According to most scientists today, before you answer a question about the past, you need to start with an assumption. That assumption is naturalism.
Naturalism – a philosophy that believes that everything in the universe is determined by natural processes we can observe with the scientific method. Physical matter and energy is all that there is, and all that controls the universe.
Naturalism is a foundational philosophy of the Theory of Evolution, which says that all living things are descendants of an original living thing – a single cell somewhat like bacteria. According to Evolution, then, the first living cell had to have come about as a result of entirely natural processes governed by laws that we can study today with the scientific method.
Before we consider what that process could have been, it’s important to lay some groundwork. Talking about the origin of life involves a lot of chemistry and biology on the molecular level. So, let’s start by defining life, and considering what the chemistry of life involves.
First of all, what sets life apart from non-life, from inanimate things? Standards may vary, but biologists tend to agree on the following things:
- All living things can sense and respond to their environment.
- All living things can reproduce.
- All living things have a means of converting matter around them and converting it into energy they can use.
- All living things have stages of growth and development.
- All living things contain DNA.
Consider a lizard. A lizard can sense if it's too hot outside, so it responds by scurrying under a rock where it can cool off. Lizards can reproduce, laying eggs that hatch into baby lizards. Lizards can convert matter into energy. We call that catching bugs (matter) and eating and digesting them (for energy). Lizards grow inside their eggs and then continue growing from baby lizards into mature adults.
A pebble does none of those things. It has no way of knowing if it's hot or cold, cannot reproduce to make more rocks, or convert matter into energy. There is a rock cycle, but there is no process for the development of rocks.
Finally, all living things (including our lizard) contain DNA. What exactly is DNA? Well, it is a molecule. Living things are chemical machines, mind you, and their cells function properly because of the well balanced and maintained chemistry inside of them. DNA is just one of those chemicals.
DNA is a long, spiral-shaped molecule at the center of every living cell, from bacteria to mushrooms to redwoods to people. It is made up of lots of smaller molecules that have bonded together. These molecules are called nucleotides, and there are four different types of them: Adenine, Cytosine, Guanine, and Thymine.
It turns out that the order of these four nucleotides is very important. These nucleotides are actually like letters of an alphabet, and the sentences they make contain the blueprint for life. DNA contains the information for every part of you: your hair color, eye color, sniffing ability, earlobe shape, height, propensity for certain diseases, metabolism, and all the internal structuring needed to keep chemical reaction going on inside your cells.
So if DNA is a code for life, how do cells read it? That is done by little workhorses called proteins.
There are many, many proteins inside of a person. Proteins are involved with how your brain responds to and thinks through problems, how your stomach digests food, how your muscles flex and relax, and how all the cells in your heart communicate with each other so that each cell is timed right in a heartbeat.
Proteins also are involved with copying DNA into a photocopy on RNA. DNA is very valuable, you see, so cells prefer to protect it in a special region and work with RNA's photocopy. Proteins then check RNA to make sure it's ok to leave the nucleus (where most cells store the DNA), then they read the RNA to make new proteins.
So that's what proteins do. What are they made of? Like DNA, proteins are also made up of lots of smaller molecules. DNA is made up of nucleotides. Proteins are made up of amino acids. There are twenty types of amino acids needed for life. And much like how the order of nucleotides codes for information in DNA, the order of amino acids in proteins gives each protein its own unique job.
Back to the question of the origin of life. In order to get that very first cell, according to the Theory of Evolution, you need a natural process to get proteins from amino acids, get DNA from nucleotides, and organize them together in the world’s very first cell. It just so happens that these organic compounds are incredibly tricky to work with.
Next Up: The Problem of Location.