The Chicken-And-Egg Problem Of The Origin Of Life

The following is an excerpt from Is Earth Exceptional?: The Quest for Cosmic Life by Mario Livio and Jack Szostak

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Is Earth Exceptional?: The Quest for Cosmic Life

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In Oscar Wilde’s play A Woman of No Importance, Lord Illingworth declares: “The Book of Life begins with a man and a woman in a garden.” To which Mrs. Allonby wittily responds: “It ends with Revelations.”

In spite of the strong religious and emotional attachment to the notion that life had to contain some extra magic or divine intervention, opinions started to change at the beginning of the nineteenth century. A step toward freeing life from the need for a “vital force” beyond the understanding of science was taken in 1828, when German chemist Friedrich Wöhler accidentally succeeded in synthesizing urea—a substance found in urine that had previously been thought to be unique to life—from common chemicals. Delighted with his success in imitating nature in the laboratory, the ecstatic Wöhler wrote to his teacher and collaborator, chemist Jöns Jacob Berzelius: “I can no longer, so to speak, hold my chemical water and must tell you that I can make urea without needing a kidney, whether of man or dog; the ammonium salt of cyanic acid is urea.”

The correspondingly dramatic leap in the understanding of biology came with Charles Darwin’s theory of evolution by means of natural selection. Whereas Darwin’s theory itself ducked the origin-of-life question altogether, saying absolutely nothing about how the first organisms came into being, in 1871 Darwin mused in a letter to his friend Joseph Dalton Hooker about how life on Earth might have started. He famously wrote: “If (and oh, what a big if) we could conceive in some warm little pond with all sort of ammonia and phosphoric salts, light, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed!”

Darwin’s prescient speculation is remarkable for no fewer than five reasons. First, it totally disposes of the need for anything supernatural in the origin of life. Second, it suggests that life may have originated in a “warm little pond,” a view that, as we shall see, is stunningly compatible with our thinking today. Third, it identifies ammonia and phosphates (compounds containing nitrogen and phosphorus) as being (potentially) necessary materials for life, again an incredible foresight.

Fourth, it proposes that some form of “protein compound” may havemplayed a role in the chemistry leading to life. And fifth, to avoid the impression that living organisms may be repeatedly springing into existence, Darwin points out that the conditions under which the first life-forms emerged no longer exist today.

This idea—that life is nothing more than a combination of highly sophisticated chemical systems—is one that was initially abhorrent to quite a few people. Life, those skeptics proclaimed, is far too cleverly contrived simply to have arisen through processes of chance, while obeying only the laws of physics and chemistry. Consequently, even many of those who were willing, in principle, to accept a chemical origin

of life used still to think that some incredibly rare chance event must have been required, to bring together in one fell swoop all the components of the first living cells.

The view of creating complexity all at once from a chaotic soup of simple building blocks was further motivated by the mind-boggling intricacy of all cellular life on Earth today. The most puzzling aspect of this convolution is that all the parts and processes of extant life depend on all the other parts and processes in a circular way. For example, a complex metabolism is needed to make the biochemicals that are required for the assembly of those protein enzymes that are needed to catalyze the reactions of . . . metabolism itself! Similarly, the nucleic acid molecules, DNA and RNA, are needed to encode the information that specifies the assembly of proteins—the workhorse molecules of life—which are required to make . . . yes, you guessed it, DNA and RNA. To make matters even more perplexing, to allow all of these molecules to accomplish their tasks, they need cell membranes that keep all the molecular players boxed together. But cell membranes are made of fatty compounds known as lipids, and those are synthesized by protein enzymes. This type of self-referential or recursive activity (reminiscent of a famous drawing by graphic artist M. C. Escher in which two hands are drawing each other) is so deeply embedded in the fabric of modern living organisms, that for many years it seemed that some miraculous event would have been required to bridge the gap between a random mixture of chemicals and the highly organized structure of a living cell. Even as late as 1981, Francis Crick, the co-discoverer of the double-helix structure of DNA, emphasized that “an honest man, armed with all the knowledge available to us now, could only state that in some sense, the origin of life appears at the moment to be almost a miracle, so many are the conditions which would have had to have been satisfied to get it going.”

Needless to say, the perception that the appearance of life on Earth might have been a freak chemical accident spelled bleak pessimism for the chances of finding life elsewhere. After all, the origin of life is that critical step that marks the transition from an extraterrestrial place being merely “habitable” to it being inhabited. As a result, very few astronomers dared in the 1950s and even the early 1960s to profess belief in the existence of extraterrestrial life in general, and extraterrestrial intelligent life in particular.

Things started to swing in the opposite direction in the late 1960s, first on the chemistry-biology front. Even so, overcoming the conceptual barriers, erected by the conviction that the emergence of life from chemistry was almost inconceivable, required no less than two Nobel Prize–winning discoveries, as well as a complete reversal in our way of thinking about the origin of life.

The first discovery involved the determination of the structure of a specific RNA molecule, the so-called transfer RNA, or tRNA, that is a part of the protein-synthesizing machinery. The complicated three-dimensional figure traced out by the strand of this nucleic acid came as a shock to the scientific community. Quite unlike DNA, with its relatively featureless and rather stiff, repetitive double helix, RNA was found to be a single-stranded molecule, intricately folded up almost like a protein. Robert Holley, a chemist at Cornell University, who was the first researcher to work out tRNA’s sequence and 2-D chemical structure, was awarded the Nobel Prize in Physiology or Medicine in 1968, together with Har Gobind Khorana, at the University of Wisconsin, and Marshall Nirenberg, at the National Institutes of Health. A bit later, Aaron Klug of the Medical Research Council in Cambridge and Alexander Rich of MIT determined the surprising 3-D folded architecture of RNA.

A few scientists, including Francis Crick himself and British chemist Leslie Orgel, realized immediately the potential implications of this striking structure—it meant that RNA might be able to act like an enzyme, a biological catalyst, just as proteins do. Orgel then came up with the breakthrough idea that the early life on Earth must have done without DNA and proteins entirely. Instead, he suggested, life started only with RNA! This was a bold speculation at the time, and the notion that RNA might be able to both carry information in its sequence and speed up chemical reactions (until then considered in biology to be the exclusive province of protein enzymes) was too much to swallow for most researchers. It wasn’t until some twenty years later that, in another dramatic Nobel-winning feat, RNA enzymes were indeed discovered by chemist Thomas Cech and molecular biologist Sidney Altman. This was the seminal step that completely revolutionized thinking about the origin of life. It meant that, in principle, RNA could act as an enzyme to catalyze even its own replication, thus potentially solving a thorny “Which came first, the chicken or the egg?” dilemma. All of a sudden, it became possible to imagine a primitive cell that was much simpler than any currently existing cell. In this putative “protocell,” RNA molecules played dual roles both as the carriers of genetic information and as the cell’s enzymes, performing the basic functions of the cell. The latter included, most importantly, the replication of the genetic information. In this novel scenario, DNA and proteins could be seen as later “inventions” of evolution, custom designed specifically for the tasks of storing information and catalyzing chemical reactions, respectively. The tantalizing conception of a simpler time in the history of life, in which RNA alone played simultaneously all the starring roles in the cast of key cellular actors—being both the “chicken” and the “egg”— became known as the RNA World.

On the astronomical side, progress lagged somewhat behind initially, but then things started to advance at breakneck speed. Specifically, on October 6, 1995, astronomers Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of a planet orbiting a Sun-like star outside the solar system. Not surprisingly, they shared the 2019 Nobel Prize in Physics for their groundbreaking discovery.

Excerpted from Is Earth Exceptional?: The Quest for Cosmic Life by Mario Livio and Jack Szostak. Copyright ©2024. Available from Basic Books, an imprint of Hachette Book Group, Inc.