Changes in the Atmosphere Causing Multicellularity


About 2.5 billion years ago, oxygen began slowly to accumulate in the
atmosphere, as a result of the photosynthetic activity of the cyanobacteria.
Those prokaryotes that were able to use oxygen in ATP production gained a strong
advantage, and so they began to prosper and increase. Some of these cells may
have evolved into modern forms of aerobic bacteria. Other cells may have become
symbionts with larger cells and evolved into mitochondria. As the amount of
oxygen and other atmospheric gasses increased, they started blocking out deadly
u.v. rays from the sun. The sunís rays made life outside of water nearly
impossible. These changes made life on land possible and evolution occurred as
prokaryotes gave rise to land living eukaryotes.
The microfossil record indicates that the first eukaryotes evolved at least
1.5 billion years ago. Eukaryotes are distinguished from prokaryotes by their
larger size, the separation of nucleus from cytoplasm by a nuclear envelope, the
association of DNA with histone proteins and its organization into a number off
distinct chromosomes, and complex organelles, among which are chloroplasts and
mitochondria. Scientists believe that eukaryotic organisms such as the protists
evolved from the prokaryotes. There are two main theories which describe how
this transition may have occurred. The first is the endosymbiotic theory, or
enosymbiosis, and the other is the autogenous theory, or autogenisis. These two
theories are not mutually exclusive, meaning one or the other could account for
different parts of eukaryotic cells. The endosymbiotic theory states that the
formation of eukaryotic cells were symbiotic associations of prokaryotic cells
living inside larger prokaryotes. The endosymbiotic hypothesis accounts for the
presence in eukaryotic cells of complex organelles not found in the far simpler
prokaryotes. Many modern organisms contain intracellular symbiotic bacteria,
cyanobacteria, or photosynthetic protists, indicating that such associations are
not difficult to establish and maintain. Endosymbiosis is said to be
responsible for the presence of chloroplasts and mitochondria in eukaryotes.
Autogenisis, the alternative to the endosymbiotic theory is specialization of
internal membranes derived originally from the plasma membrane of a prokaryote.
Autogenisis could be responsible for structures like the nuclear membrane and
endoplasmic reticulum in eukaryotes.
There are two scenarios for which multicellularity may have occurred. The
first is unicellular organisms came together to form a colonial organism, then
some tissue developed specialized functions and the cells became differentiated,
forming a multicellular organism. The other scenario starts with a coencytic
organism forming cellulorization with individual cells developing membranes,
then tissues became more specialized forming a multicellular organism. There
are some advantages of multicellularity such as having specialization of cells
which creates a division of labor, leading to greater efficiency. Another
advantage is multicellular organisms have a larger size which provides
protection from predators. Fungi are large and have a large surface area to
volume ratio, allowing them to absorb nutrients more efficiently.

Category: Science