Cell replacements myths and facts

In "The God Delusion", Richard Dawkins quotes a writer who claims that hardly any of the atoms that made up our brains when we were children are still in place in our bodies. He was quoting Steve Grand, who said:

"[think]of an experience from your childhood. Something you remember clearly, something you can see, feel, mabye even smell, as if you were really there. After all, you really were there at the time, weren't you? How else would you remember it? Be here is the bombshell: you weren't there. Not a single atom that is in your body today was there when that event took place. . . Matter flows from place to place and momentarily becomes you. Whatever you are, therefore, you are not the stuff of which you are made. "

I've heard that claim so many times, in various forms. But is it true?

Snopes looked at it, and one commentator came up with what I think is a very good counter-example:

"When thinking about having all of the atoms in your body replaced in 1 or even 10 years all I can think it that it is impossible. Radium dial workers were found to have significant amounts of radium in their bodies up to 15years after their exposure and even many years after death their bodies were radioactive. Lead, which is treated as calcium by the body, is deposited into bone tissue and can remain there for 20-30 years. Meaning a young girl with lead poisoning will pass lead onto her future children when she breastfeeds. If the concept of having all of the atoms in your body replaced held true then that means both the dial workers and people who were lead poisoned would have to be constantly exposed to the same level of chemicals. This is obviously not true. The dial workers only high level of contact with radium was while they worked at the plants. And children who have lead poisoning often have abatement done to reduce their future exposures. True, both radium and lead have half lives in the body and can eventually be cleared but to replace, say 10ppm of lead in the bone (I guessing at numbers here) they would need to be exposed to high levels all the time."

Someone else managed to find a proper scientific reference, in Brendon S. Noble and Jonathan Reeve (2000) "Osteocyte function, osteocyte death and bone fracture resistance" :

"In adult humans and many other vertebrates, although not normally in the rat, cortical bone is remodelled throughout life by an internal process termed haversian remodelling. Cutting cones of osteoclasts traverse the cortex longitudinally and the spaces created are filled in by a wave of osteoblastic bone formation. The new bone formed, as well as the cells associated with it, are supplied with nutrients by a central capillary. The remodelling of cancellous (trabecular) bone is similar in principle, although it almost invariably occurs on the bone surface. Consequently, in adult humans, the youngest bone is found superficially and the older, interstitial bone is found sandwiched between two layers of younger material. The rate of bone remodelling in the adult cortex can be quite slow, as little as 2% annually in the distal radius ranging up to 50% annually in the rapidly remodelling trabecular bone of the ilium. "

At last some hard science. This is more like it; not only does it clearly have some kind of background facts against which it is stated, but also makes it clear that different kinds of bone, for example, can change at different rates, and how it is done. So it is not a complete myth mentioned, there is a nugget of truth there, but it is a very, very misleading statement, and is virtually an urban myth; it is certainly a complete falsehood as stated by Grand, and taken over lock, stock and barrell by Dawkins, who just cites this somewhat woolly statement as if was gospel truth. Not the kind of mistake the great source checker, Stephen Jay Gould, would have been likely to make!