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| A stylized Eukaryotic cell |
The average human body is estimated to contain between 50 and
100 trillion cells (that’s potentially as high as 100,000,000,000,000).
The strange thing about these trillions upon trillions of cells is that at
their most basic level they are, in essence, pretty much all the same. This
isn’t just a characteristic of our cells either; the same is true for all
complex life on the planet. At the most basic level there is little difference
between me, you, a giraffe or a whale. These building blocks of complex life
are known as ‘eukaryotic’ cells (from the Greek meaning ‘a good kernel’ –
having a nucleus) and due to their ubiquity among all complex life, Eukaryota (organisms
made of eukaryotic cells) are termed a domain of life (or an Urkingdom).
Eukaryota are one of the three domains to which all life on the planet can be
classified and are the youngest of the three. What I want to look at with you
here is where the Eukaryota domain came from.
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| A stylized Prokaryotic cell |
Alongside the Eukaryota Urkingdom, the two others are known as the
Prokaryota (again taken from Greek, this time having no nucleus) and Archaea
(‘ancient things’). These two Urkingdoms are thought to be a similar age and
date back to between 3.5 and 3.8 billion years ago – the birth of cellular life
on earth. Both kingdoms are mostly made up of simple, single-celled organisms.
Prokaryotes are bacteria such Escherichia
coli, Staphylococcus aureus, Neisseria meningitidis and so on. Archaea were
once thought to be bacteria and it wasn’t until the late 1970s that this was
shown to be incorrect. Before the 70s the best way to look at differences
between cell types was down a microscope to consider the morphology of the
cells. Archaea and Prokaryotes appear very similar, for instance bacteria do
not have a membrane-bound nucleus and neither do Archaea (whereas our cells do).
However in the 70s, work was conducted by Carl Woese and George Fox
to
look at molecular differences instead of morphological differences in what had
been classically thought of as bacteria and they found striking differences in
some organisms, which they termed the Archaea. Archaea are defined by their
ability to survive in extreme conditions (for example halophiles which live in
extremely salty conditions) and are thought to be the oldest form of life, in
part due to the harsh conditions present on Earth around 3.5 billion years ago.
So around 3.5 billion years ago the
Earth was inhabited by simple, single-celled life forms, the Archaea and Prokaryotes.
These life forms dominated the planet for about the next 1.5 billion years
until a giant evolutionary leap was made. This giant leap was of course the
formation of the eukaryotic cell. It is proposed that around 2 billion years
ago, an archaeal cell adept at phagocytosis (the process of taking smaller molecules
into itself) took in a smaller bacterial cell that was adept at using oxygen to
produce energy. These two cells then entered a symbiotic relationship (a
partnership in which both parties benefit) with the archaeal cell providing
nutrients for the bacterial cell, which in turn was acting as the power source
for the archaeal cell. Due to this new internal source of energy, the cell was
able to grow much larger and become more complex. The formation of this
symbiotic union was first proposed by Lynn Margulis in 1966 and is known as the
‘endosymbiotic theory’. The proposal was that, as a result of the energy
supplied by this bacterium, the cell was able to undergo a 200,000-foldincrease in its genome size, allowing it to
rapidly evolve and become more complex. It didn’t take long (in the grand
scheme of things) for the single cells to become multi-cellular (fossil records
indicate the first multi-cellular eukaryotic cell to be around 1.8 billionyears old) and from then on the only way was forward, becoming ever more
complex, until 2.5 million years ago the Homo
genus was formed (along with all other complex life). The rest, as they say, is
history.
As I mentioned, all the cells that
make up our bodies are eukaryotic cells, and you may be wondering what happened
to the bacteria that entered that ancient cell to drive the evolution of
complexity: in fact - we still have them. All the cells in our bodies (with a
few exceptions) contain a powerhouse known as mitochondria. Mitochondria are
the source of respiration, where the food we eat and the oxygen we breathe
react together to produce the energy we need to survive. These tiny energy
producers are the descendants of the bacterial cell engulfed by an archaeal
cell around 2 billion years ago. This theory is backed up by the fact that the
mitochondria in our cells have a genome of their own – that’s right, our cells contain
two genomes, our human one and a mitochondrial one which resembles a bacterial
genome (highlighted by the fact that the mitochondrial genome is circular, as
with many bacterial genomes, not linear like ours). Analysis has of course been
done on the mitochondrial genome and it was found to have an ancient relative
with bacteria of the α-proteobacterial genus, supporting the chimeric origin of
our cells.
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| A mitochondrion |
Further evidence to support the
endosymbiotic theory for the origin of eukaryotic cells can be found through
molecular comparisons of Archaea, Prokaryotes and Eukaryotes. An example of this
can be seen in the formation of proteins. Protein formation uses molecules
known as RNA, which are thought to be some of the oldest molecules, if not
actually the oldest – some believe the earliest form of life was a simple RNA
strand (a hypothesis gaining more support). Eukaryotes have a form of RNA used
to start the production of a strand of protein known as Met-tRNA. Prokaryotes
on the other hand have the addition of a small chemical group giving
formyl-Met-tRNA. Archaea have Met-tRNA, like eukaryotic cells, indicating the
close link between archaeal and eukaryotic cells.
So around 2 billion years ago, an
event so monumental occurred that it shaped the planet from that moment until
this, and will continue to do so for many more years to come. This event was
the beginning of complex life and it all stemmed from one simple moment, the
engulfment of a small cell by a larger one (something that occurs throughout
our body on a daily basis). I think we will be hard pressed to find such a
small event that has had such a giant evolutionary consequence as this one has.
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