Chapter 1: The Origins of Photosynthesis

When photosynthesis first emerged, it was not the process we recognize today; it was characterized by a purple hue. Welcome to Part 16 of the ongoing series detailing the History of Earth.

Purple DM, © the Author

The theory, proposed by biologist Shiladitya DasSarma in 2007, suggests that the early Earth was indeed purple. However, to understand this concept fully, we must examine earlier stages of Earth's history.

In Part 12, we explored the topic of bacteria, highlighting that all living organisms possess a metabolism, which necessitates the consumption of nutrients. But why is this essential?

Living organisms must create organic molecules within their cells. To achieve this, they need to extract carbon, hydrogen, and oxygen from their surroundings. Essentially, it boils down to basic chemistry.

Humans achieve this by consuming carbohydrates produced by plants; we are classified as heterotrophs, with 'hetero' meaning other and 'troph' signifying food. In contrast, bacteria and plants are known as autotrophs, as they can produce their own sustenance from fundamental elements.

The earliest bacteria were likely chemosynthetic, deriving carbon, hydrogen, and oxygen from minerals. They then converted these into carbohydrates using energy from chemical reactions. Presently, certain bacteria thriving in hydrothermal vents at the ocean's depths utilize hydrogen sulfide to synthesize carbohydrates, producing sulfur as a byproduct.

Yet, the most apparent source of energy comes from our sun—the large, radiant ball in the sky.

It was inevitable that bacteria would eventually harness solar energy to facilitate chemical reactions within their cells. This remarkable process is known as photosynthesis, where the energy for creating carbohydrates is derived from light ('photo') instead of chemical reactions ('chemo'). In essence, these bacteria transformed from being battery-powered to solar-powered.

Today, we often associate photosynthesis with plants, which absorb carbon dioxide and release oxygen as a byproduct—a process termed oxygenic photosynthesis. However, photosynthesis can also occur in anoxygenic forms, where oxygen is not produced as a waste product. The earliest organisms to utilize photosynthesis may have employed this anoxygenic method, generating sulfur similar to chemosynthetic organisms.

Anoxygenic photosynthesis likely originated around 3.4 billion years ago. In contemporary plants, chlorophyll is the molecule responsible for photosynthesis, capturing energy from the red and blue light spectrum while reflecting green, giving most plants their characteristic color.

Interestingly, some primitive microbes, known as Halobacteria, utilize a different molecule called retinal to harness solar energy. A theory suggests that this molecule—or a similar variant—might have facilitated the initial form of photosynthesis. Retinal-based organisms absorb green light, reflecting the red and blue portions of the spectrum; hence, they appear purple to the human eye. This leads to the intriguing Purple Earth Hypothesis, which posits that wherever photosynthetic life existed 3.4 billion years ago, the landscape was likely purple.

While this notion may remain speculative, I find it a delightful thought. After all, who doesn't appreciate the color purple?

This article is Part 16 in a series of 50 that chronicles Earth's history, with each piece covering a 100-million-year period. If you wish to explore all the articles, please consider following along.

Chapter 2: The Role of Photosynthesis in Earth's History

In order to grasp the significance of photosynthesis, we must consider its role in shaping life on Earth.

The first video, "What Would The Earth Be Like Without Photosynthesis?" dives into the vital functions and implications of photosynthesis in sustaining life.

The second video, "Dr. Robert Blankenship - Evolution of Photosynthesis on Earth," provides an insightful look at the progression of photosynthesis throughout Earth's history.