Some Applications of the Fluorine Test

Kenneth Oakley

The Archaeological News Letter 1949

[1] Fluorine is a gaseous element which in the form of fluorides is fairly widely distributed in nature. It occurs as a trace in most ground-water, usually less than one part in a million. When "atoms" (or rather ions ) of fluorine come into contact with crystalline calcium phosphate–the mineral matter in bones and teeth–they enter the ultra-microscopic mesh of the crystals and become locked in. (Dentists by the way are interested in fluorine, because when there are unusually large traces in drinking water it becomes fixed in the enamel of the growing teeth to such an extent that they become mottled; in small amounts it is beneficial, making the enamel resistant to decay.)

If a bone or tooth lies for thousands of years in a moist gravel or sandy formation, it gradually absorbs wandering fluorine ions from the ground-water. Once they enter the bone substance they are not released, unless the whole bone becomes dissolved. The process goes on continuously, and the fluorine-content of the bone or tooth increases in course of time. This fact provides rather a neat means of distinguishing fossilized bones of different ages occurring at a particular place. Of course it does not make it possible to date bones in terms of years, or even to give a relative date to isolated bones. Thus, bones buried in gravels where there is a fair amount of fluorine in the ground-water accumulate it much more rapidly than others buried in gravels where there is very little fluorine in the water. If, however, one happens to be interested in separating bones of different ages at one locality, estimation of fluorine-content is helpful. For example, when human bones are found in ancient river gravels, doubt sometimes arises as to whether they were embedded at the time when the gravels were laid down, or whether they represent a later interment by a grave-digger. If fossil animal bones undoubtedly contemporary with the gravel are available for comparison, fluorine-analysis will clearly differentiate bones which have been interred at a sufficiently later date.

The Geological Department of the British Museum (Nat. Hist.) has for some time been interested in the possibility of settling the age of fossil remains of doubtful antiquity by fluorine-analysis; but as the required estimations would have to be made on extremely minute samples of material it seemed doubtful if the method was practicable. However, the Department of the Government Chemist agreed to help in exploring the possible applications of the test, and members of the staff of that department succeeded in determining the fluorine contained in a carefully selected series of small samples of fossil bone and teeth. Most of the samples used for analysis were obtained by a fine dental drill applied to the broken edge of each bone or tooth until a small but sufficient quantity of the "bone dust" had been cored out. The test has proved most successful in certain cases. The work is still in progress, but the results already obtained in the case of the Galley Hill Skeleton, the Swanscombe Skull and the Piltdown Skull are of considerable interest.

The Galley Hill Skeleton was found 8 feet deep in gravels at Swanscombe, Kent, in 1888. The gravels are Middle Pleistocene, that is to say of early palaeolithic age; and there has been a long con[2]troversy as to whether the human bones had been naturally buried in the gravels when they were laid down by the Thames, a quarter of a million years ago, or whether they had been buried artificially at a comparatively recent date. We collected a number of fossil animal bones from these early palaeolithic gravels, a number from later palaeolithic (i.e. Upper Pleistocene) deposits in neighbouring pits, and some from recent deposits, including part of a Saxon skeleton. These were analysed in the Government Laboratory and it was found that all the undoubted early palaeolithic bones contained around 2 per cent. fluorine, the later palaeolithic around 1 per cent., and those from recent deposits 0.3 per cent. or less, down to 0.05 per cent. Some spare scraps of the Galley Hill Man had been left in the Museum Collection by one of the original investigators, and we submitted these for analysis. They showed around 0.3 per cent. fluorine. Yet the skeleton had been found in gravels in which the genuine fossil animal bones show 2 per cent. fluorine. Clearly the skeleton is not a quarter of a million years old as has been alleged, but is a comparatively recent burial, almost certainly less than 10,000 years old.

To check the validity of the test we had it applied to the bones of the Swanscombe Skull found deep in the same gravels by Mr. A. T. Marston 13 years ago, and accepted by geologists as certainly contemporary with the containing deposit, which is rich in Acheulian hand-axes. It proved to contain 2 per cent. fluorine. Its early palaeolithic age has thus been confirmed by the test. (Incidentally it is now the oldest known human cranium in Europe and although older than Neanderthal Man approaches more closely the species to which we belong. This point is discussed by Professor Le Gros Clark in his "History of the Primates," reviewed on p. 106).

One of the outstanding problems presented by the remains known as Eoanthropus dawsoni or "Piltdown Man" also depends for its solution on the separation of bones of different ages found in a single deposit. In this case, of course, there is no question of artificial interment.

The remains of Eoanthropus were found in a 'pan' of water-laid gravel resting on an erosion surface little more than 50 feet above the Ouse, at Piltdown, near Fletching, Sussex. The water which deposited this gravel swept together bones and teeth of various degrees of antiquity. Some of the animal remains in the gravel, e.g. the bones and teeth of beaver and red-deer are probably not much older than the period when the gravel finally settled (perhaps not more than 100,000 years ago); but others, such as the teeth of the extinct elephant Mastodon, date from the beginning of the Pleistocene, say, half a million years ago or more and must have been already fossilized when embedded in the gravel.

Thus fossil bones of at least two ages are mixed together in the Piltdown gravel. They all look much alike, being deeply stained by iron oxides. It has for long been a matter of controversy as to whether the bones of "Piltdown Man" belong to the Lower Pleistocene or to the later Pleistocene group. In fact those who claim that the Piltdown jawbone, or mandible, is that of an anthropoid ape accidentally associated with the brain-case of a primitive man, have rather pinned their faith to the probability that the jawbone belonged to the Lower Pleistocene group, while the fragments of brain-case went with the later group.

With the co-operation of the Department of the Government Chemist, the British Museum (Nat. Hist.) has had all the available Piltdown material tested for fluorine. The animal remains of undoubted Lower Pleistocene age all showed high fluorine-content, while those of later Pleistocene age in the same bed showed a much lower fluorine-content. All the remains of Eoanthropus – and some 20 micro-samples were analysed–showed extremely little fluorine. It is evident that fluorine has been deficient in the Piltdown ground-water since the gravel accumulated; but nevertheless the test has shown conclusively that none of the bones and teeth attributed to Eoanthropus belongs to the Lower Pleistocene group. The jawbone and associated brain-case are contemporaneous. It is probable that they date from the time of final settling of the gravel, which from physiographic and other evidence is now considered to be not earlier than the last interglacial period.

It is still open to anatomists to argue about the naturalness of the association of an ape-like mandible with a typically human brain-case, but in the light of the new dating evidence it appears more probable that they belong to the same creature. Consideration should perhaps be given to the possibility that "Piltdown Man" was not primitive in the strict sense, but a specialized forest type which evolved in isolation and became extinct at the beginning of the last glacial period, less than 100,000 years ago.

It is perhaps worth noting in parenthesis that none of the alleged implements from the Piltdown gravel appears to stand up to critical scrutiny–a question which has already been dealt with briefly in a handbook on the tools of fossil man recently issued by the British Museum (Nat. Hist.). However, if Eoanthropus dawsoni did in fact belong to an aberrant side branch of the human family, it is not inconceivable that the material culture of this species would have been very crude, or even negligible.

In conclusion it must be emphasized that the fluorine-test is strictly limited in application. It is a valuable means of differentiating bones of widely different ages, e.g. in the case of a natural mélange such as at Piltdown, or in the case of a comparatively recent skeleton interred in a Pleistocene bone-bearing gravel. On the other hand it does not provide a means of close relative dating. A given bone or group of bones shows a certain range in fluorine-content. Unless the difference in age between the bones which are being compared is considerable (e.g. 10,000 years), there is usually an overlap in the range of their fluorine-contents. For this reason it would probably be impossible by this method to differentiate clearly between say a Saxon and a Neolithic skeleton; whereas it would enable one, for [3] example, to distinguish bones of Neolithic or later age from others of say Acheulian age, when both occur under similar conditions at the same locality.

Note.–A detailed report on the fluorine-dating of the Galley Hill and Swanscombe remains is to be published in Bull. B. M. (N.H.), Geol. I, 2, forthcoming, while a preliminary report on the work carried out on the Piltdown material is expected in Nature early in 1950.

Note.–The analyses were carried out in the Government Laboratory by Mr. R. H. Settle, Dr. C. R. Hoskins and Mr. E. C. W. Maycock.