Monday, June 7, 2010

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.


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.


occamseraser said...

Good insights, scientifically and politically. Thanks.

I thought White et al.'s debunking of the "savanna hypothesis" was just another straw (grass?) man. Or straw chimp. Who of late has championed this old notion for the origins of bipedalism? Some even argue today that bipedality was a truly arboreal adaptation (Pickford and Senut; Thorpe and Crompton). It seems pretty obvious that australopiths liked trees too (for food, sleeping, and escape) and traveled bipedally on the ground from one forest patch to the next.

Marcel F. Williams said...

Since the White team also showed a strong morphological connection between Ardipithecus and Sahelanthropus, this is really another attack on the hominin status of Sahelanthropus. The existence of Sahelanthropus pretty much throws the extremely flawed molecular clock hypothesis out the window.

Ken Weiss said...

The Mermaid's Tale Facebook page is alive with comments today. Most of them are recipes, and rather imaginative ones at that. One that isn't mentioned, however, is smoked Ardi (internally smoked, that is--the Indonesian boy). Perhaps with a side salad or something.

Holly Dunsworth said...

Definitely a side salad! And get that kid a multi-vitamin!

Ken Weiss said...

On a more serious note, for those of us who taught what seemed to be one of the obvious things about human origins--yes, the savanna hypothesis--this is a sobering lesson. It is one of our MT hobby horses: the energy with which biologists and anthropologists advocate evolutionary Just-so stories as just so.

The savanna hypothesis was alive and well at least as far back as when I was a graduate student (yes, really, that far back!). If now even that ain't necessarily so, then we as a profession should be even more circumspect about less secure inferences--the majority of our inferences.

Perhaps we evolved to be BS artists. Perhaps that took the form of bluffing when confronted by a lion, and those who were circumspect or restrained became lion-meat.

Holly Dunsworth said...

With White et al.'s interpretation of Ardi, the savanna hypothesis could remain in a modified form: Savanna ecology was a major factor driving the evolution of bipedalism, but earlier than that, when homs first split from pan, the savanna had nothing to do with it, including the impetus for bipedalism.

Marcel F. Williams said...

Oreopithecus was already bipedal 7.6 million years ago, and it lived in a swamp forest.

Ken Weiss said...

I claim no expertise in these details, but I would say that unless someone were to argue that every case of bipedalism had to involve coming down from the trees into the savanna, so to speak (even ostriches and birds?), then the Oreopithecus example would be irrelevant.

The hypothesis was that African apes, at some point, came down from the trees to move into the savanna, and this explains our particular bipedalism. Whether true or not, it would not (in itself) imply or be implied by whatever happened with Oreopithecus.

The more important thing about the Ardi argument (to me) is the clarity with which we all thought, for decades, that the savanna hypothesis was basically correct, and the degree to which it can be even questioned, or perhaps over-turned, by a new find.

The point, again to me, is the need for us all to be more circumspect in arguing for ideas, making it clear that we view them as possibilities rather than clearly proven.

But, of course, others may disagree about any or all of these points.

occamseraser said...

Oreopithecus was a highly suspensory, arboreal ape with phenetic and functional affinities to orangs and even giant extinct sloth lemurs. The "evidence" for bipedalism (and tool-making!) is less than compelling and ignores the basic biomechanics of bipedalism. Like Ardi, Oreo exhibits none of the adaptive hallmarks of terrestrial bipedalism. PNAS should be spanked for publishing the Oreo nonsense in the first place -- a classic failure of cronyism in "peer-review".

Ken Weiss said...

Holly will have to respond to this if she chooses. I daren't say a word, as it would quickly prove my ignorance of the issues! But it's good that you raise the issues from a position of knowledge