Wednesday, October 7, 2009

Why Bacteria Invented Sex

Bacteria are the ultimate survivors. They've been around for three and a half billion years. Longer than the continents, and all the mountain ranges on Earth. In contrast, the animal kingdom has been here only a paltry six hundred million years – one sixth of that time.

Bacteria are the simplest of creatures being as they were the first of all creatures to exist. They are single-celled. They are so small,that they are invisible to the naked eye. But they are not solitary creatures and they live in huge communities that are sometimes visible in the form of coloured blobs of slime, the kind you might see in a petri dish or on rotting food. It's easy to look down on them as disgusting and smelly but without them we would not exist.

It is bacteria, more than any other creature that has altered the face of the Earth – breaking down it's rocks, and adding to it's atmosphere gases such as carbon dioxide - which is crucial to Earth's temperature regulation, methane, and oxygen – without which we animals could not survive.

The technical name for bacterial cells is “Prokaryotes” , which refers to the fact that bacteria don't have a nucleus. All “Eukaryotic cells” - the kind of cells that we have in our bodies – have a nucleus. The nucleus contains all our genetic material wound up like the rubber strands in a golf ball only way tighter – if the DNA in the nucleus of one of your cells was unwound it would stretch to the moon and back several times.

That extra genetic material and the protective membrane means that eukaryotic cells can do more things and make up more complex multicellular organisms like us.

In contrast bacteria don't have as much genetic material. And the genes are looser, not packed as tight because they are not surrounded by a nuclear membrane.

But, bacteria have an advantage over eukaryotic cells. Bacteria are the original party animals. They love to hang out in huge numbers, and they're not so particular that they have to hang out with their own species. They love to mix and when they mingle they can easily exchange genetic material with whoever they please. They have no shame and the whole thing is over in seconds. Which means that bacterial evolution has been fantastically quicker than the evolution of eukaryotes

This bacterial promiscuity is the basis for genetic engineering, by the way. Because it's so easy to get bacteria to take on different genetic material bacteria can be given genes that will manufacture just about anything we want. That kind of stuff is just not possible for eukaryotic cells, thank God.

Technically speaking, “sex” is the exchange of genetic material between different organisms. It isn't necessarily tied to reproduction the way it is for bigger eukaryotic creatures. Bacteria aren't male and female because they pre-date reproductive sex.

It's immaculate conception. Bacteria can split all by themselves, And go on making millions of exact copies of themselves. They don't need love. But they need protection from toxins.

Ultraviolet light coming from the sun is hazardous to life. It can damage biological molecules. It damages DNA molecules,the genetic material for all forms of life. In Microcosmos, Lynn Margulis and Dorion Sagan's book about microbial evolution, the authors hypothesize about this connection: “The pressure to patch up damaged DNA or die induced the development of DNA repair systems. Sometimes instead of using healthy copies of their own genetic material, crowded bacteria borrowed DNA from their neighbours.”

“By adapting to life under harsh light the microcosm had invented sex. Though this first sex was different from the kind of sex animals are involved in, it was sex all the same...”

This was probably their most important means of evolving rapidly in the face of environmental danger. We now know why it was so easy for bacteria to aquire resistance to deadly toxins like anti-biotics. Bacterial sex takes advantage of the natural variety in the population to provide resistance to new toxins.

It's easy enough to say that bacteria should have just kept away from the sun's dangerous rays, and then they would have survived. Because if that's all they ever did they would never have developed photosynthesis and then we wouldn't exist. It's autopoieses. Life maintains itself. And life that survives over time does so because it adapts to Earth's changing environment. In next week's article we will discover how life can also change the odds for itself in a positive direction by altering the environment.

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