Deep-sea hydrothermal vents, water-storing craters...where is our "cradle"?

Deep-sea hydrothermal vents, warm pools of volcanic hot springs on land, meteorite craters that store water... Where did life on Earth originate? This has been a hot topic of controversy in the scientific community for many years.

Recently, an international research team led by scientists from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, jointly published an article in the latest issue of the internationally renowned academic journal "Nature Communications", stating that iron sulfide in the terrestrial hydrothermal springs of the early Earth provided the key chemical basis for the origin of life.

Verifying Darwin's "conjecture"

In 1871, Darwin wrote to British botanist Hooker: "We imagine a situation where there are various ammonium and phosphate substances in some warm small ponds. Under the conditions of light, heat, electricity, etc., protein compounds are formed chemically, and then more complex changes will occur."

Later, American scientist Miller's experiment proved Darwin's foresight. But where is the small pond proposed by Darwin? In 1977, the US Alvin submersible dived into the eastern Pacific Ocean at a depth of 1650-2610 meters, and researchers discovered incredible "black chimneys" and various hydrothermal organisms surrounding them. In extreme environments with high temperature, high pressure and rich reducing substances, there are obvious chemical concentration gradients and water temperature gradients, especially at hydrothermal vents.

Therefore, the living environment of microorganisms in deep-sea hydrothermal vents is very similar to the environment in the early formation of the earth. In particular, the ultra-high temperature-loving microorganisms found in hydrothermal vents are the root microorganisms of the "evolutionary tree" (in biology, used to represent the evolutionary relationship between species). Therefore, the hypothesis that life was born in deep-sea hydrothermal vents is still popular today.

The "black chimney" hypothesis is difficult to justify

In 2020, Chinese scientists published an article stating that supercritical carbon dioxide (containing a large amount of nitrogen and organic molecules) was discovered for the first time in a deep-sea hydrothermal area in the western Pacific Ocean. The discovery of nitrogen indicates that the origin of life may come from the deep sea.

However, the "black chimney" hypothesis has some problems that are difficult to justify. First, when scientists simulated the synthesis of organic matter in submarine hydrothermal vents, they found that this type of chemical reaction is more dependent on an alkaline environment, but in fact, the environment near the submarine hydrothermal vents is a typical acidic environment. Therefore, the ocean may be useful for maintaining life, but it is not an ideal place for the origin of life.

Secondly, scientists have also found that no matter what kind of organism the cells belong to, they all contain a lot of phosphate, potassium and other metals, and the potassium content in the cells is 10 times that of sodium. The sodium ion concentration in seawater is high and the potassium ion concentration is low. Obviously, this does not support the hypothesis that life originated from the ocean. The geothermal ponds near active volcanoes have all the metals in the cells mentioned above. In addition, some scientists have noticed that ribonucleic acid (RNA) can remain stable under ultraviolet light, which means that life may have first appeared on the surface of the earth rich in ultraviolet light, rather than in the depths of the ocean.

Volcanic hot springs are exposed to sunlight, which not only receives solar energy to provide energy for chemical reactions, but also helps to accumulate the molecular materials needed for life. When water evaporates from volcanic hot springs, even dilute solutions become extremely thick, which is conducive to the polymerization of simple molecules into complex molecules. Although there are dry and wet cycles in hot springs, which will dehydrate primitive cells and create a favorable environment for the generation of complex organelles, the variables in an environment such as seafloor hydrothermal fluids are not enough to support the formation of diverse organelles.

In recent years, scientists have also proposed that life may have originated from water-storing craters and hydrothermal systems similar to hot springs produced by asteroid collisions. However, whether it is a hydrothermal vent on the seabed or a volcanic hot spring on land, the exploration and controversy about the origin of life will continue, step by step approaching the truth.

(The author Feng Weimin is a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and the honorary curator of the Nanjing Paleontological Museum)