Plants gained the ability to photosynthesize through a process called endosymbiosis, approximately 1.5 billion years ago. Single-celled organisms called protists engulfed photosynthesizing bacteria. This wasn't a simple act of eating; the bacteria were incorporated into the protist cell, establishing a symbiotic relationship. Over time, this symbiotic relationship became permanent, with the bacteria evolving into the chloroplasts found in plant cells today. These chloroplasts are the organelles responsible for carrying out photosynthesis.
The Endosymbiotic Theory
The endosymbiotic theory explains how this remarkable event occurred. It proposes that:
- An ancestral protist engulfed a photosynthetic bacterium.
- Instead of digesting the bacterium, the protist and bacterium formed a mutually beneficial partnership. The bacterium provided the protist with energy through photosynthesis, while the protist provided the bacterium with protection and nutrients.
- Over millions of years, the bacterium and the protist's genetic material became integrated, leading to the evolution of chloroplasts as permanent organelles within plant cells.
This process is not unique to plants; it's a significant event in the evolution of eukaryotic cells, cells with a nucleus and other membrane-bound organelles. Similar endosymbiotic events are believed to have led to the development of mitochondria, the energy-producing organelles found in almost all eukaryotic cells.
Evidence Supporting Endosymbiosis:
- Chloroplasts have their own DNA, separate from the plant cell's nucleus, mirroring the DNA of free-living bacteria.
- Chloroplasts have a double membrane, consistent with the engulfment process.
- The size and structure of chloroplasts are similar to that of photosynthetic bacteria.
This "theft," as described in the provided text, highlights a key mechanism of evolutionary innovation: the incorporation of existing biological components to create new functionalities. This remarkable event fundamentally changed the course of life on Earth, enabling the flourishing of plants and the oxygenation of the atmosphere.
