doi:10.1038/nindia.2008.340 Published online 23 December 2008
Did life on Earth arrive from outer space or evolve from spontaneous interaction of simple molecules dissolved in the lakes or oceans of the prebiotic world?
"It is beyond the realms of science to pinpoint when and where precisely life had its beginning in the universe," and how it all began on Earth, says an Indian researcher after a detailed assessment of the prevailing theories on origin of life.
Ashwini Kumar Lal, Deputy Adviser to the Indian ministry of Statistics and Programme Implementation, examined over 60 relevant research papers in the fields of genetics, astrobiology, and space science before concluding that science has no clue about how life began1.
He says, "Ever since Alexander Oparin in Russia and J. B. S. Haldane in England initiated the modern theory of life's origin from non-life (abiogenesis) in 1930s, we have learnt much about how life operates, but almost nothing about how it originates. It is a puzzle whose mystery will perhaps remain unknown to humanity ad infinitum."
Lal finds that prevailing theories like abiogenesis, RNA-World (the self-replicating RNA molecules), iron-sulphur world (deep-sea-origin of life) and 'panspermia' (life arrived from outer space) fail to provide clues on the exact origin of life. According to Lal, there is general consensus among biologists that bacterial cells cannot evolve from non-living chemicals in one step and therefore there must be some form of pre-cellular life.
Among pre-cellular life theroeis, in the early stages of evolution of life on earth, the most popular contender today is 'RNA-World', which says all current life-forms could have evolved from self-replicating RNA or ribonucleic acid. But Lal says none of the experiments involving biologically produced RNA has provided concrete proof of RNA-World being the pathway between non-life and life. "Also, how the RNA came into being remains unknown so far," says Lal — a conclusion shared by Gerald Joyce of the Scripps Research Institute in California.
Lal's article notes that 'panspermia' theory propounded by Fred Hoyle and Chandra Wickramasinghe at Cardiff University 35 years ago as an alternative to abiogenesis is currently the most favoured theory of origin of life. But even this, he says, fails to address uncertainties over "the exact timing and location for commencement of the process of evolution of life on the primitive Earth and elsewhere in the universe."
According to panspermia theory, primitive life may have originally formed extra-terrestrially and been transported to Earth by comets during the Hadean Epoch (4.5-3.9 billion years ago) when Earth is believed to have undergone heavy meteoric bombardment for about 700 million years.
In an email interview, Wickramasinghe, director of the Cardiff Centre for Astrobiology, agrees with Lal's arguments against life originating in Earth. "The theory of life starting de novo (through abiogenesis) on Earth is less secure now than it has ever been," he told Nature India. "The first evidence of life on Earth during the Hadean epoch essentially squeezes the period of terrestrial abiogenesis into a vanishingly thin slice of geological time."
On the other hand Wickramasinghe says, "Abiogenesis has an incomparably better chance of being accomplished on a cosmic scale, perhaps on one of a trillion cometary ponds in the galaxy. And once life is formed in a process of cosmic abiogenesis, its survival and spread (by comets, interstellar dust and meteors) seems to be guaranteed by what we know from the study of terrestrial extremophiles."
Extremophiles are microbes that can survive in the harshest of environment on the earth. Given their potential to survive in highly inhospitable environments, possibility of life on extraterrestrial bodies (and their transportation by comets) could not be ruled out. Wickramasinghe says that panspermia theories, advocated by Hoyle and himself, were once considered heretical but are now "gradually slipping into the domain of respectable science".
Lal says though panspermia provides a satisfactory explanation, it fails to address the long-standing riddle as to when and where precisely life originated first in the universe. Nor does it provide any clue as to how prebiotic matter transformed into primitive bacteria in the early epoch of the earth. "Evidence from meteorites/comets and experiments that simulate the conditions on the early Earth suggests that probably a combination of terrestrial and extraterrestrial factors were responsible for jump-starting the process of transforming prebiotic organic compounds into entities that we call 'life' on earth," he says.
Lal concludes that origin-of-life investigators have proposed many theories, but evidence in favor of each of them is fragmentary at best. "Despite development of sophisticated biotechnology tools in recent years, scientists still have not been successful in transforming inanimate matter into life in the laboratory," he says. The fact that all organisms share the same DNA-based mechanism for genetic transmission suggests that life emerged on Earth only once during the planet's entire history but it's ironic this "crucial timing remains unknown to mankind".