Dikika

The coming attractions bin at Journal of Human Evolution includes a paper by Kaye Reed, reviewing the evidence of paleoenvironment in the Hadar formation:

Habitat reconstructions of 12 submembers of the Hadar and Busidima formations (˜3.8-2.35 Ma) are presented here along with faunal differences in these submembers through time. Habitats with medium density tree and bush cover dominated the landscape through much of the earlier time period in the Hadar Formation. The lowermost Sidi Hakoma Member is the most closed habitat. The Denen Dora Member shows the influence of frequent floodplain edaphic grasslands with high abundances of reducin bovids. There is an influx of ungulates in the Kada Hadar Member (˜3.2-˜2.96 Ma) that indicates a more arid habitat populated by mammals that were recovered from earlier deposits further south in Ethiopia and Kenya. In the younger deposits from the Busidima Formation at Hadar, the landscape was open wooded grassland with some floodplain environments. The fossil assemblages from the Busidima Formation show a substantial species turnover. Although high numbers of A. afarensis specimens are associated with the lower Sidi Hakoma Member, they clearly inhabited a variety of habitats throughout the entire Hadar Formation. Australopithecus afarensis from Laetoli through Hadar times appears to have been a eurytopic species.

This is a nicely detailed paper, focusing on the amount of wooded/bush habitat, the relation of the hominids to those habitats, and the relative lack of early faunal exchanges with areas further to the south.

The discussion focuses on the range of paleoecologies in which fossil A. afarensis has been found -- including not only Hadar but also nearby Maka and Dikika, and more distant Koobi Fora and Laetoli. Altogether, these localities cover a long time (from before 3.5 up to around 2.9 million years ago). From the range of paleoecologies reconstructed in this paper at Hadar, Reed concludes that A. afarensis did not have a "narrow" habitat preference. It is found in relatively closed woodland, open woodland/bush, and wet grassland/marshland.

There are some differences between localities. At Koobi Fora, relatively few specimens of A. afarensis have been found in the Tulu Bor Member, despite the fact that it occupies the same time as the Hadar sequence. Based on the paleoecological data, Reed suggests that Hadar was a wetter, more closed woodland habitat than Koobi Fora at that time -- Koobi Fora would have included more scrubland punctuated with wetlands and floodplains (here she cites her own 1997 paper).

The early end of the A. afarensis sample is represented at Laetoli. Reed gives a brief review of the paleoecology of that site, which has been interpreted differently by different authors but broadly appears to have had a fairly high amount of rainfall and some patches of forest amid closed woodland:

Thus, the earliest known A. afarensis material was found in deposits showing habitats in which trees and or bushes were fairly plentiful. It is also interesting to note that while the deposits of A. afarensis at Laetoli and Hadar share some perissodactyls, giraffids, suids, and proboscideans, the bovid taxa and those primates other than A. afarensis are not very similar.

Reed concludes that A. afarensis was a "eurytopic" species -- one that inhabited a wide range of habitats and moved broadly across space. It contrasts with the more habitat-selective ("stenotopic") species, which include most of the bovids.

White et al. (1993) suggested broad habitat tolerance for A. afarensis, and indeed, the species has thus far been recovered from regions in which the reconstructed habitat ranges from closed woodland through more open, but wet woodland and shrubland. There is no direct evidence that A. afarensis only existed in riverine forests or grassland habitats, or that they preferred one habitat over another. It is tempting to equate the aridification in the Kada Hadar Member with the extinction of A. afarensis. However, sediments at Hadar are sparse or missing altogether from ˜2.90-2.35 Ma thus obscuring details of the species' demise. All that can be said is that they are no longer present at 2.35 Ma and most of the fauna, including hominins, has been replaced.

References:

Reed KE. 2008. Paleoecological patterns at the Hadar hominin site, Afar Regional State, Ethiopia. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.08.013

The online companion site to Scientific American is running commentaries by a few paleoanthropologists on the importance of the new DIK-1-1 skeleton. It's an experiment in online publishing, and it has turned out some very good material, including commentaries by Owen Lovejoy, Ralph Holloway, Diana Roman, and, well, me.

These other commentators and the press of the last week are hard to follow, but actually my second thought about the skeleton has turned out to be fairly original:

The new Dikika skeleton has me wondering one thing: did Selam sink Kenyanthropus?

If you can guess the connection, you've been reading entirely too much about early hominids!

Congratulations to Kate Wong on a really innovative idea -- they have an opportunity for other folks to contribute as well, so dive in!

With apologies to the late Frank Livingstone, I couldn't help but wonder this as I read this passage from Bernard Wood's comment on the Dikika skeleton:

I am especially intrigued by the detailed morphology of the hyoid bone in the throat of the fossil. Does the open space in the body of the hyoid mean that A. afarensis had air sacs in its neck? In the absence of large canines, these air sacs might have been a way in which males established a dominance hierarchy, and females judged the quality of a potential mate.

Most apes have air sacs associated with their larynx, and these were lost sometime during human evolution, because we lack them. So it wouldn't be especially surprising if australopithecines retained this ape-like character.

Hewitt, MacLarnon and Jones (2002) suggested that the loss of air sacs in human evolution was pertinent to fine control of breathing related to language:

A possible function of laryngeal air sacs in apes and gibbons was investigated by examining the relationships between air sac distribution, call rate, call duration and body weight in a phylogenetic context. The results suggest that lack of sacs in the smaller gibbons and in humans is a derived feature. Call parameters in primates, such as rate and duration, scaled to resting breathing rate (and therefore to body weight) only in species without air sacs, which appear to modify these relationships. Apes and larger gibbons may be able to produce fast extended call sequences without the risk of hyperventilating because they can re-breathe exhaled air from their air sacs. Humans may have lost air sacs during their evolutionary history because they are able to modify their speech breathing patterns and so reduce any tendency to hyperventilate.

If this were true, there would really be no reason to expect australopithecines to differ from chimpanzees and gorillas in this regard. One might even imagine call sequences to be more important to them considering the possibility of higher predation (and the adaptive value of alarm calls) in more open habitats.

I don't think there is any need to infer a different function in australopithecines than in apes, based on the anatomy. Here is what Alemseged et al. wrote about it:

The hyoid of DIK-1-1 is only the second example in the hominin fossil record, and this element was previously unknown for any species earlier than Neanderthals. Its similarities with Pan and Gorilla hyoids suggest that the bulla-shaped body is the primitive condition for African apes and humans, rather than the more shallow, bar-like body shown by modern humans and Pongo. The bulla-shaped body almost certainly reflects the presence of laryngeal air sacs characteristic of African apes. However, the function of these structures is not well understood.

Fitch and Hauser (2003) wrote about ape laryngeal air sacs thusly:

A related possibility, the "accessory lung" hypothesis, is proposed here for the laryngeal air sacs of the great apes. Chimpanzees, orangutans and gorillas all have voluminous air sacs (6 liters in orangs, Schön-Ybarra 1995), that can be inflated with air from the lungs. The air sacs connect to the larynx via a long thin-walled channel that opens directly above the vocal membranes and vocal folds. The air sacs extend into the subdermal space in the pectoral region, and are overlain by the sheetlike platysma muscle. Thus, an ape could inflate the air sacs via lung pressure, and then forcibly deflate them by tensing the platysma and other pectoral muscles (or by pounding the chest, as in Gorilla). This anatomy suggests that great ape air sacs may act as "accessory lungs", providing an additional source of expiratory air flow and thus of energy into the source. This hypothesis seems more plausible than that offered by Negus (1949), who suggested that ape air sacs act as storage sites for oxygen during vigorous activity. Because the sacs are inflated with exhaled air that has already been in the lungs, and thus will be low in oxygen and high in CO2, such an air reserve would be of dubious respiratory value (Fitch and Hauser 1995) (Fitch and Hauser 2003:95).

Well, we're not looking at a 6-liter air sac for australopithecines -- but then it is odd that the orang and human hyoids should be similar considering the huge air sac in orangs and the lack of any in humans.

Aside from the problem of figuring out how apes actually use these laryngeal air sacs, I guess there's not much of a story here. But I'm beginning to appreciate the lack of surprises!

References:

Alemseged Z, Spoor F, Kimbel WH, Bobe R, Geraads D, Reed D, Wynn JG. 2006. A juvenile early hominid skeleton from Dikika, Ethiopia. Nature 443:296-301. Abstract

Fitch WT, Hauser MD. 2003. Unpacking "honesty": vertebrate vocal production and the evolution of acoustic signals. In Acoustic Communication, edited by Simmons AM, Popper AN, Fay RR, Springer-Verlag, New York. pp. 65-137.

Hewitt G, MacLarnon A, Jones KE. 2002. The functions of laryngeal air sacs in primates: a new hypothesis. Folia Primatologica 73:70-94. DOI link

Wood B. 2006. A precious little bundle. Nature 443:278-281. Full text (free)