Chapter 5 Where Do We Come From? 1 Carbon-Based Life
Telling stories to children is to make them fall asleep,
Telling stories to adults is to wake them up.
To understand the earth, we must start with carbon-based life.
Carbon-based life refers to life forms that are built on carbon elements. Not only humans but all life forms on Earth are carbon-based. The evolution of all life is based on a common element, which is carbon. Looking back at the "Periodic Table" of chemistry textbooks (see Figure 5.1), we see that there are many elements on Earth, so why did life evolve based on carbon?
Figure 5.1: Periodic Table of Chemical Elements
We know that the core of chemical reactions is "balance", and the formation process of compounds is the "balancing" process of chemical reactions. The rightmost column in dark green on the periodic table is called "Noble gases". Noble gases are "inert" because they are stable, do not react with other elements, and always exist in atomic form. The outer electron shell of noble gas atoms is completely filled with electrons, meaning they neither release valence electrons nor absorb valence electrons from other atoms, forming an ultra-stable structure.
Elements in the "Periodic Table", except for noble gases, are very different. Take oxygen atoms, for example. The outer electron shell of an oxygen atom has only 6 electrons and lacks 2 more to form a stable structure. Therefore, when an oxygen atom encounters hydrogen atoms, they combine. For the oxygen atom, adding 2 hydrogen electrons results in a stable structure with 8 electrons in the outer shell. Similarly, each hydrogen atom gains 1 electron from the oxygen atom (forming a stable structure with 2 electrons in their single electron shell), thus stabilizing hydrogen as well (see Figure 5.2).
Figure 5.2: Electron diagram of hydrogen (H) atoms and oxygen (O) atoms in a water (H2O) molecule
Let's take a look at the 14th column of the "Periodic Table" (the column where carbon is located). The elements in this column share a common feature: their outer electron shell has only 4 electrons. A carbon (C) atom has a total of 6 electrons, with 2 in the inner shell and 4 in the outer shell, allowing it to form up to 4 covalent bonds.
Although the carbon atom occupies an unremarkable position in the "Periodic Table", it is the most versatile "magician" among all the elements. Carbon can form very complex structures by itself. Common allotropes of carbon include diamond, graphite, graphene, fullerene C60, and carbon nanotubes. These are all different structural forms of carbon, each with distinct physical properties. Graphite is very soft, while diamond is the hardest natural mineral known, not to mention graphene, which has exceptional properties.
The unique properties of the carbon atom are the key factors making it the foundation of life. Carbon atoms can form 4 covalent bonds with other atoms, creating various stable organic molecules. These molecules can form millions of different "carbon-based structures", such as chains, rings, and branched structures, participating in numerous chemical reactions to form complex organic compounds. These organic compounds are the basis of life and play crucial roles in living organisms, such as proteins, nucleic acids, purines, pyrimidines, and carbohydrates. Organic molecules can combine to form larger molecules, such as polypeptides, polysaccharides, and nucleotides. These macromolecules can fold and arrange in specific ways to form functional biological macromolecules or polymers, such as enzymes, antibodies, and DNA. These biological macromolecules can further interact in specific ways to form biological units with defined structures, such as cell membranes, nuclei, and mitochondria, thus supporting the diversity and complexity of life.
Speaking of DNA, it stands for Deoxyribonucleic Acid. In 1953, James D. Watson and Francis Crick from the University of Cambridge discovered the "Double-helix structure" of DNA, one of the greatest discoveries in human history. They not only found the code of genetic information but also revealed the essence of life: the purpose of genetics is to transmit information. DNA is composed of nucleotides, with the main component being a "five-carbon sugar". As the name suggests, it is a ring structure formed by 5 carbon atoms connected by covalent bonds, where the carbon element plays a decisive role. Because human DNA carries a vast amount of information, we have become the most advanced carbon-based life form on Earth.
In the "Periodic Table" (14th column in Figure 5.1), it's not just carbon atoms that can form 4 covalent bonds; there are several other atoms that can do so as well. Can silicon atoms, which are below carbon in the table, form silicon-based life?
Figure 5.3: The electronic layer of carbon (C) and silicon (Si)
Silicon, like carbon, has four electrons in its outermost shell and can form four covalent bonds, indicating that these two elements have similar chemical capabilities. So, what is the difference? Looking at the distribution of electron shells (Figure 5.3): carbon (C) has only 6 electrons, while silicon (Si) has 14 electrons. Carbon has only 2 electron shells, whereas silicon has 3 shells, which means that the outermost electrons of silicon are farther from the nucleus. The covalent bonds formed by the outermost electrons of silicon are much weaker than those formed by carbon. Comparatively, carbon-based molecules are much more stable than silicon-based molecules.
Another crucial reason carbon-based molecules can form life is the omnipresence of water (H2O) on Earth. Carbon-based molecules can dissolve in water, enabling the transport, metabolism, and reactions of chemical substances within living organisms. For instance, water constitutes about 60% to 65% of an adult's body weight, and it is a major component of various body fluids such as blood, lymph, and cellular fluid. Water transports nutrients like glucose, amino acids, and oxygen to different parts of the body, providing cells with energy and nourishment. Therefore, scientists generally believe that life cannot exist without water.
Why is life dependent on water?
The RNA World Hypothesis posits that: the origin of life on Earth can be traced back to approximately 4.5 to 3.8 billion years ago when early life forms appeared in water on a planet covered by it. This suggests that, for a considerable period, water was the most abundant substance on Earth's surface.
So, did water give rise to life, or did life adapt to water due to the environment?
Scientists are still unable to provide a definitive answer. However, from the history of life, it is evident that water provided a stable chemical environment for carbon-based life, allowing it to flourish. Water molecules play various roles, such as catalysis, dissolution, transport, and dilution. Because water is readily available on Earth, life never evolved the ability to synthesize water independently during the evolutionary process. Consequently, humans and all life forms on Earth must rely on drinking or absorbing water to survive. To date, scientists have not discovered any form of carbon-based life that does not require water. Although a few microorganisms can survive for extended periods under extremely dry conditions, they still need water to maintain life.
The biggest difference between silicon-based and carbon-based life is that silicon-based compounds have lower chemical reactivity with water and generally do not react with it. Although silicon atoms can also form four covalent bonds, silicon-based life cannot efficiently utilize water and, therefore, cannot naturally arise as silicon-based life.
In conclusion, the emergence and evolution of life, although seemingly coincidental, undoubtedly contain elements of inevitability. Ultimately, the environment at the time determined this. The emergence and development of carbon-based life are inextricably linked to the earth's environment, making it the only choice for life on Earth.
Imagine, could humans create silicon-based life or semi-silicon semi-carbon life?
This doesn't seem entirely impossible.
In 1813, French biologist Jean-Baptiste Lamarck described a tiny organism called "Diatoms" in his monumental work "Flore Françoise". Remarkably, diatoms have a cell wall composed of silicon dioxide (SiO2), which is the main component of quartz, sand, and glass. Diatoms live in houses of glass that they create themselves—how fascinating! One explanation suggests that when rocks and soil are weathered and eroded, silicon dioxide enters the water and dissolves into "silicic acid". Diatoms have a "silicon transporter protein" that can collect silicic acid from the water and bring it into a structure called the "deposition vesicle", where it is converted back into silicon dioxide. As silicon dioxide accumulates, it forms extensive "silica walls". Scientists still do not fully understand the exact chemical reactions that enable diatoms to combine their cell walls with silicon. However, despite the miraculous "glass clothing", diatoms remain carbon-based life forms on Earth.
In 2016, Prof. Frances Arnold, a Nobel laureate in Chemistry, used DNA from proteins of bacteria collected from Icelandic hot springs, re-encoded it, and induced DNA mutations. She discovered that the mutated enzymes had the ability to synthesize organosilicon compounds, forming covalent bonds between silicon and carbon: silicon-carbon bonds. This was the first time humans have enabled a biological system to spontaneously form silicon-carbon bonds.
A more profound and intriguing question is: Are carbon-based life forms truly the only type of life on Earth?
In the study of extraterrestrial life, scientists have long proposed the possibility of multiple forms of life. Besides the carbon-based life forms we see around us, there could also be silicon-based life forms or life forms existing in the form of energy (such as electromagnetic waves).
The most obvious characteristic of silicon-based life is that its DNA is composed of silicon elements, giving it greater chemical stability and radiation resistance. This allows silicon-based life to survive in harsh environments, similar to the "Sand-man" in the "Spider-Man" movie. With the rapid development of artificial intelligence, if robots were to gain emotions and subjective consciousness, they could potentially become silicon-based life forms, and would likely have much longer lifespans than humans and require significantly less energy.
Another perspective suggests that Earth may host not only carbon-based life forms, but also other forms of life that ordinary humans have not yet detected or experienced. In 1988, the internationally renowned UFO expert, chairman of the "Taiwan UFO Society", and nuclear physicist Mr. Lv Yingzhong, in his book "Under the Shadow of the Stars—Traces of Alien Visits to Earth", cited numerous "unsolved cases" of alien visits to Earth from ancient texts and modern human archaeological history. He proposed the hypothesis that civilization originated from extraterrestrial intelligence, challenging traditional evolution theories and human civilization history. According to a 2008 report by the British Daily Mail, the 6th American astronaut to walk on the moon, Edgar Mitchell, stated that aliens had contacted humans multiple times, but the truth had been concealed for 60 years. The "Discovery Channel" aired several episodes exploring potential collaborations between humans and higher civilizations along the 37th parallel in the United States. Subsequently, Ben Mezrich published "The 37th Parallel: The Secret Truth Behind America's UFO Highway".
Stan Romanek (USA) authored "Messages: The World's Most Documented Extraterrestrial Contact Story" in which he captured clear photos of UFOs and aliens on videotape, detailing 11 thrilling encounters between 2001 and 2008, including overnight healing, breeding with an unknown woman, and teaching interstellar travel secret equations. In 2021, an American association MUFON published the "The Extraterrestrial Species Almanac: The Ultimate Guide to Greys, Reptilians, Hybrids, and Nordics", which detailed 82 extraterrestrial species that populate the universe, including their origins, physical characteristics, and current residences.
Of course, these supernatural events discovered by ordinary people might be part of a "mysterious conspiracy" or perhaps just the "tip of the iceberg". What we can easily see is what others want us to see. What we know might not be what is happening, and what is really happening might not be disclosed to us. To borrow a famous quote from Sherlock Holmes, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
