This is a table that makes thousands of middle school students ache. Since Mendeleev first made it 153 years ago, the number of members of the elemental family has almost tripled to 118 at present, and it is uncertain how many more will be added in the future.
Now, everything within our gaze's reach is made up of the elements on this table, even including complex life.
At present, it is generally believed that there are 28 elements essential for life on earth, but the most basic and most abundant are four main ones, namely "oxygen, carbon, hydrogen and nitrogen".
And there is one element that dominates the entire life of the earth with absolute status, and it is carbon.
Therefore, life on Earth is also known as "carbon-based life".
Some people may wonder, clearly most of the earth life in the body, the highest content of the element is oxygen, not carbon, for example, the human body oxygen content of up to 65%, while the content of carbon is only 18%.
But why is life on Earth still called "carbon-based life" and not "oxygen-based life"?
In fact, the reason is simple: in most earth organisms, the element oxygen does not exist as a single substance, but is combined with hydrogen to form water.
While water is important for the cells in the body, it is carbon that really plays a key role in cell structure and function.
So why did life on Earth choose carbon over other elements? It also starts with the structure of the atom.
We all know that atoms are held together to form molecules because of chemical bonding, specifically, the difference in the number of outermost electrons.
For atoms, the stable state is their lifelong quest, which is to get the number of outermost electrons to 2 or 8.
For example, the oxygen atom has 8 electrons at home and only 6 in its outermost layer. In order to reach a stable state, it desires 2 more electrons, making the outermost layer 8.
Its choice at this point is to grab an electron from each of its two next-door hydrogen atom neighbors.
But the two robbed hydrogen atom neighbors were not happy, and while protecting their own electrons, the neighbors decided to do the same to each other and started to rob the oxygen atom's electrons as well.
They get closer and closer, and eventually the homes of the three elements overlap, the middle wall merges into one, and the robbed electrons become the shared electrons of all three.
For the oxygen element, it gains two more electrons and reaches the steady state, while for the two hydrogen atoms, they also gain one electron each and also reach the steady state.
As their hands, both holding each other's electrons in a death grip, they can no longer be separated.
Such a bond is called a "covalent bond" in chemistry, and the hand that holds them together is a "chemical bond".
After having some understanding of chemical bonding, we can simply imagine chemical bonding as, according to the example just given, each element grows arms, and different elements have different numbers of arms depending on the number of outermost electrons.
For example, the hydrogen atom has only one hand, so it can only hold hands with one atom at a time, while the oxygen atom grows two hands, so it can hold hands with up to two atoms at the same time.
And carbon is awesome, it has four arms, which means it can hold hands with up to four atoms at the same time.
Imagine that each element of carbon has four hands, and if they are joined together, hand in hand to form a three-dimensional structure, they create a thousand different kinds of carbon skeletons - which form the basis for a wide variety of organic compounds.
In the scientific community, scientists call such molecules containing carbon "organic molecules", and chemistry, which specializes in the study of organic matter, has thus acquired an alias - "carbon chemistry".
From the simplest hydrocarbon, methane, to the complex polymeric organic substance, DNA, compounds based on the element carbon provide a rich material basis for the formation of carbon-based life in an extremely large number of constituent forms.
Like building blocks, nature uses the magical element carbon as a framework to build colorful carbon-based life, and this process is the magnificent history of the evolution of life on earth, as well as the only way for nature to select life forms.
At this point, someone may have to say, carbon is only four hands, there are many elements in nature are more than it, if these elements as the basis, can not be combined into a more diverse molecular structure?
Yes, the more elements that have more arms, the greater the number of molecular structures that can be combined. But the world of chemistry is about coincidence, not quantity. The complexity of the molecular structure is increased, but it becomes less stable.
Because of this, carbon, with its single-handed complexity and stability, has become the element with the largest variety of compounds, with more than 10 million pure organic compounds known, and this is only the theoretical tip of the iceberg in the world of compounds.
In addition to the complex diversity, the reaction rate of compounds based on carbon elements at room temperature is an important basis for natural selection.
Any activity of life, such as metabolism, reproduction and various responses to environmental stimuli, depends on chemical reactions.
Take the response to environmental stimuli as an example, from the cheetah's speedy pursuit of prey, to the chameleon's instantaneous change of body color, from the pupil contraction of the eye when encountering bright light, to the plant's phototropism, all are biological instincts to avoid harm.
And all these reactions are actually supported by chemical reactions in the organism. The speed of these chemical reactions largely determines the speed of biological reactions, while the activity of organic molecules ensures that these chemical reactions can be carried out in a timely and rapid manner in response to a number of complex changes that may occur in the environment.
If the description of carbon is a sufficient condition for life to choose it, then its huge volume is a necessary condition for life to choose it.
While carbon ranks 15th in the Earth's crust, it plummets to fourth place in the entire universe, after hydrogen, helium and oxygen.
Such abundance gives carbon a competitive edge from the start that other elements can only look up to, so we can say that carbon is the cornerstone of life on earth, the chemical root of carbon-based life.
The Earth is all about carbon-based life, so could there be life forms based on other elements beyond the Earth?
In 1891, Julius Sheiner, an astrophysicist at the University of Potsdam, first proposed the concept of silicon-based life.
In 1893, James Emerson Reynolds, a British chemist, pointed out the possibility of silicon-based life in a conference.
For example, "silicon" and "carbon" belong to the same group of elements, and they have similar basic chemical properties, just as carbon can combine with four hydrogen atoms to form methane (CH4), silicon can similarly form silane (SiH4), silicates are analogs of carbonates, and both elements can form long chains, or polymers, etc.
So silicon, in all likelihood, could be a substitute for carbon atoms in the organic compounds that make up life.
But this view, too, is not shared by everyone.
Opponents point out that although silicon, like carbon, has four hands, which means it can form four chemical bonds at the same time, the four hands of silicon are inherently ineffective and obviously not as safe and reliable as the four hands of carbon.
This is mainly due to the fact that the silicon atom has one more electron layer than the carbon atom, resulting in its control over the outermost four electrons, which is much less than that of the carbon atom, making the compound with it as the base skeleton extremely unstable and prone to fracture.
What's more, the content of silicon on Earth is more than a thousand times that of carbon. If silicon-based life can really exist in reality, then it is not carbon-based life that rules the Earth now.
Therefore, most researchers believe that the likelihood of silicon-based life in the universe is very low, and that if life exists on other planets, the odds are that they are also carbon-based.
However, as the famous astronomer Carl Sagan said, carbon-based life uniqueness, centralism, is likely to greatly limit the human exploration and imagination of extraterrestrial life, which is a complete and utter "carbon chauvinism".
After all, the universe is far larger than we know, and perhaps in places beyond human imagination, there is a group of "impossible" silicon-based life living peacefully.
Astrophysicist Victor J. Stenger's view is even more radical, arguing that the idea that life consists of molecules is actually a kind of "chauvinism", and that in a universe of different nature, atomic nuclei, or other structures, could be assembled in completely alien ways, resulting in forms of life beyond our perception.
If, according to Carl Sagan, carbon-based life is not necessarily the only form of life in the universe, what would make them different from us, as the most likely silicon-based life?
The first thing we can be sure of is that the respiratory system will certainly be extremely different.
In an aerobic environment, when carbon is oxidized during the respiration of carbon-based organisms, carbon dioxide gas is formed, which is readily excreted from the body.
And when silicon is oxidized, it forms silica, or glass or sand, and handling such solid material can pose a great challenge, or difficulty, to the respiratory system of silicon-based life.
Unless the temperature on a planet is so high that the solid silica can become gaseous or liquid, it is not possible for them to breathe like Earth organisms.
Second, their environmental temperature requirements are much "wilder" than those of carbon-based life on Earth.
For example, some silicon-based compounds have very high thermal stability, such as silicon-oxygen bonds that can withstand temperatures of about 600 K and silicon-aluminum bonds that can withstand temperatures of nearly 900 K.
For such silicon-based organisms, 200 degrees or even to 400 degrees would be comfortable for them, and they would probably freeze to death at the room temperature we find comfortable.
Finally, and most obviously, the appearance of silicon-based life is certainly very different from that of carbon-based life.
Most researchers believe that silicon-based life looks more like crystals, which can be understood as living, moving blocks of crystal.
The astronomer Terence Dickinson has briefly depicted silicon-based life in his book Extraterrestrials: A Field Guide for Earthlings.
This depiction of him basically satisfies the researcher's, initial imagination of natural silicon-based life.
In fact, in addition to silicon-based life, there are many, possibly existing life forms in the universe.
For example, the famous science fiction writer Asimov, in his article "Not as we know it: On the chemical forms of life", described six other life forms from the perspective of biochemistry, such as arsenic (shēn) based life, boron based life, and even sulfur based life.
As you can see, some of you have already discovered that our various conjectures about non-carbon forms of life chemistry are, in essence, just an extension of our knowledge of the basic principles of carbon-based life chemistry.
This is mainly due to the fact that our complete ignorance of any other form of life limits our eyes and imagination to a great extent.
At the same time, it can lead to a sad situation - in space exploration, humans encounter extraterrestrial life, but do not recognize them.
In fact, according to this possibility, we can think about it in the context of Fermi's paradox. Could it be that human beings cannot find aliens because different forms of life are born in different environments, and we lack the means to distinguish them, which is why we see a "silent" universe?
Conversely, is it possible that the dominant form of life in the universe is not carbon-based life, but some form completely unknown to humans, and that we are the "alien" that no one else can discover or understand?
