Pardon me while I wipe the drool off my chin…
As John Lynch and Kelly Hale have mentioned, there has been a new discovery of Au. anamensis fossils that greatly clarifies the relationships between Ardipithicus ramidus, Au. anamensis and Au. afarensis. In addition to clarifying the relationships between the above, the paper also has some other interesting things to say about hominid (here refers to the human clade subsequent to divergence from our common ancestor with chimpanzees) evolution.
But first let’s talk about the fossils themselves. Approximately 31 specimens have been recovered consisting of a number of teeth, several partial maxillas, a metatarsal shaft, a distal foot phalanx, an intermediate hand phalanx, four vertebral fragments and a partial femur and some cranial fragments.
a, ASI-VP-2/334 rightmaxillary dentition. b, ARA-VP-14/1 maxilla with dentition. Alignment of right and left maxillary arcades is approximate. c, Au. anamensis (KNM-KP 29283 and KNM-ER 30745, left and middle, respectively; casts, reversed) and Au. afarensis (A.L. 200-1, right) dentitions. d, Comparison of the
ASI-VP-5/154 right femoral shaft with the smaller but otherwise morphologically similar left proximal femur of A.L. 288-1 (Lucy;Au. afarensis).
Before going further you can find the original paper on Ar. Ramidus here and some interesting background on Au. anamensis can be found here.
Considering that the find is a little on the small side (although it represents at least eight individuals), why is it important? It has long been suggested that Ardipithecus, Au. anamensis and Au. afarensis formed a chronospecies – that is one evolved into the other – with Ardipithecus occuring prior to 4.4 MYA, Au. anamensis occuring between 4.2 and 3.9 MYA and Au. afarensis occuring between 3.9 and 3 MYA. The new finds date to approximately 4.41 MYA. What is interesting is the chronological and geographical relationships between the species:
Palaeoenvironmental circumstances surrounding Au. anamensis, 1,000 km to the south in Kenya have been described for Allia Bay as a mixed assemblage sampling aquatic, forest, grassland and bushland… Nearby Kanapoi conspecifics were found in another mix of environments described as dry, possibly open, wooded, or bushland conditions with a wide gallery forest in the vicinity… Habitat preferences in such mixed assemblages are difficult to ascertain despite the assertion… that hominids “favored mosaic settings”. In contrast, the Ethiopian occurrence of Au. anamensis described here allows its tight spatial and temporal placement in a vertebrate assemblage withtaphonomic integrity. Its relative abundance suggests that it was a regular occupant of a wooded biome that appears to have persisted in this part of the Afar during the 200,000-yr interval subsequent to
Ar. ramidus at Aramis. At Aramis, the lone hominoid and largest primate was Ar. ramidus (109 of 6,156 identified specimens so far). No trace of Australopithecus has been recovered in this (4.4Myr ago) or any contemporary or older African deposit… Furthermore, Ardipithecus has not been found at Asa Issie (,4.1-4.2Myr ago) or in any other contemporary or younger fauna. Thus, Ardipithecus and Australopithecus are, so far as is known, mutually exclusive in temporal distribution. Defining the first appearance datum of Australopithecus
is hazardous given the incompleteness of the geological record, but its first appearance in the Turkana basin at three separate sites (Kanapoi, Allia Bay and Fejej) is coincident (within age constraints) with its appearance in the Afar Rift at Aramis, Asa Issie and possibly Galili…
This is where things get interesting. Whte et al site several papers by Ward, Leaky and Walker that makes certain predictions about Au. anamensis morphology. They state:
One study [the one linked to above, in point of fact - afarensis] predicted that when found, the Au. anamensis femur would be similar to that of Au. afarensis.
And what did they find in the femur?
There is no linea aspera, but only relatively blunt outlines of the adductor attachments both medially and laterally. At the shaft’s approximate midpoint, these two minimal ridges are separated by about 11 mm, a distance of considerable breadth given the probable original length of the bone. The Asa Issie femur is thereby similar to the ‘minimal linea aspera’ morphology of the posterolateral femur that characterizes the smaller A.L. 288-1femur … In this sense, the older Asa Issie specimen is on the presumably primitive end of the considerable range of variation in Au. afarensis with respect to this character.
So, like the discoverers of Tiktaalik roseae we have another succesful prediction of evolutionary theory!
They go on to point out that the Afar material (coming from a single stratigraphic succession) constitute a strong test for the hypothesis of an anagenetic relationship between the three hominids (one which is not refuted by their evidence).
What more could you ask for? How about some Stephen Jay Gould? In The Structure of Evolutionaey Theory Gould says:
We can distinguish the punctuations of rapid anagenesis from those of branching speciation by invoking the eminently testable criterion of ancestral survival following the origin of a descendant species. If the ancestor survives, then the new species has arisen by branching. If the ancestor does not survive, then we must count the case either as indecisive, or as good evidence for rapid anagenesis–but, in any instance, not as evidence for punctuated equilibrium. (p. 795).
The degree to which the fossil record of human evolution supports punctuated equilibrium has always been somewhat debatable (see Stanleys’ Macroevolution: Pattern and Process pgs 80-82 for example). White et al specifically address this issue and conclude:
For the origin of Australopithecus, phyletic evolution with a burst of rapid directional change during the 200,000-yr period between 4.4 and 4.2Myr ago remains plausible given the geographic, temporal and morphological relationships of Ar. ramidus and Au. anamensis and our understanding of primate dental anatomy and development… Indeed, given the available evidence, the origin of Australopithecus could well turn out to be a case of “punctuated gradualism”… or “punctuated anagenesis”… rather than rectangular evolution sensu Stanley… Only the recovery of additional fossils from dated contexts will allow a more accurate and precise determination of the mode and tempo of early hominid evolution on the African continent.
Finally, White et al performed cladistic analysis of the material (it’s in the supplementary material) the resulting consensus tree is below.
I’ll probably have more to say on the actual morphology tomorrow (I need a little more time to digest it all)…
Added later: Indian Cowboy and John Hawks also have interesting things to say about the new find.