Around two billion years ago a single bacterium had a meal that changed the direction of life on this planet. It's a story of slavery, co-dependence and power.
By Bernie Hobbs
The first supper: complex life could not have evolved if one bacterium hadn't eaten another one, two billion years ago.
Life on Earth was nothing to write home about for the first billion or so years. It was bacteria and archaea (single-celled creatures that made bacteria look flash) as far as the non-existent eye could see.
But about two billion years ago there were some new kids on the block. Eukaryotes. They were still just single cells, but they were huge, and they were totally souped up. They were the cells that all animals, plants and fungi evolved from.
Instead of just being a walled cell with a circle of DNA, like their primitive bacterial/archael neighbours were, eukaryotic cells were organised into sub-compartments. They kept their DNA in a nucleus, and had separate membrane-bound compartments for making and putting the finishing touches on their proteins. And they also had the first known slaves: mitochondria.
Mitochondria provide the power in almost all eukaryotic cells. They use oxygen to rip the energy from food, and then use the energy to make ATP, the universal fuel for life.
Mitochondria are not the only option cells have for getting energy from food, but in terms of efficiency they leave the other methods for dead. And every eukaryote that's ever existed owes its mitochondria to a single ancient act of bacterial enslavement.
Two billion-odd years ago, one of the most important meals in history took place. One bacterium swallowed another one. But instead of being digested, the swallowee survived. And it kept doing what it had always done: using oxygen to rip apart food molecules, and then using the energy released to make ATP. So the bacteria that did the swallowing suddenly had this little lump inside it that leaked ATP, which the swallower could use to power its own cellular reactions. It was a match made in thermodynamic heaven.
And this crazy hybrid was the great grandmother cell that all eukaryotic cells evolved from. The mitochondria in your cells, mine and every plant, animal and fungi on the planet are descendents of that meal. It's like slavery, but with benefits.
From studies of the DNA in mitochondria we know that the swallowee was an α-proteobacteria, a lot like the kind of primitive bacteria (Rickettsias) that cause typhus today. And the swallower was probably one of the many bacteria around at the time that couldn't use oxygen to break down its food. They're called anaerobes, and their energy-mining methods come a distant second to the oxygen-using variety.
The world had recently become saturated in oxygen (around 2.4 billion years ago), possibly because another kind of bacteria (cyanobacteria) had been spitting the stuff out as it photosynthesised.
Oxygen is one very reactive molecule, and it chews through cell membranes, so swallowing something that can not only soak up the oxygen, but also give off energy was a total bonus. And for its part, the α-proteobacteria got a nice supply of nutrients and moisture from the host cell. Over time (of a geological scale) the descendents of the happy couple became more co-dependent, until eventually one couldn't survive without the other.
The cycle of life
This swallowing for mutual benefit is called endosymbiosis. And it's not just the story of how we got our mitochondria.
The other powerhouse organelle here on Earth is the chloroplast. Plant cells are loaded with them — the green chlorophyll pigment in chloroplasts gives plants their colour, as well as soaking up energy from sunlight.
Chloroplasts are built to photosynthesise, which is almost exactly the opposite of what mitochondria do. While mitochondria rip food apart with oxygen to release energy, chloroplasts use the sun's energy to make food and spit out oxygen. (Actually they use the sun's energy to make ATP, and because ATP doesn't store well they use it to make food, which we and a bunch of other animals and bugs proceed to eat, and turn back into ATP. It's the cycle of life).
Chloroplasts have got more in common with mitochondria than being able to make ATP — they ended up in plants and algae because of the second-most important meal in history: when a eukaryotic cell swallowed a cyanobacterium.
Cyanobacteria can photosynthesise, so when a eukaryotic cell swallowed a cyanobacteria somewhere in the distant past, it suddenly had a lump inside it that could make food as long as the sun did shine. Not only that, the lump spat out oxygen at the same time — oxygen that the eukaryote's now well-established mitochondrian could soak up to release energy. And it was a lovely green to-boot!
So the two great energy suppliers of animals and plants — mitochondria and chloroplasts — are as ancient as life itself, and if they hadn't started living with the enemy eukaryotic cells all the complex beings that followed couldn't exist.
It's not exactly slavery because both the swallower and the swallowee benefit.
Where's the evidence?There's no single piece of killer evidence that proves the case for the bacterial origins of mitochondria and chloroplasts, but there's no shortage of smoking guns.
For starters, mitochondria and chloroplasts look a lot like bacteria — they've got a single circle of DNA, an inner membrane of the same type as a bacterial cell wall, they're about the same size, and their genomes closely resemble those of Rickettsia and cyanobacteria. On top of that, they reproduce the same way bacteria do (by splitting in two), and not just when the cell does. And the cell can't make a chloroplast or mitochondrian — if you remove them it doesn't have the genes to build new ones.
What is the theory of how eukarytotic cells came to have mitochondria and chloroplasts?
Why was it necessary for eukaryotic cells to have a way of removing oxygen?
Explain the term endosymbiosis to the evolution of eukaryotic cells with chloroplasts and mitochindria.
A photosynthetic cyanobacteria gave rise to
What evidence exists to support the endosymbiotic theory for the evolution of mitchondria and chloroplasts?