The question on your mind today, no doubt, is the same one troubling humans everywhere: Is Ardipithecus ramidus a bona fide hominin?
That is, is it a member of the evolutionary lineage that is exclusively human?
Remember, the discovery team (White et al.) said that it is. They based their assessment on anatomical traits, which is standard practice. And their conclusion has many significant implications, some of which are outlined here.
However, in last week’s issue of Science, not only was Ardi’s habitat questioned, but in a separate Technical Comment, Esteban Sarmiento argued that Ar. ramidus is NOT a hominin.
According to Sarmiento, White et al.’s “analysis of shared-derived characters provides insufficient evidence of an ancestor-descendant relationship and exclusivity to the [hominin] lineage. Molecular and anatomical studies rather suggest that Ar. ramidus predates the human/African ape divergence.”
As with the habitat paper, White et al. published a reply to Sarmiento’s paper alongside it.
Here is a rough outline of the issues that are being debated.
What was the LCA like? Did you infer it correctly? Can you infer it at all?
In the initial paper, White et al. made a list of “inferred” traits of the LCA (hypothetical last common ancestor of chimpanzees and humans) to better assess the evolutionary trends seen in the hominin fossil record and determine where Ar. ramidus fits.
Sarmiento didn’t like that because they didn’t describe how they determined these character states. Furthermore, he says that the LCA traits are based on an assumption that the chimpanzee condition is the primitive one and the human condition is the derived one. Their LCA is too chimpy. This is an interesting accusation considering that much of the media and popular hoopla after the announcement of Ardi was about how chimpanzees could be more derived than we think and that chimps may not be good living models for the LCA!
In fact, this was so much the takeaway message in that flurry of Ardi papers that a separate group of scientists, led by Andrew Whiten, felt compelled to write a piece defending the significance of chimpanzees for understanding human evolution and the LCA!
What links ardipiths to australopiths? Are the traits exclusive to
According to White et al., Ar. ramidus shares features with later australopiths (A. anamensis and A. afarensis). But Sarmiento says that White et al., “fail to show that the common Ardipithecus/Australopithecus characters provide evidence of an ancestor-descendant relationship and are exclusive to the hominid lineage and shared-derived with humans.”
Since over half the traits are linked to the canine-premolar complex, Sarmiento says this makes for a weak argument given its historically deceptive nature (he cites his own work here) and given how much variation there is in the hominoid (ape) fossil record in the complex. He goes on to hypothesize that this trait, given how it varies, could have experienced multiple shifts along the continuum of nearly absent to strong presence.
But here’s the rub: Sarmiento argues that a human-like canine-premolar complex is NOT diagnostic of early hominins and does not indicate a strong relationship between ardipiths/australopiths any more than it suggests a strong ancestor-descendent relationship between oreopiths/ardipiths or sivapiths/ardipths since those fossil apes also show some so-called derived features in the canine-premolar complex.
White et al. point out that this trait in ardipiths has been firmly established and accepted in the 15 years since the first ardipiths dental remains were published. Hmm. So, what’s left then for diagnosing early
Beyond the canine-premolar complex, says Sarmiento, the evidence is also weak for Ar. ramidus’s link to australopiths, and hence, its hominin status. He says that “none of the eight postcranial characters [...] are useful because they are not exclusive to humans or even shared-derived with humans. Moreover, the other four craniodental characters are just as useless for the same reasons. Sarmiento goes on to argue that the traits that White et al. uses are present to some degree or another in earlier fossil apes (i.e., Oreopithecus and Dryopithecus), “and have appeared independently in other primate lineages.”
So that pretty much, according to Sarmiento, renders all the traits that were highlighted by White et al. useless.
You can’t help but wonder what, if anything, in the anatomy of a fossil is helpful for diagnosing its evolutionary position! And this sentiment seems to be behind the rebuttal-challenge of White et al.: Where are the new ideas and analyses, beyond the dismisses and disses?
Was Ardi really bipedal?
The interpretation of Ar. ramidus bipedality is a problem for Sarmiento. He briefly mentions that he’s not convinced by the foot morphology and notes that the femur and pelvis are so fragmentary that they are “open to interpretation” which seems like a nice way to put “they could show anything a person wants them to show so they should be ignored.” Sarmiento says that the bipedal traits cited in Ardi, “also serve the mechanical requisites of quadrupedality, and in the case of Ar. ramidus foot-segment proportions, find their closest functional analog to those of gorillas, a terrestrial or semiterrestrial quadruped and not a facultative or habitual biped.”
Are there features that indicate that Ardi was not a hominin?
Yes, says Sarmiento. He lists some primitive African ape traits in the wrist and cranium which were claimed by White’s team to be exclusively hominin traits.White et al. don't address the wrist anatomy but in their discussion of the cranial anatomy they argue that Sarmiento's interpretation of the character states is not parsimonious.
Just look at the molecular clocks!
Sarmiento writes, “Over the past 40 years, a multitude of independent biomolecular studies based on different methods, some analyzing millions of DNA base-pair sequences, have arrived at a minimum human/ African ape divergence date of ~3 to 5 million years before the present.”
If you're furrowing your brow, you’re not alone. Go to timetree.org. Type in humans and chimpanzees. Do you arrive at a consensus of ~3 to 5 million years? Ken didn’t. Neither did I.
Sarmiento writes, “With a 4.4-million-year geologic age, Ar. ramidus probably predates the human and African ape divergence.”
Uh, then this means that Orrorin and Sahelanthropus (two strong candidates for early homs) are also too old to be hominins and this means that we can ignore the majority of molecular clock studies that estimate the split to be much earlier like 5.6 to 7 Ma. White et al., point all this out in their rebuttal.
Conclusions on the ‘No’ side.
Sarmiento ends with food for thought, “Even if Ar. ramidus was an exclusive member of the human, chimpanzee, or gorilla lineages, given its proximity in time to this divergence date, it would be difficult to unambiguously recognize it as such.”
- Clearly it’s difficult to figure out, otherwise there wouldn’t be cause for debate.
“It therefore seems premature to use Ar. ramidus to directly infer LCA ecology and locomotor anatomy or the origin of supposed human social systems, selection strategies, and sexual behaviors.”
- Good cautionary advice, but it won’t stop people from trying. The evidence in the fossil record is now there - why not use it?
“Human evolutionary studies are not a new science where every new find revolutionizes interpretations of our past. In fact, what is known of LCA anatomy and ecology is based largely on comparative studies of human and nonhuman primates. These same studies allow us to classify fossils and recognize ancestors. A purported fossil ancestor that must overturn nearly all we know about our evolution to fit into our lineage is unlikely to be such an ancestor. In this regard, it is curious that in a century-old race for superlative hominid fossils on a continent currently populated with African apes, we consistently unearth nearly complete hominid ancestors and have yet to recognize even a small fragment of a bona fide chimpanzee or gorilla ancestor (29).”
- This echoes a bit of what Whiten et al. wrote and sounds like a lot of grumbles about Ardi.
Conclusions on the ‘Yes’ side.
White’s team ends their reply with, “The character distributions we noted in the pelvis, C/P3 complex, and basicranium are consistently indicative of a sister relationship of Ar. ramidus with Australopithecus (and later hominids). For Ar. ramidus to be a stem species of the African ape and human clade as Sarmiento advocates, its highly derived C/P3 complex morphology, basicranial shortening, and iliac structure must have first emerged in some yet-unidentified Miocene ancestor before then reverting to an African ape–like condition. Such multiple, nonparsimonious character reversals are highly unlikely.”
Basically they say that their interpretation of Ar. ramidus is the most, and potentially only, parsimonious one. But somehow, in doing this, they change the rules of parsimony for phylogenetic interpretations of fossil apes that that precede Ardi and this is sort of what Sarmiento is getting at and this is what we discussed.
What’s primitive? What’s derived? Where’s parsimony? Where’s homoplasy? How much can converge and how often?
I’m getting a head ache, are you?
Both teams do a lot of citing themselves in their papers. Not to discredit or to diminish, by any means, any of their previous research, but this behavior suggests a narrow, potentially polemical view of the field and of how to conduct paleoanthropological research.
And this leads me to wonder....
How could we remove personality, personal stakes, reputations, etc. from paleoanthropology?
A Modest Proposal: Blind Paleoanthropology
Other scientific fields have strategies for removing subjectivity and potential bias from scientific research. Why not implement blind studies on fossils? This goes beyond what number-crunching, taxon-free statistical analyses can do. I’m talking about the hands-on part.
Here's a preliminary outline of the process:
1. Discover an important fossil.
2. Immediately make high quality replicas and take high quality photographs.
3. Send them to a central clearing house.
4. While you begin your analysis, the clearing house begins its duties.
5. The clearing house finds a suitable researcher or team of researchers to analyze the specimens and they, obviously, agree to do it.
(These researchers will need to have access to comparative data, both extant and fossil. This may be the trickiest part, but it still doesn't shoot down this idea totally.)
6. The clearing house sends off the replicas and photographs to the blind team and it's free of all other information. Nothing about identity of discoverers, site or even geographic region, age, associated artifacts, fossils, etc. is included. Nothing. The only thing the blind team knows is that these things came from somewhere on Earth.
7. Discovery team and blind team perform their taxonomic and morphological analyses and come to preliminary conclusions about the species designation (obviously only the discovery team is permitted to name the new species if one is required) and about the individual’s morphology, posture, locomotion, etc…
8. Through the middle-man clearing house the two teams publish in concert.
The Result: Paleoanthropology is more objective, more scientific, more accessible and, potentially, more efficient.
Who would serve as the clearing house middle-man/woman? It could be a rotating position within AAPA. People who want to be the blinds can sign on to a list where they describe the kinds of resources that they have or could have if given the opportunity to be the blind. There are certainly plenty of us to fill this roster.
I don’t think this strategy undermines the important role of experience. If your experienced team finds one thing and a “blind” postdoc finds a completely different thing, then experience may indeed prevail until further studies are performed to support one or the other interpretation.
However, the blind will have an invaluable opportunity to see the anatomy plain and simple. What it is and nothing more. I'm jealous of these hypothetical "blind" people. It sounds like a thrill. Plus, their results will have the potential to be completely stimulating hypotheses for future workers to test!
Questions for MT readers:
Has this already been done?
How could we improve this proposal?
Or are there too many limitations to implementing such a process?
Blind paleoanthropology: Why not?
Esteban E. Sarmiento. Science 328, 1105-b (2010); Comment on the Paleobiology and Classification of Ardipithecus ramidus.
Tim D. White, et al. Science 328, 1105-c (2010); Response to Comment on the Paleobiology and Classification of Ardipithecus ramidus.