A ladder, not a bush?

Tim White and colleagues (2006) report on new fossils from Aramis and a new site, Asa Issie, with estimated dates between 4.1 and 4.2 million years ago.

In addition to the paper, there are articles in the New York Times (by John Noble Wilford), the Associated Press (by Seth Borenstein), and BBC (by Paul Rincon).

The story is being played as another "missing link" -- this one between Ardipithecus and Australopithecus. From the Times:

Tim D. White, a paleontologist at the University of California, Berkeley, who was a leader of the team, and his colleagues said the 4.1-million-year-old fossils were anatomically intermediate between the earlier species Ardipithecus ramidus and the later species Australopithecus afarensis, the Lucy family. The newfound bones and teeth are the earliest remains of the most primitive Australopithecus, known as anamensis.
"This new discovery closes the gap between the fully blown australopithecines and earlier forms we call Ardipithecus," Dr. White said in a statement. "We now know where Australopithecus came from before four million years ago."

The fossil specimens are a partial maxilla from Aramis, ARA-VP-14/1; two partial maxillary dentitions from Asa Issie numbered ASI-VP-2/2 and ASI-VP-2/334; and a large femur shaft fragment, ASI-VP-5/154. There are also several postcranial bones -- phalanges, vertebrae, a metatarsal -- that are pictured in some of the press accounts and briefly discussed but not pictured or numbered in the paper. The postcanine teeth in the maxillary specimens are larger than the known sample of Ardipithecus, but the canines are larger and more mesiodistally elongated than in Australopithecus afarensis. The best anatomical match for these features is with the Kanapoi and Allia Bay samples assigned to Australopithecus anamensis, and White and colleagues assign the new fossils to that species.

So why are these fossils important? On the surface, there isn't very much to them. Three piecemeal upper dentitions don't tell much. They have big molars and big canines, both within the range of Au. anamensis. Neither they nor the femur shaft extend the known range of variation in early hominids.

Remembering that every fossil fragment is a precious relic of a bygone age, the main importance of these is that they may address hypotheses about the biogeography of Early Pliocene hominids. The maxillae show that a large-molared hominid existed in the same geographic location at a later time than the small-molared Ardipithecus. That could be interesting, and it is the hook for the news stories and the team's press statement.

The strongest part of this story is the geographic -- finding them in the Middle Awash instead of Kenya -- and the paleoenvironmental. There is some suggestion in the paper that there may be a paleoenvironmental difference at the sites that currently have evidence of Au. anamensis:

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 the 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 with taphonomic 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 (White et al. 2006:887-888).

This points to two salient facts about the Australopithecus lineage: they were able to disperse effectively across relatively long distances, and occupy at least those habitats where wooded cover and resources were available.

On the other hand, the fossils don't really "fill a gap" between Ardipithecus and Australopithecus, because they are pretty firmly within the time range of known Au. anamensis, being around the same age as the Au. anamensis sample from the Lake Turkana area -- the oldest Kanapoi hominids may be between 4.1 and 4.2 million years old also. The paper points out the other East African examples of Australopithecus at or above 4 million years ago; but it omits the Sterkfontein Member 2 remains, which are also conceivably in the age range of Au. anamensis. Or, for that matter, the Lothagam mandible, which might be the earliest australopithecine even if its date weren't as high as the >5 Ma estimate.

The paper attempts to close off -- for the moment -- the idea that there were allopatric species of early (ca. 4 Ma) australopithecines with differing dietary adaptations. But the paper cannot reject this hypothesis without caveats:

Two phylogenetic hypotheses concerning the origin of Australopithecus can be offered to account for the available data. The first hypothesis derives Au. anamensis phyletically from Ar. ramidus within a 200,000-yr interval [i.e., between 4.4 and 4.2 Ma]. The second involves cladogenesis of Au. anamensis from an ancestor (presumably Ardipithecus or some close relative) even deeper in the Pliocene or Late Miocene. Under the latter hypothesis, Ar. ramidus would represent a relict species in an ecological refugium (White et al. 2006:888).

This latter alternative is the only "bushy" interpretation -- the idea that known species of Ardipithecus can't really be the direct ancestors of Australopithecus, but that there must be some as-yet-undiscovered hominid (or better yet, hominids) that are the common ancestors, cousins, and other bushy relatives of the known species. White and colleagues cannot reject it, but they clearly do not favor it.

In its place, they suggest Ardipithecus ramidus as a lineal, possibly anagenetic ancestor of Au. anamensis, and Au. anamensis as the anagenetic ancestor of Au. afarensis. It's a ladder from primitive to derived, small-molared to big-molared, big-canined to small-canined.

I tend to think this is the null hypothesis -- we have sampled adaptations that differ because of evolution in what is essentially a single lineage of successive species. I say "essentially" because there was not necessarily a wholesale transformation of one species to another across its entire range. Instead, dispersals of new adaptive packages by population movements were probably important biogeographic aspects of evolution in these early hominids. But I think it important to recognize that one species can indeed be the ancestor of a later species.

People who like their phylogenies bushy and their speciations punctuated can take solace in that 200,000-year gap. The finding of Au. anamensis within the already-known time range of Au. anamensis means that the new fossils haven't really added much to the question of phylogenetic diversity in early hominids.

As a postscript, I have a nomination for "most significant sentence" in the paper:

At Aramis, the lone hominoid and largest primate was Ar. ramidus (109 of 6,156 identified specimens so far) (White et al. 2006:888, emphasis added).

References:

White TD and 21 others. 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature 440:883-889. DOI link