There have been some new discoveries in the Afar region of Africa that fill the gap between A. anamensis and A. afarensis. This is great news. The story, from the Cleveland Museum of Natural History is long but worth the read. This is the fun part:
The fossil hominids from the Woranso-Mille area sample a time period that is poorly known in human evolutionary studies. An outstanding question in the study of early human evolution, says Haile-Selassie, relates to the relationship between the earlier Australopithecus anamensis (4.2 - 3.9 million years) and the later Australopithecus afarensis (3 - 3.6 million years). Researchers have hypothesized an ancestor-descendant relationship between these two species based on their similarities. However, there has been no fossil record from the 3.6 - 3.9 million years time frame thus far to test, confirm, or falsify this relationship. Haile-Selassie adds that the fossil hominids from the Woranso-Mille study area dated to between 3.5 and 3.8 million years ago sample the right time and play a major role in testing the hypothesis with fossil data. The Woranso-Mille fossil hominids from the deposits younger than 3.5 million years extend the geographic distribution of Australopithecus afarensis further to the north of Hadar, where the species is best documented.
Hopefully, these specimens (40 so far) will give us some much needed information about this time period. This is a great example of hypothesis testing using predictive models. It is also a great test of evolutionary theory. Stay tuned.