directed panspermia.
A meteorite blasted off from the surface Mars about 15 million years ago was found in Antarctica in 1984 by a team of scientists on an annual United States government mission to search for meteors. The meteor was named Allan Hills 84001 (ALH84001). In 1996 ALH84001 was shown to contain structures that may be the remains of terrestrial nanobacteria. The announcement, published in the journal Science by David McKay of NASA, made headlines worldwide and prompted United States President Bill Clinton to make a formal televised announcement marking the event and expressing his committment to the aggressive plan in place at the time for robotic exploration of Mars. Several tests for organic material have been performed on ALH84001 and amino acids and polycyclic aromatic hydrocarbons (PAH) have been found. However, most experts now agree that these are not a definite indication of life, but may have instead been formed abiotically from organic molecules or are due to contamination from contact with Antarctic ice. The debate is still ongoing, but recent advances in nanobe research has made the find interesting again.
The announcement of the discovery of evidence of life on ALH84001 sparked a surge in support for the theory of panspermia. People began to speculate about the possibility that life originated on Mars and was transported to Earth on debris ejected after major impacts (see ballistic panspermia).
On April 29, 2001, at the 46th annual meeting of the International Society for Optical Engineering (SPIE) in San Diego, California, Indian and British researchers headed by Chandra Wickramasinghe presented evidence that the Indian Space Research Organisation had gathered air samples from the stratosphere that contained clumps of living cells. Wickramasinghe called this "unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 kilometers, well above the local tropopause, above which no air from lower down would normally be transported". A reaction report from NASA Ames doubted that living cells could be found at such high altitudes, but noted that some microbes can remain dormant for millions of years, possibly long enough for an interplanetary voyage within a solar system.
On May 11, 2001, Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci from the University of Naples announced the finding of extraterrestrial bacteria inside a meteorite estimated to be over 4.5 billion years old. The researchers claimed that the bacteria, wedged inside the crystal structure of minerals, had been resurrected in a culture medium. They asserted that the bacteria had DNA unlike any on Earth and had survived when the meteorite sample was sterilized at high temperature and washed with alcohol. The bacteria were determined to be related to modern day Bacillus subtilis and Bacillus pumilus bacteria, but appear to be a different strain.
On April 21, 2008, renowned British astrophysicist Stephen Hawking spoke about panspermia during his "Why We Should Go Into Space" lecture for NASA's 50th Anniversary lecture series at George Washington University.
In a virtual presentation on Tuesday, April 7, 2009, Stephen Hawking discussed the possibility of building a human base on another planet and gave reasons why alien life might not be contacting the human race, during his conclusion of the Origins Symposium at Arizona State University. Hawking also talked about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia, which says that life in the form of DNA particles can be transmitted through space to habitable places.