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Friday, May 28, 2010

Ardipithecus ramidus paleoenvironment: Not a paradigm shift after all

The current issue of Science contains two letters addressing Ardipithecus ramidus ("Ardi") as described by White’s team in Science last October. Both have a similar message: contrary to the claims made by authors of October’s Ardi papers, Ardipithecus and its surrounding environments do not change everything we know about the origins of exclusively human ancestors. Esteban Sarmiento addresses the skeletal evidence of Ardi itself and raises many of the issues I raised when I blogged about it.

But what I didn’t address at the time was the environment. White and colleagues reconstructed Ardi’s environment as a fairly humid and closed woodland. This, they argued, overturns the conventional ‘wisdom’ that hominids originated in a savannah environment.

However, Cerling and colleagues present their reinterpretation of the data presented by White and colleagues, and reach the opposite conclusion. They find, rather, that all lines of evidence point to a relatively more open (i.e. grassland) habitat for Ardi. First of all, ancient soil analyses give a strong signal that Ardi’s habitats contained a very large C4 plant component – plants like grasses rather than woody vegetation like trees. Even compared to other hominid sites, Aramis appears to have one of the highest C4 (i.e. grass) biomasses. Second, the water deficit value of Aramis (1500 mm) is completely consistent with a dry woodland in a riparian (river) environment. In fact, the Aramis value “is similar to values in some of the hottest and driest parts of eastern Africa today,” Cerling and colleagues note. Third, there are more species of grazing (on grass) than browsing (on leaves) hooved animals at Aramis. Finally, Cerling and others argue that the way other data were presented or interpreted by the Ardi team were misleading and/or downplayed the evidence for a more open, grassy environment.

So where did Ardipithecus live - an open grassy environment, or a more closed and forested one? What does its reconstructed habitat mean for hominid origins? This is an important question, because as I've argued before it's not too clear that Ardi is actually a hominid, or whether it is a hominid that had already diverged from later hominids that were ancestral to us humans. The apparent misrepresentation of Ardi's environment, and the stress placed on hominid traits for which there is no direct or unambiguous evidence (i.e. lumbar curvature inferred even though no lumbar vertebrae or a sacrum are known for Ardi), suggests that all the hype around Ardipithecus is just that - hype. Indeed, a sexy find that 'overturns everything we know about hominid origins' is bound to garner glorious funding.

References

Cerling T et al. 2010. Comment on the Paleoenvironment of Ardipithecus ramidus. Science 328: 1105-d.

Sarmiento E. 2010. Comment on the Paleobiology and Classification of Ardipithecus ramidus. Science 328: 1105-b

1 comment:

Marcel F. Williams said...

This is really just a backlash against those who question the validity of the molecular clock. Anyone who takes any molecular clock estimate seriously is fooling themselves. Molecular evolution may be phylogenetically useful, but a clock, it is not!