This is without doubt one of the great landmarks in science in the late 20th century. It is a turning point in our understanding of one of the greatest of mysteries - Life.

It is the complete genetic code of a multi-celled animal, a transparent worm or nematode called Caenorhabditis elegans. Under the microscope as it sweeps its head gracefully from side to side, it is not hard to see how the tiny creature got the "elegans" part of its name.

Yet this tiny spec of protoplasm holds the same secrets of life as other animals and it is soon to yield them. Reading its complete genetic code is an event that will have profound implications for biology and medicine. Someday we will all owe something to that tiny worm.

Why a nematode?

C. elegans has achieved its place in history because it was chosen to be a so-called "model organism". Along with fruit flies, zebra fish and mice, it can be studied in depth to reveal the basic principles of life.

The small worm is like us in many ways

An organism's collection of genes is called its genome. The human genome consists of about 100,000 genes. Genes are strands of DNA that make proteins and it's the proteins that work together to make us what we are.

The DNA in our genome contains all the genetic instructions to make all the tissues out of which we are made and all the chemicals that enable us to function.

In a real sense, an organism's genome is its blueprint.

A simple life

Compared to humans, C. elegans is simple. It has less than 20,000 genes, but "it's a little gold mine" according to Dr Cynthia Kenyon of the University of California.

"Basically, it's an animal that does everything animals do."

The idea that C. elegans could be used this way came from Dr Sydney Brenner of Cambridge University, one of the pioneers in working out the genetic code. He chose it because the tiny worm was small, transparent and easy to grow.

The project began in 1965 and it took nine years to prove to other scientists that useful biology could come from its study.

Growing fascination

At the first scientific meeting devoted to C. elegans in 1977, only 70 scientists turned up. At a similar meeting last year, there were 974 - almost one for each cell in the adult worm.

More genomes are being sequenced year by year

That is what makes this tiny worm so fascinating. It is made up of 959 cells, not 960 or 958 but 959 exactly, and each cell has a name.

Mapping the way it develops from one cell which divides and alters to build the complete worm has taken Dr John Sulston of the Sanger Centre in Cambridge and colleagues some 15 years to chart.

The result is a view of how a living multi-celled creature develops. Nothing like it is available for any other animal.

A common ancestor

It may be just a worm, but its kinship to humans is breathtaking. Most human genes that have been discovered have a counterpart in this tiny worm.

So similar in fact are many of our genes that they must have come from the ancient common ancestor of humans and worms many hundreds of millions of years ago.

After aeons of separate evolution, biologists have been able to replace a C. elegans gene with a human version of that same gene.

Staggeringly, the worm gets along just fine with its human replacement transplant. It goes to show just how related we are. Obtaining the entire genetic blueprint of C. elegans is unquestionably a landmark in science.