GEOLOGISTS have identified 鈥渇ossils鈥� from the time when non-living matter was taking its first tentative steps towards life. The ancient grains of radioactive rock are coated with an oily gloop, suggesting they could perhaps have driven the first chemical reactions that were necessary for life to evolve.
Before life got started, complex organic molecules must already have existed on Earth. Simple organic molecules such as amino acids showered down on early Earth, and the atmosphere at the time was rich in methane. But how these chemicals took the next step on the ladder of complexity still mystifies biologists.
鈥淭he basic problem with the origin of life is that we understand the chemistry that makes simple molecules such as amino acids, but not the complex molecules,鈥� says geochemist Iain Gilmour of Britain鈥檚 Open University. The prime suspect for bringing this about is ionising radiation.
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Early Earth was bathed in cosmic rays, which could have provided the energy to convert small, simple molecules into complex polymers. Now John Parnell of the University of Aberdeen in Scotland suggests that radioactive mineral grains could have done the job.
The grains, commonly found in ancient sedimentary rocks, have been eroded from granite. They are made from the minerals thorite and monazite, which are rich in the radioactive element thorium, or uranium-rich uraninite. But being a petroleum geologist, Parnell was more interested in their gloopy coating. He studied the coating of grains dating from after life started and found evidence of long-chain hydrocarbons, similar to those found in crude oil.
This means that more ancient grains, which are similar, were a potential source of the chemical complexity that eventually led to life, Parnell told an astrobiology meeting in Cambridge last week. He believes that before life started, the grains could have acquired a coating after coming into contact with simple organic molecules; radioactivity from the minerals could have converted these simple molecules into viscous polymers which would have stuck firmly to the surface of the grains.
He is cautious about his discovery. 鈥淚鈥檓 not saying that mineral radioactivity is the cause of the origin of life,鈥� he says. He points out that he only saw oily molecules, not biomolecules such as protein or DNA. But, he says, these molecules could be formed by the same process; a possibility he is now testing in the lab.
The mineral grains idea has many advantages over the cosmic-ray theory. Mineral surfaces would be an attractive location for chemical evolution because the surface itself can catalyse polymer formation.
What鈥檚 more, monazite contains phosphate, an important ingredient of biomolecules such as DNA. And while cosmic rays are spread across the Earth鈥檚 surface, eroded mineral grains tend to get washed up by the sea and concentrated on beaches, leading to stronger doses of radioactivity focused in one place.
Geochemists are sure that such radioactive beaches existed on Earth before life got started. The conditions needed for their formation 鈥� granites, erosion and tidal oceans 鈥� were present as long as 4.4 billion years ago.