Why Are Leaves Green? The Story of a Purple Earth
While biology teachers might explain the green color of leaves through chlorophyll and photosynthesis, there's a more profound story rooted in the ancient history of life on Earth. The green color itself is a consequence of plants reflecting green light, which seems counterintuitive given that green light is the most abundant in sunlight. Why would photosynthetic organisms reject this plentiful resource?
The Purple Earth Hypothesis
In 2007, microbiologist DasSarma proposed the "Purple Earth Hypothesis" to explain this phenomenon. This hypothesis suggests that early Earth, billions of years ago, was dominated by archaea that used a pigment called retinal to harvest sunlight. This retinal pigment strongly absorbs green light, giving these organisms a purple hue and painting the early Earth purple.
The First Cells and the Quest for Energy
The story begins in the deep-sea hydrothermal vents, where the first two major branches of life, bacteria (true bacteria) and archaea, originated. As these early cells moved away from the vents, they faced the challenge of finding new energy sources. While resources like carbon dioxide and sulfide were available in the primordial oceans, the energy from hydrothermal vents wasn't.
Retinal and the Rise of Archaea
Evolution led to the discovery of retinal by archaea. Retinal allows archaea to use solar energy to maintain a proton gradient across their cell membranes, mimicking the chemical gradients found in hydrothermal vents. This process fueled their cellular activities. This light-harnessing ability enabled archaea to flourish and spread across the globe, creating a world dominated by purple organisms.
The Green Shift: Bacteria and Chlorophyll
Beneath the "purple blanket" of archaea, bacteria struggled to access the limited sunlight. They evolved a different type of pigment called chlorophyll. Chlorophyll weakly absorbs green light, thus appearing greenish-blue. This pigment allowed bacteria to utilize the remaining light energy, though sparsely available, under the purple archaea layers. Early bacteria also relied on hydrogen sulfide, but access was scarce under the archaea.
Cyanobacteria and the Great Oxidation Event
Eventually, cyanobacteria emerged. These bacteria developed a complex photosynthetic system allowing them to split water molecules (H2O) and extract hydrogen for organic matter production, releasing oxygen as a byproduct. This marked the dawn of true photosynthesis and ignited a major turning point in Earth's history. Over billions of years, the cyanobacteria slowly released oxygen into the environment.
Oxygen as a Poison and a Catalyst for Change
The increasing oxygen levels eventually exhausted the Earth's reducing agents like iron and sulfide. Around 2.6 billion years ago, free oxygen began accumulating in the atmosphere, leading to the Great Oxidation Event. Oxygen, toxic to anaerobic life, wiped out many organisms, including the purple archaea. Oxygen also destroyed methane, a potent greenhouse gas. This led to a significant drop in global temperatures and a period of intense glaciation, the Huronian glaciation.
Adaptation and the Rise of Complex Life
Despite the widespread devastation, some organisms adapted to the oxygen-rich environment. They developed aerobic respiration, a more efficient energy-generating process. This advancement paved the way for the evolution of complex cells and multicellular life. Furthermore, the accumulating oxygen formed an ozone layer, protecting Earth from harmful ultraviolet radiation and allowing life to colonize land.
The Legacy of the Purple Earth
Today, the legacy of the purple Earth can be found in certain extreme environments like the salt ponds near San Francisco Bay. These ponds are colored pink by halophilic archaea, descendants of the ancient purple archaea. These archaea still use retinal to harvest energy from green light and survive in conditions where their bacterial competitors cannot. They are a living reminder of a forgotten era, a time when Earth was bathed in purple light.