More than a century ago, Charles Darwin considered that life might have started when simple molecules linked together into more complex ones in some "warm little pool". Researchers since then have been mixing different ingredients in the laboratory in the hope of reinventing that primordial soup.
Now British-born Leslie Orgel, of the Salk Institute for Biological Sciences in San Diego, California, and his colleagues, claim researchers may have been looking in the wrong place. The problem with the primordial soup is that reactions with water tend to split molecules off faster than they can adhere to the growing chain of complexity.
Instead, Dr Orgel suggests, Earth made the transition from being lifeless to having the first self-replicating entities on the surface of rocks, minerals or clays, rather than in Darwin's pool. Life began quite literally clinging to the surface of our planet and has its roots in clay.
The researchers report in today's issue of the scientific journal Nature that they have been able to persuade strings of nucleic acids - the molecules of genetic inheritance - to grow better on the surface of clay than in watery solution. The basic building blocks of proteins, the second important class of biological molecules, also assemble themselves better on mineral surfaces.
According to Gunter von Kriedowski, of the Ruhr-Universitat in Bochum, Germany, writing in the same issue, the experiments show that the complex molecules of life, known as polymers, "were more likely to have been baked like pre-biotic crepes than cooked in a primordial soup". Just as "French crepes are prepared by pouring liquid dough over a hot stone plate, causing it to dehydrate and solidify" so the polymers of life may have condensed together on the surface of stones and clays - although the process would not have been carried out at baking hot temperatures.
But Dr von Kriedowski believes that some sort of soup would still have been necessary, if only to provide the ingredients splashed on to the rocks and clays - a little like the tomato sauce spread on the surface of a pizza, perhaps. The primordial soup (or sauce) model "is thus compatible with a surface-mediated origin of life. The common message is that the earliest forms of life may have proliferated by spreading on surfaces".
The details of how life got from long chains of nucleic acid to self- replicating molecules and then to the even greater complexity of single- celled organisms have not yet been worked out. But Dr Orgel is also one of the leading advocates of the idea that before the present diversity of life evolved, based as it is on DNA, there was a shadowy "RNA-world". Life in this world used a molecule that is chemically related to DNA to carry genetic information down the generations. But RNA is different in two ways. Its most important property is that, in some forms known as ribozymes, it can splice itself thus promoting its own replication - a vital step in the development of life.
RNA is also less stable than DNA, so it is unlikely to have produced even an organism as simple as a bacterium. Therefore, had life not switched, some 3 billion years ago to putting its eggs in the DNA basket, the RNA world would never have had the richness and diversity of life on Earth today.Reuse content