Endosymbiosis describes a close, long-term biological relationship where one organism lives inside another. This arrangement benefits both organisms, with the internal partner, called an endosymbiont, receiving protection and nutrients, while the host gains new capabilities. This process is a fundamental concept in biology, explaining major evolutionary steps in the development of complex life forms, including the intricate cells of plants, animals, and fungi.
The Origin of Mitochondria
Mitochondria, often called the powerhouses of the cell, are believed to have originated from ancient aerobic bacteria. These bacteria were engulfed by early anaerobic eukaryotic cells, which lacked the ability to efficiently use oxygen for energy production. The host cell provided a stable environment and nutrients to the bacterium.
In return, the engulfed bacterium provided the host with a more efficient method of generating energy through aerobic respiration, producing adenosine triphosphate (ATP). Over time, this relationship deepened. The bacterium gradually lost many of its independent functions and genes, transferring some to the host cell’s nucleus, evolving into the mitochondrion.
This transformation is supported by several distinct features of mitochondria. They possess their own circular DNA, much like bacterial chromosomes, and separate from the linear DNA in the cell’s nucleus. Mitochondria also contain ribosomes structurally similar to bacterial 70S ribosomes, unlike the larger 80S ribosomes in the host cell’s cytoplasm. Furthermore, mitochondria reproduce by binary fission, a process characteristic of bacteria, rather than through the host cell’s own division mechanisms.
The Origin of Chloroplasts
Chloroplasts, the organelles responsible for photosynthesis in plants and algae, also arose through a similar endosymbiotic event. This occurred after the initial acquisition of mitochondria, meaning the eukaryotic host cell already possessed the ability to perform aerobic respiration. An ancestral eukaryotic cell engulfed a photosynthetic cyanobacterium, a type of bacterium capable of converting sunlight into energy.
This engulfment led to a mutually beneficial relationship. The host cell gained the ability to photosynthesize, while the cyanobacterium was protected within the host environment. Over time, this cyanobacterium underwent significant changes, becoming the chloroplast.
Evidence for the cyanobacterial origin of chloroplasts includes their distinct circular DNA, resembling bacterial DNA and 70S ribosomes. Chloroplasts also contain internal membrane structures called thylakoids, highly similar to photosynthetic membranes in free-living cyanobacteria.
Evidence Supporting Endosymbiosis
Multiple lines of scientific evidence support the endosymbiotic theory for the origins of both mitochondria and chloroplasts. A significant piece of evidence comes from the genetic material within these organelles. Both mitochondria and chloroplasts possess their own DNA, which is circular, mirroring bacterial chromosomes, and distinct from the linear DNA in the eukaryotic nucleus.
Beyond DNA, the machinery for protein synthesis within these organelles also points to a bacterial ancestry. The ribosomes found in mitochondria and chloroplasts are 70S ribosomes, smaller and structurally similar to bacterial ribosomes. This contrasts with the larger 80S ribosomes in the eukaryotic host cell’s cytoplasm.
Another piece of evidence is their mode of reproduction. Mitochondria and chloroplasts replicate by binary fission, a process characteristic of bacteria, where a cell divides into two identical daughter cells. This replication occurs independently of the host cell’s mitotic division. Furthermore, both organelles are enclosed by a double membrane. The inner membrane is thought to be derived from the original bacterial cell membrane, while the outer membrane likely formed from the host cell’s engulfing membrane.
Impact on Life’s Diversity
Endosymbiosis shaped the diversity and complexity of life on Earth. The acquisition of mitochondria provided early eukaryotic cells with an efficient means of energy production through aerobic respiration. This increased energy availability was a prerequisite for the development of larger, more complex eukaryotic cells, paving the way for multicellularity and the diversity seen in animals, fungi, and various protist groups.
Similarly, the endosymbiotic acquisition of chloroplasts transformed life by enabling photosynthesis in eukaryotic lineages. This ability to capture energy directly from sunlight led to the evolution of plants and algae, which increased primary productivity on Earth and altered the planet’s atmosphere by releasing oxygen. These photosynthetic organisms now form the foundation of nearly all terrestrial and aquatic food webs. This event fundamentally reshaped life, leading to the evolution of all plant and algal lineages, which form the base of most food webs on Earth. Endosymbiosis was not a singular historical event; it continues to be a mechanism for evolution and adaptation, including secondary endosymbiosis and other ongoing symbiotic relationships.