Where did life on Earth come from?

By: Zuri Sullivan

This fundamental question fascinates and frustrates scientists and non-scientists alike, and scientists across many fields have spent centuries trying to answer it. In biology, for example, we address this question through the study of evolution. This particular branch of biology allows scientists to draw inferences about past organisms through examining certain characteristics of current organisms. By comparing and contrasting the species that exist today, and investigating their relationships to one another over evolutionary time, biologists can make predictions about what some of Earth’s earliest life forms may have looked like.

These predictions are made possible through our understanding of natural selection, which is the process by which random variations that make an organism more likely to survive and reproduce are passed on to subsequent generations, gradually becoming more frequent in the population. In other words, natural selection is “survival of the fittest.”  Through this process, advantageous variation in very simple systems slowly gave rise to more complex ones. From single-celled organisms like bacteria slowly emerged more complicated single-celled organisms, like yeast. From this class of organisms, called single-celled eukaryotes emerged simple multicellular organisms, of which sea sponges are a modern example. Gradually, over hundreds of millions of years, increasing layers of complexity were built upon one another, giving rise to the diverse array of highly sophisticated organisms (including ourselves) that we observe today. This doesn’t necessarily mean that simpler life forms haven’t been able to survive over all of these millions of years (in fact, the vast majority of living organisms today are unicellular). Rather, evolutionary biology tells us that the common ancestor of all extant organisms was a single-celled organism that could have resembled some of the bacteria we see today.

The insights we gain from evolutionary biology are extremely powerful, but the question of the origin of the original life form upon which all this sophistication was built remains elusive. However, a recent study published in Nature Chemistry, led by John Sutherland of the UK Medical Research Council, provides important clues as to how this original life form could have emerged. Now you may be wondering—if we’re talking about the origins of life, and biology is the study of life, then why were chemists investigating this question? In order to understand how life began, it is necessary that we examine the individual building blocks that are needed for life, and organic chemistry provides the tools necessary to study these building blocks.

So what are these most fundamental building blocks for life? They’re called macromolecules, and include nucleic acids (like DNA or RNA), proteins, lipids (or fats), and carbohydrates. Each of these macromolecules is made of even smaller building blocks: nucleic acids are made of nucleosides, proteins of amino acids, fats from fatty acids, and carbohydrates from monosaccharides (simple sugars). The names aren’t important, but the fact that life is built upon macromolecules, which are built from small precursor molecules, transforms our question about the origin of life from the realm of biology to the realm of chemistry. Instead of asking, “where did life on Earth come from?” the more fundamental question is “how were the building blocks of life first assembled?”

Chemists have been asking this question experimentally since the 1800s, and have made a number of important discoveries. Chemists have figured out ways that amino acids, complex sugars, and certain nucleosides could be synthesized from the simplest possible building blocks that are believed to have been on Earth before life emerged. Scientists interested in these questions often refer to the hypothetical composition of pre-life molecules and water as the “primordial soup.” The issue with these studies, however, has been that the complex reactions needed to produce each macromolecule were incompatible with the reactions needed to synthesize other macromolecules. In other words, no one has been able to create a set of conditions under which all of life building blocks could be synthesized. 

This is the problem that the Sutherland lab set out to address—are there a set of conditions under which all of these macromolecule precursors could have been synthesized? Using three simple molecules that could have existed on Earth before life began, the group showed how the combination of water and ultraviolet radiation from sunlight could have produced a set of chemical reactions that gives rise to building blocks for the carbohydrates, lipids, proteins, and nucleic acids that we know today. As it was put in a commentary that covered this study, the Sutherland group uncovered “a primordial soup that cooks itself.”

As is always the case in science, this study led to more questions than it did answers. One caveat to their complex synthesis reaction is that certain molecules needed to be added at particular times in the reaction. Returning to the soup analogy, the recipe would have relied on a cook standing over the pot and slowly adding certain ingredients at the right moment. The authors of the study put forth an additional hypothesis to address this, suggesting that rainfall could have introduced these molecules at the right moment in the synthesis reaction. Seems plausible, but I’m not a chemist.