Some of the oldest cells on Earth are single-cell organisms called bacteria.
Fossil records indicate that mounds of bacteria once covered young Earth. Some
began making their own food using carbon dioxide in the atmosphere and energy
they harvested from the sun. This process (called photosynthesis) produced enough
oxygen to change Earth's atmosphere. Soon afterward, new oxygen-breathing life
forms came onto the scene. With a population of increasingly diverse bacterial
life, the stage was set for some amazing things to happen.
The Endosymbiotic Theory
There is compelling evidence that mitochondria and chloroplasts were once primitive bacterial cells.
This evidence is described in the endosymbiotic theory. How did this theory get its name? Symbiosis
occurs when two different species benefit from living and working together. When one organism actually
lives inside the other it's called endosymbiosis. The endosymbiotic theory describes how a large host
cell and ingested bacteria could easily become dependent on one another for survival, resulting in a
permanent relationship. Over millions of years of evolution, mitochondria and chloroplasts have become
more specialized and today they cannot live outside the cell.
It's Just a Theory
In everyday speech, people use the word theory to mean an opinion or speculation not necessarily based on
facts. But in the field of science, a theory is a well established explanation based on extensive experimentation
and observation. Scientific theories are developed and verified by the scientific community and are generally
accepted as fact.
Mitochondria Have DNA
Mitochondria and chloroplasts have striking similarities to bacteria cells. They have their own DNA, which is
separate from the DNA found in the nucleus of the cell. And both organelles use their DNA to produce many
proteins and enzymes required for their function. A double membrane surrounds both mitochondria and chloroplasts,
further evidence that each was ingested by a primitive host. The two organelles also reproduce like bacteria,
replicating their own DNA and directing their own division.
Drawing the History of Evolution
Mitochondrial DNA (mtDNA) has a unique pattern of inheritance. It is passed down directly from mother to child,
and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA
has provided important clues about evolutionary history. For example, differences in mtDNA are examined to estimate
how closely related one species is to another.
Archaea: Models for Astrobiology
Conditions on Earth 4 billion years ago were very different than they are today. The atmosphere lacked oxygen, and
an ozone layer did not yet protect Earth from harmful radiation. Heavy rains, lightning and volcanic activity were
common. Yet the earliest cells originated in this extreme environment. Today, a group of single-celled organisms called
archaeabacteria, or archaea, still thrive in extreme habitats.
Astrobiologists are now using archaea to study the origins of life on Earth and other planets. Because archaea inhabit
places previously considered incompatible with life, they may provide clues that will improve our ability to detect
extraterrestrial life. Interestingly, current research suggests archaea may be capable of space travel by meteorite.
Such an event could have seeded life on Earth or elsewhere.
Genetic Science Learning Center. (2010, September 2) The Evolution of the Cell.
Retrieved May 22, 2019, from https://learn.genetics.utah.edu/content/cells/organelles/
The Evolution of the Cell [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2010
[cited 2019 May 22] Available from https://learn.genetics.utah.edu/content/cells/organelles/
Genetic Science Learning Center. "The Evolution of the Cell." Learn.Genetics.
September 2, 2010. Accessed May 22, 2019. https://learn.genetics.utah.edu/content/cells/organelles/.