Wednesday, 21 September 2011

In the beginning evolution created the eukaryotic cell

If you take the book of Genesis literally, then 6000 years ago God took all of 6 days to create the earth and everything on it, including man. Now I’m pretty sure most people reading this blog will agree with me when I say that we shouldn’t take the book of Genesis literally. The earth is in fact closer to 4.6 billion than 6000 years old and it wasn’t until about 2.5 million years ago that the genus Homo (to which we belong – Homo sapiens) was first seen. However, unlike in the book of Genesis, humans did not just appear - there were many tiny evolutionary steps along the way. The biggest and arguably the most important, was the formation of the building blocks of all complex life.

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.

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.
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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