The Endosymbiotic Theory
The Origins of Prokaryotic and Eukaryotic Cells
Some of the earliest life forms on Earth were prokaryotic cells, which first appeared around 4 billion years ago. Eukaryotic cells appeared around 1.8 billion years ago, and many scientists believe that all eukaryotic cells evolved from prokaryotic cells.
The Role of Organelles in Eukaryotic Cells
Eukaryotic cells are powered by organelles—specialized structures within the cell that act like batteries. All eukaryotic cells contain an organelle called the mitochondrion, which generates energy to power the cell. In plant cells, the energy-producing organelle is called a plastid, which functions like a solar battery by using sunlight to produce energy.
Mitochondria and Plastids: Evidence of Prokaryotic Origins
Scientists discovered that both mitochondria and plastids have their own DNA, separate from the cell’s main DNA. A detailed analysis of their genes revealed that they are more similar to those found in prokaryotic cells, supporting the idea that organelles are closely related to ancient prokaryotes.
Lynn Margulis and the Concept of Endosymbiosis
In 1967, Lynn Margulis published an article titled “On the Origin of Mitosing Cells” in the Journal of Theoretical Biology. Margulis proposed that three organelles—mitochondria, plastids, and basal bodies—were originally free-living cells that became part of primitive eukaryotic cells in a process called endosymbiosis. This symbiotic relationship gave rise to the energy-producing mitochondria and photosynthesizing plastids found in algae and plants.
How the Endosymbiotic Theory Explains Human Health and Evolution
The Endosymbiotic Theory also suggests that red blood cells evolved from plant cells through symbiosis. It provides insights into human health by explaining how aerobic bacteria enabled anaerobic cells to perform oxidative metabolism, and how photosynthetic bacteria gave cells the ability to photosynthesize.
The Relationship Between Humans and Plants
The theory highlights how humans have evolved to depend on plants. Around 400 B.C., Hippocrates said, “death sits in the bowels” and “bad digestion is the root of all evil.” Plants play a vital role in human health by biosynthesizing three of the nine essential amino acids that the human body cannot produce.
The Agricultural Revolution and Its Biological Impact
About 12,000 years ago, humans began domesticating plants and animals for agriculture in regions like Turkey and the fertile crescent of Mesopotamia. Beyond providing food, plants also produce chemical defenses that have shaped human biology and evolution over time.
