It would have first been thumbed through 150 years ago, almost certainly by an open mouthed reader shocked to the core by its contents.Considering the transformation it would lead to in man's understanding of God, creation, history and science, one would not have blamed the astonished reader if he or she had dropped and damaged it.

It would have first been thumbed through 150 years ago, almost certainly by an open mouthed reader shocked to the core by its contents.

Considering the transformation it would lead to in man's understanding of God, creation, history and science, one would not have blamed the astonished reader if he or she had dropped and damaged it.

But fortunately all these years on this first edition, first issue of Charles Darwin's seminal work On the Origins of Species by Means of Natural Selection remains in what has been described as 'fine' condition by a Norfolk expert.

And next month it will be up for public sale at an Aylsham auction expected to attract interest from around the world.

Already a special year for Darwin experts and academics because it is the bicentenary of the scientist's birth and the 150th anniversary of the publication of Origin of Species, 2009 is set to be long remembered at Keys' auction house for the sale, which could see a bidding war beat the expected value of �20,000.

For Keys' book specialist Andrew Bullock, the sale will be particularly significant: 'After 26 years in the business, I know it will be the most expensive book I have ever sold at auction.'

Private collectors, academic institutions and specialist book traders are expected at the auction on April 30.

Mr Bullock said he felt 'utterly privileged' to be handling such a book on behalf of its anonymous north Norfolk owner.

'It is widely held to be one of the 10 most important books ever written - some regard it as the most important.

'Certainly it is the most important biological book ever.

'It is one of those rare occasions when the white gloves really do go on before you pick it up and you handle it with reverence!'

The first run of Origins of Species in 1859 saw just 1,250 copies printed. It sold out immediately, with its controversial evolutionary theory subject matter causing a social furore.

'It is a definitive work and what every serious bibliophile wants is a first edition, first issue copy and this is a must-have for the very best shelf, ranking alongside a first edition of Ulysses,' said Mr Bullock.

'A book like this only comes along very rarely and when it does it is considered so important it is often acquired by institutional libraries from around the world.'

Another important factor in the volume's value is the fact it is still bound in its original publisher's cloth.

'Buyers often sent an important book off to be rebound so they matched the other volumes in their library - but that would reduce the value to about �5,000,' said Mr Bullock.

He described the book's condition as 'fine' with just a small amount of wear on the binding.

'It clearly hasn't been read over and over - and it has not been annotated at all.'

The book belongs to a private client of Keys, who bought it in a collection of books several years ago.

'The client realised some time ago that 2009 would be a good year to sell it,' said Mr Bullock.

'He is right - almost every time you switch on the television or open a newspaper it is all Darwin, Darwin, Darwin.'

The retired vendor knew the book was very valuable, said Mr Bullock, and he had not been shocked at the �20,000 estimate, but he had been 'pleasantly surprised'.

The volume will be the centrepiece of Keys' book sale on April 30. Internet and telephone bidding facilities will be available on sale day. Viewing arrangements are to be confirmed.