From Food for a Future by Jon Wynne-Tyson, published 1979 by Universe Books, 381 Park Avenue South, New York, N. Y. 10016, U.S.A.
pages 110-111, footnote page 114:

It is interesting and possibly very relevant that lifelong vegans seldom suffer from B12 deficiency. Where deficiency has been established, it has been mostly in people who have switched, perhaps too abruptly, from a diet based on animal products. This has helped to prompt the suspicion that as children brought up from birth on a vegan diet are able to absorb the B12 made by the bacteria in their colons, it may be that the perversion of the body's chemistry through meat eating has made it impossible for a small number of people to switch to the more natural vegan diet later in life without showing signs of B12 deficiency. (2)

This is not to say, of course, that symptoms due to inadequate intake of B12 are not to be found in malnourished people the world over, whether they are omnivorous or more selective in their eating habits. An inadequate diet will be deficient in a great many elements, whether or not meat is included. Although volumes have been written about B12, it is in our knowledge of man's adaptational abilities that we have far to go; the uncertainties surrounding a clearly inadequate intake (irrespective of absorptional capacity) are less in dispute.

It is known that the bacteria in the human colon synthesise B12, but that in most cases this is not available for absorption by the large intestine because the absorption takes place in the lower end of the ileum (or small intestine). This absorption depends upon the presense of the 'intrinsic factor', a protein-like substance produced in the stomach. In the case of pernicious anemia, for instance, this intrinsic factor is absent.

There is some evidence that the large intestine of vegans can absorb B12 (Ellis, F.R.; Wokes, F., 'Vitamin B12', the Vegan, Winter 1966/67) - a point that may well be given more prominence as research continues.

[...]* To put the horse more squarely before the cart, there is a good deal of support for the theory that man lost his ability to manage without dietary B12 when he took to meat eating, which caused the bacterium Escherichia coli to move further down his intestinal tract to beyond the point at which B12 could be absorbed (see A. G. Long and F. Wokjes, 'Vitamins and Minerals in Plants', Plant Foods for Human Nutrition, Vol. 1, No. 1, May 1968). The upward movement of the Escherichia, it may be, is encouraged by a return to that vegan pattern we abondoned. However, the inner reaches of our guts are not of universal fascination and most of us prefer to leave the problems of bacterial balance to the scientists. I must leave it to the relaly keen reader to explore for himself in greater depth.