Relative Dating of Fossils

There are two types of dating in anthropology. The first is absolute dating, "which ties a fossil or artifact to an absolute time scale in years" (Conroy, 2005). It can also be referred to as chronometric dating. Absolute dating involves the analysis of radiocarbon, potassium/argon, argon/argon, electron spin resonance, and luminescence, among others. However, "before the advent of absolute dating techniques, paleoanthropologists were limited to relative dating methods, [...] which seek to place fossils and artifacts into a temporal context with other locally associated archaeological, faunal, and floral materials" (Conroy, 2005). When fossils are discovered in sites that contain rocks or sediment that will not yield dateable samples by use of absolute dating techniques, relative dating must be used. Many of the sites in South Africa are best dated by these means. Most important to relative dating is that the fossil, when found, remain in provenience until properly recorded. This is due to the fact that relative dating relies on geologic correlation to measure time and relationship. From here, a paleoanthropologist can refer to the basic law of superposition - which states that the lower the layer, the older the strata - and/or biostratigraphy. Biostratigraphy is valuable in that "when nonhuman fossils of known age are found in association with hominin fossils, we may infer the age of the hominins" by means of correlation (Park, 2008). With the use of biostratigraphy, the same can be said of stone tools.

One useful application of relative dating exists in its ability to determine a chronological timeline:

Cultural traditions evolve just as biological systems do; therefore accurate dating is imperative in order to understand the connections between biological evolution and cultural evolution (Conroy, 2005).

For example, in relation to the statement above, at Sterkfontein some of "Africa's earliest stone-cultures" were discovered at Member 5 (Conroy, 2005). The Oldowan tools by themselves are noteworthy, but when we consider the fact that Homo habilis was discovered at Member 5, as well, it is possible to conclude with some degree of probability that Homo habilis was using them (Conroy, 2005). Further, since cultural traditions do often evolve with biology, an anthropologist can create a chronological timeline based on what is above and below H. habilis and the Oldowan tool culture in varying strata (Conroy, 2005). Notice, in Member 4 at Sterkfontein, the remains of A.africanus were discovered, but we find no evidence of stone tools (Conroy, 2005). Hence, we can assume that A. africanus was not a stone tool user and, if enough evidence presents itself, that some sort of biological/cultural transition had been made between the transition of the two species. One crucial point to remember, of course, is that evolution (cultural or biological) is not always linear. Evolutionary events do occur in both time and space, but they do not always follow a set direction.

There is no doubt that Sterkfontein has been a problematic cave site. The reasons for this, according to Berger et. al. (2002), are as follows:

1) A relatively poor understanding of taphonomic and geochemical processes relating to dolomitic caves and their fossil assemblages.

2) Damage from mining activities.

3) The lack of volcanic tuffs within the systems, so that the argon/argon technique or other radiometric methods cannot be applied (p. 192).

Nonetheless, by means of relative dating, Berger et. al. have managed to establish some degree of accuracy for the timelines of various members at Sterkfontein. Of particular note, is the following: "If Equus, however uncommon in the assemblage, is definitely in situ, this supports the suggestion that Sterkfontein Member 4 could be older than ca. 2.36 Ma" (p. 194). If one is willing to accept the appearance of Equus specimens in Member 4 (Kuman and Clarke did raise some objection to this and believe all Equus specimens should be moved to Member 5), the appearance of these specimens in Member 4 state a relative age based on their correlation to other specimens in other parts of the African continent that are more easily dated by absolute means.

References

Berger, L., Lacruz, R., & de Rulter, D. (2002). Brief Communication: Revised Age Estimates of Australopithecus-Bearing Deposits at Sterkfontein, South Africa. American Journal of Physical Anthropology, 119, 192-197.

Conroy, G. (2005). Reconstructing Human Origins. New York: Norton.

Park, M. (2008). Biological Anthropology. New York: McGraw-Hill.