All living organisms share a common ancestor; most likely, a population of microorganisms that lived almost 4 billion years ago was the last universal common ancestor (LUCA) of life on earth. This common ancestor was itself the product of a long period of prebiotic assembly of nonliving matter, including organic molecules and water, to form self-replicating units. All living organisms retain a fundamental chemical composition inherited from their ancient common ancestor.
The origin and maintenance of life on earth depend critically upon water. Water is the most abundant of all compounds in cells, forming 60% to 90% of most living organisms. Water has several extraordinary properties that explain its essential role in living systems and their origin. Water has a high specific heat capacity and a high heat of vaporization, it has a lower density as a solid than as a liquid, it has high surface tension but low viscosity, it is an excellent solvent, and it participates in many chemical reactions in living organisms (such as hydrolysis and condensation reactions).
Chemical evolution in a prebiotic environment produced simple organic compounds that ultimately formed the building blocks of living cells. The term “organic” refers broadly to compounds that contain carbon. Carbohydrates are compounds of carbon, hydrogen, and oxygen. Familiar examples include sugars, starches, and cellulose. Carbohydrates are usually grouped into monosaccharides (simple sugars), disaccharides (double sugars), and polysaccharides (complex sugars). Lipids are fats and fatlike substances. The three principal groups of lipids are triglycerides, phospholipids, and steroids. Proteins are large, complex molecules composed of 20 kinds of amino acids. Many proteins function as enzymes, the biological catalysts required for almost every reaction in the body. Nucleic acids are complex polymeric molecules whose sequence of nitrogenous bases encodes the genetic information necessary for biological inheritance. The two kinds of nucleic acids in cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They are polymers of repeated units called nucleotides, each of which contains a sugar, a nitrogenous base, and a phosphate group.
The reducing primeval atmosphere was conducive to the prebiotic synthesis that led to life’s beginnings. It has been proposed that early organic molecules accumulated in the primitive oceans to form a “hot dilute soup”. To produce a chemical reaction between the simple gaseous compounds present in the early atmosphere, a continuous source of free energy, such as ultraviolet light or lightning, must be supplied. One hypothesis maintains that life originated not on the surface of the earth, but deep beneath the sea in or around hydrothermal vents. Experiments have shown that highly reactive intermediate molecules are formed when a reducing mixture of gases is subjected to a violent energy source. These molecules react with water and ammonia or nitrogen to form more complex organic molecules, including amino acids, fatty acids, urea, aldehydes, sugars, and nitrogenous bases. The next stage in chemical evolution involved the joining of amino acids, nitrogenous bases, and sugars to yield larger molecules, such as proteins and nucleic acids. Our strongest hypothesis for prebiotic assembly of biologically important polymers is that they occurred within the boundaries of semipermeable membranes formed from small amphiphilic molecules.
The fossil record reveals that life existed 3.8 billion years ago; therefore the origin of the earliest form of life can be estimated at approximately 4 billion years ago. The first living organisms were protocells, autonomous membrane-bound units with a complex functional organization that permitted the essential activity of self-reproduction. The principal problem in understanding the origin of life is explaining how primitive chemical systems could have become organized into living, autonomous, self-producing cells. In a later stage of evolution, nucleic acids began to behave as simple genetic systems that directed the synthesis of proteins, especially enzymes. The earliest enzymes could have been RNA, and the earliest self-replicating molecules could have been RNA. Investigators are now calling this stage the “RNA world“. Once the protocellular stage of organization was reached, natural selection would have acted on these primitive self-replicating systems. Evolution of the genetic code and fully directed protein synthesis followed. The system now meets the requirements for being the common ancestor of all living organisms. The earliest postulated microorganisms are sometimes called primary heterotrophs because they relied on environmental sources for their food and existed prior to the evolution of any autotrophs. Because chemical evolution had supplied generous stores of organic nutrients in the prebiotic soup, the earliest organisms would not have been required to synthesize their own food. Evolution of autotrophic organisms most likely required acquisition of enzymatic activities to catalyze conversion of inorganic molecules to more complex ones, such as carbohydrates. Autotrophy evolved in the form of photosynthesis. As the atmosphere slowly accumulated oxygen gas, a new and highly efficient kind of metabolism appeared: oxidative (aerobic) metabolism.
Fossil data indicate that oxygen-producing cyanobacteria became prominent in the world’s oceans approximately 2.5 billion years ago. An important product of evolution in an oxygenated atmosphere was the eukaryotic cell. Eukaryotes have cells with membrane-bound nuclei containing chromosomes. Fossil evidence suggests that single-celled eukaryotes arose at least 1.5 billion years ago. Biologists have proposed that eukaryotes did not arise from any single prokaryote but were derived from a symbiosis of two or more types of bacteria. The endosymbiotic theory proposes that a population ancestral to eukaryotic cells, derived from anaerobic bacteria, evolved a nucleus and other intracellular membranes from infoldings of the cell membrane. Cells of this population acquired, by ingestion or parasitism, aerobic bacteria that avoided digestion and came to reside in the host cell’s cytoplasm. The endosymbiotic aerobic bacteria would have metabolized oxygen, which is toxic for their anaerobic host, and the anaerobic host cell would have given its aerobic residents food and physical protection.
—May 2021
—May 2023
Lance Jones 