Researchers call B.S. on Ardi’s habitat

Raise your hand if you think I’m talking about this Ardi.

Well, I’m sure that there are plenty of people calling B.S. on his parents, but that’s not the Ardi I’m talking about.

This is the Ardi that I’m talking about.

Ever since the papers were published by White’s team in Science last fall, we’ve been waiting for the public debate to unfold and now it's well underway with two Technical Comments in Science last week.

The first paper questions the phylogenetics and functional morphology of Ardipithecus (to be discussed in a later, separate post) and the second, by Cerling, Levin et al., questions the paleoenvironmental interpretations for the creature from Aramis, Ethiopia. Both were published alongside rebuttals by White’s team.

Let’s talk about habitats first…

White’s team originally claimed that Ardipithecus habitats ranged from woodland to forest patches and that, “…early hominids did not evolve in response to open savanna or mosaic settings.”

“So what?” you might be asking.

Well, it's a big what! If Ardipithecus is indeed an early biped, then bipedalism did not evolve in a grassy place, but rather a wooded one.

Ever since the 1960s, the prevailing hypothesis, known as the “savanna hypothesis,” has more or less been used to explain the evolution of human-ness.

Savanna-living, according to the savanna hypothesis, is what separated hominins—the exclusively human branch on the evolutionary tree—from the other African apes. Moving about on the ground, rather than in the trees, was a selective force in the evolution of bipedalism. And, of course, there are all of the other evolutionary ramifications of a shift towards savanna ecology. So if Ardipithecus did not live in a savanna, then this is some major evidence refuting the savanna hypothesis. But if White’s team is wrong, then the hypothesis lives on.

Well, what’s a savanna? It’s actually much broader than “grassland” and can include wooded aspects as well. Here is how the United Nations Scientific and Cultural Organization (UNESCO) classifies and defines African vegetation (as summarized in Cerling et al.):

1. Grassland is land covered with grasses or other herbs, either without woody plants or the latter not covering more than 10 per cent of the ground. (What people call “savanna.”)

2. Wooded grassland is land covered with grasses and other herbs, with woody plants covering between 10 and 40 per cent of the ground. (Also what people call “savanna.”)

3. Scrub woodland has a canopy height less than 8 m, intermediate between woodland and bushland. As proportions of bushes, shrub, and grasses increase, woodlands grade into bushland/thickets or wooded grasslands (above). (Savanna-ish)

4. Woodlands have trees with canopy heights of 8 to 20 m; their crowns cover at least 40% of the land surface but do not overlap extensively. Woodland ground layer always includes heliophilous (sun-loving, C4) grasses, herbs/forbs, and incomplete small tree and shrub understories.

5. Closed woodlands have less continuous canopies and poorly developed grass layers.

6. Forests have continuous stands of trees with overlapping crowns, forming a closed, often multistory canopy 10 to 50 m high; the sparse ground layer usually lacks grasses.

Okay, first of all, both teams (both Cerling et al. and White et al.) agree that open savanna grassland was not the environmental context of Ardipithecus. In other words, number 1 is out.

However, White’s team says that the conditions at Aramis were between numbers 4-5, but Cerling and Levin’s group prefer 2-3 which include a patchy riparian forests set within a dry, savanna landscape.

[What’s at stake here is the issue of interconnectivity: Could a hominin move from tree to tree through the forest canopy without coming down to the ground, or not? In choices 5 and 6 above, it could, but in 1-4, it couldn’t.]

Researchers on both sides of this debate address the gamut of information used to piece together the scene...

Some Major Sources of Paleoenvironmental Evidence

1. Stable carbon isotopes in paleosols (ancient soils)
2. Oxygen isotopes of mammalian tooth enamel used to determine paleoaridity
3. Carbon isotopes in mammalian tooth enamel
4. Relative abundance of phytoliths (microscopic silica particles from plants)
5. Relative abundance of micromammal fossils
6. Vertebrate species: types of birds, large and small mammals, gastropods and fossil wood preserved in association with Ardipithecus.
7. Vertebrate morphology: For example, Browsers (tree and shrub eaters) vs. Grazers (grass eaters)
8. Geology and taphonomy of fossil assemblage (How the fossils got there)

However, Cerling and Levin’s team emphasize the importance of 1-4, while White’s team emphasizes 5-8.

Regarding 1-4, there is a fundamental disagreement between the two camps over what samples, both ancient and modern, to include in the analyses. Both have used the same data, but with different comparison data or on different scales (regional vs. small-scale habitat) and have come to very different conclusions.

White et al. end their rebuttal with a comment on the implications of their results on the savanna hypothesis.

“The vision of apes trekking bipedally between increasingly isolated forest patches has maintained its allure across decades of research. The repeated discovery of obligatorily bipedal, megadont Australopithecus in later open habitats erroneously reinforced this notion of bipedality’s beginnings. It was perhaps inevitable that proxy records reflecting global shifts in carbon isotope values would be postulated as the missing piece of the puzzle of hominid origins (22).”

That sounds a little like a dig at Cerling’s team’s work, citing their 1997 Nature paper. But I could just be overly sensitive to these things. Anyway, White et al., continues…

“By focusing too coarsely on the regional environment, Cerling et al. seem to overlook evidence that differentially and consistently links Ardipithecus to a woodland habitat and thereby distinguishes it ecologically from Australopithecus. We contend that compared with Ar. ramidus, Australopithecus was more ecologically flexible, probably ranged more frequently and further into the open environments that Cerling et al. term “tree or bush savanna,” and evolved remarkably distinct and highly derived dietary and locomotor adaptations to this end. After assessing the totality of the pertinent environmental and ecological evidence, we concluded that Ar. ramidus preferred the more wooded habitats among the available spectrum in the regional geography: “...the integration of available physical and biological evidence establishes Ar. ramidus as a denizen of the closed habitats along this continuum”. The Aramis evidence is not easily accommodated by an environmentally deterministic view that involves globally shrinking forests spawning savanna-striding hominids. We contend that this narrative is now undermined by the totality of data from 4.4-Ma Aramis. These rich, diverse data are spatially and chronologically intimately associated with Ardipithecus, thereby providing an unparalleled view of the early hominid niche within the larger geographic setting.”

Until others can grasp the “totality of data” from 4.4-Ma Aramis in a similar way, I suppose this new view of early hominin evolution will remain something to “contend” rather than something for others to readily see and accept.

References

Thure E. Cerling, et al. Science 328, 1105-d (2010); Comment on the Paleoenvironment of Ardipithecus ramidus.

Tim D. White, et al. Science 328, 1105-e (2010); Response to Comment on the Paleoenvironment of Ardipithecus ramidus.