Lesson 3: The Essence of Life 3.1 Life Is All Around Us Figure 3.1.1 Life is all around us! One of the great discoveries of childhood is that plants and animals produce food for people. In all cultures, this fact has been a powerful motivation to learn about the living things around us. That we are not just consumers but part of a web of living support systems is less readily obvious, especially in an environment where food comes in highly processed form, in boxes and cans. Practically all of our food (excepting salt) comes from organisms, mostly from domesticated plants and animals, but also from microbes, at least in part (yeast, yogurt, cheese, beer, wine). A few types of plants are prominent in providing food (of American origin: potato, maize). Grass seeds are especially dominant (rice, wheat, rye, etc.). Much of the plant material we eat can only be processed by our guts with the help of bacteria. Among animals, poultry and mammals dominate the scene (eggs, milk, meat). Whatever our cattle eat is processed with the help of bacteria. Fish and other seafood is important locally. Some of the earliest known money consisted of sea shells, and some of the earliest tools (in the so-called "stone age") were made of wood and bones and antlers, while houses were made of plants and of bones. In summary, we are entirely dependent on other life for sustenance, and these forms include plants and animals and also different types of bacteria. In turn, the plants and animals we depend on rely on other organisms to maintain them: plants rely on fungi and bacteria for nutrients, animals on plants for food and on bacteria for digestion. We are not some isolated freak species on this planet. We are part of a network, an entire system. What sets us apart is having an unusual number of unusually active nerve cells hungry for processing information. So, we gather information from the farthest corners of the visible universe. Before we can assess what are the chances of Life being present elsewhere in the Universe, we should contemplate how Life came to be present around us. Let us examine this question step by step, in the simplest possible manner. First, the presence of Life in the Universe, like that of stars, or dust or gas, is a fact. We know so because we are alive. Thus, it is a fact that matter can organize itself in such a way as to produce living things. Second, the life on this planet is a form of carbon chemistry. It starts with photosynthesis, which takes carbon dioxide and water and makes it into complicated carbon compounds, which get incorporated into living things. After death, complicated carbon compounds get degraded into simpler ones, whereby many bacteria make a living. The processes associated with the conversion of carbon compounds are tied to the use and release of energy. Photosynthesis uses sunlight, solar energy. The reverse process (combining carbon compounds with oxygen) releases energy. (We note this when getting overheated while running, and also in the warmth generated within a compost heap.) Figure 3.1.1 One of the unique features of carbon is that is forms "chiral" molecules. A "chiral" molecule comes in a right- and left-handed form, each the mirror image of the other. As can be seen in the animation above, there is no way to rotate the forms so that they can be superimposed. Many of the important molecules for living things (including DNA) are chiral. Third, there is plenty of carbon around. It is one of the common elements produced in stars (its nucleus contains precisely three helium nuclei). It readily combines with other elements (with hydrogen to make methane, with oxygen to make carbon dioxide, with nitrogen to make cyanide) and with itself, to make chains and rings and branching molecules and complicated structures made of such parts.All of these have a exceptional stability that is not found with any other element! Carbon is special in the Universe. No other life forms, built on a fundamentally different kind of chemistry, have been found. Thus, carbon chemistry is most likely the basis for any life forms elsewhere. Fourth, there is currently no spontaneous generation of life, as far as we know. All life forms are generated by one or more parent organisms. Until a few hundred years ago it was believed that maggots could spontaneously appear in rotting meat. (This was easily observed, actually.) The Italian physician Francesco Redi (1626-1697) made an experiment to show that appearances are deceiving. He prevented flies from getting to the meat. The meat still rotted, but no maggots. Maggots only come from fly eggs, which come from flies, which hatch from maggots. Much later, the celebrated French chemist Louis Pasteur (1822-1895) (who had found out earlier why wine can turn sour) proved that decay is only produced by microorganisms, not by some "vital principle." In a simple but clever experiment he admitted air to boiled meat extract in a bottle, but gave access only by way of a long narrow neck bent into a U-shape. This allowed access to any "vital principles" in the air but prevented dust particles (and microbes) from entering the flask. No decay. No "vital principle." Fifth, at some point prior to perhaps 3.8 billion years ago, life appeared on Earth. The scientific study of how this happened constitutes the field of exobiology, and we shall examine its progress later in this course. Either life originated on Earth or it came here from elsewhere. The latter hypothesis is called panspermia, especially in the case of "seeds of life" permeating the universe. To exobiologists, this is simply transposing the event to another location. The problem is still the origin of life, here or elsewhere. The chemistry (carbon-based, using the same amino acids and replication techniques) and other factors such as the "handedness" of life's molecules strongly suggests that all surviving life originated from a single parent replicating molecule. Life is all around us, and in us, because we are an intimate part of it.