“In our world,” said Eustace, “a star is a huge ball of flaming gas.”
“Even in your world, my son, that is not what a star is, but only what it is made of.”
― C.S. Lewis, The Voyage of the Dawn Treader
Darwin did it
Two hundred years ago, the origin of life was easy. Flies came from dead meat and worms from dirt. Experiments performed by Louis Pasteur showed that living things are only made by other living things.
Except for the first time.
Charles Darwin speculated that life first arose in a “warm little pond”. John Haldane and Alexander Oparin suggested that lightning could be a source of energy for the pond. In 1953, Stanley Miller set off a shock in a mixture of ammonia, methane and water vapor, and formed small amounts of a half dozen amino acids. And the field of experimental prebiotic chemistry was born.
Why is the production of amino acids so interesting? Living organisms use proteins to control the rates of chemical reactions. Without this control over its own chemistry, the organism cannot stay alive. Proteins are molecular machines that are built out of amino acids. Life also uses DNA and RNA to store and translate the instructions for how to build proteins. The information contained in DNA and RNA is copied many times. DNA and RNA are built out of nucleotides. All of these molecules are contained within membranes. Membranes are a semi-permeable boundary of the cell. Membranes are built out of phospholipids.
Each of these molecules are involved in many more functions than I listed. The functions are closely integrated to carry out a complex web of chemistry. How did this web of chemistry first arise? Where were the first building blocks of RNA, DNA, proteins and membranes first formed, and how were they first assembled?
There have been several conversations about origins of life on Unbelievable? Most recently the Big Conversation with Paul Davies and Jeremy England. I am a scientist working on the question of how life originated on Earth, and how the origins question is connected to the prevalence of life elsewhere in the universe. I am also a Christian. This guest blog is a brief overview of my perspective on origins. It touches on several of the points raised by Paul Davies and Jeremy England, but it is written to stand on its own.
Two Different Questions
A lot of Christians are interested in origins of life research, even Christians with no background in Chemistry. One reason why many Christians are interested in science in general, and origins in particular, is that the creation can teach us about the Creator. For those of us who believe in God, Creator of all things seen and unseen, there are two important aspects of the origins question. Who made the first life? How did he do it?
Consider this example. I believe that God knit every child together in their mother’s wombs (Psalms 139:13). I also believe that there is a reasonably complete biological explanation for the origin of children from conception until birth. It is this second kind of answer that I seek for the question of how life first originated on Earth.
There are several signs of God’s hand in the science of origins, both to bring about life and to do so in a way we can hope to discover. Here is my favorite sign: Many of the molecules involved in prebiotic chemical pathways fall apart in a matter of weeks to months. If these molecules stuck around for centuries or for millions of years, it might be easier for life to originate. But it would be much harder to discover these pathways in the lab. Prebiotic chemistry seems very well-suited for scientific investigation.
Though there was no chemistry lab where life started, I believe there was a chemist. Not just a chemist but the Author of Chemistry. It is a joy to try to piece together how God assembled the first living organisms here on Earth. The rest of this blog discusses the field in general; where it’s been, and where it’s going. Perhaps you are not interested in the second question. That’s fine! Though in this case, you may want to stop reading here.
Origins of Life is a Chemistry Problem
Life is made of molecules. The behavior of molecules is the subject of chemistry. When things bigger than molecules enter the story, like organisms or ecosystems, this becomes the subject of biology.
Organisms by their nature can evolve by random genetic mutation and natural selection. The original transition from a non-living group of molecules to a living group of molecules cannot be explained by evolution because only living things evolve and life did not precede its first instance.
The question of how the first self-contained self-copying group of molecular machines arose from available molecules on the surface of the ancient Earth is a question of chemical synthesis. And so the origins of life is a chemistry problem.
Origins of Life is not just a Chemistry Problem
If the origins of life were only a chemistry problem, then it could be solved by creating life in the lab from simple commercially available precursors. Even this is a task beyond our present capabilities. It took over ninety world-leading chemists almost twelve years to synthesize vitamin B12. Synthesizing life will be much harder.
To make life in a lab only proves that it is possible to make life in a lab. The ancient Earth had no chemistry labs. It had streams, impact craters, geothermal pools, and countless other local environments. What kind of chemistry happened in these environments, and could that chemistry have resulted in life?
To answer these questions requires contributions from many different scientific fields: astrochemistry, biology, mathematics and geochemistry, to name just a few. Here is an example from physics:
Robert Shapiro, a chemist who worked in one of the labs where vitamin B12 was synthesized, compared synthetic prebiotic chemistry in the lab to a game of golf played without a golfer. He said,
“The analogy that comes to mind is that of a golfer, who having played a golf ball through an 18-hole course, then assumed that the ball could also play itself around the course in his absence.”
Although Shapiro was skeptical of the origins endeavor, I find his analogy useful. The golf ball represents the initial chemical state of the system. The course represents the chemical environment. The flag is far away and represents life. Surrounding the course is a steep cliff that drops off into the sea. The sea represents chemical equilibrium. Equilibrium equals death. Unless and until the ball ends up in the sea, it will not stay still.
Thermal forces, like heating a chemical solution, can throw the ball into the air in a random direction. It almost always lands into the sea.
But a far-from-equilibrium force, like ultraviolet light, could act like a puff of air, blowing the ball closer to the flag. If the ball is in the right place to start and if the landscape is the right shape, and if the tuft of air is strong enough to move the ball. The physics of ultraviolet light can help the right chemistry along. That light can make chemistry do this is one of the important insights of Jeremy England’s work (1). The inclusion of photochemistry in origins experiments is already yielding promising results; a veritable “let there be light” moment in the field.
Lots of Progress
The picture has changed a lot since Stanley Miller’s experiment in 1953. We now know how to make about half of the building blocks of DNA, RNA, proteins and membranes. They can be synthesized from simple molecules without the use of biochemistry. Reactions have been discovered that are efficient. This is important because life is chemically complex and a long series of reactions is needed to go from simple molecules accessible within a local environment to the big molecules life uses. If twelve reactions in water were needed to go from simple molecules to a strand of RNA, and each step is 1% efficient, then the final product will be less than one molecule for every milliliter of water. This is no longer chemistry. This is homeopathy.
Chemists have discovered several different ways to achieve high and selective yields of the building blocks of life from simple starting conditions (2). I think that some recently discovered pathways which involve ultraviolet light are especially promising (3).
The prebiotic chemistry that uses ultraviolet light is not 100% efficient. There are by-products. Remarkably, some of these by-products will react with the building blocks and link them together in the same way living organisms link these building blocks together. These by-products can link deoxyribonucleotides and ribonucleotides together to make a hybrid structure of RNA and DNA (4). They can link amino acids together to make polypeptides, precursors to proteins. They can link phospholipids together to make containers that resemble cell membranes.
Many of these reactions have been discovered in the last five years. This is a lot of progress, but there is a very long way left to go.
A Long Way to Go
In my opinion, the biggest problem facing prebiotic chemistry is getting rid of the chemistry lab. There was no chemistry lab on the ancient Earth. There were many different kinds of local environments. To connect lab work to these local environments, we need the answers to three questions:
- What was the global environment of Earth like, before there was life?
- What local environments can exist, given this global environment?
- How does lab chemistry work in these environments?
These questions reveal a vast chemical landscape that we have just started to explore. Even the question of the global environment is hard to answer and is a topic all its own. So far, no early Earth environment has yet been identified where any chemical pathway has been shown to work to completion. We have just started looking.
This is one of dozens of problems that will need to be solved in order to start to solve the origins of life problem. We are at the very beginning of a long journey. We have some early success and progress is speeding up, but there’s a lot more to do.
It can be tested
Much of science is storytelling. When starting out, it does not matter if the story is true. It only matters if the story is interesting and if it comes into contact with some facts that can be checked in the lab or in the rocks or among the stars.
One of the most exciting new developments in origins of life research is the ability to connect it to planetary and exoplanetary science. This connection allows us for the first time to test different stories of how life originated.
The story that is easiest to test is also the story that I think is the most promising: the story that involves ultraviolet light. I have published two papers that try to predict where life would start, based on the starlight (5).
This particular kind of ultraviolet photochemistry will be tested soon with the Mars Perseverance rover. The rover will explore whether some of this ultraviolet light chemistry might have happened there (6). If this chemistry happened on Mars, and if we are fortunate enough that the chemistry is preserved, we may have to wait for years before this chemistry is confirmed. Even if the chemistry is confirmed, this does not mean that the chemistry resulted in life.
Light and Life
One of the few things we know about the path from simple molecules to life is that it will be a long and arduous one. Whether life is common or rare depends on how many paths there are from chemistry to life and how difficult the paths are to traverse, weighed against the billions of local environments on billions of planets enduring for aeons, throughout which these experiments have been and are being attempted, all orchestrated by a Master Chemist.
Though I cannot demonstrate that the most likely path from chemistry to life is guided by starlight, there are encouraging signs in the experiments, and soon, possibly, in the Martian geochemical record. There is also a poetic beauty to this hypothesis. In a sense, if light drives prebiotic chemistry, then life is found in the light that “shines in the darkness, and the darkness has not overcome it.” (John 1:5)
This blog post was a review of The Big Conversation between Paul Davies and Jeremy England on the Origins of Life
Paul B. Rimmer has a postdoc in the Cambridge University Department of Earth Sciences with affiliations at Cavendish Astrophysics and the MRC Laboratory of Molecular Biology. The beliefs expressed here are his own and do not necessarily reflect the beliefs of his employers.
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(1) England, J.L., 2015. Dissipative adaptation in driven self-assembly. Nature nanotechnology, 10(11), 919.
(2) For example, Benner, S.A., Kim, H.J. and Biondi, E., 2019. Prebiotic chemistry that could not not have happened. Life, 9(4), 84. Becker et al. 2016. A high-yielding, strictly regioselective prebiotic purine nucleoside formation pathway. Science, 352(6287), 833.
(3) For example, Xu et al. 2018. Photochemical reductive homologation of hydrogen cyanide using sulfite and ferrocyanide. Chemical Communications, 54(44), 5566.
(4) Gavette et al. 2016. RNA–DNA chimeras in the context of an RNA world transition to an RNA/DNA world. Angewandte Chemie, 55(42), 13204. Xu et al. 2020. Selective prebiotic formation of RNA pyrimidine and DNA purine nucleosides. Nature, 582(7810), 60. Liu et al. 2020. Harnessing chemical energy for the activation and joining of prebiotic building blocks. Nature chemistry, 12(11), 1023.
(5) Rimmer et al. 2018. The origin of RNA precursors on exoplanets. Science advances, 4(8), eaar3302. Rimmer et al. 2021. Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions. Astrobiology. https://www.liebertpub.com/doi/full/10.1089/ast.2020.2335
(6) Sasselov, D.D., Grotzinger, J.P. and Sutherland, J.D., 2020. The origin of life as a planetary phenomenon. Science Advances, 6(6), eaax3419.