Abiogenesis: Videos & Practice Problems

Life on Earth began through a process known as abiogenesis, which refers to the natural origin of life from nonliving simple molecules. This theory posits that life emerged in Earth's oceans approximately 3.8 billion years ago. The foundational elements for this process were prebiotic molecules, which were abundant on early Earth, including hydrogen gas, water, ammonia, carbon dioxide, and methane.

The transition from these non-biological molecules to the first biomolecules is explained by several theories. One prominent theory is the atmospheric conversion theory, which suggests that energy from the sun and lightning facilitated the transformation of prebiotics into simple biomolecules. Alternatively, the hydrothermal vents theory proposes that biomolecules formed in the depths of the ocean, where sunlight and lightning could not reach, with hydrothermal vents providing the necessary energy for this conversion.

To illustrate this process, consider the formation of biomolecule monomers, such as amino acids. In the first theory, solar radiation and lightning act as catalysts to convert prebiotic materials into these monomers. In contrast, the second theory emphasizes the role of hydrothermal vents in generating the energy required for this transformation in environments shielded from sunlight.

While both theories have substantial experimental support, the exact mechanism of how the first biomolecule monomers were formed remains uncertain. However, it is clear that these monomers were crucial in the development of more complex biomolecules, setting the stage for the evolution of life. Future discussions will delve into how these initial biomolecule monomers combined to form the first biomolecule polymers.

Understanding the formation of biomolecule polymers is crucial in exploring the origins of life. Initially, biomolecule monomers, which are the building blocks of these polymers, likely formed through natural processes. The subsequent polymerization of these monomers occurred on charged mineral surfaces, specifically positively charged clay. This interaction is driven by the attraction between negatively charged biomolecule monomers and the positively charged surfaces, allowing for proper alignment. When conditions are favorable, covalent bonds can form between neighboring monomers, resulting in the creation of biomolecule polymers. Experimental evidence supports the notion that RNA and other polymers can polymerize in this manner, suggesting that this was a fundamental step in the emergence of complex biomolecules.

Once these biomolecule polymers were established, the next significant development was the formation of membranes, which played a vital role in the evolution of protocells. Membranes are essential as they enclose biomolecule monomers and polymers, preventing them from diffusing away and thereby increasing the likelihood of interactions among these molecules. The formation of membranes is primarily driven by the hydrophobic effect, a phenomenon that will be explored in greater detail later. In an aqueous environment, phospholipids, which are key components of membranes, aggregate to form a lipid bilayer. This process effectively creates a phospholipid membrane that encapsulates the biomolecule monomers and polymers, transforming them from a state of free diffusion into a more organized structure.

The enclosed environment created by the lipid membrane is referred to as a protocell. While a protocell is not a fully functional cell, it represents a crucial precursor that possesses the potential to acquire characteristics over time, ultimately leading to the development of true cellular life. This foundational understanding sets the stage for further exploration into the origins of life, including the connection between protocells and the double origin theory of life.

The double origin theory proposes that life originated from two distinct systems: a coding system and an enzyme catalysis system. These systems are believed to have developed in separate protocells, which later merged to form a single protocell capable of supporting life. This merging allowed for the integration of both systems, leading to the evolution of the first living cells.

In this context, RNA is identified as the first coding material, surpassing DNA in functionality during the origin of life. RNA possesses both encoding and catalytic abilities, making it a versatile molecule. This is significant because it suggests that a single molecule could perform multiple roles, which would have been advantageous in early life forms. While RNA can catalyze reactions, it does not function as an enzyme in the same way proteins do. Enzymes, which are proteins, are more efficient catalysts than RNA, highlighting the importance of the enzyme catalysis system that developed in a separate protocell.

As life evolved, DNA emerged later in the timeline, complementing the existing RNA coding system and the enzyme catalysis system. The first cells likely contained an RNA coding system, an enzyme catalysis system, and eventually DNA, illustrating a complex interplay of molecular evolution. This synthesis of systems underscores the significance of both coding and catalytic functions in the development of early life forms.