Don Johanson and Tim White, writing in their 1979 paper on the phylogeny of early hominins (and introducing Australopithecus afarensis as an ancestor of later hominins) [1]. They faced the problem of showing that similarities between the Laetoli and Hadar samples are indicative of a single species, while similarities among other samples may not be so:
Of course, morphological and metrical comparisons should not be expected to unerringly place every single individual along an evolving lineage. Our interpretation of the South African gracile australopithecines is based on a consideration of the available sample characteristics for the fossil hominids. We are fully aware that individual traits and even single specimens can be matched in samples that we consider to represent different evolutionary entities and ultimately taxa. For example, the matching of individual specimens and demonstration of overlap between the samples from Sterkfontein and Swartkrans serve to point out the general similarities of these groups, but at the same time conceal real and biologically meaningful differences which we consider to have phylogenetic significance.