Africa

OK, I'm going to live-blog this show. I've been looking forward to it for a while -- I loved the old NOVA series with Don Johanson and have often showed it in classes but I had to stop several years ago because it's getting out of date. These are great overview-type programs, unlike the more special-purpose one-topic shows.

The producers gave me the opportunity to review the program's script a few months ago (that's explains the acknowledgement at the end), so I'm not expecting any unpleasant surprises.

The pre-credits opening: Naked people smiling. Naked chimps grooming...

7:01: "What set us on the path to humanity? The questions are huge, but at last, there are answers..."

"For millions of years, many human-like species coexisted on our planet, until one day, there was only us."

7:03: "Apes that had walked on four legs stood up and walked on two." We see apish CGI hominins. Then, to the Sahara to see Toumaï. Michel Brunet is describing the skull.

"We, Homo sapiens, are the first ever to be alone."

7:06: To the Afar, explaining the Rift Valley and its erosive contexts. The Insta-Zoom effect across the desert is actually kind of cool. We see Zeresenay Alemseged driving an SUV, then walking in badlands with scattered bones. Nice photographs of the Dikika skull in context.

7:09: Zooming backward into a timeline, as if the years are sucking us back, the program explains the timespan of human evolution as a series of doublings backward in time.

7:10: Alemseged is in the National Museum of Ethiopia, preparing the skull. It's a nice video treatment, shoing the slow preparing with dental drill. The long shots of the postcranial elements are very illustrative -- this is a good demonstration of how the anatomy informs us about the developmental schedule and lifeways.

7:13: Don Johanson is explaining how he found AL 129-1. Then, he explains the difference between the chimpanzee and human pelvis. Too bad they couldn't have included Ardipithecus; it would be interesting.... I'm really liking the fact that you have people interacting with actual casts instead of lots of CGI images. You have a much better impression of the scale

7:15: Now the scene moves to Kenya, this is going to be about paleoenvironments. Yannic Garcin and Daniel Melnick are describing how the now-desert landscape was once much wetter. We go back to the Afar, with Alemseged explaining the fauna that's just eroding up out of the ground (wonder how set up that scene was...).

7:18: Bipedalism. It's like Saturday Night Fevur. Brian Richmond appears to explain theories about why bipedality was adaptive. This is all accompanied by contemporary dancers wiggling around. Chimpanzee-like ancestors are illustrated with video of actual chimpanzees (wonder what Lovejoy is thinking...). Dan Lieberman is talking about energy budgets. People and chimps on treadmills hooked up to oxygen meters.

7:22: Mark Stoneking explains the molecular clock. "The dates that one almost always gets are 5 to 7 million years ago for when humans and chimpanzees shared a common ancestor."

7:24: We go to Chad. Brunet explaining why they needed to recover fossils from somewhere other than East Africa. "Everyone said 'no', there just aren't any [human-like] fossils there."

7:26: "There were no bones apart from the skull..." Er...

7:27: The skull is reconstructed with a CT scanner and then cast. Oops...the rest of the shots of casts are all taken directly from the skull, not the 3-d scan version. Nice artist's rendering of Toumaï here.

7:30: I'd hate to be one of the dancers walking by on the screen with the voiceover, "Walking upright didn't mean that they had big brains."

7:33: Brain growth in Selam. Hints of a longer childhood -- of course, at 330 cc, it's almost the size of a full-grown chimpanzee. Todd Preuss is discussing the evolution of the brain, showing us actual pickled brains of human and chimpanzees. Lunate sulcus -- was Selam like a human or a chimpanzee?

7:35: Ralph Holloway is describing the brain reorganization -- great shot of him with his collection of endocasts. The conclusion is that the lunate sulcus was human-like.

7:37: Now we have stone tools appearing, Brian Richmond explains how we recognize tools. Unlikely they were made by Australopithecus, because they didn't make them earlier. Skip forward to KNM-ER 1470, "the dawn of a new era, beginning around 2 million years ago." Tools were used for meat processing. Homo habilis was small in body size, but had a much bigger brain than Australopithecus.

7:41: Viktor Deak is reconstructing Homo habilis. I like it, more apish than the usual rendering.

7:43: "Africa's gradual drying trend was punctuated by bursts of rapid climate fluctuation." We see Rick Potts explaining the stratigraphy of a lake alternating with desert and volcanic layers over time. The idea of "variability selection" is explained.

7:45: Analyzing diatoms in layers of rock -- the species tell the alternation of shallow and deep lake levels. It's a record of strong fluctuations. We see rapid clips of three different scientists (Potts, John Kingston, and Mark Maslin) talking about water fluxes. It's a good way of explaining the climate instability -- although they could have gone a bit further: when they mention "Lake Victoria-sized lakes appearing and disappearing", for example, they might have pointed out that Lake Victoria itself has appeared recently.

7:48: Dust from ocean cores. Once again, it comes down to tiny sea creatures whose anatomy correlates with date.

7:50: We get a rapid montage reviewing the climate instability idea. Hmmm...I have to say that the very fast cutting of clips and louder music doesn't really add to the credibility of the idea -- it seems like something is being left out.

7:51: Rick Potts restates the variability selection argument. "Simple but revolutionary idea -- human evolution is nature's experiment with versatility...we are creatures of climate change."

That's the end. I think the paleoenvironment story was well done. The shots of how this science is done were very illustrative -- from the field to the lab, the program showed the fine layers of sediment and careful study of microscopic creatures.

On the other hand, the show may have gone a little too far in the "climate made everything happen" direction. I don't think the "variability selection" idea explains the origin of Homo, and while the program did briefly list alternative views about the adaptive value of bipedality, it left no doubt that African desiccation and loss of forest was the ultimate cause.

I think everything with actual fossils, dirt, or rocks was well done. In particular, we got a good view of most of the Selam skeleton, with the notable exception of the hyoid bone. These are the best available images of the specimen to date. Holloway's descriptions of endocast evolution were well done, placed in the middle of a big table of fossil casts. I like the solidity with which the program showed the fossil record. Hopefully the next two segments will also follow this technique -- much preferred over the CGI-reconstruction technique.

I will be out of the country for the next two parts of the trilogy, so I'll have to see if I can get them online. The NOVA Evolution website has the first episode online now, so there's some hope.

On occasion, I point out interesting findings from archaeological chemistry and microscopic study of site formation processes. Last month, I pointed to the ability to distinguish animal and plant fat residues on ancient artifacts. Before that, there was the discovery of flax fibers from the Upper Paleolithic of Dzudzuana Cave, Georgia.

In July, a paper by Paul Goldberg and colleagues described the "micromorphology" of the sediments from Middle Stone Age levels of Sibudu Cave, South Africa. The excavations at Sibudu have been able to distinguish many distinct stratigraphic units with distinctive spatial locations and compositions. Micromorphology involves looking at these sediments in microscopic detail, picking out small grains of crushed bone, charcoal, plant fibers, phytoliths, and other materials.

Goldberg and his colleagues were able to make some very cool observations. For one thing, they have charred drips of broiled grease:

Two types of amorphous organic combustion remains were identified in samples from Sibudu: a type with a typically vesicular texture and a type with a cracked texture. The first type was found as isolated bodies, subrounded with a diameter of 10 µm to 1 mm, and they exhibited no evidence of cell structure. Bubbles or vesicles give the bodies a highly porous nature, and they are often thin walled. The microstructure of these homogenous or finely heterogeneous isotropic particles and their droplet-shaped occurrence suggest that these bodies were originally fluid and that they underwent a degassing process but have since hardened. These bodies resemble char and are probably derived from the burning of flesh or animal fat (104-105).

Mmmmm...MSA barbecue. The other type of "amorphous organic combustion remains" are charred resins, from trees or seeds. Mmmm...MSA smokehouse.

Second, they have beds.

Because of its long fibrous nature, it seems that this material consists of herbaceous plants, possibly some type of sedge, reed, or grass. There is no evidence to suggest that this plant would have grown naturally in the rock shelter, and the presence of clay aggregates derived from the river valley found in association with the laminated plant fibers implies that the grass or reed was transported to the cave from the nearby Tongati River by the shelter’s inhabitants.

The compact and laminated structure of the organic fibers in this microfacies also suggests that, once brought to the cave, the grass or reed was subjected to compaction, most likely through trampling. Further evidence supporting the interpretation of trampling is seen in the stringers of charcoal, clay aggregates, and burnt bone that define horizontal and subhorizontal surfaces on top of and within the laminated organic fibrous material. Pieces of éboulis and lithic fragments also define surfaces wi thin the microfacies. The fact that this grass or reed was transported to the cave by humans and that once there it was influenced by human trampling suggests that this microfacies represents a type of constructed bedding. If this is the case, then Sibudu contains the oldest evidence for constructed bedding, significantly older than that reported at the open-air site of Ohalo in Israel (Nadel et al. 2004).

The bedding material was in some instances burned, in some instances swept or trampled in such a way that the regular alignment of the phytoliths was jumbled and disrupted. They interpret this as efforts to "maintain" the site -- in other words, housekeeping:

What seems a likely and reasonable scenario is that the original organic matter of this laminated layer of sedges, grass, or reeds was completely combusted, resulting in total ashing of the organic material. The calcitic ash in this microfacies was transformed through phosphatization, as evidenced by the presence of a few remnant pockets of phosphate in this microfacies. The fact that large crystals of gypsum often form directly below these phytolith layers provides suggestive evidence for the downward leaching of CO₃- or P-rich solutions.

Just an aside -- that is such interesting chemistry, like the organic materials and ash are melting down into the underlying deposits.

The association between microfacies 2 and 4 suggests that the sedges, grass, or reeds that were brought into the cave for bedding were usually burned and probably by humans when they no longer used the bedding. This observation explains the sequence seen in samples SS-6 and SS-5 of laminated nonburnt fibrous organic material grading into laminated burnt fibrous organic material with phytoliths (microfacies laminated type 2B); the sequence is finally capped by a layer of laminated phytoliths.

Why did they burn the stuff? The authors guess that they were trying to cut down on parasites:

Together, this evidence shows that not only were the occupants of Sibudu bringing grass or reeds into the cave—likely for the construction of bedding—but they were periodically burning them, possibly as a means to remove pests or insects that had colonized the beds. (Smoldering goat dung and organic matter can be observed in many parts of the Middle East, including Hayonim, where tick removal is one of the important objectives; P. Goldberg 1992, personal observation.)

The MSA at Sibudu dates to between 45,000 and 65,000 years ago, with the best evidence for bedding in the units that OSL puts around 50,000 years ago. The implications of the "site maintenance" and spatial characteristics of the site are mentioned in the paper's conclusion:

Organization of living space, and particularly a deliberate use of space, has been suggested by Wadley (2001) and also Binford (1996) as an important trait of culturally modern behavior, reflecting a more complex social organization. While the evidence from the laminated units at Sibudu may reflect such organization, the lack of evidence for such spatial organization, such as is the case for the lower homogeneous layers at Sibudu, should not automatically suggest that occupation in these units was any less complex.

If spatial organization of living space is a "modern" behavioral feature, it is one shared by Neandertals (I noted that briefly in a 2006 post). But then, it's shared by any number of invertebrates, also. I think the interpretation of this kind of behavior will have to wait until we have more sites investigated with comparable methods. As the introduction to the current paper points out, a lot of spatial information could be brought out of these micro-scale studies, if they were conducted routinely.

References:

Goldberg P, Miller CE, Schiegl S, Ligouis B, Berna F, Conard NJ, Wadley L. 2009. Bedding, hearths, and site maintenance in the Middle Stone Age of Sibudu Cave, KwaZulu-Natal, South Africa. Archaeol Anthropol Sci 1:95-122. doi:10.1007/s12520-009-0008-1

Today is Ardipithecus day. Eleven papers in tomorrow’s issue of Science describe the research on one exceptional skeleton (numbered ARA-VP-6/500, nicknamed “Ardi”) as well as more than thirty other individuals, mostly represented by isolated teeth with a few partial sets of teeth.

I have a lot of material to share about these papers and how they change things in paleoanthropology – so much that I can’t possibly fit it all into one post.

So I’m starting out with a basic overview of the main points, organized as an FAQ. Over the next few days I’ll be exploring some of the most central issues in closer detail: in particular,

How we now interpret the earliest hominins in light of Ardipithecus.

What the skeleton means for our understanding of the human-chimpanzee common ancestor.

How Ardipithecus relates to the first australopithecine, Australopithecus anamensis.

How the crushed pelvis became a 3-d model, and whether we should believe it.

Can Ardipithecus be consistent with genetic estimates of human-chimpanzee divergence time?

What was the locomotor adaptation of Ardipithecus really like?

How was the "Discovering Ardi" documentary feature?

I expect I’ll be posting a new story every day for the next week or so. This initial post will be the central location for the series, and here I’ll try to give the most general-interest information.

I will also have a short article coming out in Seed sometime this week, I will post a link when that is up.

I will be editing this post recurrently – I’ve been speed-writing for the last couple of days and so I have some work yet to do on adding references, fixing typos, rephrasing, etc. This will be a stable document after the first week.

UPDATE (2009-10-03): OK, it's been a couple of days, so I'm closing out the post and adding a jump. I'll continue to update links inside as I round out my reactions to the papers.

What’s the big deal?

If you want a basic description of the facts, here they are. Today’s series of papers is basically unprecedented in paleoanthropology. There are eleven papers in total, giving comprehensive coverage of the anatomy, paleoenvironment, and evolutionary interpretation of a new skeleton of Ardipithecus ramidus and dental remains representing more than 30 additional individuals. They have been published simultaneously in a coordinated effort including excavation, faunal correlation, microscopy, palynology, CT-scanning, three-dimensional reconstruction, isotopic analysis, and lord knows what else.

It’s the closest thing we’ll ever see to a big science effort in the little field of human evolution – like Tim White was building a supercollider under everybody’s noses.

The skeleton has been nicknamed, “Ardi” and it is 4.4 million years old. The site is Aramis, Ethiopia, in the Middle Awash field research area. The skeleton includes most of both arms, except the humeri, both hands, both feet, the right leg, the left ox coxa and part of the right ilium, a bit of sacrum, a couple of vertebrae, and a near-complete skull and dentition. It’s a bit more complete than Lucy, although preserving different parts.

The skull and pelvis were badly crushed, both of these were given CT-assisted reconstructions which are presented in separate papers (Suwa et al., 2009; Lovejoy et al., 2009c). Additionally, the series includes three papers on the paleoenvironment, a complete description of the dentition, and separate papers on the forelimbs and feet. The central paper in the series, by White and colleagues 2009b, is accompanied by a summary paper by Lovejoy examining the human-chimpanzee common ancestor in light of Ardipithecus.

As one of the papers puts it (White et al., 2009b), it represents a previously-unknown “adaptive plateau” for the hominins. Considering that really only three such “adaptive plateaux” were known before this – roughly, australopithecines, robust australopithecines, and humans – that gives some impression of the amount of difference evident in these remains from later hominins.

As I’ll describe, some substantial ambiguities and questions remain, which will no doubt shape the progress of paleoanthropology for many years to come.

Why did it take so long?

White and colleagues 2009b give a detailed overview of the state of preservation of the skeleton

Still, it’s possible to overstate this explanation. Bad preservation of remains is not uncommon in archaeological contexts. In this case extreme care was obviously warranted. But just as important may have been the opportunity to interpret and guide the reconstruction of the fossil using CT scanning and other technological enhancements.

To me, that is the central message of today’s announcement and papers. The big science version of paleoanthropology is one that brings an interdisciplinary and technological approach to fossil remains right from the very start. Coordinating such an extensive interpretive project requires time – in this case, fifteen years.

I can see a shiny nugget of goodness in that depressing span of time. The initial publication of the distorted Sahelanthropus skull led to substantial disagreement about the anatomy of the skeleton. Later CT reconstruction appeared to clarify some aspects of the anatomy. Arguably, it would have been better to delay publication until the CT reconstruction could be done. Obviously White and his team wanted to minimize the opportunity for error in their interpretations. They’ve covered their bases.

But that example also shows the danger of the wait-and-see approach, in that it tends to silence skeptical inquiry. Are there morphological details of the Ardipithecus skeleton that are obscured rather than clarified by reconstruction? At the moment, we don’t know.

What was the story before today?

Back in the Cold War, CIA analysts and other folks would read carefully through Pravda and other Soviet publications, parsing every word to look for the barest hint of the Politburo’s intentions. There was a word for those people: “Kremlinologists.” It seems to me that somebody quoted in Ann Gibbons’ book, The First Human said that paleoanthropologists basically do the same thing with Ardipithecus — poring through every publication or interview, looking for hints about the fossils hidden from the field for fifteen years. I don’t remember right now who, and I don’t have the book in front of me (I reviewed the book here in 2006).

On the other hand, there are people who follow every Twitter about their favorite celebrity, recording the GPS coordinates of sightings, and “running into” them at the openings of exclusive clubs. There’s a word for these people, too: “stalkers.”

Paleoanthropologists for the last fifteen years have been a little bit of both. It’s hard to help it – Tim White let slip ten years ago that the skeleton’s locomotor style was like something out of the Star Wars cantina and, well, let’s just say that some people hear his voice and think of Weird Al Yankovic.

If you weren’t following paleoanthropology in 1994, you may not remember Ardipithecus at all. For a brief, shining moment, it was the earliest hominin. Well, except for the Lothagam mandible, but nobody ever seems to remember Lothagam. It doesn’t even show up in the current series of papers.

Then, the species fell under a veil of secrecy. The initial find was from a locality called Aramis, within the Middle Awash field concession worked by Berkeley paleoanthropologist Tim White and colleagues. The news escaped that further fossils from Aramis had been found, including a partial skeleton. After initial examination of the skeleton, White and colleagues (1995) submitted a brief comment to Nature in which they changed the genus name of the first specimens. Instead of Australopithecus ramidus, they would henceforth be Ardipithecus ramidus. After that, silence.

Research at other localities in the Middle Awash uncovered earlier remains, which Haile-Selassie and colleagues 2001; 2004 attributed to a new species of Ardipithecus, Ar. kadabba. These were never the earliest hominins (predated at their initial discovery by Orrorin and later by Sahelanthropus, but at 5.5 million years old they were not far off. In their 2004 paper, Haile-Selassie and colleagues even suggested that all of these terminal Miocene hominins actually represent variations of a single species. An unstated implication is that the species would then be called Ardipithecus tugenensis.

Sileshi Semaw and the Gona Research Project 2005 found Ardipithecus downriver from Aramis, at a locality called As Duma. This represented approximately the same age as the Aramis horizons, and showed that Ardipithecus ramidus was not just a one-off. But the remains were only a few fragments. Based on the paleoecology of the immediate find, they suggested that the species had lived in a ”mosaic” of environments, bringing together elements of the fauna from both woodland, wetland and grassy woodland facies. That interpretation becomes a point of contention in the current series of research articles.

Other hints about Ardipithecus morphology have been dropped over the years. In a key 1999 paper, Owen Lovejoy along with Martin Cohn and Tim White described the Ardipithecus pelvis. They didn’t show it or say it was Ardipithecus, but there it was nonetheless. The interpretations of tooth size in the other, more fragmentary Ardipithecus remains (referred to as “relatively small”) made the body size of the skeleton fairly clear, which enabled interpretation of an radius earlier found at Aramis as a relatively long forelimb. And so on, the main conclusions have been foreshadowed elsewhere.

One thing stood out as a surprise. Ardipithecus had a grasping foot.

Did Ardipithecus really have a grasping foot?

Short answer: Yes.

The paper about the foot remains, by Lovejoy and colleagues 2009a, is full of just the kind of impenetrable prose you’d expect for a paper about foot remains. I have a lot of affection for people who know feet, but all the “fulcrumating” has me fulminating.

If we hack through the verbiage, the feet send a clear message:

“Unique among primates.” I hate it when they say that.

From the point of view of a foot specialist, this foot has many interesting aspect that can illuminate the evolution of stance and locomotor behavior in Miocene apes and the ancestors of the hominins.

From an Anthro 101 point of view, it’s an ape foot.

Still, Lovejoy and colleagues 2009a; 2009b describe the anatomy of the Ardipithecus foot as clearly different from Australopithecus, but different from chimpanzees also. The confusing thing is that it isn’t intermediate between those two forms. In their account, chimpanzee feet are specialized for more grasping, while the Ardipithecus foot retained a more generalized form. The confusion comes from parallelism in apes after Proconsul, which left Ardipithecus resembling monkeys more than apes in certain aspects of its foot anatomy, but more recent apes more than early apes in others.

The metatarsus of Ar. ramidus, chimpanzees, and gorillas presents a striking contrast to their metacarpus. Like the foot phalanges, the metatarsals also appear to have been universally shortened in all hominoids subsequent to Proconsul. The basis of this universal shortening, however, is somewhat unclear, because tarsal evolution contrasts dramatically in hominids and African apes. The modern ape foot has obviously experienced functional reorganization into a more hand-like grasping organ. The Ar. ramidus foot did not. This suggests that substantial elements of a more lever-based, propulsive structure seen in taxa such as Proconsul and Old World Monkeys [robust plantar aponeurosis; retained quadratus plantae; robust peroneal complex] were preserved in the GLCA/CLCA. These structures were sacrificed in both African ape clades to enhance pedal grasping for vertical climbing (Lovejoy et al., 2009b, 102)

That may be all I want to say about the foot for now. You can see that this is one of the most important anatomical aspects of the specimen in terms of understanding the origins of bipedality. Ardipithecus was not an obligate biped in any sense applied to Australopithecus.

OK, it wasn’t a biped, then. So how do you explain the pelvis?

The pelvis of Ardipithecus, as reconstructed by Lovejoy and colleagues 2009c, is intermediate between the chimpanzee and australopithecine morphology. In particular, the blade of the ilium is short and relatively curved compared to the long, flat chimpanzee ilium. But it does not approach the pelvis of Lucy or Sts 14 in those aspects, and the ischium is very chimpanzee-like in shape. The pubic symphysis was shorter than the long chimpanzee morphology, and the auricular surfaces appear consistent with a relatively shorter sacrum than in chimpanzees.

In reconstruction, it looks like a blend of hominin-like and chimpanzee-like anatomies. Lovejoy and colleagues further argue that the proximal femur indicates that a somewhat humanlike gluteus maximus insertion was in fact primitive for apes, with chimpanzees and gorillas having a derived non-humanlike form.

So what does this mean for locomotion? In their description (Lovejoy et al., 2009b), the African ape pelvic morphology is derived as a way of stiffening the lower back, in coordination with shortening the lumbar spine. If the African ape gluteal morphology is also derived (are you counting parallelisms yet?), then neither the ilium nor the proximal femur (excepting the possibility of bone distribution data not observable in Ardi) are useful markers of bipedality.

In other words, even though the Ardipithecus pelvis may look intermediate between chimpanzee and australopithecine morphologies, it’s not indicative of bipedality. Ardipithecus might have the locomotor morphology of the human-chimpanzee common ancestor.

To me, that seems shocking. More on this later.

What was Ardipithecus’ environment like?

The Middle Awash field team was able to do a very interesting thing in its paleoenvironmental reconstructions. The layer at Aramis containing the Ardipithecus skeleton and other remains is essentially a 3 to 5 meter thick series of paleosol, alluvial silt and fossilized bone and wood of various kinds. It is underlain by a glassy tuff and above by a basaltic tuff, which presumably represents some kind of pyroclastic event that swept through the area. The two tuffs are statistically indistinguishable in age, and the team guesses that the time between them represents something like a thousand years, maybe an order of magnitude more or less. So what they have is a thin sandwich of paleoenvironments, spread over the extent that the twin tuffs cover.

Now, this sandwich outcrops across roughly 9 km of linear distance (White et al., 2009b). So the team could sample distinct localities across this entire transect. What they found was that the line represented a range of habitats from open and grassy at one end to closed and wooded for (most) of the rest. They found Ardipithecus exclusively in associated with the wooded environment – complete with fossil wood, lots of monkeys and tragelaphines. They found no Ardipithecus at all in the localities representing more open environments. White and colleagues 2009a argue that this is a very strong test of habitat preference for Ardipithecus — it liked the trees.

That reconstruction makes sense with the locomotor anatomy. It also makes sense with the isotopic data on diet. After sampling carbon and oxygen stable isotopes in five Ardipithecus individuals, they conclude that it had a C4 plant consumption much less than later australopithecines, while higher than the very minimal value in chimpanzees, and that it habitually lived in mesic (not too wet, not too dry) habitat.

What does Ardipithecus tell us about hominin origins?

The paper by Owen Lovejoy, “Reexamining human origins in light of Ardipithecus ramidus” is possibly the most interesting in the collection. It will take me some more reflection to figure out what I think about the whole paper, but here I can abstract out the main ideas.

Much of the paper is speculative, concerning the “reproductive biology” of the human-chimpanzee common ancestor. In this paper, Lovejoy uses the acronym CLCA for ”chimpanzee last common ancestor,” which I find totally confusing. Since this is so close in time to the human-gorilla common ancestor, I’ll just take advantage of the new taxonomic scheme and call these ancestors the “stem hominines.” Lovejoy’s interest in reproductive biology is longstanding, as it formed the centerpiece of his 1981 article on human origins.

In many ways, this current article is an update of that one, because they arrive at the same singular focus: the association of canine reduction with increasing bipedality. Canines, in Lovejoy’s description, are principally a function of mating biology, and so any indirect evidence we have about the evolution of mating systems in humans or chimpanzees becomes very relevant to the factors that caused hominin origins.

Ardipithecus clearly shows that the canine reduction came first, bipedality later. Lovejoy integrates this fact into his earlier model, that the change in mating biology caused the change in locomotor strategy, as males substituted provisioning and food sharing as modes of mating competition in the place of aggression.

However, I think this is short-sighted. We already know that some degree of canine reduction occurs in several Miocene ape lineages, and that mating competition is highly variable among living apes and primates generally. What Ardipithecus shows, if we assume a connection between it and earlier candidate hominins like Sahelanthropus and Orrorin, is that the reduction of the canines preceded the evolution of effective bipedality by more than three million years. It is very difficult to conceive of mating biology as a cause of the locomotor evolution, when it is so removed from the change in time. It’s as if we stubbornly insisted that bipedality was the cause of stone tool transport.

The most interesting part of this paper is what Lovejoy says about the relevance of chimpanzees. This was also anticipated in an earlier paper, this one by Sayers and Lovejoy 2008, which argued that a chimpanzee model is too restrictive as a way of understanding the initial biology of hominin ancestors. Here, Lovejoy makes that view explicit in terms of the arboreality of Ardipithecus:

Lovejoy and colleagues discuss this concept in more detail in the paper outlining the Ardipithecus postcrania (Lovejoy et al., 2009b). I will be spending much more time on this paper, which makes several provocative assertions about developmental biology. But the conclusion of the paper

OK, I don’t remember Darwin saying anything about the neighbors in our crown. But you get the point – the stem hominines weren’t like chimpanzees or gorillas.

What about the skull?

Gen Suwa headed the cranial reconstruction (Suwa et al., 2009). Most of Ardi’s skull is represented on one side or the other, except for the basicranium. The team did have the temporal bones from another specimen, ARA-VP-1/500 (previously described by White and colleagues 1994). These temporal bones were too big to fit together with Ardi’s skull, so they digitally shrank them – sort of like reducing on a photocopier.

First of all, they did a really cool thing – they reconstructed the spatial relation of the two temporal bones by aligning the semicircular canals. Those tiny structures of the inner ear are like a miniature three-dimensional coordinate frame – part of the vestibular system that senses the position of the head. I’m sure they’re the first to do that, but it’s pretty neat to be able to align two temporal bones with no contact points between them.

I mention that because their reconstruction of the temporals determines the position of the line between the carotid canals on the base of the skull – the bicarotid line. This element of anatomy was very important in our consideration of Sahelanthropus (Wolpoff et al. 2006), as the measure between the bicarotid line and basion was a key indicator of the position of the foramen magnum. Jim Ahern found that this distance actually overlaps substantially between humans and chimpanzees, and most australopithecine crania actually fall into the chimpanzee range. That makes the trait questionable as an indicator of habitual head posture. Here, Suwa and colleagues found that the basion-bicarotid distance in Ardipithecus is as low as seen in the lowest known australopithecine cranium.

Suwa and colleagues advance an old hypothesis for this basicranial form. It’s not about upright posture, it’s about the brain.

I say, “old” because that was Raymond Dart’s interpretation of the Taung endocast — it was humanlike because of a neural reorganization.

Are they right? I’ll say this: if the basicranium does not reflect posture in these fossils, then there is no compelling evidence for posture at all.

If brain reorganization was underway in these ancient species, there’s no indication of it in the size of the brain. Ardi, like Toumaï, had a small brain — they estimate only 300–350 ml for its endocranial volume.

The rear of the skull – the nuchal plane of the occipital bone – was not preserved, so the most important remaining comparison with Sahelanthropus is the supraorbital region. This is small in Ardi, and extremely large and thick in Toumaï. Suwa and colleagues propose that this morphology matches the assessment of female sex for Ardi, which seems entirely reasonable. In the context of later Australopithecus, Ardi’s supraorbital torus might even be large for a female of its size. The difference between Ardi and Toumaï in browridge size would be surprisingly large considering the relatively slight difference in size between the two skulls. Still, with only two specimens to compare, a species with very large browridges in males might show this kind of difference on occasion.

Why don’t any of the papers have a cladogram?

This is an interesting omission, no? There’s no cladogram. What we get is this weird phylogenetic diagram that looks like a sectioned spinal cord:

If we could find a way to repair it, Homo would regain feeling.

Setting aside the aesthetics — which I’m sure were a lot of work — this set of scenarios is very unsatisfying. In all three, some version of Ardipithecus is the stem for later hominins. They haven’t shown that at all. None of the scenarios include chimpanzees or gorillas — yet no matter what you think about genetic estimates of divergence, the stem hominines were large and diverse populations with long-term interactions. Maybe these stopped before 6 million years ago, but none of the genetic data suggest that now at all.

Cladograms would oversimplify some aspects that should be considered complex, but maybe we could have one?

White and colleagues 2009b give a long table of “derived” characters in Ardipithecus and Australopithecus, but they are “derived” only with reference to their inferred state in the human-chimpanzee LCA. But elsewhere in these papers, they argue that some of these “derived” characters are actually primitive morphologies for apes, for which chimpanzees are independently derived. For many of the dental features, if we supposed a Miocene ape ancestor, the broadened mandibular body, thicker enamel and so on would look primitive, not derived. In the table, they list upper and lower canine traits separately, and break them up into six or more for each. That’s a quick way of making one morphological change look like twelve or more instances of independent evolution. Talk about atomizing traits!

So I wonder if a real cladistic analysis might not place Ardipithecus with the australopithecines. Especially if it included a proper sampling of Miocene ape taxa.

Maybe worse, a real cladistic analysis that did place Ardi with Australopithecus would probably put the earlier Ardipithecus kadabba as an outgroup to both. That would make Ardipithecus paraphyletic.

I wouldn’t typically care, because I don’t think taxonomic rules should direct the science. But it does seem like a delicious taxonomic dilemma. The likely solution would be to lump Ar. kadabba with either Orrorin or Sahelanthropus, or both. Orrorin kadabba would have priority.

But if one were feeling saucy, she could publish the cladistic analysis on the teeth, point out the dilemma, and then offer a novel genus name for the Kadabba sample. Maybe somebody’s already thought of that — there are a lot of journals out there.

Developing...

References

I want to share a paper that might not get a lot of attention but that I think makes an interesting contribution to understanding the ecology of early Homo after its initial dispersal from Africa. The paper is by José Joordens and colleagues, in the early bin at Journal of Human Evolution, titled, "Relevance of aquatic environments for hominins: a case study from Trinil (Java, Indonesia)".

The authors plowed through the old collections of fossil fauna from Trinil, originally from Dubois' excavation, looking to characterize the paleoenvironment in terms of hominin habitat preferences. The fauna are Early Pleistocene in age, possibly as old as 1.5 million years ago, but some uncertainty surrounds that date assessment, which is possibly under a million. What they found was some strong hints that the Trinil humans may have been eating shellfish and using other resources from the swampy lowlands in which the site was formed.

If aquatic resources such as molluscs and fish were available for hominins on Java, what is the probability that they indeed consumed these resources? In coastal areas, terrestrial predators often consume aquatic foods and have a considerable impact on the local aquatic ecosystem (Polis and Hurd, 1996; Roth, 2003). Systematic, often seasonal, predation by non-human terrestrial mammals on freshwater and marine fauna occurs widely. Carlton and Hodder (2003) reviewed occurrence of terrestrial mammals as predators in marine intertidal communities and documented 121 records of intertidal predation among 38 species of terrestrial mammals. For instance, mice, rats, pigs, chacma baboons, brown bears, black bears, striped and spotted hyenas, coyotes, domestic dogs, grey and red wolves, jackals, and foxes in coastal habitats catch and consume crabs, molluscs, fish, and other aquatic fauna (Carlton and Hodder, 2003; Smith and Partridge, 2004).

Terrestrial predators and omnivores eat fish, crabs, crayfish, turtles and shellfish when they get the chance. But the long list here muddies the water, so to speak. We often find hominins in lacustrine and riverine contexts, both because they inhabited those places and because of preservation biases. Those environments often yield evidence of consumption of aquatic organisms, especially shellfish but also fish, crocodiles and aquatic mammals. Somebody ate those animals, and the list above gives a bunch of suspects that aren't hominins.

So maybe we should redirect our null hypothesis -- instead of demanding proof of every instance of Early Pleistocene exploitation of aquatic foods, we should assume they ate the foods available to them. But with 30 or more species noshing on clams, crabs or fish at the shoreline, it's going to be tough to diagnose cases where humans may have been involved.

Well, anyway, what would this reorientation mean for our understanding of the behavioral breadth of early Homo?

The literature cited above shows that systematic aquatic exploitation (either year-round or seasonal) by terrestrial mammals is normal and predictable mammalian behavior when the mammal is 1) omnivorous, 2) living in a coastal marine or freshwater habitat, 3) where nutritious and catchable aquatic prey is available. Considered in the perspective of aquatic exploitation by terrestrial mammals in coastal habitats, the systematic and seasonal aquatic exploitation by Homo sapiens (Marean et al., 2007) and H. neanderthalensis (Stringer et al., 2008) does not differ from that of other mammals. Also, transport of aquatic prey to a base (such as a cave, in the case of H. sapiens and H. neanderthalensis) is not ‘‘modern’’ behavior. For example, Navarrete and Castilla (1993) reported that Norway rat burrows in coastal Chile contain remains of w40 intertidal prey species such as limpets, bivalve molluscs, crabs, and fish. Erlandson and Moss (2001) provide many more examples of terrestrial omnivorous animals transporting aquatic food (remains) to dens, nests, burrows, and caves on land. A label of ‘‘modernity,’’ if applicable at all to aquatic exploitation, should perhaps be reserved for aquatic exploitation with evidence of advanced technology such as fish hooks and boats. The assumption, that early hominins living in a coastal habitat with catchable nutritious aquatic fauna were restricted to eating terrestrial resources, does not agree with published accounts of common mammalian behavior. Therefore, instead of having to provide evidence of aquatic exploitation before it is considered as a realistic option, we propose that the default assumption in hominin evolutionary research should be that omnivorous hominins who lived in coastal habitats with catchable aquatic fauna could have consumed aquatic resources (Joordens et al. 2009).

I like this point a lot: It is a bad sign when archaeologists use a definition of behavioral modernity includes rats but not Neandertals.

Joordens and colleagues suggest that the Trinil faunal collections may already contain evidence of waste heaps (they say, "midden-like" accumulations) of shells:

[T]he presence of a relatively large number of only adult, large-size Pseudodon shells, excavated from a very limited area (Hauptknochenschicht in Trinil), in both the Dubois and Bandung collections, is a discrepancy in the aquatic assemblage that merits further attention for these shells.

But there aren't literally heaps of shells in the records; they have the shells and can only infer their original locations within the excavation at a relatively course grain. So the persuasive parts of the assemblage require us to see through the differnet biases that might have affected the collection. After dismissing a number of possible objections, they continue:

The fact that many of the Pseudodon valves are still paired and well-preserved would suggest that the molluscs were not dead and transported by water before fossilization but were buried in live position. However, the complete absence of small, juvenile shells as well as the mixed occurrence of two different (but equally large-sized) shell forms argues against interpretation of burial of a live population (Van Benthem Jutting, 1937). Instead, the discrepancies suggest that the Pseudodon shells could have been brought together, prior to fossilization, by a size-selective collecting agent who may have used them for consumption of molluscan flesh (Joordens et al. 2009: 13).

They found a similar pattern for another species:

The Elongaria assemblage from Trinil, just like Pseudodon, appears to indicate collection by a selective agent for the purpose of mollusc consumption. The Pseudodon and Elongaria assemblages from Trinil have the characteristics of shell middens (e.g., Waselkov, 1987; Rosendahl et al., 2007): large adult shells only, many complete shells, no signs of damage due to water rolling, signs of damage due to being deliberately opened, presence of human (hominin) bones in the same layer. We conclude that they represent a subtle clue of possible aquatic predation by non-hominins or by hominins (ibid.).

This hypothesis may not be testable further, unless signs of deliberate modification are found on one or more shells. Joordens and colleagues write that such a study is "currently underway". The only other thing to do is apply a higher standard of rigor to possible shellfish features in other Early Pleistocene contexts. Early Pleistocene surface collections dug by vertebrate paleontologists (as opposed to archaeologists) sometimes discard or leave fish bones unidentified, and it is not always clear whether a loose association of shells would be recognized as a possible hominin-accumulated feature.

The paper cites the work of Stewart (1994), who argued for fish consumption at Olduvai Gorge. From that abstract:

Fish remains are associated with many early hominid sites, and five sites at Olduvai Gorge are examined here in detail. The patterns of fish exploitation seen in Late Pleistocene archaeological sites are manifested in three of the Olduvai Gorge sites, making a strong, although not absolute, case for early hominid fish procurement. The implications for early hominid behaviour of fish procurement are several, and include timing of the early hominid seasonal round to exploit spawning or stranded fish, and group size larger than the nuclear family unit, with greater social interaction.

These examples bring to mind the challenge of identifying chimpanzee nutcracking archaeologically. The chimpanzee pattern of behavior is barely systematic enough to pick out from the background. Yet archaeologists have devised some ways to find that slim signal, at least in contexts where they expect chimpanzees to have been active. Late in prehistory, some shell middens were vast and highly-recognizable. Those populations put together the elements of recurrent (or constant) occupation of a site and recurrent transport of shellfish to that site for processing. When we look further into the past, even Neandertals rarely put together those elements in a recognizable way. At the Italian sites where Mary Stiner showed Mousterian shellfish consumption, the presence of shellfish is just at a level where the signal can be picked out due to the lack of other credible transport agents for shellfish remains.

A hitch: Suppose we accept that early humans commonly exploited aquatic resources, even in the absence of specialized archaeological traces of such dietary sources. That does seem to create a problem for the interpretation of stable isotope ratios in fossil humans. I wrote about this issue regarding Neandertal diet ("Neandertals: gone fishin' or not?", see also "Shellfish use by Neandertals" and "Neandertal diet was not dolphin-safe"). It doesn't take much fish to increase the nitrogen-15 composition of bone, making it hard to test hypotheses about the proportion of different terrestrial prey species, and even behavioral interpretations such as weaning age may be thrown off. The problem is too many independent dietary parameters to test with only one estimate. Ignoring the possibility of aquatic resource use helps simplify the interpretation, but that doesn't necessarily make it better.

References:

Joordens JCA, Wesselingh FP, de Vos J, Vonhof HB, Kroon D. 2009. Relevance of aquatic environments for hominins: a case study from Trinil (Java, Indonesia). J Hum Evol (in press) doi:10.1016/j.jhevol.2009.06.003

Stewart KM. 1994. Early hominid utilisation of fish resources and implications for seasonality and behaviour. J Hum Evol 27:229–245.