Searching for the origin of life involves essentially trying to work out how chemistry gave rise to biology, and many think that deep sea hydrothermal vents are likely to be the place where this occurred. Adding credence to this hypothesis, a new study in the journal Astrobiology has shown that the precursor to DNA spontaneously forms within artificial vents grown in a laboratory.
Today, the building blocks of life are both DNA – which stores genetic information – and proteins, such as enzymes, that speed up chemical reactions. Neither can form without the other, so they both must have had a chemical ancestor.
RNA is the prime candidate: Although it’s less efficient than both DNA and proteins, it can both encode genetic data and form proteins. Many viruses still use RNA today, but at one point the world was dominated by it.
The researchers behind this new study thought that the type of “chemical garden” that once existed within ancient hydrothermal vents may represent the birthplace of RNA. In order to verify this, they decided to replicate their own vent systems in a laboratory setting.
The types of minerals that may have helped to first synthesize RNA are commonplace within alkaline hydrothermal vents, such as those found in the Lost City Hydrothermal Field in the Atlantic Ocean. These minerals, particularly metal hydrides, could “direct” inorganic chemicals to form organic compounds – including RNA.
In addition, these vents also feature large chimney-like structures rich in iron, silicon, and sulfur compounds. Newly-formed organic molecules may be concentrated along the walls of these chimneys, perhaps giving rise to RNA.
Chimney-like spire structures grown in a lab under high concentrations of iron, sulfur and silicon chemical solutions. Burcar et al./Astrobiology
Using a mixture of these compounds, the researchers encouraged small chimneys to grow within a laboratory setting in conditions representing that of a very early Earth. Both small mounds with hollow centers and complex spires with multiple tubes formed within an artificial, acidic ocean rich in iron and nitrogen.
Remarkably, RNA did indeed form, but this wasn’t just down to the specific chemical mixture. In a separate experiment, when the chemical mixture was placed within a simulated ancient ocean without any chimneys, RNA didn’t appear. “The chimneys, and not just their constituents, are responsible for the [formation of RNA],” said study co-author Linda McGown, an analytical chemist and astrobiologist at Rensselaer Polytechnic Institute in New York State, in a statement.
Despite heating seawater to 400°C (750°F), it’s been well documented that viruses, bacteria and archaea live in these extreme environments. Many of them take advantage of particular chemical compounds there in order to produce energy in a process known as chemosynthesis. Researchers think that this may represent a very ancient energy-production mechanism, meaning that these organisms are sometimes seen as being evolutionarily “primitive.”
As this study suggests that RNA was able to form in underwater chimneys in an early Earth ocean, it is a sound assumption to make that these RNA molecules may have given rise to the earliest microbial lifeforms – ones that are similar to the primitive types seen in modern-day hydrothermal vents.
Significantly, this study also implies that any planet or moon with this type of hydrothermal vent system – perhaps within the ice-covered oceans of Europa or Enceladus – may hold reservoirs of RNA, or even simple life itself.
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