My family vacationed in the California redwoods and the giant sequoias this past summer. Seeing these majestic trees towering above the surrounding landscape invokes a sense of awe and wonder along with a feeling of smallness. Just to give a perspective on the size of a sequoia, I calculated that one tree has enough wood to make 10 billion toothpicks! I still enjoy visiting the scenic groves of sequoias and redwoods, but I am reminded that for much of Earth’s history, nothing remotely resembling such amazing life could exist.
When Earth formed, it started as a wasteland, hostile to life. Even as life began to flourish on the planet, Earth lacked an ingredient essential to everything but single-celled life-free oxygen. In fact, more than two billion years elapsed until the atmosphere contained any oxygen. Yet even as oxygen established a permanent foothold on Earth around 2.5 billion years ago, it would take another two billion years for large, multicellular life to appear in abundance. Two recently published studies provide some reasons for this long gap.
Abundant evidence points to a Great Oxidation Event 2.5 billion years ago that introduced a permanent oxygen reservoir in the atmosphere. However, it appears that the amount of oxygen present at different times exhibited quite a bit of instability—with the amount often remaining below the requirements for multicellular life.
One study looked at cerium concentrations in 1.87-billion-year-old carbonate deposits. Cerium reacts readily with oxygen, forming compounds that will precipitate out of water. Changing oxygen levels will produce anomalies in the amount of cerium in the carbonates. Using this tool, the team of researchers found that the amount of atmospheric oxygen during this time period was less than 0.1% of current levels and that the oceanic oxygen was confined to the top 50–100 meters. Such low levels of oxygen preclude any multicellular life.1 Additionally, it appears that the oxygen levels remained low for most of the Proterozoic period (between 2.5 billion and 542 million years ago).
The second paper helps explain the low oxygen levels by studying oxygen isotopes in 1.4-billion-year-old sedimentary sulfates. Comparing the ratios of 18O, 17O, and 16O, the researchers could determine the amount of oxygen produced by photosynthesis. This value measures the activity of the primary producers (organisms that convert light into stored energy via photosynthesis). The 1.4-billion-year-old sulfates show the smallest ratio of 17O anywhere in the Proterozoic except for the period of extreme glaciation from 720–635 million years ago. This result means that the low fraction of atmospheric oxygen happened because there were fewer photosynthetic organisms living.2 The episodic pulses of complex life in Earth’s history can be linked to the presence of available oxygen brought on, in part, by energy-converting organisms.
Scientists know that oxygen plays a critical role in the ability of complex, multi-cellular life to live and thrive. However, the reverse is also true, the abundance of life influences the amount of oxygen in the atmosphere. These studies add more data to support the idea that Earth’s amazing capacity to host a thriving biosphere requires a complex interaction of biological, atmospheric, astronomical, and geological (think plate tectonics) processes.