Defined Words: amino acids, proteins, lipids, liposomes
Before we can talk about the actual story of life’s formation, it’s important to be aware of what actually makes up living things. We classify all the molecules that life is made of into four groups: carbohydrates, lipids, proteins, and nucleic acids. Living things utilize these four types of molecules in order to perform functions that are necessary to them continuing to live.
When describing the origin of life, we do not have all of the answers but that doesn’t mean that our theory is wrong; it is merely incomplete.
There are many theories of how life got started and currently we do not have a definitive answer to explain the exact process. Nobody was around at the time so we have no written accounts, nor have we been able to successfully create life in laboratory settings. However, back in the 1950’s two scientists tried to do just that using only the molecules that were in the atmosphere at the time that we suspect life to have started, approximately 3.7 billion years ago (give or take 100 million years).
We can approximate the composition of our atmosphere at a time before life using geochemical cycles. An electrical signal recreates the energy that could have come from escaping heat through hydrothermal vents on the ocean floor or lightning caused by a buildup of electrons diffusing from clouds.
Two scientists, Miller and Urey, took ocean water and ran it through some water vapor, ammonia, methane, and hydrogen gas as well as an electrical spark. The experiment was designed to recreate the molecules that made up early life.
Success! Kind of…
The naturally occurring compounds, in the presence of energy, spontaneously formed simple amino acids. Amino acids are the building blocks of proteins which are the building blocks of you!
Granted, they didn’t create life itself but this is still a huge deal! While you might be saying, “big whoop, they made a part of a part of a living thing,” this is only one part of the process! We’ll just take the findings of the experiment as a single statement: amino acids form spontaneously in early Earth conditions.
Next, we’ll look at lipids. Lipids are mostly long chains of carbon atoms with hydrogen atoms filling in the bonding gaps. We know that carbon atoms are capable of bonding with up to four other atoms and therefore we can expect that hydrocarbon chains also form spontaneously. But there’s more to lipids than just their formation: their reaction to water is where the real fun begins.
Lipids have two sides to them, a polar “head” and a nonpolar “tail.” You may already know that polar compounds stick together and nonpolar compounds stick together. Because lipids have a nonpolar side, they tend to group up and form bubbles with their polar heads facing the outside, polar water.
If the lipids are in a container where the bubble can reach the sides, the lipids will line up to create a sheet. The heads will face the water outside and the tails will remain buried inside. If there is no container, the lipids will fold on themselves and create a bubble with an inner cavity. These bilayer bubbles are called liposomes and they form spontaneously!
Finally, a recent study has proposed a mechanism that would explain the spontaneous genesis of a molecule very similar to RNA. I say “very similar” because current genetic material, RNA and DNA, is much more advanced than what we would expect to form spontaneously. This early-RNA molecule had an ability to self-replicate.
I honestly have no idea how these things are able to replicate themselves, but that’s okay. We are allowed to have gaps in our story because there’s still more to discover. Not knowing all of something does not take away what you do know.
The reactions that are required in order to form amino acids, carbohydrates, and early-RNA all utilize catalysts that could be found in rocks at the time. This means that it is very likely that these organic molecules formed in different places and were washed away by rainwater to a common pool. We’re going to go with this theory because it seems most likely without being too serendipitous.
Now we have to turn to our friend, probability. If a huge, we’ll call it near infinite since we don’t know the time scale, amount of molecules are flooding into a common space, they are bound to run into each other. After enough collisions, we end up with a bunch of liposomes with random assortments of amino acids, carbohydrates, and nucleic acids inside the bubble. This is when natural selection begins.
Some bubbles will, by pure luck, have a collection of amino acids that can form structures to help prevent its destruction. The rest of them will do nothing until they are destroyed. This is a difficult thing to imagine because of the huge scale of probability, but just know that 99.9999% of these bubbles didn’t do anything and therefore did not “survive.”
All it takes is one. The one bubble with a collection of beneficial organic molecules “survived” and kept on existing. The longer a bubble was able to exist, the more likely it was to absorb even more organic molecules that could eventually give the bubble the ability to perform “life-like” functions.
Our definition of what is alive gets a bit skewed here since there is no definitive moment when a bubble becomes alive. The spontaneous formation of life was a gradual process of replication and natural selection.
I’ve reclaimed the term “miracle” to refer to occurrences that are so improbable that they should not have happened. The random collisions of molecules and the assortment of the right stuff created the first cell and life itself. This was a statistical miracle; there is no disputing it.