If you do not accept the Biblical account of the beginning of Earth and how life started on our planet, naturalistic science is turned to by many for answers. There are many technical details in the secular theories about the origin of Earth and of life. Earth is generally thought by most scientists to be about 4.56 billion years old and the origin of life, though debated, is put at more than 3 billion years ago. So without a Creator-God to bring about life on our planet, how would it start? Evolutionary biology theories will not help answer this question, because there cannot be evolution till after life exists! The question is more about how the first simplest life forms could form from chemicals and survive on Earth. If life could start, there is a large presumption that biological evolution could proceed from then forward. Many scientists would also take this logic to the next step and say that if life could form on Earth from nonliving chemicals that the same could happen on extrasolar planets orbiting other stars. In the secular scenario for our planet, Earth began very hot and was bombarded for a long time with objects from space. These impacts provided the raw materials. Only after the impacts slowed down and the Earth’s surface cooled off could life have had a chance to get started and survive. Life as we know it requires organic chemicals. So scientists have proposed that at least some of the chemical compounds necessary for life could have come from space. The composition of comets and asteroids have been studied by astronomers, and meteorites found on Earth have been studied similarly. So this brings us to the questions about life starting on our planet. Could comets or meteorites help explain the origin of life?
The term “organic” used to mean substances that only came from living things. But as scientists learned more about chemistry they learned how to make organic compounds, like those that are in living things, but make them from basic constituents like water, gases in our atmosphere, and other compounds that are considered “inorganic.” Organic compounds are carbon compounds, that usually have four elements in them, including carbon, hydrogen, nitrogen, and oxygen. Organic compounds in a living cell have a three-dimensional arrangement that is important to the functions of molecules in the cell. There are often multiple forms of many organic compounds. Amino acids make up proteins and enzymes in living cells. But in living things, out of the many amino acids that are possible, only 22 of them are used to make proteins, and there is another very special requirement. Only the left-handed forms of these amino acids can be used in living things.
This “handedness” in organic molecules is very analogous to our hands. Both of our hands have the same parts, but because of the different arrangement of the parts of our left and right hands, they are mirror images of each other. The atoms in an organic molecule work the same way. This property of “handedness” is referred to with the technical terms “chirality” or “stereoisomerism,” and a few other terms. Sugars are also needed by living things and living cells require only the right-handed sugars. The terms “right-handed” and “left-handed” has to do with the order and three-dimensional arrangement of the atoms in the molecule. Amino acids are relatively small organic molecules with generally around 3 to 11 carbon atoms. But, proteins can be made up of a sequence of several hundred or more amino acids. In a real living cell, the organic compounds are often much larger molecules and they are much more complex. DNA contains a complex information code. So do a number of other molecules in the cell.
What Chemicals are in Comets?
Comets are made up of water ice, dust, and some rock. The ice in a comet is not only water ice but can include frozen organic compounds. As a comet gets near the Sun, it will form a tail. The ices in a comet are easily driven away and vaporized into space, unless trapped inside the object. In recent years there have been multiple missions to comets (and some to asteroids) by robotic spacecraft to get samples of the materials coming from a comet. Some chemical compounds detected in comets are listed below. The significance of these chemicals is in how they can take part in organic chemical reactions that form other organic molecules more important for life.
Carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), methanol (CH3OH), glycine (the simplest amino acid), formaldehyde (H2CO), Hydrogen Cyanide (HCN), Ammonia (NH3), also sulfur dioxide and sulfur monoxide
Organic chemicals have also been found in some meteorites. An example would be the Murchison meteorite, from Australia. It was observed to fall in Sept 28, 1969. It broke up into multiple fragments totaling 220 lbs. The Murchison meteorite was like a carbonaceous chondrite asteroid and contained a variety of carbon compounds, including both left and right-handed amino acids. It contained these four amino acids found in living things: glycine, alanine, glutamic acid, and isovaline. A total of 80 or more different amino acids have been found in this meteorite and scientists believe many organic compounds could possibly be in such an object. Carbon compounds with up to 9 carbons have been identified in the Murchison meteorite. There have been other meteorites with organic materials in them also but the Murchison meteorite is one of the most well known and most studied. An important point to understand is this. Even if amino acids found in living things are found in a meteorite or a comet, that does not mean that it came from living things in the comet or meteorite. There are known processes that can form these molecules in space.
How do organic chemicals get into comets?
Carbon is an amazing element because it allows so many kinds of bonds to make a virtually endless variety of organic molecules. When hydrogen, carbon, nitrogen, and oxygen are present then simple organic molecules can form but it depends on the conditions. It usually requires an energy source to drive or enable the reaction that forms an organic molecule. In space there is not usually much heat but ultraviolet light is often present and it can drive reactions that form organic chemicals. Saturn’s moon Titan contains many organic compounds, but they are pretty simple molecules. Titan actually rains methane and ethane in much the same the way Earth rains water. Guess which kinds of organic compounds are not in Titan’s atmosphere–amino acids and other bio-relevant compounds are absent. Some amino acids can be formed in space, such as especially glycine. What about the left and right-handed molecules? Outside a living cell, in inorganic reactions, handed molecules normally are found in a 50-50 mix, which is called a racemic mixture. If a chemist wants to create some L-alanine (left-handed) that has no right-handed molecules (the “D” form) in it, they can but it is not a simple process. Chemical reactions happening in nature would not normally give you more L forms than D forms of the molecule.
This is why scientists have searched for processes in nature, especially in space, that can cause an “excess” of either the L or the D form. There is at least one effect than can do this, which is what’s called circularly polarized light (CPL). Without explaining exactly what this is, it can be in the radiation from some stars. So if there is a nebula in space with amino acids that start as a racemic mixture, exposure to this polarized light can destroy more of one amino acid form than the other and you could get slightly more L amino acids than D amino acids, for example. If this happened in space, then the amino acids could later get trapped into a comet or other object when the nebula collapses into a solar system object. But the “excess” measured in these nebulas, for the L or D form of the molecule is typically only around 1%. The actual truth is that no one saw the comets or the asteroids form in the beginning, so it makes many assumptions to think that the organic compounds in a comet were once in a nebula in space. However, for the sake of the argument let’s assume this scenario is correct, that organic compounds from a nebula found their way into a comet or a meteorite that fell on the Earth about 3 billion years ago.
What happens to chemicals in comets or meteorites?
A comet near Earth in space would usually already have a comet tail due to its nearness to the Sun. So the organic chemicals in it, would tend to come out even before it got to the Earth’s atmosphere. So these organic chemicals would scatter across space but they could fall onto Earth’s atmosphere. Once in Earth’s atmosphere, they may just float around in Earth’s atmosphere, or they may eventually drift to the surface in a few years time. If the comet (or a meteor) made it to Earth’s atmosphere without all the organics being driven out, then the comet or meteor object would be strongly heated as it falls through Earth’s atmosphere at high speed. So this heat would also drive out organics and would probably change them. They would tend to be decomposed into smaller molecules from the heat, unless they were protected inside it. But, even if some of these organic chemicals were not destroyed as the object fell through the atmosphere, one comet or meteorite would not put a lot of chemicals on the surface of the Earth. So it would require many many comets and meteorites. This is the story modern science is telling, that there were millions of years of impacts that made organics accumulate. After it reaches the surface (or the ocean) rain and other processes would tend to wash the organics out of the object or scatter them on Earth.
So on Earth organic chemicals would get naturally diluted in many ways. Ultraviolet light from the Sun tends to destroy organic chemicals on the Earth, even if they are in shallow bodies of water. In order for simple organic chemicals from a comet or meteorite to form some molecule larger and more important for life, the organic compounds have to get concentrated somehow and there has to be a source of energy to drive the chemical reactions. There may be heat available in certain places on Earth. But when you mix a variety of organic chemicals together, they have a tendency to break each other down, or they form something that is not related to what’s needed for life. If there is nothing to control the process, what you get from the chemical reactions is not something more like a cell but is more like useless chemical junk. The chemicals used by living things have very specific requirements that make them difficult or impossible to form under natural conditions. If a chemist were forming a compound relevant to life, such as a protein made up of left-handed amino acids, they would have to follow very specific steps and it would be very challenging. Many of the molecules in living cells are something even very accomplished chemists would not know how to make in a lab. Chemists have never made anything remotely approaching the complexity of a real living cell. Science related nontechnical articles written for the public often make a lot of exaggerated claims about what scientists have actually done related to the origin of life. So the organic chemicals in a comet or meteorite can form some other organic chemicals under the right conditions, but they are a long way from being like the molecular structures that make up a cell.
The Jump from Chemicals to Life
All the above is to make the point that it is very unlikely for biologically relevant molecules to form even if the raw organic materials did come from space. However, for the sake of the argument, let’s skip this problem and assume that the important biomolecules did accumulate and somehow get concentrated together under conditions conducive to life on the early Earth. There is still a huge problem, which is the information and organization in living things. Even if you had all the right molecules, how can they form in the right sequence? Proteins are a good example. Proteins in living things are formed from many amino acids combining in a certain sequence. The sequence makes a complex code, similar to a language. If you have many of the exact same molecule placed end-to-end this is referred to as a polymer. But in a polymer, there is little information content because the individual units are all alike and they don’t have to be in a particular order. Pop beads are similar to a polymer because all the beads are alike and they go together the same way. But a protein molecule is different. First, out of the many amino acids that exist, only 22 different amino acids make up the proteins in living things. Also, right-handed amino acids won’t work, they must all be left-handed amino acids. Chemical reactions do not normally naturally form only left-handed molecules exclusively like this. Also in a protein, the sequence of the amino acids will determine the three-dimensional shape of the molecule. Organic molecules often fold, and the shape of the molecule after it folds will determine what it can be used for in the cell. If the sequence is wrong, the shape is wrong and then it may be useless in the cell, or it can even be destructive.
If the sequence for so many molecules in a cell is so specific, where does the information for the sequence come from? Science cannot answer this kind of question. It requires a great intelligence, like an infinite Creator-God. The laws of chemistry and physics cannot provide a source for the how-to-make-it information. The information in DNA is vastly complex beyond human understanding. Scientists have mapped the raw sequence of human DNA, but that does not mean the information in it all is totally understood. In living cells, DNA has multiple types of codes, not just one. The cell also mechanisms to detect if the DNA code is incorrect and then correct it. This is fantastically complex.
So if organic chemicals came to Earth on comets and meteorites (or asteroid impacts long ago), does this explain how life formed on Earth? No. We should be honest about what science cannot do. Biblically the raw materials for life on Earth do not really have to come from far away in space. God created the Earth before other solar system objects, so if Genesis 1 is true, how could life on Earth come from space? In the creation week in Genesis, plant life was created before the Sun, Moon, and stars. But animals were formed after the Sun, Moon, and stars. When Earth was formed (Day 1 of creation week), there were no comets or asteroids yet, apparently. The first life came from God by miraculous creation, not from natural processes over billions of years.