by Don Lincoln
For instance, alien life (and especially Alien life) will require a complex chemistry. Chemicals that perform analogous tasks to our familiar carbohydrates, proteins, DNA, and so on, will have to form molecules consisting of many, interlinking atoms. So two important considerations in the chemistry of life will be to identify atoms that (1) can make many connections to neighboring atoms and (2) can make strong enough connections so that the molecules are stable.
Students of chemistry have long been required to learn about valences, which is essentially the number of bonds the atom of any particular element can make. In order to make complex molecules, an atom will have to be able to connect to many nearby atoms. This can be made incredibly clear by considering the noble gas elements (helium, neon, argon, etc.), which inhabit the rightmost column in figure 6.2. These elements do not interact with other atoms. Each atom of the noble elements stands alone. They just don’t participate in chemistry at all. Consequently, we can be certain that these elements do not play a substantial role in any life-form’s metabolism and certainly do not have a structural role in any form of life.
FIGURE 6.2. The atoms that make up matter each have a personality, with varying abilities to make stronger and weaker bonds and even different numbers of bonds. This variation between the elements is central to understanding all of matter, including life itself. Chemistry students will find the location of hydrogen (H) to be a little strange, being used to seeing it head the column that includes lithium (Li) and sodium (Na). However, each hydrogen atom can be seen as able to donate or accept an electron to form a bond, thus it could naturally be put in either location.
We can then consider the column immediately to the left of the noble elements. This column—which includes hydrogen, fluorine, and chlorine— consists of atoms that can form one bond with a neighboring atom. Since all of these elements act similarly, we can illustrate the point considering just hydrogen. It’s kind of like a room full of one-armed people. They can hold hands with only one other person at a time. In a world in which hydrogen is a building block of life, you can only make very simple molecules, specifically ones consisting of identically two atoms. If hydrogen can form only one bond, then one atom of hydrogen bonds to a second atom. Both atoms form a single bond and the result is a two-atom molecule, as shown in figure 6.3. This is true for all elements in that column.
Moving one column to the left, we encounter the two-bond elements. The lightest example of these atoms is oxygen. Since oxygen can form two bonds, it can take on two hydrogen atoms. This is how water is formed, with an oxygen and two hydrogen atoms. Invoking our example of arms, oxygen is a two-armed element. It can hold hands with two hydrogen atoms or hold two hands with another oxygen atom. Moving again one column to the left, we encounter the three-bond elements. In a similar way, a nitrogen atom can connect with three hydrogen atoms and make ammonia.
FIGURE 6.3. This is a couple of ways to represent how hydrogen atoms (H) combine to make a hydrogen molecule (H2). The electrons of the two atoms are shared between them. On the bottom, we see a shorthand, with the atomic symbol standing in for the atom and a long dash (–) to represent the bond.
However, the column that allows for the most intricate molecular structures is the carbon one. Carbon and other elements in that column can form four bonds. Continuing our exploration of bonding with hydrogen, a carbon atom bonded with four hydrogen atoms makes a methane molecule. In our analogy of arms, nitrogen has three arms, while carbon has four.
Carbon (like any atom) can connect with more than simply hydrogen atoms. It can combine with other carbon atoms, as well as all of the other atoms of the periodic table. Mind you, this is also true of the nitrogen and oxygen columns, but it is the ability to make four bonds that allows the most complex molecules to be created. Figure 6.4 gives just a sense of the kinds of structures that become available when one has atoms that have this many bonding possibilities. These are the molecules of life on Earth.
Now you’ve probably already gotten ahead of me and thought, “But what about the other elements in that column?” After all, silicon can also form four atomic bonds. Is silicon-based life possible?
Certainly silicon atoms can compose complex molecules; however the situation is more difficult than simply replacing carbon atoms with silicon ones. As a simple example, consider the common carbon dioxide that we exhale as we breathe. Carbon dioxide is a gas, which makes it easy for the fluid (i.e., blood) in our bodies to transport it. In contrast, silicon dioxide is a solid, known by the more common name of “sand.” We will return to silicon-based life at the end of the chapter.
FIGURE 6.4. The different elements can participate in a different number of bonds, ranging from zero to four. The more bonds in which a specific element can participate has a large effect on the complexity of the molecules that can be formed.
Bond Strengths
While the number of bonds in which an atom can participate is a very important consideration, of equal importance is the strength of the bonds. The molecular and atomic world is a frenetic place, with constant motion being the norm. Due to simple heat, atoms vibrate, bounce into one another, and undergo a continuous stream of collisions. If the bonds aren’t strong enough, these atomic and molecular collisions could rip apart the molecules of life, just like a hard tackle in football can cause a fumble. Without a stable molecular environment, surely no life could exist.
We can understand this point in a visual way by considering one of those reality television shows where they come up with ridiculous competitions. Suppose this show is called “Togetherness” and the point is that two people are tied together somehow and they are to stay together for the entire season. If their connection fails, they are disqualified. Suppose one couple is tied together with ordinary sewing thread, while another is connected with the kind of rope that mountain climbers use. It doesn’t take much imagination to realize that the couple connected by a thread has a serious disadvantage. Just in the day-to-day to and fro of life, with walking around, brushing one’s teeth, sleeping, and so on, something is going to break that thread. In contrast, there is very little that the rope couple will encounter that will cause them to be separated.
There are a couple of ways that atoms can be bonded together, but the strongest is called a “covalent bond.” In a covalent bond, some of the electrons in each individual atom are shared between the two atoms. In a sense, the two atoms sort of fuse together into a single molecular unit. And these bonds are really strong. To give a sense of scale, two hydrogen atoms can bond this way to form a hydrogen molecule. The bond is so strong that if you took hydrogen gas at room temperature and pressure, you’d need a volume of gas the size of the Milky Way galaxy to have a 50% chance of breaking apart a single molecule into its two constituent atoms. These molecules are really hard to break apart. If they weren’t, a volume containing that many atoms would have many broken molecules.
Getting back to the question of which atoms are most likely to have a significant role in life, we can ask if different elements can form stronger or weaker bonds. It turns out that the lower-mass elements can form much stronger bonds than the heavier ones. The reason is a little subtle, but luckily not too hard to understand. It all boils down to the degree to which the atoms overlap one another. The larger fraction of overlap, the more those two electrons are shared and the stronger the bond. This point is illustrated in figure 6.5.
This figure is simplified, but has some valuable features. Atoms consist of a nucleus and then a swarm of electrons around the outside. The electrons closest to the nucleus (or in the lowest energy states, if you’ve taken a chemistry class) are not generally available to form bonds, while the outer few electrons are. In figure 6.5, I’ve chosen to represent the core, noninteracting, portion of the atom as a black dot. The outer white circle is intended to represent the electrons available to form bonds. You’ll note that I drew a small and large atom. For both atoms, the thickness of the white area is the same.
I then
graphically made molecules, by connecting two atoms together. To a degree, one can say that the atoms share the electrons in the region between the two atoms where the white areas overlap. This overlap region is indicated in gray. Now compare the gray region to the white region in small-atom molecules and large-atom ones. You see that in the small-atom molecules that the gray area is a larger fraction of the white area. Smaller atoms share their electrons with their neighbors a greater fraction of the time, which is the basis for the much stronger bonds in the lighter elements.
FIGURE 6.5. The strength of a covalent bond depends a lot on just how much the electrons from the atoms overlap. The larger the fraction of time they overlap, the stronger the bond. Here, the white area represents the electrons available for bonding, while the gray area represents the region of overlap. In smaller molecules, the gray area is a larger fraction of the white area.
These simple considerations show why it is somehow natural for life to be formed of carbon. Carbon can form four strong bonds with neighboring atoms, allowing the formation of complex molecules. Other light atoms cannot form as many bonds, reducing the complexity of the possible chemistry, while other heavy atoms cannot form as strong a bond, thereby reducing the probability that the molecules will be stable. Carbon is an optimum element for complex molecular chemistry.
It is, perhaps, unsurprising that we carbon-based life-forms would conclude that carbon was an ideal basis for forming life. This is called “carbon chauvinism.” We will return to this point when we’ve finished our overview of the important components of life and consider alternative chemistry.
Oxygen
All multicellular life on Earth uses oxygen as part of its respiration system, although this is not true of all forms of life. The role of oxygen is that it is a receptor of electrons. The movement of electrons is the source of the energy of life, so an element that can accept electrons is facilitating the flow of energy. Oxygen is a superlative acceptor of electrons.
Is the use of oxygen a necessary feature of life in the universe? Well, the answer is pretty clearly no, given that we know of life on Earth that uses other substances to breathe. In fact, we are quite confident that the first forms of life on Earth would have been killed by the presence of oxygen. So what is it about oxygen and why has it become such a ubiquitous presence on Earth now? Does the universal usage of oxygen by multicellular Earth life mean that oxygen breathing is universal?
FIGURE 6.6. How it is believed that the first living organism began and underwent speciation is shown here. Eventually all of the early branches of life died out except for one organism that was the last universal common ancestor, or LUCA. This diagram shows only the most basic points, as cross-species genetic mixing is thought to have occurred when the organisms were simpler.
It doesn’t, of course, but it’s worth spending a little time learning about the essentials of the role of oxygen in the history of life on Earth. We don’t know very much about the first life on Earth. Life formed and many species evolved and became more complex. As is usual with evolution, some species thrived, while others became extinct. It is thought that one of these complex organisms is the parent of all existing species, while the others died out. This parent being is called the last universal common ancestor, or LUCA. A family tree showing how life might have branched out is depicted in figure 6.6.
Working backward from today, biologists are quite confident that mankind shared a common ancestor with chimpanzees. That common ancestor shared an even earlier ancestor with other primates. The primates shared a common ancestor with other mammals. Moving backward in time, we now believe that each of the domains, kingdoms, phyla, classes, and so on, mentioned in the previous chapter originated from a common ancestor, whose descendants varied slightly and consequently set into motion the physical and biological differences we observe now in these different divisions of life. Each of the domains of Prokarya, Eukarya, and Archaea had a different common ancestor, although modern research suggests that Eukarya was formed by a mixture of earlier Archaea and Prokarya ancestors.
Taking the pattern one step further, there presumably was an organism who was the ancestor of all forms of life on Earth. Now this ancestor (the last universal common ancestor, or LUCA, mentioned above) was not the first form of life the Earth saw. Using comparative genetics and biochemistry, scientists have learned a lot about LUCA. For instance LUCA used DNA and a couple of hundred proteins to live. LUCA was already a very complex organism, quite different from the earliest form of life. It’s hard to know which adaptation from LUCA gave it the edge to survive and thrive, while all of its cousin contemporaries were doomed to extinction. But survive it did and here we are.
LUCA probably didn’t depend on oxygen for its respiration. While our understanding of LUCA’s biochemistry is incomplete, it seems to be true that iron was an important part of its metabolic pathways. This fact is pretty conclusive evidence that LUCA lived before the Earth’s atmosphere had a lot of oxygen in it. We know this as iron really loves to combine with oxygen into a form that is extremely insoluble in water. If there was a bunch of oxygen around, the iron would get gobbled up and pulled out of the ecosystem in the form of rust. As you’ve no doubt experienced, rust doesn’t dissolve and, once the iron is in the form of rust, it is unavailable for future use. In order for an organism to depend a lot on iron means that it must exist in an anoxic (low/no-oxygen) environment.
While the date of the formation of life on Earth is an ongoing topic of debate, the period of about 3.5 billion years ago is a credible position, and the evidence grows increasingly stronger after about 2.7 billion years ago. Studies of the isotopic composition of early rock suggest that before about 2.4 billion years ago, there was very little oxygen in the atmosphere. However, at 2.4 billion years ago, the amount of oxygen in the atmosphere began to rise. The source of the oxygen was presumably early photosynthetic bacteria. For about half a billion years, the iron in the ocean absorbed oxygen and settled out on the ocean floor. This process went on until the iron was entirely used up and is the source of the iron mines we now exploit.
Once the iron was used up, the oxygen in the atmosphere began to rise much more rapidly. As I mentioned, the source of oxygen was photosynthetic bacteria that had existed since the earliest forms of life, but, given oxygen’s reactive side, the oxygen was quickly bound to other substances in the ocean and eventually on the land. However, once these oxygen-loving materials in the sea and on the land were saturated, the oxygen concentration in the atmosphere increased. As the concentration of oxygen in the atmosphere grew, it encountered ultraviolet light from the sun. This led to the formation of ozone, which shields the Earth’s surface from ultraviolet light (and makes land-based life possible). Without ozone’s protection, the ultraviolet light would sterilize the surface of the planet, just like we use ultraviolet light to sterilize surgical instruments and to kill algae and parasites in fish tanks.
About 800 million years ago, the amount of oxygen in the atmosphere began to rise rather rapidly. This increase in oxygen is an oft-cited contributor to the origins of multicellular life (and, especially relevant to the idea of Aliens, animal life). The oxygen provided a large reservoir of a substance in the atmosphere that was an excellent acceptor of electrons and whose use in respiration and metabolism could generate lots of energy.
So oxygen is ubiquitous on Earth and plays a central role as part of all animals’ energy budget. The question when we think about Aliens is “is oxygen necessary?” We know of life on Earth that uses other substances as electron acceptors, with ferric iron, nitrates, sulfates, and carbon dioxide to name a few. However, these alternative forms of respiration are found in microbes, not multicellular animals, suggesting that the benefits of oxygen respiration are substantial and that evolution will likely nudge biochemistry in that direction if possible.
Even on Earth, the mechanism whereby oxygen is used to give energy to organisms isn’t a simple process but rather a multistep affair. Therefore it
is possible that on a planet with an anoxic environment, evolution would invent a multistep process to get the required level of energy necessary to support Alien life. However, given the benefits of oxygen, it seems plausible that life would eventually find out a way to exploit it if it is present. This brings us to the next point.
Chemical Abundances
The chemistry we have been discussing is partially academic at this point. For instance, it may well be that carbon is the perfect atom from which to build life, but, if there is no carbon around, then it won’t be used. Similarly, if there is no oxygen present, it makes it kind of hard to use it to breathe. So we need to add to our knowledge which elements are most present in the universe. To understand how certain elements are more or less common, we need to understand their origins.
Current theory is that the universe began just shy of 14 billion years ago in a cataclysmic event called the big bang. While the physics of the big bang is a fascinating topic, for our purposes, we merely need to know that the universe was once so hot that atoms couldn’t exist; indeed individual protons and neutrons couldn’t form, as the temperatures didn’t allow them to coalesce out of the bath of energy and subatomic particles that existed at the time.
As the universe expanded, it cooled in a way that is analogous to more familiar explosions, and very early in the history of the universe, protons and neutrons came into existence, followed by the elements hydrogen and helium. For all intents and purposes, no other elements existed. Following our discussion above, life couldn’t possibly form in that universe. Helium doesn’t form molecules, and hydrogen makes simple molecules consisting of two atoms. If that were the whole story, we wouldn’t be having this discussion. There must be more we need to consider.
Every morning as the sun rises, we are reminded of a seemingly trivial, but important, thing. The sun is bright and gives off heat. It does this because very dense collections of hydrogen and helium can undergo nuclear fusion. And nuclear fusion is one of the purest forms of scientific magic mankind has ever encountered and understood.