Evolution and the Primordial Soup

One of the biggest challenges presented to evolutionary biologists is the question of how it all started, how the supremely complex molecular engines we call life emerged from seemingly lifeless chemicals. Yet, as more and more is learned about the natural arrangements that arise every day all over the universe, it becomes increasingly apparent that the emergence of complex organic molecules that can order themselves and reproduce, and therefore the development of life, was an inevitable consequence of a chemical universe. First, we can start with the proportions of elements found in the universe, and their relative importance to life.

Top 10 Elements in Order of Abundance in the Universe (atomic number): Hydrogen (1), Helium (2), Oxygen (8), Carbon (6), Neon (10), Iron (26), Nitrogen (7), Silicon (14), Magnesium (12), Sulfur (16). The reasons for the relative abundance of these elements in relation to the other hundred or so known to exist becomes apparent when you realize that new elements are made in the searing fusion at the heart of stars. As young stars burn and consume their fuel (our own sun consumes about 4 trillion tons of hydrogen per day) they form heavier elements in relation to the number of protons contained in their nucleus (since the heart of the sun is so hot, electrons exist as a part of the roiling plasma). So, as stars burn their fuel, they build heavier atoms in stages; first by combining hydrogen to make helium, then when the fuel is exhausted, by burning helium to get other elements in multiples of 2, (such as oxygen and carbon), with a few oddities thrown in like a nitrogen, which forms through the interaction of odd numbers of protons. Throughout this process, elements are continuously cast off into space to speed around the universe, mixing with other castaways, adrift in the cosmos, and generally disturbing the carefully established equilibrium. Stars do not make any elements heavier than iron with any particular abundance until they eventually go supernova and explode with an intensity greater than 10 trillion suns. This is because the cost of iron fusion, and therefore the formation of anything larger than iron, is an endothermic process with a net energy consumption. This means that iron-based stars are nearly dead (but again, “dead” generally means “ready to blow up in a spectacular fashion”).  But I digress.

If we look closely at this list one more time, we find five of them absolutely essential to building the full palate of organic molecules: hydrogen, oxygen, carbon, nitrogen, and sulfur. Three of them are quite important to the function of complex life: iron, magnesium, and silicon. And the remaining two are noble gasses, which don’t do much of anything. Now, it can be seen that an abundance of hydrogen and oxygen means one important thing, water is absolutely everywhere in the universe. The similar abundance of oxygen and carbon and nitrogen means basic organic molecules form easily in gas clouds all over the place. And since it is from these gas clouds that planets, moons, asteroids, and comets are formed, it should be no surprise that the earth was likely rich in organics building blocks even before it cooled.

I will skip the next billion or so years, when the earth was struck with so many meteors, so geologically active, and so constantly bathed in interstellar radiation as to preclude the formation of all but the most simple molecules from forming. Although, these events also gave rise to the stratified nature of our planets elemental organization and probably brought most of the water we enjoy so much today down from the great beyond. But, on to the reason for my post. Scientists have recently observed ribozymes (RNA based molecules with catalytic properties) that can reproduce themselves in a nutrient rich environment. Not only this, they have demonstrated that these molecules can evolve on their own when presented with novel environmental conditions, adapting and competing with each other for resources. They do not call these molecules life, as they do not demonstrate “…the capacity to invent a whole cloth of functions,” but this is merely semantics. The molecules provide some of the most compelling evidence for the RNA world hypothesis of early life on earth; an exciting piece to the evolutionary puzzle.