One of the greatest mysteries on Earth is the origin of life. Abiogenesis is the subject of biology and chemistry that focuses on the emergence of life from non-living matter.
While many aspects of abiogenesis are not yet fully understood and many of the specific hypotheses of how life arose are undergoing a lively debate, scientists researching abiogenesis agree on several key points regarding the origin of life.
First, what is “life”?
Scientists agree that even cells, the basic and simplest unit of life, are staggeringly complex. Cells living today demonstrate this complexity through the structure of their genome and the often visually astounding molecular “machinery” made of protein and sometimes also ribonucleic acid (RNA). In order to understand how life could have arose, understanding towards what biological end these molecular machines serve and, just as importantly, how these molecular machines are constructed within the cell is a useful endeavor.
Some properties of life are common throughout all organisms (viruses excluded):
- Cells require a source of raw materials and energy, as well as a method to utilize these sources for their own growth and development.
- Regulation of a more or less steady internal state is just as important to the cell as acquiring raw materials; this regulation is called homeostasis.
- Cells also demonstrate the ability to self-repair, and can detect and respond to signals in the outside their environment.
- Finally, cells are able to reproduce and grow, and reproduction affords a population of cells not only a means of providing a new generation, but also the opportunity for these populations to evolve through variation and natural selection.
Two of the most important properties of cells with respect to abiogenesis are replication and metabolism. Metabolism may be divided into two more or less distinct pathways. Catabolic metabolism is the cell’s use of complex materials, such as sugars, lipids, and protein, to produce energy by breaking these materials down into their simpler components, such as water and carbon dioxide.
Anabolic metabolism is the cell’s use of simpler materials, such as amino acids and nucleic acids, to construct complex molecules called macromolecules (literally: large molecules) that provide function to the cell. The four major classes of macromolecules are carbohydrates, which range from simple sugars to complex polysaccharides, protein, nucleic acids such as DNA and RNA, and the diverse class of macromolecules called lipids, which includes fatty acids and cholesterol.
Where did the raw materials for life come from?
There are two possible sources of raw materials, also called organic compounds (“organic” in chemistry means “carbon-containing”). These compounds either originated on Earth, having a terrestrial origin, or they originated in space, having an extraterrestrial origin. These two sources are not mutually exclusive, however, as some organic molecules could have originated on Earth while others originated in space.
Extraterrestrial organic molecules could have been delivered by objects falling to Earth, while organic molecules of terrestrial origin may have been produced by a number of energy sources on Earth. Ultraviolet (UV) light, electrical discharge, geothermal vents, and impact shocks of extraterrestrial objects falling to the ground may have all played a role in the origin of organic molecules on the planet.