Rain helped form the first cells and may have been the beginning of life as we know it

Billions of years of evolution have made modern cells incredibly complex. Inside cells, there are small compartments called organelles that perform specific functions essential for cell survival and activity. For example, the nucleus stores genetic material and mitochondria produce energy.

Another important part of the cell is the membrane that surrounds it. Proteins embedded in the surface of membranes control the movement of substances into and out of cells. This sophisticated membrane structure made possible the complexity of life as we know it. But how did the earliest and simplest cells hold everything together before elaborate membrane structures evolved?

In a study recently published in Science Advances, my colleagues at the University of Chicago and the University of Houston and I raise the intriguing possibility that rainwater plays an important role in stabilizing early cells, paving the way for life’s complexity. I looked for it.

origin of life

One of the most interesting questions in science is how life began on Earth. Scientists have long wondered how inanimate objects such as water, gas, and mineral deposits are transformed into living cells that can reproduce, metabolize, and evolve.

University of Chicago chemists Stanley Miller and Harold Urey conducted experiments in 1953 to demonstrate that complex organic compounds (meaning carbon-based molecules) could be synthesized from simpler organic and inorganic compounds. . These chemists used water, methane, ammonia, hydrogen gas, and an electric spark to produce amino acids.

Scientists believe that the earliest forms of life, called protocells, emerged spontaneously from organic molecules present on early Earth. These primitive cell-like structures likely consisted of two basic components: a matrix material that provided the structural framework and genetic material that carried instructions for the protocell to function.

Over time, these protocells appear to have gradually evolved the ability to replicate and carry out metabolic processes. For the essential chemical reactions to occur, certain conditions are required, such as a stable energy source, organic compounds, and water. It is important that the compartments formed by the matrix and membrane provide a stable environment in which the reactants can be concentrated and protected from the external environment, allowing the necessary chemical reactions to occur.

Two important questions therefore arise. What materials were protocell matrices and membranes made of? And how did these early cells achieve the stability necessary to transform into the sophisticated cells that make up all living organisms today? Were you able to maintain its functionality?

bubbles and droplets

Scientists have proposed that two different models of protocells – vesicles and coacervates – may have played pivotal roles in the early stages of life.

Vesicles are tiny soap-like bubbles in water. They are made of fatty molecules called lipids that naturally form thin sheets. When these sheets roll into a ball, they form vesicles that encapsulate chemicals and protect critical reactions from harsh environments and potential degradation.

Like tiny pockets of life, vesicles resemble the structure and function of modern cells. However, unlike the membranes of modern cells, vesicular progenitor cells are thought to lack specialized proteins that selectively allow molecules to enter and exit the cell, allowing communication between cells. Masu. Without these proteins, folliculogens have a limited ability to interact effectively with their surroundings, limiting their potential for life.

Coacervates, on the other hand, are droplets formed from the accumulation of organic molecules such as peptides and nucleic acids. They form when organic molecules stick together due to chemical properties that cause them to attract each other, such as electrostatic forces between oppositely charged molecules. These are the same forces that cause a balloon to stick to your hair.

Coacervates can be imagined as droplets of cooking oil suspended in water. Like oil droplets, coacervate protocells do not have a membrane. Without a membrane, the surrounding water can easily exchange materials with the protocell. This structural feature helps coacervates concentrate chemicals and accelerate chemical reactions, creating a bustling environment for the building blocks of life.

Therefore, due to the absence of a membrane, coacervates appear to be better protocellular candidates than vesicles. However, the lack of a membrane also has a significant drawback: genetic material can leak.

Unstable and leaky protocells

A few years after Dutch chemists discovered coacervate droplets in 1929, Russian biochemist Alexander Oparin proposed that coacervates were the earliest model of protocells. He argued that coacervate droplets provide a primitive form of compartmentalization important for early metabolic processes and self-renewal.

Scientists have since discovered that coacervates can be composed of oppositely charged polymers, long chains of molecules with opposite charges, similar to spaghetti on a molecular scale. When polymers with opposite charges are mixed, they tend to attract each other and stick together, forming filmless droplets.

The lack of a membrane posed challenges. Droplets rapidly fuse together, similar to how individual oil droplets in water combine into larger clumps. Furthermore, the lack of a membrane allowed rapid exchange of RNA, a type of genetic material thought to be the earliest form of self-replicating molecules essential for the early stages of life, between protocells.

My colleague Jack Szostak showed in 2017 that rapid fusion and exchange of material can lead to uncontrolled mixing of RNA, making it difficult to evolve stable distinct gene sequences. . This limitation suggested that coacervates may not be able to maintain the compartmentalization required for early life.

Compartmentalization is a strict requirement for natural selection and evolution. If coacervate progenitor cells were to continually fuse and their genes would be continually mixed and exchanged, they would all be similar to each other with no genetic variation. Without genetic variation, a single protocell would not have a high probability of surviving, reproducing, and passing on its genes to future generations.

However, life today thrives on a variety of genetic materials, suggesting that nature has somehow solved this problem. Therefore, a solution to this problem must exist, probably hiding in plain sight.

Rainwater and RNA

Research I conducted in 2022 demonstrated that immersion in deionized water (water without dissolved ions or minerals) stabilizes coacervate droplets and avoids coalescence. The droplets release small ions into the water, presumably allowing the surrounding oppositely charged polymers to approach each other and form a mesh-like skin layer. This mesh “wall” effectively prevents droplet coalescence.

Next, with colleagues and collaborators such as Matthew Tirrell and Jack Szostak, he studied the exchange of genetic material between protocells. Two separate populations of protocells treated with deionized water were placed in test tubes. One of these populations contained RNA. When the two populations were mixed, the RNA remained trapped within each protocell for several days. The mesh-like “wall” of the protocell prevented RNA leakage.

In contrast, when untreated protocells were mixed with deionized water, RNA diffused from one protocell to the other within seconds.

Inspired by these results, my colleague Alamgir Karim wondered if rainwater, a natural source of ion-free water, could do the same thing in the pre-living world. Together with another colleague, Anusha Bontedu, I discovered that rainwater actually stabilizes primitive cells against fusion.

We believe that rain may have paved the way to the first cells.

Work across disciplines

Studying the origin of life addresses both scientific curiosity about the mechanisms by which life arose on Earth and philosophical questions about the nature of our place and existence in the universe.

My current research is closely investigating the beginning of gene replication in primitive cells. Without modern proteins to make copies of genes in cells, the prebiological world relied on simple chemical reactions between nucleotides, the building blocks of genetic material, to make copies of RNA. It would have been. Understanding how nucleotides come together to form long strands of RNA is an important step in deciphering pre-living evolution.

To address profound questions about the origin of life, it is important to understand the geological, chemical, and environmental conditions of the early Earth some 3.8 billion years ago. In this way, revealing the beginning of life is not limited to biologists. Chemical engineers like myself and researchers from various scientific fields are exploring this fascinating existential question.

This article is republished from The Conversation under a Creative Commons license. Read the original article.