Shining a Light on Earth’s Oldest Toxic Threat?

Abstract

Between 3.4 and 3.8 billion years ago, life happened. While scientists debate whether life first emerged on the planet’s surface in an organic “primordial soup,”¹ was helped along by meteorites,² or evolved in the ocean’s black smokers,³ there is little doubt that it developed on a planet flooded with massive amounts of ultraviolet radiation (UVR). UVR is a highly energetic and destructive force that may have actually aided evolution. UVR-induced mutations induced in deoxyribonucleic acid (DNA) struck at the very heart of life, and when not lethal, may have accelerated the pace of change. (UVR may even have played a key role in the formation of RNA nucleotides, possible precursors to DNA in the earliest stages of life’s formation.)⁴ In either case, high mutation rates would have resulted in precarious conditions requiring protection if life was to evolve into the complex forms known today. In a highly coordinated process essential for both reproduction and survival of all living things (and viruses), DNA bonds break and reform, transferring, capturing, and storing chemical energy. Owing to their molecular structure, nucleotides like DNA and RNA are chromophores. That is, their chemical nature causes them to readily absorb UV light (more than, say, proteins), and so they are particularly susceptible to UVR, which, in sufficient amounts, breaks DNA’s bonds and modifies life’s genetic code.⁵ If not repaired, these UVR-induced changes cause permanent mutations or death. Today we rely on the earth’s stratospheric ozone layer to protect us from excessive UVR, but more than three billion years ago, when life first emerged, there was no ozone layer. The threat posed by UVR was far more hazardous then than it is now. This potentially lethal interaction between life’s basic genetic code and UVR makes it one of the first known physical toxicants still relevant today. As such, it is a good place to begin our exploration of the evolution of toxic defense.

To humans, the Earth is a photobiologically protected haven encircled by a fragile O₃ shield that has been perturbed by the activity of industry. This view of the recent perturbation of the O₃ column is certainly accurate, but it is clear that the Earth has been subjected to quite varied UV regimens throughout history.

Charles S. Cockell