Sir, - I write in response to the letter from Mr Denis Wall (September 24th) regarding my article in the "Science Today" column of September 9th. Mr Wall claims that a statement I made in that article is flatly contradicted by a statement I made in a previous column of July 24th, 1995 (Mr Wall is keeping a sharp eye on me).

He quotes from my recent article, in which I said that there was a scientific consensus that life arose spontaneously on earth. He also quotes from my 1995 article, where I said: "I cannot see how the first living cell spontaneously arose". But the next sentence in my 1995 article said: "Of course, neither can anybody else - the mechanism is unknown."

I further added: "I am prepared to await the development of a scientific mechanism to explain how this step (the spontaneous birth of the first living cell) occurred." In another section of the 1995 article, I clearly stated that science holds the working hypothesis that life spontaneously arose on earth. Mr Wall quotes none of this.

Nobody knows the details of the mechanism whereby life spontaneously arose. Nevertheless, great progress is currently being made in understanding how it is possible for order to arise spontaneously in random systems maintained far from equilibrium.

I expect that, in time, science will uncover a natural mechanism to explain how life arose spontaneously on the early earth. However, as a biochemist, I am very conscious of the complexity of life, and I am therefore sensitive to the enormity of the intellectual challenge involved in developing a detailed understanding of how life might have arisen spontaneously. Indeed, there is no guarantee that science will ever uncover a credible mechanism for this event. - Yours, etc.

Biochemistry Department,

University College. Cork.