ZOOHCC - 602: Evolutionary Biology (Theory) unit:1
Origin of photosynthesis
Photosynthesis is the process by which organisms convert light energy from
the sun into chemical energy in the form of glucose. Scientists believe that
the origin of photosynthesis dates back over 3 billion years ago and that it
was the first major energy-converting metabolic pathway to emerge on Earth.
Although the exact origin of photosynthesis is still debated, the prevailing
theory is that it evolved in ancient bacteria called cyanobacteria, some of
the earliest organisms on Earth. Unlike other bacteria, cyanobacteria have
the ability to perform oxygenic photosynthesis, which splits water molecules
to produce oxygen as a byproduct. This process is thought to have played a
critical role in the oxygenation of Earth's atmosphere and the evolution of
complex multicellular life.
Explanation
Photosynthesis is the only significant solar energy storage process on Earth
and is the source of all of our food and most of our energy resources. An
understanding of the origin and evolution of photosynthesis is therefore of
substantial interest, as it may help to explain inefficiencies in the
process and point the way to attempts to improve various aspects for
agricultural and energy applications.
A wealth of evidence indicates that photosynthesis is an ancient process
that originated not long after the origin of life and has evolved via a
complex path to produce the distribution of types of photosynthetic
organisms and metabolisms that are found today (Blankenship, 2002; Björn and
Govindjee, 2009). An evolutionary tree of life based on small-subunit rRNA
analysis. Of the three domains of life, Bacteria, Archaea, and Eukarya,
chlorophyll-based photosynthesis has only been found in the bacterial and
eukaryotic domains. The ability to do photosynthesis is widely distributed
throughout the bacterial domain in six different phyla, with no apparent
pattern of evolution. Photosynthetic phyla include the cyanobacteria,
proteobacteria (purple bacteria), green sulfur bacteria (GSB), firmicutes
(heliobacteria), filamentous anoxygenic phototrophs (FAPs, also often called
the green nonsulfur bacteria), and acidobacteria (Raymond, 2008). In some
cases (cyanobacteria and GSB), essentially all members of the phylum are
phototrop2hic, while in the others, in particular the proteobacteria, the
vast majority of species are not phototrophic.
Overwhelming evidence indicates that eukaryotic photosynthesis originated
from endosymbiosis of cyanobacterial-like organisms, which ultimately became
chloroplasts (Margulis, 1992). So the evolutionary origin of photosynthesis
is to be found in the bacterial domain. Significant evidence indicates that
the current distribution of photosynthesis in bacteria is the result of
substantial amounts of horizontal gene transfer, which has shuffled the
genetic information that codes for various parts of the photosynthetic
apparatus, so that no one simple branching diagram can accurately represent
the evolution of photosynthesis (Raymond et al., 2002). However, there are
some patterns that can be discerned from detailed analysis of the various
parts of the photosynthetic apparatus, so some conclusions can be drawn. In
addition, the recent explosive growth of available genomic data on all types
of photosynthetic organisms promises to permit substantially more progress
in unraveling this complex evolutionary process.
While we often talk about the evolution of photosynthesis as if it were a
concerted process, it is more useful to consider the evolution of various
photosynthetic subsystems, which have clearly had distinct evolutionary
trajectories. In this brief review we will discuss the evolution of
photosynthetic pigments, reaction centers (RCs), light-harvesting (LH)
antenna systems, electron transport pathways, and carbon fixation pathways.
These subsystems clearly interact with each other, for example both the RCs
and antenna systems utilize pigments, and the electron transport chains
interact with both the RCs and the carbon fixation pathways. However, to a
significant degree they can be considered as modules that can be analyzed
individually.
Evolution of eukaryotes
Coacervate:
Coacervates are aggregates of colloidal particles that spontaneously form
under certain conditions, such as changes in pH or salt concentration. They
are thought to be a possible model for the formation of the first cells on
Earth. Coacervates are non-living structures that can form spontaneously in
aqueous solutions. They are typically composed of a mixture of organic
molecules, such as amino acids, sugars, and lipids.
Protobiont:
Protobionts are the hypothetical precursors to living cells. They are
believed to have formed from coacervates by acquiring a selectively
permeable membrane. This membrane would have allowed the protobiont to
maintain a distinct internal environment, separating it from its
surroundings.
Prokaryotes:
Prokaryotes are single-celled organisms that lack a membrane-bound nucleus
and other complex cell structures. They are the simplest and most ancient
forms of life, with fossils dating back billions of years. Prokaryotes are
found in a wide range of environments, from soil to water to the human body.
They are divided into two groups: bacteria and archaea.
Eukaryotes:
Eukaryotes are organisms that have cells with a nucleus and other
membrane-bound organelles, such as mitochondria and chloroplasts. They are
more complex than prokaryotes and are found in a wide range of environments,
including water, soil, and the human body. Eukaryotes include animals,
plants, fungi, and protists. The origin of eukaryotes is still a topic of
debate among scientists, but it is believed that they evolved from a
symbiotic relationship between prokaryotes. This theory is known as the
endosymbiotic theory.
Endosymbiotic origin of eukaryotes
( Endosymbiosis means is a type of symbiotic relationship between two
different species of organisms in which one organism lives inside the other.
In the case of the endosymbiotic theory of the origin of eukaryotic cells,
it refers to the idea that mitochondria and chloroplasts were originally
free-living bacteria that were engulfed by other cells, and over time
evolved a mutually beneficial relationship with their host cells. The term
"endosymbiotic" is derived from the Greek words "endo," meaning "inside,"
and "symbiosis," meaning "living together." )
The endosymbiotic theory is a widely accepted explanation for the origin of
eukaryotic cells. According to this theory, eukaryotic cells evolved from a
symbiotic relationship between two different types of prokaryotic cells,
specifically an archaeon and a bacterium.
It is believed that the first step in this process was the uptake of a
bacterium by an archaeon. The bacterium was most likely a member of the
alpha-proteobacteria, which are known to be closely related to the
mitochondria found in eukaryotic cells. Once inside the archaeon, the
bacterium was able to survive and reproduce, providing the archaeon with an
additional source of energy in the form of ATP (adenosine triphosphate.
Over time, the bacterium became dependent on the archaeon for protection and
nutrients, and the archaeon in turn became dependent on the bacterium for
energy. This mutual dependence led to the formation of a stable symbiotic
relationship between the two cells.
Through a process called endosymbiosis, the bacterium eventually became
integrated into the archaeon's cell membrane, forming a new type of cell
known as a eukaryotic cell. The bacterium eventually evolved into the
mitochondrion, which is responsible for producing energy in eukaryotic cells
through aerobic respiration.
Another symbiotic relationship may have led to the development of the
chloroplasts found in photosynthetic eukaryotic cells. It is believed that a
eukaryotic cell engulfed a cyanobacterium, which was then retained as a
permanent endosymbiont. This symbiotic relationship eventually led to the
evolution of chloroplasts, which are responsible for photosynthesis in
eukaryotic cells.
The endosymbiotic theory provides a compelling explanation for the origin of
eukaryotic cells and the complex cellular structures that define them. It
also highlights the importance of symbiosis in the evolution of life on
Earth.
