Rex Dalton has a great two-page article in Nature about the bush vs. ladder dispute. It keys off of the Middle Awash Australopithecus anamensis article by White and colleagues from a couple of weeks ago.
If you recall that one, White et al. posited that Ardipithecus was likely ancestral to Au. anamensis, and that the two did not overlap in time. Here's the key exchange in the Dalton piece:
This month's Nature paper makes a bold argument, and shows the Awash team seeking to put its mark on the record. Others in the
field are impressed. "When you find 30 new hominid fossils, you are allowed a certain amount of conjecture," says Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC. "As always, they have done a fantastic job."
But he and others are unconvinced by the Awash team's conclusion: "This is only the first half of the rugby match," says Wood. Meave Leakey, lead author on the Au. anamensis discoveries in Kenya, is more blunt. "I don't believe this," she says. "We do not have the specimens to fill the gaps."
Leakey and Wood are among those who believe that other, as yet undiscovered hominid species may have lived at this time, from 4.4 million to 2.9 million years ago. The existence of other species would cloud or eliminate the argument for a direct lineage. "My prejudice is there are more lineages rather than fewer -- more diversity," says Wood. "I have to concede these new data are dramatic. But we should beware coming out with a complete explanation when we don't have all the
evidence."
This argument frustrates White. "There were Martians there back then too," he says. "And spacecraft all over the Pliocene -- we just haven't found them yet."
Waiting for Monte Cassino
In a series of articles since 2000, White and colleagues have laid out a systematic attack on the "bushy" phylogeny model. Their arguments have extended across four million years and seven species, with a breadth that rivals the Allies breaking the Winter Line.
Consider the angles of attack:
1. Au. anamensis -- Au. afarensis. Everyone basically accepts that Au. anamensis is a direct ancestor of Au. afarensis. And the two species are really not very different from each other -- for instance, they are more alike than either is to Ardipithecus. The transition between these species would look to be a simple case of anagenesis, except...
...for Kenyanthropus (Leakey et al. 2001). This small-toothed, flat faced hominid needs an ancestor, too. Au. anamensis might have been the common ancestor of Kenyanthropus and Au. afarensis. If so, then both these later species originated by cladogenesis from Au. anamensis. A similar argument might be made for other species, like Australopithecus bahrelghazali (Brunet et al. 1996) or the Sterkfontein Member 2 hominids. But Au. bahrelghazali is only known from a partial mandible and only differs from Au. afarensis by a three-rooted premolar, which is considered by many to be weak evidence, and the Sterkfontein Member 2 sample has not yet been taxonomically assigned -- they might turn out to be Au. afarensis, for example. Kenyanthropus remains the strongest case for cladogenesis (i.e., a "bush"). Yet...
...White (2003) denied that the Lomekwi skull KNM-WT 40000 was a distinct species. In particular, he argued that the extensive postmortem deformation of the skull made it impossible to substantiate an anatomical difference from Au. afarensis, and even if it was different, the anatomical diversity of living hominoid species is so great that it would probably encompass the difference between KNM-WT 40000 and known Au. afarensis crania.
2. Earliest hominids. At the moment, the earliest putative hominids include three genera: Orrorin (Senut et al. 2000), Sahelanthropus (Brunet et al. 2002), and Ardipithecus, represented in the Late Miocene by Ar. kadabba (Haile-Selassie 2001, Haile-Selassie et al. 2004). Evidence for obligate bipedality has been challenged (by different researchers) for each of these three (I'm one of those who has questioned bipedality for Sahelanthropus).
So far the only comparable anatomical parts from all three samples are teeth...
...which were examined by Haile-Selassie, Suwa and White (2004). They concluded that the variation among these three genera
is no greater in degree than that seen within extant ape genera. Despite claims of molar enamel thickness differences among these late Miocene fossils, we question the interpretation that these taxa represent three separate genera or even lineages. Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus (Haile-Selassie et al. 2004:1505).
Additionally, Ohman, Lovejoy and White (2005) challenged the interpretation of the internal anatomy of the Orrorin femur, which had been suggested to be more derived than that of Au. afarensis. They wrote:
We agree that the Lukeino femur's external morphology suggests some form of bipedality. Yet the more detailed original scans appear to show a distinct superior cortex different from Australopithecus and humans, with the cortex distribution being more primitive than that seen in any other hominid, including Australopithecus.
The relevance of this argument to the phylogenetic diversity of early hominids depends on the anatomy of the Ardipithecus femur, which none of the rest of us are in a position to know. But one may speculate that if all these early "hominids" had femora with similar morphology, it would further reinforce the interpretation that they belong to a single lineage.
3. Ardipithecus -- Au. anamensis. This is the current example. Here's how Dalton discusses it:
The latest Afar discovery is exciting experts because it shows that the three hominids existing in the same area, but in successive time periods. Tim White of the University of California, Berkeley, co-leader of the Awash team, believes this points to a direct lineage between the three -- a process called phyletic evolution. The new Au. anamensis fossils are only 300,000 years younger than Ar. ramidus, meaning that if one became the other, the changes would have had to happen that fast. But the key point, says White, is that fossils of Au. anamensis and Au. afarensis have never been found in sediments the same age as those containing Ar. ramidus. If fossils of the different species were found together, that could show that they belonged to multiple lineages existing simultaneously.
Finding remains of all three species in the same area but not from the same time period suggests they did not coexist, says White.
...
The specimens also provide anatomical clues to evolutionary history. "The new Au. anamensis fossils are anatomically intermediate between the earlier Ar. ramidus and the later Au. afarensis," says White. For example, the teeth of the newly discovered Au. anamensis fossils seem adapted to chew tougher and more abrasive foods than Ar. ramidus. The researchers believe this shows that Au. anamensis had a broader diet. "All this strengthens the view that there is phyletic evolution from Ar. ramidus through Au. anamensis," says White. He believes he has nailed down the relationship between the two later species, although he says that further specimens are needed to prove the earlier link (Dalton 2006:1100).
Of course, it would help matters if we knew in more detail what Ardipithecus looked like. But one must imagine that the stage is being set for its revelation. The unilineal interpretation places Ardipithecus at the critical point as an ancestor to the major mid-Pliocene australopithecine lineage. Extending the unilineal interpretation earlier into the Late Miocene would make Ardipithecus the earliest hominid as well.
It is not necessary to think that taxonomic uniformity means anatomical uniformity, though. Ardipithecus already encompasses a trend of decreasing canine size and less sectorial P3 for example. A trend toward fuller skeletal adaptation to bipedality may also be imagined. But in that context, it is important to note that the time interval between the Orrorin femur and the unpublished Aramis skeleton is longer than the time between Aramis and Hadar. Those relative times may become quite important in thinking about the evolution of those postcrania.
The Winter Line was broken at Monte Cassino, after many failed attempts from different approaches. The Aramis fossils are either the heavy shoe waiting to drop, or they are the uncomfortable foot that all this talk about phyletic evolution is meant to shoehorn into place.
Commentary
If all these cases are added together, they imply a single evolving lineage encompassing at least four anagenetic taxa, Ar. kadabba -- Ar. ramidus -- Au. anamensis -- Au. afarensis. This last would presumably be followed by a cladogenesis into a robust australopithecine species (Australopithecus aethiopicus) and Australopithecus africanus.
One could add Homo erectus to this list, since White and colleagues argued in their description of the Daka skull (Asfaw et al. 2002) that the Asian and African samples represent one cosmopolitan species.
But then one species sticks out as a surprising exception to the pattern: Australopithecus garhi (Asfaw et al. 1999). It will be interesting to see a close argument showing why this species is really different from South African Au. africanus. Say, more different than KNM-WT 40000 is from the Hadar crania. It's quite glaring, really, that this species should be there mucking up such a simple phylogeny.
I have to say, after reviewing all these papers in one sitting -- this entire bush vs. ladder thing is getting very tiresome! I mean, isn't there something else that we could organize early hominid discoveries by? These are all papers in the top journals, and this is the (fairly specialized) discussion that has been promoted as the central issue in the field!
The subtitle of the Dalton piece suggests that it is merely a philosophical difference:
Deciding whether our ancestors evolved as a single lineage may depend more on philosophy than fossils.
But that's not really true. There is a clear null hypothesis here, quite directly drawn from William of Ockham:
entia non sunt multiplicanda praeter necessitatem
Which of course means:
Sometimes fossil samples really do form ancestor-descendant relationships.*
(*) It doesn't really. It means "Entities should not be multiplied beyond necessity."
References:
Asfaw B, Gilbert WH, Beyene Y, Hart WK, Renne PR, WoldeGabriel G, Vrba ES, White TD. 2002. Remains of Homo erectus from Bouri, Middle Awash, Ethiopia. Nature 416:317-320. DOI link
Asfaw B, White T, Lovejoy O, Latimer B, Simpson S, Suwa G. 1999. Australopithecus garhi: A new species of early hominid from Ethiopia. Science 284:629-635. DOI link
Begun DR. 2004. The earliest hominins -- is less more? Science 202:1478-1480. DOI link
Brunet M. and 37 others. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145-151. DOI link
Brunet M, Beauvillain A, Coppens Y, Heintz E, Moutaye AHE, Pilbeam D. 1995. The first australopithecine 2,500 kilometres west of the Rift Valley (Chad). Nature 378:273-275. DOI link
Dalton R. 2006. Feel it in your bones. Nature 440:1100-1101. DOI link
Haile-Selassie Y. 2001. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412:178-181. DOI link
Haile-Selassie Y, Suwa G, White TD. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505. DOI link
Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410:433-440. DOI link
Ohman JC, Lovejoy CO, White TD. 2005. Questions about the Orrorin femur. Science 307:845. DOI link
Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino formation, Kenya). Comptes Rendus 332:137-144.
White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996. DOI link
