After hundreds of years of research, the molecular spark that triggered life still puzzles scientists, writes Paul Davies
In Mary Shelley’s novel Frankenstein, the monster is brought to life by a bolt of electricity. This procedure fitted in with the 19th-century view that living matter is somehow distinct from non-living matter, and that an organism acquires its remarkable properties only when infused with a vital spark.
Such vitalistic ideas have been discredited, but scientists still find it hard to put their finger on what it is that characterises life. Near the end of World War II, the Austrian physicist Erwin Schrdinger wrote a book called What Is Life? His ideas foreshadowed the revolution that would take place in molecular biology and genetics. Schrdinger recognised that living organisms behave unlike any other physical system, and the secret of their unique qualities probably lies at the molecular level.
With the discovery of the structure of DNA in the early 1950s, and the unravelling of the genetic code, it seemed to many scientists that the answer to Schrdinger’s question would soon be at hand. But in spite of impressive strides in our understanding of the molecular processes within the living cell, the essential nature of life remains out of reach.
The problem is confronted most starkly when it comes to the origin of life. Charles Darwin’s theory of evolution explains how new species can arise from old, but leaves open the question of how life started. If vitalism – the doctrine that life cannot be explained in purely mechanical terms – is rejected, there has to be a natural means for non-living chemicals to turn themselves into a living organism. But what is it?
Darwin conjectured that the required chemical transmutation might have taken place over millions of years in a “warm little pond”. His idea eventually matured into the theory of the primordial soup, according to which a chemical broth driven by an energy source gradually accumulated ever more complex molecules, until some primitive form of life finally emerged.
Over the years, there have been many variants of the primordial soup theory. The most promising scenario is that the chemical brew lay not in a pond, but deep beneath the sea bed, in the pores of volcanically heated rocks. From this Hadean pressure cooker, life was forged nearly four billion years ago.
However, even if we can identify where life began, elucidating the actual process is another matter. Most researchers have focused on chemistry. Since it is inconceivable that life sprang into being in a single monumental reaction, scientists have tried to reconstruct a plausible sequence of steps, beginning with simple molecular building blocks, and ending with a complex self-replicating molecule such as RNA.
Some individual steps in synthesising RNA can be accomplished in the laboratory under carefully controlled conditions. What is far from clear is how Mother Nature might achieve the same results under the rough and ready conditions of a primordial soup.
Biochemists have struggled for decades to discover an easy route to life, without success. Nevertheless, many scientists, especially those who work on Nasa’s astrobiology programme, are convinced that life will form readily when conditions are suitable.
Wesley Huntress, Nasa’s associate administrator, says: “We used to think that life was fragile, but wherever liquid water and chemical energy are found, there is life. There is no exception.”
According to this thinking, known as biological determinism, a watery medium infused with organic substances such as amino acids will inevitably incubate life after a few million years. Since water and organic molecules are common throughout the cosmos, this assumption implies that the universe is teeming with life.
It could be that the universe does have bio-friendly laws that can fast-track chemical mixtures towards life, but if so, those laws are not yet known to us. However, the essential quality of living organisms is not their chemical complexity, but their information content. At root, life is an information processing and copying system; it is complex because it is information-rich.
Take genomes, the molecules that store the genetic algorithm needed to make the organism. Genomes are chain molecules of DNA or RNA. The genes are instructions on how to make proteins that carry out the business of life, like metabolism and the construction of cells.
DNA is sculpted like a twisted ladder, with four varieties of rungs.
Just as human instructions can be stored by arranging letters into words, so biological instructions are stored by appropriate sequences of rungs, using a four-letter alphabet. Proteins are made from chains of amino acids, of 20 different types. The precise order of amino acids determines the chemical properties of the protein. The sequence of rungs in DNA specifies the corresponding sequence of amino acids. To translate from the four-letter alphabet of DNA into the 20-letter alphabet of proteins requires a mathematical code. There are a vast number of possible codes, but all organisms on Earth (with some rare exceptions) use the same one. The existence of a coded data link between DNA and proteins is vital because chemically they are scarcely on nodding terms. By relaying the instructions in code, using software, the chalk of DNA can speak to the cheese of proteins. If the genetic databank dealt directly with the assembly line, without the intermediary of a software link, life would be at the mercy of chemical affinities.
An analogy helps make the point. Compare flying a radio-controlled plane with a kite. The kite is hard-wired to the controller, whereas the plane is guided via radio signals from the ground. The radio waves themselves don’t pull the plane about, as do the kite wires. Instead, they transmit information that harnesses other forces to do the work. That is how life does it, using energised molecules to drive chemical reactions that would otherwise be impossible, and deploying customised enzymes to speed things along.
The enigma of life is how such software emerged spontaneously from hardware. Adding complexity isn’t enough. No amount of complexification of a kite will turn it into a radio-controlled plane. In the same way, the emergence of a genetic code isn’t just a matter of more chemistry, but something totally novel.
The problem of where biological information comes from originally remains unanswered. Biological determinists often compare the formation of life with crystallisation. But a normal crystal has a simple periodic structure, so it can’t encode much information. A genome is information-rich because it is not periodic; the four letters of its alphabet can be distributed in an enormous number of ways.
Mathematically speaking, the letters within a gene have no pattern. But only a fraction of such patternless combinations will be biologically relevant. It is hard to see how such a state can arise dependably from either law or chance acting alone.
Does this mean that life began with a stupendously improbable accident, or a miracle? I do not believe so. The theory of evolution shows how a felicitous blend of chance, in the form of random mutations, and law, or natural selection, can elaborate an already-existing genome.
Could it be a case of Darwinism all the way down, with selection operating at the molecular level? Or might there be of self- organisation that can garner information from the environment and etch it into a molecular structure like RNA?
Researchers are split on this issue, partly because scientists have been confusing the medium with the message. Life is not primarily about complexity or chemistry or replication, it is about a remarkable form of information. We shall not know how life began until we understand how this information came into existence.
Paul Davies is a physicist and writer living in South Australia. This article is based on his new book, The Fifth Miracle: The Search for the Origin of Life