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Thursday, November 19, 2009

Bridging the gap: Australopithecus from Woranso

Recently discovered Australopithecus fossils from the Ethiopian site of Woranso-Mille help fill a gap between parts of the early hominin fossil record (Haile-Selassie et al, in press). The fossils date to between 3.8-3.6 million years ago (Ma), and consist of several teeth and a jaw fragment. These specimens show a number of features that are intermediate in morphology between the earlier Au. anamensis (4.2-3.9 Ma) and later Au. afarensis from Laetoli (~3.7-3.5 Ma). As a result, the Woranso fossils lend support to the hypothesis that Au. anamensis and Au. afarensis represent a single evolving species (i.e. Kimbel et al. 2006).


I think this is exciting for two reasons. First, the fossils bridge the morphological gap between the older anamensis and younger afarensis fossils. As a result, we get to ‘see’ anagenetic evolution—changes within a single lineage. One topic in evolutionary biology is about the mode and tempo of evolution: are species fairly constant, then evolve into multiple ‘daughter’ species (“punctuated equilibrium”); or does evolutionary change tend to occur more within individual lineages (“anagenesis”)? Obviously neither is mutually exclusive, rather evolution is probably best characterized variously by both processes. Still, in the world of paleoanthropology, where many researchers argue for rapid and constant species turnover within the human lineage, it is cool to see a convincing argument for anagenesis. However, this ignores the meager (but intriguing) K. platyops material (Leakey et al. 2001), dating to around 3.5 Ma, possibly indicating the proliferation of at least two hominin species shortly after 4 Ma.


Second, the morphological intermediacy of the Woranso fossils allow a look at the patterns of evolutionary change within the anamensis-afarensis lineage. The authors note that the teeth of the Woranso hominins are generally more similar to anamensis, but have some derived characters of the later afarensis teeth. If we truly have a glimpse of dental evolution within a single lineage, we can ask questions about the evolution and development ("Evo-Devo") of teeth. Are changes in these teeth correlated in a way that could be predicted by certain developmental models? Or is selection acting independently on various tooth traits?


References
Haile-Selassie Y, Saylor BZ, Deino A, Alene M, and Latimer BM. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and Taxonomy of Early Australopithecus. American Journal of Physical Anthropology, in press.

Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, and Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of Human Evolution 51: 134-152.

Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, and McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410: 433-440.

2 comments:

The Monkeyman said...

Prof Haile-Selassie gave a talk on this in london, must have been about two months ago now, and it's certainly exciting stuff. I actually liked his world view, especially that he didn't rush to name a new bloody species!

I do wonder if surface scans of teeth, and comparisons based on that, might be the way to go.

I suppose the assumption (regarding systematics and macro-micro evolution) is that teeth are shaped too much by activity/diet/epigenetics or are too homoplastic to get a strong idea about evolution within and between lineages, but I think that's BS.

zacharoo said...

I don't know as much about teeth as I probably should. But I think they've been pretty well-used in taxonomy. Being so GD abundant it's not really surprising that people have figured out patterns to look at to infer taxonomy (enamel thickness, cusp proportions, non-metric trait frequency, etc).

The more I think about it, what really stands out about the paper is the utter neglect of Kenyanthropus. Whether Kenyanthropus is really another species (or genus for that matter) does not prevent anamensis from being ancestral to afarensis, but it does put the kibosh on the idea of an anagenetic lineage spanning over 1 million years.