The Evolution of the Cell
The Cells That Changed the Earth
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.
Bacteria are single-celled organisms with
a circular DNA molecule and no
organelles.
The Endosymbiotic Theory
There is compelling evidence that
mitochondria and chloroplasts were once
primitive bacterial cells. This evidence is
described later in the article. 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.
We think that around 2 billion years
ago,the only living organisms on Earth
were prokaryotes, single-celled organisms
lacking membrane-bound organelles.
Let's look closely at three single-celled
organisms living at the time. One was a
big, simple blob-like cell with the ability to
absorb things by wrapping its cell
membrane around them. Another was a
bacterial cell that converted solar energy
into sugar molecules through photosynthesis. A third used oxygen gas to break down materials like sugar
and release its energy into a form useful for life activities. The blob cells would occasionally absorb the
little photosynthetic bacteria. These bacteria then lived inside the blob and divided like they always had,
but their existence became linked. If you stumbled upon this living arrangement, you might just think that
the whole thing was one organism, that the green photosynthetic bacteria were just a part of the blob that
performed one of its life functions, just like your heart is a part of you that performs the function of
pumping your blood. This process of cells living together is called endosymbiosis, one organism living
inside another. But the endosymbiosis didn't stop there. What would happen if the other bacteria moved
in, too? Now the cells of this species started becoming highly complex.They were big and full of intricate
structures that we call chloroplasts and mitochondria.These structures work together to harness sunlight,
make sugar, and break down that sugar using the oxygen that right around this time started to appear in
the Earth's atmosphere.Organisms absorbing other organisms was one way species adapted to the
changing environmental conditions of their surroundings. This little story highlights what biologists call
the endosymbiotic theory, the current best explanation of how complex cells evolved.
Evidence of the Endosymbiotic Theory
There’s a lot of evidence that supports this theory, but let’s look at three main pieces. First, the
chloroplasts and mitochondria in our cells multiply the very same way as those ancient bacteria. If you
destroy these structures in a cell, no new ones will appear. The cell
can’t make them, they can only make more of themselves.
Mitochondria and chloroplasts therefore can replicate their own
DNA and direct their own division. This means that within a
eukaryotic cell, mitochondria and chloroplasts can make additional
copies of themselves if necessary.
The second piece of evidence… Chloroplasts and mitochondria
both contain their own DNA and ribosomes. There DNA has a
circular structure that is strikingly similar to the DNA of the ancient
bacteria and it also contains many similar genes. The ribosomes also
look and function the same way as those found in ancient bacteria.
Lastly, think about the membranes involved in the engulfing
process. Chloroplasts and mitochondria both have two membranes
surrounding them, an inner and outer membrane. Their inner
membrane contains some particular lipids and proteins that are not
present in the outer membrane. Why is that significant? Because
their outer membrane used to belong to the original bacterial cell.
When they were engulfed in the endosymbiosis process, they got
wrapped in that membrane and kept their own membrane as their
inner one. Surely enough, those same lipids and proteins are found
on the membranes of the ancient bacteria.
Biologists now use this theory to explain the origin of the vast variety of eukaryotic organisms.