Australopithecus before Lucy (White et al. 2006)

[Daryl Habel]

To all,
Appearing in tomorrow's issue (13 April 2006) of the journal Nature, but appearing online today on the Nature website, is an article describing new 4.1-4.2 myr-old fossils of Australopithecus anamensis from the Middle Awash, Ethiopia and their relationship vis-a-vis the origin of Australopithecus afarensis.

Asa Issie, Aramis and the origin of Australopithecus. Nature 440, 883-889 (13 April 2006) | doi:10.1038/nature04629; Received 22 September 2005; ; Accepted 2 February 2006.

Tim D. White, Giday WoldeGabriel, Berhane Asfaw, Stan Ambrose, Yonas Beyene, Raymond L. Bernor, Jean-Renaud Boisserie, Brian Currie, Henry Gilbert, Yohannes Haile-Selassie, William K. Hart, Leslea J. Hlusko, F. Clark Howell, Reiko T. Kono, Thomas Lehmann, Antoine Louchart, C. Owen Lovejoy, Paul R. Renne, Haruo Saegusa, Elisabeth S. Vrba, Hank Wesselman and Gen Suwa.

Abstract.
The origin of Australopithecus, the genus widely interpreted as ancestral to Homo, is a central problem in human evolutionary studies. Australopithecus species differ markedly from extant African apes and candidate ancestral hominids such as Ardipithecus, Orrorin and Sahelanthropus. The earliest described Australopithecus species is Au. anamensis, the probable chronospecies ancestor of Au. afarensis. Here we describe newly discovered fossils from the Middle Awash study area that extend the known Au. anamensis range into northeastern Ethiopia. The new fossils are from chronometrically controlled stratigraphic sequences and date to about 4.1–4.2 million years ago. They include diagnostic craniodental remains, the largest hominid canine yet recovered, and the earliest Australopithecus femur. These new fossils are sampled from a woodland context. Temporal and anatomical intermediacy between Ar. ramidus and Au. afarensis suggest a relatively rapid shift from Ardipithecus to Australopithecus in this region of Africa, involving either replacement or accelerated phyletic evolution.

Full article available to Nature subscribers at: CLICK HERE

or from the link at the bottom of the editor's summary HERE
Added later: Thanks to a tip from Michael Hamilton I was able to access the full text in html and also download the pdf free today, although I'm not a full Nature subscriber, being registered only for the free content.  Who knows how long it will stay that way, so you take your chances if you wait until later.

At the same time, the discovery has been announced in the media, with an Associated Press news story from MSNBC.com containing photographs of the fossils and discovery site, at: CLICK HERE

Tim White is quoted in the MSNBC (AP) news report:

“This appears to be the link between Australopithecus and Ardipithecus as two different species,” White said. The major noticeable difference between the phases of man can be seen in Australopithecus’ bigger chewing teeth to eat harder food, he said.

While it’s looking more likely, it is not a sure thing that Ardipithecus evolved into Australopithecus, he said. The finding does not completely rule out Ardipithecus dying off as a genus and Australopithecus developing independently.

The connections between Ardipithecus and Australopithecus have been theorized since an anamensis fossil was first found in Kenya 11 years ago. This draws the lines better, said Alan Walker of Penn State University, who found the first anamensis and is not part of White’s team.

Dar

[Daryl Habel]

Added later: Thanks to a tip from Michael Hamilton I was able to access the full text in html and also download the pdf free today, although I'm not a full Nature subscriber, being registered only for the free content.  Who knows how long it will stay that way, so you take your chances if you wait until later.

Free access lasted two days, and the article is no longer free from the Nature website.  I'd guess they made a mistake and have now corrected it.

You snooze, you lose.  But there are other options if you know where to look.
Dar

[ScottB]

Nature's Rex Dalton had a news article in last week's issue about the implications of White et al. 2006.

http://www.nature.com/news/2006/060424/full/4401100a.html

I also found a paper in the JHE to be a useful read for another independent perspective.

Abstract:
We tested the hypothesis that early Pliocene Australopithecus anamensis was ancestral to A. afarensis by conducting a phylogenetic analysis of four temporally successive fossil samples assigned to these species (from earliest to latest: Kanapoi, Allia Bay, Laetoli, Hadar) using polarized character-state data from 20 morphological characters of the dentition and jaws. If the hypothesis that A. anamensis is ancestral to A. afarensis is true, then character-state changes between the temporally ordered site-samples should be congruent with hypothesized polarity transformations based on outgroup (African great ape) conditions. The most parsimonious reconstruction of character-state evolution suggests that each of the hominin OTUs shares apomorphies only with geologically younger OTUs, as predicted by the hypothesis of ancestry (tree length = 31; Consistency Index = 0.903). This concordance of stratigraphic and character-state data supports the idea that the A. anamensis and A. afarensis samples represent parts of an anagenetically evolving lineage, or evolutionary species. Each site-sample appears to capture a different point along this evolutionary trajectory. We discuss the implications of this conclusion for the taxonomy and adaptive evolution of these early-middle Pliocene hominins.

Kimbel et al. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of Human Evolution in press.

[Daryl Habel]

Nature's Rex Dalton had a news article in last week's issue about the implications of White et al. 2006.

http://www.nature.com/news/2006/060424/full/4401100a.html

Thanks for bringing this up.  I hadn't realized the Dalton article was free online.  John Hawks' weblog has some interesting things to say about the Dalton article, CLICK HERE.

I also found a paper in the JHE to be a useful read for another independent perspective.

Abstract:
(snip)
Kimbel et al. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of Human Evolution in press.

Thanks for this also. I had not yet noticed this 'in press' JHE article.  The full list of authors is interesting:
William H. Kimbel, Charles A. Lockwood, Carol V. Ward, Meave G. Leakey, Yoel Rak and Donald C. Johanson.  I can't remember the last time I read a paper co-authored by Donald Johanson and a member of the Leakey family (Meave, in this case).  The conclusion of this paper, which looked only at specimens of A. anamensis and A. afarensis, supports the hypothesis that A. anamensis and A. afarensis represent parts of an anagenetically evolving lineage, but this is nowhere near as controversial as the idea that Ardipithecus ramidus also is a part of this evolving lineage.  As you may have noticed in the Dalton article, regarding the White et al position that Ardipithecus ramidus is ancestral to A. anamensis, Meave Leakey (co-author of the 'in press' JHE article you referenced) is quoted as saying, "I don't believe this....We do not have the specimens to fill the gaps."   So, although many are willing to accept the "ladder" for A. anamensis and A. afarensis,  when it comes to A. ramidus, the "bush" is not dead. 

I had to chuckle at Tim White's response, "There were Martians there back then too...And spacecraft all over the Pliocene -- we just haven't found them yet."   But we're all still patiently waiting for White to publish full descriptions for Ardipithecus ramidus specimens (including a reported partial skeleton) which his team found way back in the early 1990s.   As Hawks notes, "...it would help matters if we knew in more detail what Ardipithecus looked like." 

Dar