Much later, in the nineteenth century, Hermann Richter put forward the idea that life has always existed in the universe, propagating itself from one place to another by means of 'cozmozoa' (germs of the cosmos). In this theory, life has existed and will exist for all eternity, and so there is no need for an explanation of its origin. Lord Kelvin and Herman von Helmholtz also took the view that life on Earth arrived from elsewhere in the universe.
In 1908, the Swedish physical chemist Svante Arrhenius put forward a new version of the cozmozoa theory, and gave it the name Panspermia originally used over 2000 years earlier by Anaxagoras. Arrenhius' contribution was a new theory of the mechanism by which life could be transported between planets; he proposed that bacterial spores were propelled through inter-planetary space by radiation pressure. Previous theories had assumed transport was by means of meteorites. This entailed difficulties associated with the very high temperatures that meteorites are typlcally subjected to on entering the Earth's atmosphere. Arrenhius' theory avoided these difficulties; bacterial spores arriving at the Earth (possibly attached to grains of interstellar dust) could fall slowly to the ground without being subjected to high temperatures due to air friction.
One of the motivations for Arrenhius' Panspermia theory was that it provided an approach to dealing with the crisis caused by Louis Pasteur's experiments showing that contemporary bacteria do not arise by Spontaneous Generation. If there was no way in which the origin of life could be explained, it was reasonable to suppose that life was an inherent property of the universe and had always existed.
Arrenhius' theory was dropped by most scientists when it became apparent that the bacterial spores would be unable to survive exposure to ultra-violet radiation on their journey through space. The development of an alternative 'Prebiotic Soup' theory was also a factor contributing to the demise of Arrenhius' ideas.
However, this was not the end of Panspermia theories. In the closing decades of the twentieth century, Fred Hoyle, Francis Crick and Leslie Orgel proposed Panspermia theories. Hoyle argued that there is evidence of bacteria and virus-like organisms in space, for example within comets. Crick and Orgel speculated in 1973, in a suggestion known as Directed Panspermia, and reminiscent of Arthur C Clarke's science fiction book '2001: A Space Odyssey', that advanced civilisations had developed elsewhere in the universe and the germs of life had been despatched to Earth on their spaceships.
More recently, it has been suggested that meteoritic rock ejected from Mars which landed on Earth may contain microfossils of bacteria-like cells. The evidence on this is still inconclusive, and the possibility that life exists or has at some time existed on Mars remains a topic of great interest to scientists and the public at large.
It must be noted that Panspermia theories do not offer any explanation for the origin of life. Arrenhius' theory provided an explanation for the arrival of life on Earth, but assumed that life had always existed in the universe and so did not have an origin. Crick and Orgel's Directed Panspermia assume's that life originated elsewhere in the universe (eg on a planet more suitable for the origin of life than the early Earth), but does not explain how this origin occurred.
The idea that life is eternal may seem odd from an early twenty-first century perspecitve. However, at the time that Arrenhius developed his theory, the idea that the universe is unchanging was generally accepted. Indeed, in 1917, nine years after Arrenhius proposed his theory, Einstein added a 'cosmological constant' to his equations of general relativity for the specific purpose of arriving at a solution which would correspond to a static universe, unchanging in time. So well established was the idea of a static universe that Einstein included the term even though it was "not justified by our actual knowledge of gravitation", as he wrote in his 1917 paper. Two years later, he wrote that the cosmological constant was "gravely detrimental to the formal beauty of the theory". Subsequently, after Edwin Hubble had shown in 1929 that the universe is expanding, Einstein referred to the cosmological constant as "the biggest blunder" of his scientific life.