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.

A chimp in our ancestry?

The recent announcement of a new 4.4 million year old human ancestor, named Ardipithecus, has led to a flood of papers, debates, and publicity. Found many years ago initially by Yohannes Haile-Selassie working in a team led by Berkeley anthropologist Tim White, this interesting set of specimens, and its unusually well-representative ecological environment (including diverse floral and faunal fossils), deserved the attention it was given. Holly described these finds, and the ensuing excitement, here.

We're triggered to write this post by having listened to the BBC radio program "Material World", in which White and Haile-Selassie were interviewed when attending a Royal Society gala meeting to announce and celebrate the finds.

Part of the discussion was about 'missing links', as is so often the case when fossil finds are the subject. The discussion eventually turned to a recent and rather sensationalized story about a truly remarkable find in Germany, a 47 Million year old primate named Ida (image above). This was touted in the media (though not in the Science article last May itself) as a fabulous missing link, our earliest known ancestor that separates apes and humans.

More recently, a find in Egypt of the fossilized jaw of a similar creature, called Afradapis (figure at left), put Ida into a broader and less sensationalized framework, of lemurs and other ancestors of monkeys, apes, and humans.

Ida was remarkable on its own, without the hype. In particular, the idea that one find revolutionizes our understanding becomes less and less tenable the more pieces of the history we have. To truly overturn a century's accumulated knowledge in paleontology (not to mention comparative embryology, genetics, and morphology) would take something that even our over-adrenalized media-run society would recognize as really, truly, genuinely new! Not this one.

It should be interesting enough that the fossil was complete, with fur and all. It didn't need to be our Lost Grandmomma. Well, actually, Ida was a yearling female, probably never reproducing and hence not the ancestor of anyone at all; that's another lesson: what we mean, if we were to speak carefully, is that Ida is a member of a group among which were distant members of lineages that survive today, such as ourselves. The lesson, one that is perhaps not in the interest of the media, or even that of some scientists to learn, is not to claim too much.

As Holly said in her earlier post, the Ardi finds present a kind of pastiche of changes in different traits, rather than a simple linear trend from our common ancestor with the chimp and gorilla. The interpretations are already controversial and debated, but that doesn't diminish the relevance of the finds.

In the interview, Tim quite properly and enthusiastically praised Charles Darwin's prescient predictions about human origins, and that earlier fossils would be more primitive than recent fossils relative to modern humans. But one needs to be careful about such statements.

When you read Darwin, you regularly see very cogent arguments, and diverse facts impressively brought to bear. Darwin's books presented a consistent, multi-faceted view and his predictions like that related to human origins could be strikingly on target as we understand things today. But we should be careful not to use a post hoc sieve to make Darwin into something other than what he was. He could be incredibly prescient, but he could also be dramatically wrong, and some of his arguments seem now to be thoroughly naive. His reputation rightly survives his gaffes (some of which were simply
extending his theory beyond the data available at the time). But, while this is not meant to denigrate Darwin in any way, he was human and selective after-the-fact praise could suggest that he was more than the already remarkable person he actually was.

There are other trip-wires in evolutionary discussions, too, that we should be wary of. The Material World moderator, Gareth Mitchell, referred to the surprising lack of resemblance of Ardi to chimps. Even Tim, easily one of the world's leading paleoanthropologists, agreed that Ardi was surprisingly unchimplike. But that was then, and chimps are now. We should be more careful: the question should more properly have been the degree to which Ardi resembled our common ancestor with the chimp, not today's chimp. Did he show the way in which those lines diverged? Were its traits more like what we would expect in a line from ancestor to chimp, as if the more distinctive human traits came along later?

These slips of the evolutionary tongue are easy to make. If one wishes to argue that humans have changed much more than chimps relative to our common ancestor, then that's what should be said. But if we're sloppy, we can rightfully be accused, by creationists for example, of saying we descended from chimps--as suggested by the famous lampoon of Darwin drawn roughly when his Origin was published.

While we note here the other issues that arose, overall this story about these specimens was excellent, including its correctly noting that the lemur fossil was improperly and prematurely hyped, yet is still an excellent addition to our knowledge. Tim and Yohannes described the arduous nature of the finds and the value of the other, less glamorous plant and animal finds--and the patience it takes to assemble a synthesis of such delicate, piece-meal, fortuitous data, something that Holly also described in her post.

The program is worth listening to as a quick, well-done thumbnail sketch of new finds. We only note here the other issues that arose.

The Many Tales of “Ardi”

The Inspirational Tale

Common metaphors in paleoanthropology are those of winning the lottery and being struck by lightning. But although fossils are rarely preserved or exposed at just the right place to be spotted, the assumption that fossil discoverers are supremely lucky is an unfair one.

Eugene Dubois decided he’d find a missing link on Java, moved his family there, and found the first Homo erectus fossils. Louis and Mary Leakey dedicated their lives to traversing East Africa for fossils and they found beautiful ones. It took most paleoanthropologists years and even decades of hunting, sometimes walking over the same doggone dust every season, just to spot a tiny speck of something spectacular.

This is the kind of tenacity it took for Tim White and his team to amass a fossil record of as many as 36 Ardipithecus individuals and to collect over 150,000 plant and animal specimens from the Middle Awash study area in the Afar region of Ethiopia. Not only that but “Ardi’s” fragile skeleton had to be prepared sliver by tedious sliver in the laboratory before the analysis could be accomplished. Last week’s special issue of Science came 15 years after the first announcement of the project because of the massive amount of collection and analysis that followed.

It's no surprise that paleoanthropologists were glued to their computer monitors last Thursday and Friday. Many of us are still carrying around our stack of 11 papers, picking our way through them every spare moment we get.

The Paradigm Shifting Tale

This is not just a new skeleton. Depending on your take, this is a renovation or an entire upheaval of the way we interpret the last 23 million years of ape evolution. Here is a list of just some of these new perspectives that Team Ardipithecus has presented to us:

1. Ardipithecus is a hominid yet it is much more primitive than chimpanzees and gorillas which are both highly derived. (This one is exciting because it links Proconsul (the 18 million-year-old ape that I work on) to hominids.)

2. Anatomical and behavioral similarities between ape species that have to do with suspension and locomotion-in-general are independently evolved. That is, they’re convergences or homoplasies. Even these traits in the fossil apes Dryopithecus and Ouranopithecus do not link them phylogenetically to living apes because they too evolved independently as homoplasies. Conclusion: Ardipithecus shows that phenotypic and maybe genotypic homoplasy is rampant in the hominoid (ape) fossil record.

3. Chimpanzees and humans share a deeper last common ancestor than the one estimated to have lived 6 million years ago (mya). Although the 6 mya hypothesis is currently “in vogue,” the last common ancestor actually lived more like 7-10 mya.

4. Even though the split between the chimp and human lineages occurred 7-10 million years ago, Ardipithecus from between 4-5 mya is our best model for the last common ancestor, better than any other fossil apes from Africa from between 6-11 mya such as Orrorin, Sahelanthropus, Chororapithecus, and Samburupithecus.

If paleoanthropologists were stifled, rather than stimulated, every time something new overturned old ideas, they would quit the field, or flunk out.

However, the suite of game-changers proposed by Team Ardipithecus are bound to raise not just eyebrows but hackles as well.

For example, they explain the ape evolutionary tree with multiple homoplasies (rather than with parsimony or the simplest explanation: that the traits evolved once in a common ancestor), but then they use parsimony analysis (which excludes homoplasies whenever possible) to claim that Ardipithecus is a hominid. Furthermore, the claims about ape homoplasies are grounded in the assumption that Ardipithecus is a hominid in the first place. Could they have done it any other way? I don't know the answer yet. But as it stands, it makes an already large pill even harder to swallow. The whole field is now charged to test these new hypotheses and we certainly have our work cut out for us.

The Snarky Tale

Tim White’s papers are always thrilling to read. Usually the science is tight and methodical, which is highly satisfying, but you can also count on enjoying a bit of snark, and when I write “enjoy” I guess I’m being snarky.

Here’s an example: As the authors explain how un-chimp-like Ardi is, and how derived chimps and gorillas are compared to hominids, they write,

“Some hold that our last common ancestors with African apes were anatomically and behaviorally chimpanzee-like, that extant chimpanzees can be used as “time machines” and/or that unique features of Gorilla are merely allometric modifications to accommodate its great body mass.”(White et al., Science 326(64): 75)

These hypotheses of "Some" paraphrased and quoted by Team Ardipithecus may not sound so strange to any of you. They don't to me. But we're being admonished. Sure, the authors can forgive Darwin for thinking this way because he was forced to work without a fossil record. But the rest of us seem to have no excuse even though up until now we’ve all had to do paleoanthropology without Ardi.

A decent hypothesis for what our last common ancestor with chimps looked like is that it looked a lot like chimps. It’s pretty simple. And it’s only a hypothesis, not a personality trait. And it’s going to remain a decent hypothesis until scientists beyond these authors also support Ardipithecus as a hominid.

If they must lash out at colleagues, the authors should wait until after we've ignored their findings or rejected their arguments. Bashing us over the head the second we get a chance to read about Ardi is something Ardi might do.

The wait is over....Ardipithecus skeleton revealed

My own comments are coming soon, but in the meantime, here are just some of the flurry of reports about Tim White and his team's long-awaited Pliocene hominin skeleton of Ardipithecus ramidus. I've been waiting my whole adult life for this announcement so the expectations are pretty high.

Time Magazine
Nature News
The Guardian
The Loom @ Discover
ScienceNOW Daily News (includes long movie featuring Tim White)
New York Times

I won't be able to read the paper until it is available to the rest of us, but until then this passage from The Guardian is terribly intriguing:

"Many of Ardi's features are far more primitive than those seen in modern apes, suggesting chimpanzees and gorillas have evolved considerably after they split from the common ancestor they shared with humans."