It is generally believed that it appeared about 4 billion to 3.5 billion years ago.
From 65438 to 0977, Barghoorn, a fossil expert from Harvard University in the United States, discovered fossils containing bacteria in rocks 3.4 billion years ago in South Africa.
So, about 3.5 billion years ago, life must have appeared on the earth.
Man lives by breathing oxygen in the air. If there is no oxygen, they will suffocate.
Therefore, probably many people think that oxygen is essential for any living thing.
The atmosphere of the ancient earth did not contain oxygen. In fact, many kinds of bacteria can't live without oxygen.
Bacteria like this are named anaerobic because they hate the oxygen-containing air on the earth now.
In addition, some species in protozoa and fungi do not need oxygen.
3.5 billion years ago, our earliest ancestor was anaerobic bacteria.
Since then, among these anaerobic bacteria, there have been bacteria that can carry out photosynthesis like cyanobacteria now.
Cyanobacteria are the original species of cyanobacteria and live on the sea surface.
Its photosynthesis, like plants, uses light energy to convert carbon dioxide and water into nutrients such as organic matter, and oxygen will be produced in the process.
Due to the increase of cyanobacteria, the oxygen on the earth gradually increased 2 billion years ago, not only in seawater but also in the atmosphere.
This is a serious environmental pollution and the earliest large-scale public hazard on earth.
Our ancestors, though only bacteria at that time, were facing the first major crisis.
Many creatures died as a result.
Fortunately, however, all living things on the earth are not completely extinct, and bacteria that can use oxygen have evolved.
Now they are named aerobic bacteria according to their preference for oxygen.
There are still some places on the earth where oxygen can't get in.
There may be no oxygen deep underground.
In such a place, the ancestors of archaea barely survived.
In fact, we are totally dependent on oxygen, without which we can't live at all.
If people were allowed to live in an oxygen-free environment, I am afraid there would not be such a prosperous scene today.
How are animals and plants produced?
Before the gradual diffusion of oxygen on the earth and the archaea crisis of our ancestors, organisms mainly obtained energy from nutrients through fermentation.
This is the method adopted by many anaerobic bacteria, yeasts, etc.
Yogurt is made from milk fermented by lactic acid bacteria.
Brewing beer also uses yeast to decompose nutrients to produce alcohol.
Aerobic bacteria, which can actively use oxygen and evolve, adopt a brand-new method-aerobic respiration-to generate energy.
Compared with fermentation, this method can produce more energy from the same amount of nutrients, which is a very effective method.
Therefore, this newly evolved aerobic bacteria has exploded on the earth.
Due to the prosperity of aerobic bacteria, although our ancestors who evolved from archaea barely survived without oxygen, they also completed two "inventions" during this period: first, a nucleus with a nuclear membrane was produced in the cell, and in order not to destroy important DNA substances, the nuclear membrane completely wrapped them in the nucleus; Second, cells have the ability to swallow other cells into their own bodies, that is, they can swallow aerobic bacteria and cyanobacteria into their own cells.
There is a story in Greek mythology: Cronus, the second great god, swallowed his children one by one.
The famous Zeus is the third generation great god. He is also a child of Cronus. He was once swallowed up by his father, but he managed to escape.
The ancestors of eukaryotes also swallowed aerobic bacteria and bacteria produced by later evolution, so some scholars called it "Clonocet" according to the myth of Cronus.
The most important thing here is to devour aerobic bacteria that can breathe aerobically.
According to the evolutionary theory of intracellular birth, this event is specially called intracellular birth.
About/kloc-0.5 billion years ago, some aerobic bacteria were swallowed into the cells of anaerobic bacteria and began to grow. Initially, anaerobic organisms could survive in aerobic environment.
After that, the swallowed aerobic bacteria become the mitochondria of the cells.
In this way, mosaic cells are produced, which are the common ancestors of protozoa, fungi, animals and plants, and this is also the root of various evolutions since then.
Eukaryotic cells with mitochondria quickly swallowed cyanobacteria.
It is very suitable for eukaryotes to obtain nutrition through photosynthesis in their own cells.
They evolved into plants today.
Animals and plants thrive with their favorable cellular abilities, which constitute the biological system on the earth now.
However, even now, we can't underestimate the ability of bacteria.
Mitochondria and chloroplasts are descendants of bacteria, and there are many other bacteria living in our bodies.
Among them, there are both bacteria that cause trouble in the intestine and pathogenic bacteria. Most of them are necessary for our life.
Therefore, if drugs that completely kill bacteria are overused, pathogenic bacteria will be killed together with other beneficial bacteria, which will also bring trouble.
There are all kinds of evidences to support * * *, among which the most powerful evidence comes from molecules.
Mitochondria and chloroplasts not only have independent DNA (different from chromosome DNA of nucleus), but also have independent protein synthesis system (different from cytoplasmic synthesis system controlled by nucleus).
If traced back to the source, our human cells should be the product of the birth and evolution of aerobic eubacteria in anaerobic unicellular organisms close to archaea.
The important role of "contingency"
When Japanese geneticist Mr Kimura published the theory of neutrality, he thought that evolution was produced to adapt to the environment, and even at the molecular level, natural selection was the most important.
Therefore, the neutral theory of molecular evolution has been strongly opposed by scientists from all over the world.
Since then, due to the development of molecular biology, many new data have appeared soon, which all support the neutral theory.
For example, if natural selection is important, genes that seem to be vital to living things can also evolve rapidly.
But in fact, these crucial genes are hard to change.
On the contrary, genes that are considered less important change rapidly.
In other words, most of the genetic changes in the process of evolution do not have the positive significance of becoming better than before, that is, most of the changes are "neutral".
Therefore, the neutral theory is gradually accepted by scientists all over the world.
1992, Dr. Kimura, like Darwin and Mendel, was included in the list of people who made outstanding contributions to the study of evolution.
Human genetic information contains about 3 billion words of DNA.
Comparing the DNA of two people should be basically the same.
However, about every 1000 characters, there is 1 character variation.
Perhaps this is considered as an extremely insignificant difference, but the DNA map consists of 3 billion characters, so there are 3 million variations in the whole DNA map.
According to the neutral theory, most of these mutations are neither good nor bad for natural selection.
For example, even if there are variations that affect appearance, they are not necessarily meaningful for human survival, and many of them are variations that have nothing to do with natural selection.
There is a disease called diabetes, which shows that carbohydrates that originally provide energy in the body cannot be used, but are excreted directly through urine.
One of the reasons for this disease is that protein, called insulin, cannot be effectively synthesized in the body.
So as long as you inject insulin, you can control the symptoms.
Recently, advances in molecular biology have enabled microorganisms to produce protein equivalent to human insulin.
Before that, insulin from cattle and pigs was used.
There are some differences in amino acid sequences between human insulin and bovine and porcine insulin, but their functions as insulin are the same.
The amino acid change (variation) of insulin from people, cattle and pigs of the same ancestor can be said to be a "neutral" variation with no difference in function.
Another problem expounded by neutral theory is that "accidental" events in biological evolution also play a vital role.
Because nature randomly chooses mutations like dice.
Take the entrance examination as an example. Instead of choosing the ones with good grades in turn, we first remove the ones with particularly poor grades (although we can live, those extremely bad mutations will be removed), and then draw lots from those who stay.
In short, the view of natural selection is: "The best creatures survive (survival of the fittest).
The neutral theory holds that "the luckiest and beneficial creatures survive (the lucky ones survive)."
Among those who study Darwin's theory of natural selection, some people think that it is just for Britain to occupy the colonies and rule the colonial people, because the winner in the competition is the best.
This is to prove that the British colonial policy and natural selection theory are abused.
Although Darwin himself opposed slavery, his theory was used for purposes that even he himself thought were incorrect.
Therefore, Heliocentrism, evolution and other scientific discoveries not only significantly changed everyone's thinking, but also affected everyone's lifestyle and social status.
In today's competitive society, will clarifying the neutral theory, which plays an important role in accidental events, also affect people's way of thinking? This question is not only very important, but also very meaningful.