In 1930, Robert Broom commented on the age of the Taung specimen. This is one of the earliest instances I have found of someone claiming that a fossil is “too recent” to be an ancestor:
The little fossil ape skull that was found at Taungs five years ago is, in the opinion of many, the most important fossil ever discovered. It is manifestly the remains of an anthropoid ape somewhat allied to the chimpanzee, and of about the same size. But it differs from both it and the gorilla in a large number of characters, and in almost all these characters it resembles man. It thus seems highly probable that it is very near to the ape from which man sprang, and possibly a representative of the very genus. One difficulty has been our ignorance of the age of the cave deposit. The bone breccia found in most caves has proved to be of Pleistocene age, and if the Taungs cave is also of this period, then Australopithecus would be too recent to be a possible human ancestor, as man is known to have existed in the Pliocene.
From today’s perspective, the Taung specimen is believed to be a bit older than 2.5 million years old, but may easily be older or younger; Jeffrey McKee revisited the fauna in a 1993 article, and debate about the exact position and depositional circumstances represented by the skull continue today.
Of course, with today’s definition of Pleistocene extending to 2.5 million years ago, a date of that age is perfectly consistent with Broom’s statement–and many anthropologists might agree that the skull is “too recent” to be a lineal ancestor of today’s humans, by the same argument. I’ll have more to say about the concept of ancestors and geological age soon.
Broom, R. 1930. The age of Australopithecus. Nature 125:814. doi:10.1038/125814a0
McKee, J. K. 1993. Faunal dating of the Taung hominid fossil deposit. Journal of Human Evolution 25(5): 363-376. doi:10.1006/jhev.1993.1055