For those of you who have read the remarkably funny books by Douglas Adams, The Hitchhiker’s Guide to the Galaxy, you know that the answer to the ultimate question about the secret to “life, the universe and everything”, is “42”. However, there are a few other details that might also matter to the secret of life, besides 42. By the “secret of life”, I’m not talking about philosophy or “how to be happy” or anything like that. I’m talking about the actual beginning of life on Earth, and this article will wind up our initial exploration of the properties of water as we examine the role that water played on the process.
We mentioned in an earlier article that the Earth is roughly 4.5 billion years old. That is a pretty solid figure, based on precise measurements of radioactive substances in rocks. We will actually be going into radioactivity and that sort of thing in a future article, so there’s little point in trying to make any sense of it right now. Just trust me that the age of the Earth has been reliably determined. What I find to be way more fascinating than that (although how the Earth came to be is a really interesting story, in itself) is that life first appeared on this planet really quickly (by geological standards) after that.
For most of my lifetime, the origin of life of Earth was determined to be about 3.5 billion years ago. That means that it “only” took about a billion years for life—very simple life forms—to develop in the incredibly hostile environment of the early Earth. However, there has been some recent research that suggests that life might actually have evolved even earlier than that—about 4 billion years ago. This new study will have to stand up to the scrutiny of the scientific community before this new information becomes accepted, but it’s a fascinating possibility.
The early Earth was a violent, hot place. There was little to no atmosphere, and what there was of the atmosphere was completely unlike the one we have now. It was mostly volcanic gasses, hydrogen sulfide (which is the really stinky “rotten egg” smell), methane and lots of carbon dioxide. There was no oxygen (because most of our oxygen only appeared after photosynthetic plants started to spread. The crust of the Earth started cooling, and by about 4.1 billion years ago, it had cooled enough for water, mostly from comet impacts (most comets are gigantic cosmic snowballs), to start to collect on the surface. This is hugely important because, as we mentioned before, chemistry generally takes place in solution, meaning the different things that might combine with one another have to be dissolved in a fluid. Water is a very good solvent.
So, at about 4.1 billion years ago, the Earth was still very hot, there was some water, and there were other chemicals like sulfur, carbon, hydrogen and nitrogen present, making what is often described as a “primordial soup”, which is a great environment for chemical reactions to occur. Atoms bonded to other atoms to form simple molecules. Simple molecules combined with other to form slightly more complex molecules, and so forth. At some point, these molecules combined to create the first very simple life forms. It’s hard to define “alive”. I will do an entire article on this later on. It’s one of my favorite subjects to mull over. However, though it’s difficult to define “alive”, we have managed to come up with the characteristics that all living things share. Some people keep it simple, with 7 or 8 characteristics. Some get a little further out in the weeds with details and say there are as many as 18 characteristics. I’m a fan of simplicity, so I’m going to say that the characteristics that are shared by all living things are: 1) they reproduce, they have a mechanism to pass information from generation to generation (heredity), they have a cellular structure, they grow and develop over their lifespan, they can respond to stimuli (such as heat or light), they adapt over time through evolution, they can maintain an internal environment that is separate from the external one, and they process and use energy.
The first life forms were very, very simple composed of only a few different kinds of molecules, but they could do all of those things, and, if the new study is correct, that process only took about 500 million years to happen on the early Earth. 500 million years is just a drop in the bucket.
Now that we’ve established a sort of timeline, let’s get back to the role of water in all this. As we mentioned, water is a great solvent, which allows all these chemicals to dissolve into it and come into contact with each other, which allows for the new molecules to form. This need for such a solvent is why the search for water on other planets and moons is such a big deal. Without water, life as we know it couldn’t develop because the biochemical reactions necessary couldn’t occur. But wait…THERE’S MORE!
Water was necessary for the primordial soup, so that’s important. However, there is something that is possibly even more important to us. If life started 4 billion years ago as these very simple forms in the hot, inhospitable environment of the early Earth, how did we get from there to here? Part of the answer is time. In 4 billion years, lots of things can happen. Remember, one of the characteristics of all life is that it adapts and evolves. Simple life forms adapt to their environments by evolving into more complex forms that are better able to survive and reproduce. Over time, the Earth cooled. The atmosphere changed as plants (mostly algae in the early times, long before land plants appeared 750 million to a billion years ago) took carbon dioxide out of the atmosphere and started to add oxygen. We talked a bit about how the hydrogen bonding of water molecules allows for water to get from the roots to the tops of plants. That was a key to land plants developing. If water couldn’t move through capillary action, plants could never have grown much over ground level. There’s one more, really important property of water that I like to think might be the secret to life as we know it.
We talked about how hydrogen bonding makes water molecules arrange themselves in an orderly fashion, while in most substances, the molecule just pack together. We’ve talked about energy, too, and how energy is most often seen in the form of heat. When you add heat to something, you add energy. The energy makes the atoms and molecules vibrate more quickly. As they vibrate more quickly, they move slightly away from each other. This is why things expand as they get hot, like a bridge getting a little longer when it’s hot than when it’s cold. The concrete of a highway or a piece of steel does the same thing. When things cool off, the molecules vibrate less and they move closer together, so the material contracts. Water does this, too, mostly. When you heat water up, the molecules move faster and faster. Eventually, at about 100 degrees C (212 degrees, F), the molecules are moving so fast that they begin to break away from the solution, forming a gas (steam) and floating away. If you capture that steam, as a still does, and cool it, it will recondense into water. If you continue to cool the water, the molecules will move slower and slower, and the volume of the water will decrease. This is just like steel or concrete or most anything else. However, right at the point of freezing, water does something amazing. Because of the hydrogen bonds, when the water molecules get closer and closer as the water gets to the freezing point, the electrical repulsion between the hydrogen atoms and between the oxygen atoms pushes the atoms slightly apart. This is why water expands when it freezes. This is why you need antifreeze in your car. This is why ice floats. Water, like everything else, gets denser and denser as it cools, right up until just freezing, when it expands again, just slightly. That makes ice slightly less dense than water. It’s just a little difference, which is why 90% of an iceberg is under water and just the tip is sticking out. It’s not a big difference, but that difference is why complex life forms (like most of us) were able to evolve on Earth. If water was like everything else, oceans, lakes, ponds and other bodies of water would freeze solid, in the cold. If that happened, everything in the water would die every winter (or every Ice Age). None of the life would have the opportunity to live long enough to evolve. But, because ice floats, bodies of water freeze from the top down. The layer of ice provides insulation to the water beneath, and although it gets cold, any large-ish body of water won’t freeze solid and life goes on, evolving year after year.
Source: Freepik
So, if someone asks you what the secret of life is, you can say, with some justification, that the answer is “ice floats”. Obviously there are all sorts of other potential answers, as well, but my first answer is always “ice floats”. My second answer is usually “42”, but there are some people who haven’t read “Hitchhiker’s Guide”, and they just look at me like I’ve lost my mind. Which can also be fun.
This brings our examination of water to a end, for now. It will come up, over and over, as we keep learning about the science and technology of the universe we live in, but that will be a different story.