Two previous articles ( How Life Began – Formation of the Earth and How Life began – Origin of Biochemicals), described the processes that created the Solar System and Earth, the existence in outer space of certain organic chemicals essential for life and how they might have been transported to the Earth.
The present article introduces the theories describing the appearance of cellular life-forms by describing a controversial suggestion that cells could have arrived from outer space.
Panspermia – Cells From Outer Space
Some scientists believe that the simplest life-forms, whole cells (especially microbial cells), have been transported to the Earth from extraterrestrial sources. In this way, a process called panspermia (means seeds everywhere) might have initiated life on Earth.
There are several variations on this theory, including:
- Pseudo-panspermia where evidence for organic chemicals on meteorites (such as the Murchison meteorite discussed in Origin of Biochemicals) and in interstellar dust supports the theory that prebiotic compounds come from space
- Arrhenius's radio-panspermia where microbes are transported through space by the pressure of light
- Crick's directed panspermia where microbes are transported by unmanned spaceships
- Ballistic panspermia where microbes are transported by meteorites
- Hoyle and Wickramasinghe's modern panspermia where microbes are transported by comets. In a stronger version of panspermia, it is suggested that in addition to the origin of life on Earth, the process also causes its evolution into higher life forms.
Support for Extraterrestrial Seeding
Many well-known and historical figures have supported panspermia. These include Anaxagoras (500-428 BCE), Hermann von Helmholtz (1821-1894), William Thomson Kelvin (1824-1897), Svante Arrhenius (1859-1927), Sir Fred Hoyle (1915-2001), Francis Crick (1916-2004) and his student and collaborator, Chandra Wickramasinghe (1939-present).
Most mainstream scientists have not supported panspermia, but early challenges have been thwarted in recent years due to discoveries such as terrestrial microbes that survive in extreme environments and incredibly aged yet viable microorganisms found in ancient rocks. In addition, water (essential for life) has been discovered on other planets and moons, and, as discussed in previous articles in this series (see links above), organic chemicals have been found on meteorites and in interstellar debris.
Nevertheless, panspermia remains a minor theory in the quest for an explanation as to how living cells and life in general came to exist on Earth. Even if it were the correct explanation, it does not clarify how the seed cells themselves originated elsewhere in the cosmos. Thus, other suggestions for the origin of cells have attracted great attention. These will be discussed in the next article, How Life Began – From Molecules to Cellular Organisms.
References:
Hoyle F. (1983). The Intelligent Universe. Michael Joseph Limited, London, ISBN 0-7181-2298-4.
Wainwright, M. (2003). A microbiologist looks at panspermia. Astrophysics and Space Science. 285(2):563-570.
Bernard Betts - I hold the degrees: BA (Biology and Chemistry, 1st class hons), MSc (Medicinal Chemistry) and PhD (Microbiology). My career has included ...
