Ardi was the missing link that was bigger than a meteor hitting the Earth or whatever, right? Nope, that was Darwinius, also called Ida (see Science by PR Blitz). This one got nowhere near the press because they didn't have a book and TV show about it before the science was even revealed.
Ardi discoverers also don't claim it is anything more than what it is - a creature phylogenetically descended from whatever humans and chimps are descended from - but they made one claim about its habitat that has brought them under fire. Geochemist Thure Cerling, from the University of Utah, is lead author of a 'technical comment' in Science that objects to the contention that Ardi lived in dense woodlands. What difference does it make?
Though Cerling says he is not advocating it, it damages the credibility of the Savannah hypothesis, when what Cerling contends is the Ardi data actually supports it.
The Savannah hypothesis, was first proposed in 1925 by Raymond Dart, who wrote
For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect - a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.and it contends that the expansion of grassy savannahs prompted human ancestors to come down from trees and walk upright. Tim White of Berkeley and others published 11 studies in Science outlining 17 years of work in which they excavated Ardipithecus fossils along with 150,000 animal and plant fossils. They characterized Aramis, Ethiopia then as woodlands or forest patches.
Not so, says Cerling and anthropologists Richard Klein of Stanford University and John D. Kingston of Emory University, along with geologists Frank Brown of the University of Utah, Naomi Levin of Johns Hopkins University, Jay Quade of the University of Arizona, Jonathan Wynn of the University of South Florida, and David L. Fox of the University of Minnesota.
Instead, they say, Ardi lived in an area that was 5-25% trees or shrubs, well below the minimum 60% to meet the definition of a closed-canopy woodland. Cerling says Ardi could have lived in a wooded river corridor, but it was a river that flowed through savannah.
"The idea that savannas were important in human ancestors starting to walk on two legs means they spent less energy getting from one food source to another than they would have if they were still moving around on all fours," says Brown. "One of the big, newsworthy items that White and coworkers put forward is that Ardipithecus walked upright on two legs, yet lived in a forested environment,. They then say the savanna hypothesis – which holds that the reduction in forest cover in Africa is one of the reasons for early man becoming bipedal – must be incorrect."
Cerling says they used three of White's Science papers for their conclusion and phytoliths, tiny grains of silica in living plants that later become fossilized, tell the tale. The abundance of phytoliths can tell (your mileage may vary) if an area area was covered by grasses or woody plants. Who did phyoliths supports? Well, both. Cerling says 83 of 84 samples in the Aramis area indicate a significant amount of tropical grasses when the soils formed and the median amount of tropical grass by biomass produced was 40 percent to 60 percent - not a forest, if you use a particular description of a forest.
Carbon-13 levels were also unclear, he says. 13C is one three carbon isotopes in nature: as you can guess, each contain the same number of protons (six), but a different number of neutrons so the isotopes have different atomic mass: 13C is important in paleoclimatological research whereas 99% of atmospheric CO2 is the less heavy carbon, 12C. The deviation of isotopic concentration of 13C in any sample (d13C) is calculated as
(13C/12C)sampled – (13C/12C)standard
——————————––––––––––––––– x 1.000
but that means the d13C of paleosoils depends largely on the type of plant that grew on them, due to discrimination during photosynthesis. The 13C levels in tooth enamel from animals they found were interpreted by White and colleagues to indicate closed-canopy forest patches but Cerling says the values are instead consistent with animals that move from river-edge woodlands to savannahs, at least when it comes to modern animals that live in dense forests.
Cerling says the index of aridity White and colleagues used for Aramis "are comparable to the driest modern sites that have been examined" but that they claim "out of the blue" that the area was forested rather than being a savannah with wooded areas along a river.
A lot of this is minutae, of course, but science is about minutae, and Cerling says he isn't endorsing the Savannah hypothesis but that it remains viable. Does anyone in the paleoanthropological community still believe savannah environments were essential in human evolution?
The interest in clarity is always welcome but it might be time the Savannah Hypothesis was put out to pasture.
White and colleague's response is at (subscription required):
Tim D. White, Stanley H. Ambrose, Gen Suwa, Giday WoldeGabriel, 'Response to Comment on the Paleoenvironment of Ardipithecus ramidus', Science 28 May 2010: Vol. 328. no. 5982, p. 1105 DOI: 10.1126/science.1185466
and says their analysis "presented the totality of the relevant physical and biological evidence, including structural geology, geochronology, sedimentology, soil isotope geochemistry, paleoflora, taphonomy, terrestrial invertebrates, micromammals, and birds" and that "isotopic data in terms of the floral habitats leads to a systematic overestimation of proportion of grassland."
Thure E. Cerling, Naomi E. Levin, Jay Quade, Jonathan G. Wynn, David L. Fox, John D. Kingston,6 Richard G. Klein, Francis H. Brown, 'Comment on the Paleoenvironment of Ardipithecus ramidus', Science 28 May 2010: Vol. 328. no. 5982, p. 1105 DOI: 10.1126/science.1185274