Find the common theme among these things: a good joke, a well-hit baseball, a long touchdown pass, a moving piece of music. Give up? They all rely on precise timing. Each clearly requires accurate execution of a task. However, if that execution happens too early or too late, the joke fails, the ball goes foul, the pass drops incomplete, and you get cacophony. On the other hand, play all the musical instruments together and you get a great symphony. Timing also plays a critical role in Earth’s capacity to support human life.
As discussed earlier, Earth started out hostile to life. The surface was dark, covered in water, and had no structure or form. We now know that the process of plate tectonics added the structure and form so critical for life today. In fact, over two-thirds of life lives on either the continents or the continental shelves. Scientists have tried to understand when and how this important process started. What they found was that it has not always operated in the same fashion and when (and how) it changed likely led to Earth’s friendliness to large, complex animal life.
Scientists have long-known that plate tectonics acts like a thermostat, regulating the surface temperature of Earth. It also affects what kinds of rocks are exposed to the atmosphere. Cyanobacteria, which produce oxygen via photosynthesis, have been around for more than three billion years. However, Earth’s atmosphere developed a permanent oxygen component only 2.5 billion years ago.
Recent discoveries may provide a reason for the delay. Looking at continental crust with ages as ancient as 3.7 billion years, scientists found that the composition changed significantly between 3 billion and 2.5 billion years ago. Continental crust older than 3 billion years contains an abundance of iron-rich and magnesium-rich (mafic) minerals. As these minerals react with water, they release chemical compounds that scour any free oxygen from the oceans and atmosphere. From 3 billion years onward, the continental crust shows a gradual transition toward (felsic) minerals enriched with silicon, oxygen, aluminum, and similar light elements. Around 2.5 billion years ago, it reached a composition intermediate between felsic and mafic, similar in composition to crust seen today. In other words, continental crust maturing from its primitive, mafic state to a more mature, felsic state coincides with the oxygenation of Earth’s atmosphere.
Scientists want to explain what might have caused this maturation of the continental crust, but still don’t have a good understanding. One possible clue arises when looking at hafnium isotopes from continental crusts (more specifically from zircons contained in the crusts) with ages between 3.2 and 3.9 billion years. Prior to 3.2 billion years ago, zircons show a depleted abundance of hafnium isotope composition compared to zircons in crust older than 3.5 billion years. The shift indicates that the actual processes generating continental crust changed and that the reservoir of material that ultimately became continental crust changed.
Even without a good understanding of why the continental crust composition changed, the timing was critical. If the shift had happened later, all the iron- and magnesium-rich material would have sequestered virtually all of Earth’s oxygen deep inside the planet. Consequently, none would remain to produce the free oxygen in the oceans and atmosphere. On the other hand, an earlier change would have hastened the great oxygenation event in such a way that Earth may have plunged into a permanent global snowball.
Starting from the initial conditions over four billion years ago, Earth has experienced extraordinary changes. The Sun was 30% dimmer compared to today. While initially lacking free oxygen, the atmosphere now contains over 20%. Earth’s early single-celled life was joined by incredibly complex, large-bodied animals. Sometimes lay readers associate plate tectonics with harm today, but these processes transformed Earth’s surface from no continents to roughly 30% continental coverage and operated in concert with the atmosphere, the Sun, and Earth’s organisms to maintain Earth’s ability to support abundant life. The timing was impeccable. It looks like all of Earth’s “instruments” knew exactly when to join the symphony.