Chemical evolution refers to the gradual process by which simple molecules and compounds transformed into more complex organic molecules and eventually gave rise to the first self-replicating systems, leading to the origin of life on Earth. It is a theoretical concept that seeks to explain how the building blocks of life, such as amino acids and nucleotides, could have arisen from simpler chemical compounds.
Scientists have reason to think that the first living cells on Earth came about through a natural process called Chemical Evolution. What is Chemical Evolution, how does it work, and how is it different from Biological Evolution? To answer these questions, We’ll first dissect the terms and then look at an example of how Chemical Evolution can take simple molecules and organize them into complex structured systems similar to those found in living cells.
What is chemical evolution?
The word “Evolution” means change over time. Biological Evolution deals with changes in things that can reproduce: living creatures make copies of themselves. The change we see in Geological Evolution is not just a random chance over time. Often it is adaptive change.
Populations become better able to survive and reproduce within their environments. Biological evolution can drive a species to develop new characteristics and abilities when conditions are right. For this to happen, biological evolution typically requires three conditions:
Many species have smooth-edged leaves. English Holly, however, is covered in spikes that protect the plant from deadly predators. How did these weapons first evolve? When a holly plant reproduces, its offspring often show a random variation. They are slightly different from their parents and slightly different from each other.
In a forest filled with grazing animals, individual plants that are harder to eat than their siblings are more likely to grow up and have children. By being challenging to survive, nature selects who gets to reproduce and pass on their new traits and who does not. In this case, mutations that caused these leaves’ vanes to extend past their edges gave a new weapon!
The discovery of biological evolution was an incredible breakthrough in science. It explained how new complex traits and abilities develop naturally in living things. The problem is Biological Evolution depends on reproduction to work. Reproduction, however, is a highly complex process in itself. This begs the question: How did reproduction first evolve?
Many scientists are looking into Chemical Evolution to try and solve this mystery. Chemical evolution refers to changes in things that need not be reproduced. Examples could be individual molecules or entire chemical systems. A chemical system is a group of molecules that interact with each other. Molecules, structures, and chemical systems almost always evolve but often evolve towards simplicity.
Solid iron corrodes into rust when it comes in contact with the water. Proteins break down when exposed to too much heat. Suppose simple chemistry is to give rise to something advanced enough to reproduce. In that case, there must be situations in which chemical systems can grow in complexity, form new structures, and gain new functions. For this to happen, reproduction, which is needed in biological evolution, can replace repetitive production with a much simpler process.
On planet Earth and throughout the universe, powerful natural events occur in regular cycles: The heating and cooling off day and night. The repetitive eruptions, volcanic geysers, and the rise and fall of ocean tides. These events repetitively produce or give birth to new molecules and chemical systems. These products increase over time and develop new abilities as they interact with their environment.
How does chemical evolution work?
The instructions are necessary to build every protein in an organism. In a process known as transcription, a molecular machine first unwinds a section of the DNA helix to expose the genetic instructions needed to assemble a specific protein molecule.
Another machine then copies these instructions to form a molecule known as messenger RNA. When transcription is complete, the slender RNA strand carries genetic information through the nuclear pore complex. The gatekeeper is for traffic in and out of the cell nucleus.
The messenger RNA strand is directed to a ribosome, a two-part molecular factory. After attaching itself securely, the process of translation begins. A molecular assembly line builds a specifically sequenced chain of amino acids inside the ribosome. These amino acids are transported from other parts of the cell.
Then they linked into chains, often hundreds of units long. Their sequential arrangement determines the type of protein manufacturer. When the chain is finished, it moves from the ribosome to a barrel-shaped machine. It helps fold it into the precise shape critical to its function. After the train is folded into a protein, another molecular machine releases it and shepherds it to the needed location.
Steps of chemical evolution
The steps of chemical evolution describe the progression from simple molecules to more complex organic compounds, which are not definitively known but are based on scientific theories and experimental evidence. Here is a general outline of the steps proposed in chemical evolution:
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 --------->---------->--------->----------->------------->--------------> Abiotic-Synthesis Polymerization Protobiont-Formation RNA-World-Hypothesis Self-Replicating-Systems Selection-and-Evolution
Abiotic Synthesis of Small Organic Molecules: The first step involves the formation of small organic molecules, such as amino acids, nucleotides, sugars, and fatty acids, through abiotic processes. These molecules can be produced through various mechanisms, including atmospheric reactions, volcanic activity, lightning discharges, and reactions in hydrothermal vents or on mineral surfaces.
Polymerization: Once the small organic molecules are present, the next step is their polymerization, forming larger macromolecules. Polymerization reactions like condensation link the smaller molecules to create more complex structures. For example, amino acids can polymerize to form proteins, and nucleotides can link to form nucleic acids.
Formation of Protobionts: Protobionts are complex aggregates of organic molecules enclosed in a membrane or membrane-like structure. These structures have some properties of living systems, including maintaining an internal environment and undergoing simple metabolic processes. Protobionts may have provided a precursor to the emergence of cellular life.
RNA World Hypothesis: The RNA World hypothesis suggests that RNA (ribonucleic acid) played a significant role in the early stages of chemical evolution. RNA molecules can store genetic information and catalytic properties, allowing them to perform enzymatic functions. It is hypothesized that RNA may have served as both the genetic material and the catalyst for early life processes before the emergence of DNA and proteins.
Emergence of Self-Replicating Systems: One of the critical steps in chemical evolution is the development of self-replicating systems. These systems would have been able to make copies of themselves, passing on their genetic information to subsequent generations. The emergence of self-replication marked a significant transition from purely chemical processes to early biological systems.
Selection and Evolution: Once self-replicating systems emerged, natural selection and evolution could come into play. Variations and errors in the replication process and environmental factors could lead to the diversification and selection of more efficient and adaptive self-replicating systems.
Process of chemical evolution
Chemical evolution is thought to have occurred on early Earth, approximately 3.8 to 4 billion years ago when conditions were favorable for synthesizing complex organic molecules. Several key factors likely contributed to this process:
Abiotic Synthesis: The early Earth had an atmosphere rich in gases such as methane, ammonia, water vapor, and hydrogen, as well as a constant supply of energy from sources like lightning, volcanic activity, and ultraviolet radiation. Under these conditions, simple organic molecules, such as amino acids and nucleotides, could have been synthesized through processes like Miller-Urey experiments, demonstrating the possibility of abiotic synthesis of organic compounds.
Concentration and Selection: The next step in chemical evolution involves the concentration and selection of certain organic molecules. These molecules could have become concentrated in specific environments, such as shallow pools, hydrothermal vents, or mineral surfaces, allowing for further chemical reactions and forming more complex compounds.
Polymerization: Over time, the organic molecules could have undergone polymerization, joining together to form larger macromolecules, such as proteins and nucleic acids. Polymerization reactions, facilitated by mineral catalysts or thermal gradients, would have contributed to the increased complexity of the chemical systems.
Self-Replication: One of the critical milestones in chemical evolution was the emergence of self-replicating systems. These systems would have been able to make copies of themselves, passing on their genetic information to subsequent generations. The development of a self-replicating system marked a transition from purely chemical processes to early biological systems.
Let’s observe the fatty acid. It’s a collection of carbon, hydrogen, and oxygen atoms stuck together in a specific pattern. Fatty acids are many complex molecules that living cells use inside their bodies. They build fatty acids with atoms they get from their environment.
Scientists used to think that living cells were the only things that consistently built fatty acids. Lab experiments have shown that simple common gases are carbon monoxide and hydrogen. They are heated with minerals in the Earth’s crust, and various complex carbon molecules, including fatty acids, begin to grow!
Living cells are not needed! It can happen naturally in underground chambers heated by the Earth’s Magma. As pressure builds, these molecules can belch up into pools of water, where a simplified version of Natural Selection takes over. Most particles blasted into water will either float or they will sink. Nature selects against them staying in the watery environment.
Fatty Acids remain suspended in warm water, growing in number as the cycle repeats. When fatty acid concentrations are high enough, they bunched together, automatically self-assembling into a stable ball! It happens because water molecules are attracted to the oxygen heads of the fatty acids, sort of like a magnet. But water repels their oily carbon tails.
When fatty acids pass near each other, water pushes their tails together, forming a ball. As fatty acid collections continue increasing, they combine to make large skins! If fluctuations in the skins make the edges touch, water forces those fused. The result is a stable hollow container similar to a living cell’s membrane or skin!
These containers have a brand-new ability. They can trap other molecules, creating a new environment for chemical evolution to continue working within! It’s important to note that these membranes are not considered living creatures. They can’t reproduce on their own the same way living cells do.
The development of these membranes and many other molecules and chemical systems that scientists have observed demonstrate a fundamental principle: Chemical Evolution can give new characteristics and abilities. Because of this, scientists hypothesize that chemical evolution could give rise to systems that are fully capable of reproduction under the right circumstances!
If correct, this will bridge the gap between Chemical Evolution and Biological Evolution, demonstrating that chemistry can give rise to life! Scientists at the Center for Chemical Evolution and other research groups worldwide are working hard to test this hypothesis.
So to sum things up, the main difference between Chemical Evolution and Biological Evolution is that Chemical Evolution can produce new characteristics and abilities without depending on reproduction. Because of this, Chemical Evolution is being investigated as a possible cause of the origin of life.
Lineweaver, Charles H. “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.” Science. 303 (2004): 59-62.
Perry, Randall S., and Vera M. Kolb. “On the Applicability of Darwinian Principles to Chemical Evolution that Led to Life.” International Journal of Astrobiology.
Rasmussen, Steen, et al. “Transitions from Nonliving to Living Matter.”
Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth.
Table of Contents