HOW LIFE ON EARTH BEGAN

Proteins are synthesized using information contained in DNA. But DNA itself must be synthesized when a cell reproduces, and this synthesis requires proteins, in particular enzymatic proteins such as DNA polymerase.

For scientists who study the origins of life, this poses a serious problem: DNA cannot have come first, because proteins are needed to make it. But proteins cannot have come first either, because information from DNA is needed to make them. To resolve this paradox, scientists had to determine how DNA might have been synthesized without enzymatic proteins, from the basic building blocks of life.

In the early 1980s, Thomas Cech and Sydney Altman discovered that certain forms of RNA could act as catalysts, just like proteins. These special forms of RNA are now called ribozymes. Ribozyme RNA molecules can play the roles of both DNA and proteins. They can thus resolve the chicken-and-egg-like paradox that scientists faced. On the basis of certain molecular markers, several researchers have concluded that RNA probably appeared on Earth before DNA. The problem of the origins of life has thus become in part the problem of the origins of RNA, which is still far from being solved. In fact, the very notion of the first molecule may not have any real meaning. Some mechanisms of protein synthesis simpler than those we now know may have existed in the past and allowed the appearance of primitive but effective forms of molecules with catalytic or autocatalytic properties.

Structure of the 5S ribosomal RNA subunit of Haloarcula marismortui. Source: 5S Ribosomal RNA Database

Likewise, any attempt at a precise definition of life itself may well be unrealistic. Life in fact might be regarded as appearing gradually in systems of increasing complexity. Thus, a pebble is inert, but crystals grow, and clays may serve as matrices that facilitate biochemical reactions. Proteins do the work of cells, and viruses can reproduce. Protozoans are autonomous, and the cells of metazoans are specialized and organized into tissues.

When we look only at the extremes on this continuum, or set excessively arbitrary criteria for what constitutes life, we often find ourselves confused and frustrated by contradictions of language. Perhaps the best understanding of life that we can hope to achieve may be limited to observing how matter behaves and how this behaviour gradually reaches higher and higher levels of complexity.

In the late 1970s, a microbiologist named Carl Woese discovered a third group of organisms living on Earth: archaebacteria. Though archaebacteria are prokaryotes, they are no closer to conventional bacteria than to eukaryotes on the universal genealogical tree. The archaebacteria group comprises a very large number of organisms that live under extreme temperature conditions. Some archaebacteria are found in hot springs whose temperatures are close to the boiling point of water. Many researchers have drawn a connection between this observation and the fact that temperatures were probably far higher on primitive Earth than they are on Earth today. For Woese, the discovery of archaebacteria meant that Darwin's cherished doctrine of a common ancestor might need to be questioned. According to Woese, there are at least three forms of primitive cells from which all current life forms originated: primitive prokaryotes, primitive eukaryotes and primitive archaebacteria, all of which could trade genes with one another.