Oldowan hunting behaviors at Kanjera South

Joseph Ferraro and colleagues have done some neat analyses of the faunal remains from Kanjera South, Kenya Ferraro:carnivory:2013. Kanjera South is an archaeological assemblage of Oldowan artifacts and associated animal bones from around 2 million years ago. The site was once a plain next to a lake, and gradually built up clay and silt sediments over years and years of flooding and soil formation. Stone tools and bones stand out in the sediments, representing recurrent activities of ancient humans over a few hundreds or thousands of years. As a result, the site has a good statistical representation of fauna that were hunted by early humans, relatively early in the evolution of our genus.

This is not the earliest site with evidence for meat acquisition by stone toolmakers. We know that people were butchering animals with stone tools around 2.6 million years ago. But the first really good evidence for hunting strategies is much more recent -- around 1.8 million years ago at Olduvai Gorge. There are actually very few Oldowan-era faunal assemblages large enough to study hunting behaviors. Kanjera South shows that the activities documented at Olduvai Gorge were happening a bit earlier, and the site helps to clarify the kind of context in which we might expect to find more evidence of hunting behavior.

Hunting versus scavenging is the tiredest chestnut in anthropologists' Oldowan arsenal. Were early hunters really competent enough to bring down a duiker on their own? Or did they steal away pieces of half-eaten zebra carcases when the lions took a break?

In reality, there is no contradiction here. Undefended meat doesn't last a day in the open, whether on the plains or near waterholes. So scavenging meat from other carnivores usually means facing them down -- not a job for an incompetent killer. Meanwhile, present-day peoples who hunt and gather rely quite a lot on "power scavenging", or taking advantage of other carnivores' successes. The present value of a dead carcass is higher than that of a live animal, as long as it may still escape you. Whether the hunter has to predict prey behavior, or the scavenger has to predict competitors' behavior, both strategies require a depth of planning. So, when it comes to Oldowan-era sites, we should expect to see a mixture of hunted and scavenged remains.

In that context, we can make some inferences about hominin hunting practices by assessing which kinds of animals they hunted, and which they scavenged. Looking at tooth mark and cutmark evidence is not a perfect way of sorting hunting and scavenging -- because both kinds of marks are rare on faunal elements in archaeological contexts. But sometimes those comparisons lead to clear results. For example, here is the chart showing the number of tooth-marked midshaft fragments from long bones at Kanjera South, in comparison to experimental bone assemblages:

Figure 3 from Ferraro et al 2013

Figure 3 from Ferraro et al. 2013. Original caption: Tooth-marked mid-shaft fragments: results from experimental assemblages and excavations at KJS. Figure follows a published model [26]. Hominin-first assemblages refer to remains initially defleshed and demarrowed by hominins, then subsequently exposed to large-bodied carnivores (primarily hyenas). Carnivore-first assemblages refer to remains initially defleshed and/or demarrowed by large-bodied carnivores (primarily hyenas and/or lions). Data for body sizes 14 [21]. Modern data (with single standard deviations where available) derived from the literature [23][26], [56][58]. KJS frequencies are from Table 2 and Table S1. Multiple symbols for KJS indicate the results of multiple analysts. Xs indicate minimum and maximum estimates of damage (see Table S1). doi:10.1371/journal.pone.0062174.g003

These are cool data. Carnivores who get to chew on bones for a while tend to leave the middle of them covered in tooth marks. If humans get access to the carcass early, they will strip off the meat from those midshafts, break them into bits, and otherwise prevent the taphonomic pathway to carnivore tooth marking. And in the graph we see that the Kanjera South faunal assemblage looks like cases where humans were the agents of defleshing and butchering.

If humans had primary access to the carcasses, then the transport decisions of ancient hunters should have shaped the bone assemblage at Kanjera South. It is very common in analyses of the fauna from African Oldowan-era sites to divide the prey animals into three size classes -- small, medium and large. The majority of prey species were bovids, ranging from small antelopes to water buffalo, although most were in the small and medium size categories at Kanjera South. Ferraro and colleagues show that for medium-sized bovids, the hominins were taking two strategies. These bovids were too big to carry wholesale to a central place for sharing. So the hunters disarticulated the animals and carried back the legs, leaving the axial skeleton for the most part behind.

Except for the heads:

But why acquire, transport, and process an abundance of medium-sized heads? In living animals, these remains contain a wealth of fatty, calorie-packed, nutrient-rich tissues: a rare and valuable food resource in a grassland setting where alternate high-value foodstuffs (fruits, nuts, etc.) are often unavailable [2], [3], [29], [49], [52], [63], [76][78]. Medium-sized heads are also relatively dense and durable elements, and their internal contents are generally inaccessible to all but hyenas and tool-wielding hominins [63], [79], [80]. As a result, they are often seasonally-available as scavengable resources in East African grasslands [63], [76], [79][83]. Additionally, bone surface modification studies at KJS clearly demonstrate that hominins accessed internal head contents: several cranial vault and mandibular fragments bear evidence of percussion striae. Considered in sum, the presumed availability of these isolated remains across the landscape, the relative abundance of these remains in the KJS assemblages, and unambiguous material evidence that hominins exploited their contents on-site is most parsimoniously interpreted as reflecting very early archaeological evidence of a distinct hominin scavenging strategy one that included a strong focus on acquiring and exploiting fatty, nutrient-rich, energy-dense within-head food resources (e.g., brain matter, mandibular nerve and marrow, etc.) [e.g., 24,63,76,82,8486].

This is John Speth's scenario for fat acquisition from lean animals. The brain is the last part of the body to become fat-depleted during times of stress. If hunters are energy-limited, further lean meat is not going to be valuable to them because protein takes energy to digest. What they need most is fat, and the most ready source of fat is the brain. Accumulation of head elements, whether from hunted or scavenged sources, is an effective behavioral strategy in those circumstances. It's one that we think Neandertals pursued at the end of winter in some parts of Europe, and a strategy followed by hunters in ethnographic and historic contexts as well.

The paper's conclusion is well-framed as a summary of the overall value of evidence from Kanjera South.

With regard to evolutionary ecology, the relative uniformity of hominin activities documented through the KJS sequence indicates an evolved foraging adaptation well-tuned to local ecological contexts. This point implies that hominin involvement with, and their presumed consumption of, animal remains had substantial fitness implications. In turn, sufficiently strong selective pressures are implicated as having favored the evolution of persistent hominin carnivory no later than 2.0 million years ago. This date is approximately 200,000500,000 years earlier than previously documented [11], [20], [33], [45], and increases the known time depth of this adaptation within the hominin lineage (range of dates reflects varied interpretations of faunal materials from Olduvai [20][42]).

This one was fun to read, because the data being built up at Kanjera South are really capable of testing hypotheses about hunting behavior in a way that some of the Oldovai Gorge assemblages have done up to now. Putting the faunal exploitation together with the stone tool evidence, we see a really interesting picture. As I reported a few years ago ("Plant processing with early Oldowan tools"), Kanjera South is one of the locations where we have good evidence of plant exploitation of some kind by Oldowan peoples. The site has also provided evidence about stone material transport decisions and the planning depth of stone flaking ("Technological sophistication of the earliest toolmakers". It is a good illustration of how deep knowledge of a single site, with teams returning to excavations over multiple seasons, can yield a richness of statistical information about hominin behavior.