Summary: One of the biggest puzzles in biology is also one of the principle challenges for astrobiology. Just how did life emerge on Earth and under what conditions might it arise on other planetary bodies?
One of the biggest puzzles in biology is also one of the principle challenges for astrobiology. Just how did life emerge on Earth and under what conditions might it arise on other planetary bodies? This is an area of research that is still highly speculative but there are clues available from the careful analysis of what we know of life on Earth today. Buried deep in the cell are chemical fossils that hint at the way simple molecules might have got together to produce the beginnings of life.
It is this period of the Earth’s pre-history, the transition from small, simple molecules to large, complex cells, that has been subjected to detailed scrutiny in “From Suns to Life: A Chronological Approach to the History of Life on Earth” edited by M. Gargaud et. al. and reprinted from Earth, Moon, and Planets, Vol. 98/1-4, 2006. This is an ambitious book put together by a number of professionals in the French astrobiology community and this review looks at just the chapter on the pre-biotic world. Starting with the newly habitable Earth and ending with the first true cell, known as the Last Common Ancestor - a period that stretches from roughly 4.2 to 2 billion years ago – “From Suns to Life” brings together the current thinking of this challenging subject.
What hits you immediately about this subject is the large amount of uncertainty and the many different possible scenarios. There is no clear idea of chronology; many different pathways from pre-biotic soup to living organism; numerous possible intermediate stages with any number of complex organic and biochemical reactions en route. It’s also clear that the biochemicals of today may have performed very different functions in the past. For example, the majority of chemical reactions are today mediated by protein enzymes but some scenarios suggest that RNA was the chief catalyst during early chemical evolution.
All life on Earth has a similar
There is general agreement about the main players -- amino acids, nucleosides and nucleotides as the small pre-cursors leading to peptides, proteins and the long polymers of RNA and DNA. Energy sources are also required and there are important supporting roles for fats, carbohydrates and inorganic ions such as magnesium, iron and sodium. But just what happened and in what order is a matter of much debate and likely to remain so for some time.
Broadly speaking, there are three main roles of biochemicals in living organisms - that is, the storage of genetic information, structure and catalysis. For example, the capture of energy involves structures fine tuned to perform their function coupled with a catalytic activity to facilitate the necessary chemical reactions. These can be clearly seen in the structure and function of modern day bacteria or mitochondria and chloroplasts. The compartments within the cell are defined and controlled by the combination of lipids and proteins that make up cell membranes, and of course genetic information is stored as long strands of DNA. One of the requirements of any description of chemical evolution is to suggest a plausible mechanism by which the evolving molecules can fit into one or more of these roles.
A 3D structure of RNA.
Three different scenarios for chemical evolution are discussed in the review; co-evolution, self-replicating peptides, and the RNA world. Co-evolution makes no specific assumptions about a sequence of events. It argues that a protein and nucleic acid based life emerged more or less fully formed from a cocktail of pre-cursor molecules. It is the simplest of the models, requiring perhaps the least detailed explanation but it is not a particularly satisfying description.
The self-replicating peptide scenario argues that short strands of protein, peptides, were the first class of large biochemical to emerge from the soup. It requires them to perform their modern functions of catalysis and structure but also to store early genetic information. Intriguingly, there is a peptide based molecule that might, in theory, have performed this function. Called a Peptide Nucleic Acid (PNA) it has a similar structure to RNA but is not a nucleic acid. However, a fundamental problem with this is that it would have to have been replaced at some later period by RNA and then DNA. There is, as yet, no convincing rationale for this transition and what’s more, there is no hint of PNA in any modern organism. While that does not rule it out, both biochemical and Darwinian evolution are expected to leave detectable traces of their heritage behind.
The model that receives the most attention and is perhaps the most convincing is that of the RNA world. In this short strings of nucleic acid, RNA, are the first complex biochemical molecules to emerge from the soup. They have to perform the three functions of structure, catalysis, and genetic storage. However, RNA does indeed perform all of these functions to some degree in modern cells – perhaps the smoking gun of molecular evolution.
For many years RNA was seen as a bit player in cellular metabolism but the last couple of decades have seen a significant re-think of its role. RNA can, like DNA, store genetic information along its length and does so in a number of modern viruses. It is also a key structural element of the ribosome, the cellular component responsible for protein synthesis. But what has become clear recently is that it can also be a catalyst for a number of chemical reactions. RNAzymes, as they are called, are now known to have all sorts of different activities, including some of those essential for the reading and writing of genetic information. So, not only can RNA perform all of the three main roles of life, it actually does.
The final steps from the RNA world to the modern is its substitution by DNA as the primary carrier of genetic information, and the replacement of RNAzymes with protein enzymes. Then, or perhaps simultaneously, there needs to be a higher level of self assembly as the many different components required for life organize themselves into cell like structures. And then there is the role of viruses. Undoubtedly ancient, they are increasing seen as crucial elements of evolution as they swap genes from one organism to another.
It is impossible to say which of these scenarios most accurately represents what really happened on an early Earth. The discussion is also far more complex than this brief race through some of the ideas. Astrobiology, though, might provide one of the few opportunities to help unravel some of the puzzle. What might we find in the oceans of Europa or even the methane lakes of Titan? Even if there is nothing that might be classified as life, could they contain elements of the pre-biotic soup? If so, which model might they support?
This book covers every element of the evolution of life from the emergence of simple organic molecules to theories on how the first cells might have got together. How did groups of chemicals and their associated reactions become compartmentalised into prototype cells? What was the involvement of inorganic matrices and, the big one, how did complexity arise from simple origins? The authors painstakingly pour over the limited evidence and make intelligent, though guarded, speculations as appropriate. Anyone who is not comfortable with biochemistry might struggle at times, but the summaries are less intense and will allow virtually all readers to grasp the concepts and uncertainties. In describing the problem of how life emerged the authors also illustrate why astrobiology might provide one of the few experimental opportunities to test the hypotheses.
By Toby Murcott