Something fishy going on

UNDER THE MICROSCOPE

I am reported to have said, in a moment of apparent passion, that it is not birth, death, or marriage but gastrulation that is the most important event in our lives. It is during gastrulation that the gut is formed, by a rather complex series of cell movements that transforms the simple early embryo into something more like its later form.

This is a vital stage in developing into a human; as we all have a decidedly fishy past. During our embryonic development in the womb we pass through a stage that is rather like that of a fish. At our head end a number of slits appear that could be taken for the gills of fish and, behind, our elongated body looks well fitted for side to side swimming movements. A story is told of the great embryologist Karl von Baer, who first discovered that mammals like ourselves come from a single cell, the fertilised egg. He apparently forgot to label two small embryos which he had preserved in alcohol, and could not determine whether the embryos were those of a bird, a lizard, or a mouse. This serves to illustrate that all vertebrates, animals with backbones, pass through a common stage during their journey through embryonic development, and only then diverge. Fins, wings and arms all have a very similar early beginning. Observations like this provide one of the strongest arguments for Darwin's theory of evolution.

Embryonic development gives rise to all the animal forms we see around us. The fertilised egg divides and the multitude of cells it gives rise to are organised into flies' wings, elephants' trunks and human brains. All this arises from the behaviour of individual cells which by "talking" to each other make decisions as to what to do and what to become. To become, for example, a nerve or a skin cell; to divide or not to divide; to move or not; and even whether to commit suicide. All these decisions are under the ultimate control of the genes which all cells in an embryo share. There are lots of them - more than 10,000 control our own development.

The genes in the embryo do not provide a blueprint for the organism that wall develop. There are no genes that provide a plan for the arm or eye. Rather the genes provide a programme for how to make an arm or eye. Origami is just like development, for the instructions on how to fold the initially featureless piece of paper are just what genes do. The final result of the folding bears no simple relation to the instructions. We are often surprised when after the final fold and tug we find we have constructed a bird or a boat. Evolution of animals can thus be seen as the modification of the embryo's developmental programme. This programme of instructions is under the control of the genes which are made of the fundamental genetic material, DNA. The only thing that changes during evolution is the constitution of the DNA, and so the instructions on how to develop.

New techniques for manipulating DNA have revealed that instructions provided by genes are similar over an astonishing range of animals. One set of genes universally provides cells with information about their position in the body. Mutations in such genes can provide misinformation; a recently- discovered gene in the fly is capable of initiating eye development even on legs. This same gene probably underlies the development of all known eyes, including ours.

Modern animals have evolved many genes that buffer them from change. Precision and reliability of development are paramount. I think I can make an argument that no interesting new animal forms could ever evolve. Evolution of form is largely over. But the dinosaurs might have come to the same conclusion. And how wrong they would have been.

! Lewis Wolpert of University College, London, is chairman of Copus (the Committee on the Public Understanding of Science). His book 'Triumph of the Embryo' is published by Oxford University Press at pounds 7.50.