Origin of Life — Introduction to Paleontology (Part VI)

Origin of Life

Based on the ratio of C12 to C13 isotopes, the earliest evidence of life on Earth is currently estimated to date back to about 3.7 billion years ago, during the Eoarchean. However, it is quite possible that life-like substances already existed even earlier during the Hadean eon. How life first emerged has long been an intriguing question, and many hypotheses have been proposed to explain its origin.

Some researchers have suggested that the origin of life came from beyond Earth. Small amounts of organic molecules such as formic acid, aldehydes, and acetylene have been detected in meteorites and comets, and comets are also thought to have delivered water to Earth, potentially creating conditions suitable for life to arise. Nevertheless, this idea has not gained widespread acceptance. In 1996, NASA scientists discovered structures resembling bacterial fossils and traces of organic compounds in the meteorite ALH84001 found in the Allan Hills of Antarctica. This discovery led some researchers to support the hypothesis of panspermia, which proposes that microorganisms exist throughout the universe and may be transported to planets by meteorites or comets. Although this hypothesis remains highly controversial, it has encouraged scientists to consider alternative perspectives on the origin of life.

The hypothesis most widely accepted today was independently proposed by the Russian scientist Alexander Oparin and the British scientist J. B. S. Haldane. They suggested that life began as a series of organic chemical reactions. According to this view, early Earth contained simple organic molecules such as methane and acetylene, and these simple molecules gradually combined to form more complex compounds. Many of these reactions require high temperatures, and the early Earth was filled with active volcanoes and geothermal activity. Among the environments proposed, alkaline hydrothermal vents on the ocean floor are considered one of the most likely locations for the origin of life.

The Oparin–Haldane hypothesis was proposed in the 1920s. Later, in 1953, Stanley Miller conducted a famous experiment in which a glass apparatus containing water, nitrogen, carbon monoxide, and other gases was exposed to electrical sparks to simulate lightning. After several days, organic molecules such as sugars, amino acids, and nucleotides—basic components of life—appeared in the mixture. During the 1960s, researchers modified this experimental design, for example by reducing the proportion of hydrogen and methane, and were able to produce polysaccharides, peptide chains, and other larger organic molecules. The American biochemist Sidney Fox even generated membrane-like structures resembling primitive cells during laboratory experiments. Subsequent studies also showed that certain lipids can spontaneously assemble into small vesicles and even produce additional vesicles, demonstrating a form of self-replication.

The key molecules of life—genetic material—exist primarily in the form of DNA in modern organisms. DNA is transcribed into RNA and then translated into proteins that carry out the biological functions of life. However, many scientists now believe that the earliest genetic material existed in the form of RNA. This idea is known as the RNA world hypothesis. RNA can both store genetic information, similar to DNA, and possess catalytic activity similar to enzymes made of proteins. DNA alone cannot produce signs of life because gene expression requires proteins such as RNA polymerase. Proteins themselves lack hereditary properties. RNA, however, can exist independently and still perform both informational and catalytic roles, making it a plausible candidate for the earliest life forms. Most importantly, many experiments have shown that RNA molecules can replicate themselves through RNA-polymerase-like activity. This evidence strongly supports the idea that RNA played a central role in the origin of life.

Scientists therefore propose that the first cells formed when RNA molecules became enclosed within lipid vesicles under certain conditions (Figure 1). This primitive cell represents the common ancestor of all life on Earth and is known as the last universal common ancestor (LUCA). From LUCA, two major lineages later evolved: Bacteria and Archaea (Figure 2).

（Figure 1）Cellular origin according to the RNA world hypothesis（水也佑製作）

（Figure 2）Tree of life（水也佑製作）

Eukaryotes

Eukaryotic organisms are characterized by a nucleus surrounded by a double membrane that encloses the DNA, and by the presence of mitochondria, which carry out aerobic respiration to generate energy. In plants, chloroplasts are also present and perform photosynthesis. Before eukaryotes appeared, bacteria and archaea had already existed on Earth for nearly two billion years.

Phylogenetic analyses indicate that, compared with bacteria, eukaryotes are more closely related to archaea. Among archaeal groups, the closest relatives appear to be the Lokiarchaeota lineage (Figure 2), and current evidence suggests that eukaryotes evolved from ancestors related to these archaea. In addition, the well-known endosymbiotic theory proposes that mitochondria and chloroplasts originated from bacteria. Both organelles possess their own DNA, and sequence analyses have shown that mitochondria evolved from symbiotic α-proteobacteria, while chloroplasts originated from symbiotic cyanobacteria. Figure 3 illustrates the evolutionary process leading to the formation of the eukaryotic cell.

（Figure 3）Evolution of eukaryotes（水也佑製作）

The earliest known eukaryotic fossils are the acritarchs Tappania plana and Shuiyousphaeridium macroreticulatum, dating to about 1.6 billion years ago. The former measures approximately 30–150 micrometers, while the latter ranges from 100–300 micrometers (Figure 4). Both are unicellular organisms. The earliest known multicellular organism with sexual reproduction is a red alga called Bangiomorpha pubescens (Figure 5).

（Figure 4）(A) Tappania plana, (B) Shuiyousphaeridium macroreticulatum（感謝Yin Leiming提供）

（Figure 5）Bangiomorpha pubescens（感謝N. J. Butterfield提供）

Author: Shui-Ye You