Although we will probably never know precisely how life started on Earth, scientists have a few different ideas that attempt to answer this intriguing but challenging question. Some believe that an asteroid or comet slamming into primordial Earth could have provided the right conditions to spark the formation of the chemical building blocks of life. And now, researchers have some tantalizing evidence to back up this idea.
Using a high-power laser to simulate a collision, scientists from the Academy of Sciences in the Czech Republic created the four essential ingredients of RNA, a close relative of DNA, from a chemical soup. While the work can’t prove how life started on Earth, it is the first time that these four molecules have all been made under one set of experimental conditions. The work has been published in Proceedings of the National Academy of Sciences.
The mystery of how life sprung from non-living matter has long eluded scientists. Since we have some understanding of the conditions present on early Earth, scientists have been recreating this environment for decades to see if they can produce some of the molecules that could have led to life.
Life appeared on our planet around 4 billion years ago, a period that coincided with a dramatic, 150-million-year event called the Late Heavy Bombardment in which Earth was pummeled with debris. While some scientists believe these collisions could have wiped out any existing early life forms, others believe they could have provided the necessary conditions for the building blocks of life to emerge.
To simulate a space rock collision, scientists fired a high-power laser at a chemical soup containing clay and a molecule called formamide. Formamide, a simple chemical produced when hydrogen cyanide reacts with water, was first postulated as a possible parent compound of the molecules needed for life back in 2001 by a duo of Italian researchers. The chemical would have been abundantly available on early Earth and has even been identified in the tail of comets. Furthermore, previous experiments have shown it is possible to create genetic building blocks using this molecule and other ingredients.
The laser pulses generated intense pressure, high temperatures exceeding 4200oC and also X-rays and extreme UV radiation; the kind of conditions we would expect if space debris slammed into Earth. When the researchers examined the resulting molecules, they found all four precursor chemicals to the building blocks of RNA: adenine, guanine, cytosine and uracil—the first three are also found in DNA. Although DNA is the molecule of life on Earth, many believe that RNA could have been the first molecule to encode genetic information.
While this work is certainly interesting, not everyone is convinced and its relevance has been called into question. Some have argued that pure formamide wouldn’t have existed on prebiotic Earth, while others have suggested that there would have been too much water to allow formamide-rich pools to exist. Furthermore, the experiments can’t tell us how these building blocks united with other molecules to form RNA. But the researchers recognize this and are planning to conduct more experiments to see if the other basic RNA ingredients can be made under similar conditions.