Tuesday, 27 January 2009

A Brief History of Life

In order to reach the level of life that we have reached, we have had to progress from more primitive forms. We are vertebrates and so have a spinal cord. The earliest known occurence of something with a spinal cord is in a type of animal which has remained unchanged for millions of years: The sea-squirt. Adult sea-squirts are pretty basic tubular filter feeding creatures which remain stuck to the same place for their whole life, but the larval form of a sea squirt looks like a tadpole and possesses the crucial spinal cord. It is believed that around 550 million years ago, some of these sea-squirt tadpoles never found a rock to attach to, and instead remained in their fishy form, went on to evolve into fish and eventually into us.

Lets take it back even further. Before we evolved to the multicellular tadpole stage, we would have had to have been a single celled organism. Cells like ours with a nucleus that contains the genome, are known to exist as far back as 2.7 billion years ago. Lets go back further, before our cell can wrap its genome in a nucleus, you need a cell with loose genetic material, like modern bacteria. The oldest fossil record of bacteria is from 3.5 billion years ago. This is only a billion years after the earths crust cooled from boiling magma to solid rock.

What we have in common with bacteria is that we are vehicles for our DNA to replicate itself. We are a lot bigger and more complicated, but we do the same thing, we duplicate our DNA and we pass it on to a new generation. So where did this DNA come from? We make DNA with proteins and enzymes that are made with instructions from the DNA, so which came first? At this point its easier for me to just jump right to the start, its not that far away now.

Once the earths crust formed the oceans began to form, natural reactions began in the salty mineral rich water and organic chemicals began to form. These organic chemicals provide the well known 'primordial soup'. This soup would collect and become concentrated in the rock pools on the coasts, in here the first amino acids would have formed, as well as the first nucleotides. Amino acids are the basic building blocks of proteins. Nucleotides are the basic building blocks of RNA and DNA. These can assemble themselves and interact with eachother, eventually some proteins will randomly have developed a structure that enabled the nucleotide chains to be copied. Other proteins could use nucleotide chains to produce a new protein. Suddenly we have the first instance of replication.

As this chemical replication proceeded, in its very early form it would have been imprecise and many errors would occur. But errors are good in biology, errors produce variation, variation produces differences in performance. The best performing combinations of proteins and nucleotides would become the most common. The complexity of the combinations would have increased with time. The single strands of RNA formed complimentary double strands, and acquired functions that outperformed the primitive proteins. Some of these RNA functions still exist within our cells. For a while the world was dominated by the RNA chains, with help from proteins.

Eventually the RNA would have found a performance boost by being isolated within a bubble of oil, the products of its work being kept close instead of washing away into the ocean. This is essentially the cell membrane that us and bacteria possess. The complexity of the replication reaction has taken another step and inside the bubble complexity grew ever greater. Sometimes however, other more primitive RNA systems might get into the bubble, and take advantage of the resources there, we call them viruses today. Later proteins became more complex and outperformed RNA which was replaced with DNA and life started to look like something we would recognise.

Thats a heavily summarised account of the most accepted theory of how life developed. But at what point can we say life actually commenced? Do simple chemical reactions count as life? Is it the basic replication where life starts? The fact is that we are the product of a basic chemical reaction that began more than 4 billion years ago, and inside us the reaction continues, growing ever more complicated as long as it enables us to reproduce ourselves better. We are only aware of our environment because awareness helps us to find food, survive and reproduce. Every aspect of human nature can be related back to how it helps us ensure successful propagation of our DNA. With the global human population nearing 7 billion, we're certainly doing quite well, but not nearly as well as those bacteria, there are a hundred trillion of them in your gut alone, and bacteria will be around long after humans are forgotten.

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