Why are microorganisms the true protagonists in the evolution of life?
There's a common misconception that only animals and plants that can be seen with the naked eye, especially humans, are the protagonists in the history of Earth and life. However, this view is completely wrong.
Life originated about 3.8 billion years ago. For the first 3 billion years of life's history, the vast majority of organisms on Earth were tiny microorganisms. Currently, scientists classify Earth's organisms into three categories based on their ribonucleic acid sequences: archaea, bacteria, and eukaryotes. Over the long history of Earth, the first 85% of the time only consisted of prokaryotes composed of archaea and bacteria, which were distributed in various environments. The known plants and animals are just a part of eukaryotes, and they only occupied a small portion of the evolutionary process and a tiny fraction of the environment. In comparison, the environmental range where microorganisms can survive is much larger. Scientists have found microorganisms living in extreme harsh conditions in rocks at depths of thousands of meters in the ocean, and their numbers are extremely high. It is estimated that their biomass accounts for one-tenth of the total biomass of organisms on the Earth's surface.
Many people believe that eukaryotes, including humans, evolved from primitive eukaryotic cells that engulfed bacteria and co-evolved with them. In addition, multicellular organisms, including the human body, are also a tiny world where microorganisms such as bacteria are important organic components. The human body contains about 100 trillion cells, and the number of bacteria is ten times that of cells. Preliminary estimates suggest that there are about 500 species of bacteria in the human body, with more than 10 trillion bacteria. Most of these bacteria are concentrated in the human digestive tract. In terms of bacterial diversity, the number of bacterial species in the intestine can be compared to the number of species in the tropical rainforest.
In terms of sheer numbers, the bacterial community in the mammalian colon is the densest on Earth. For large herbivores, if it weren't for the help of bacteria, they would be unable to digest the semi-cellulose and cellulose in their food. Although cows, sheep, and horses seem to enjoy eating grass, they cannot completely digest fibrous foods with their own digestive enzymes. Instead, they rely on symbiotic microorganisms to ferment and break down the semi-cellulose and cellulose in the digestive tract before they can be utilized.
According to biologists' estimates, there are approximately 1030 microorganisms on Earth, most of which are bacteria. Microorganisms are the main force in transforming the Earth, and their ecological processes affect almost all elements in the periodic table of chemical elements. Microorganisms are also important players in the evolution of life. They have been silently "working" on Earth for nearly 4 billion years. In a sense, they are the true protagonists of the Earth.