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Human Evolution and the Stone Tool "Problem"

5/27/2015

4 Comments

 
PicturePhotographs of some of the artifacts from LOM3 (Harmand et al. 2015:Figure 4).
The recent announcement of the discovery in stone tools in Kenya dating to 3.3 million years ago (MYA) has been greeted with a lot of fanfare.  I first heard the story at some point earlier in the academic year, and I know there was a lot of buzz about it at the SAAs and Paleoanthropology meetings in San Francisco in April.  The publication of a formal paper in Nature last week (“3.3-Million-Year-Old Stone Tools From Lomekwi 3, West Turkana, Kenya,” by Sonia Harmand and colleagues) led to a flurry of stories in the popular media.  Many of those stories (for example this one in the L. A. Times) framed the discovery as one that "hints that anthropologists may have had the wrong idea about the evolution of humans and technology."

Spoiler alert:  The stone tools from Lomekwi 3 are an important finding, but not a surprising one.

Hyping and over-simplification by the popular media of scientific findings  are a fact of life, and I understand the need to find an "angle" for a summary story.  I find the media's coverage of the Lomekwi paper particularly annoying, however, because of the general implication that the discovery of tools of that age somehow caught us all by surprise.  It didn't.  Anyone who has been paying attention to the field for the last few decades will not be surprised at all by the claims that: (1) there are stone tools that pre-date Oldowan; (2) those tools were probably not made by members of the genus Homo; and (3) the use of stone tools can be traced back to at least 3.3 MYA.

Let me be clear:  this is a very important finding, just not a particularly surprising one.  The tool assemblage from Lomekwi 3 (LOM3) fits very comfortably within an emerging picture of tool use pre-dating Oldowan and Homo.  That picture has been coming into focus for decades now, thanks to a lot of hard work by many different scientists.  The LOM3 tools make a significant contribution to that picture by providing a line of direct evidence that was previously absent.  For the first time, we get some idea of what pre-Oldowan stone technologies might have been like.  I think it was only a matter of time, however, and there will be a lot more coming down the road.

Why did we expect stone tools pre-dating Oldowan to be found?

First, as pointed out in the LOM3 paper, the 3.3-million-year-old age of the tools is consistent with the 3.4 MYA cutmarked bones from Dikika, Ethiopia that were reported several years ago. Not everyone accepts those cutmarks as legitimate (here is a John Hawks' post about the critique), however.  I'm not a cutmark expert, so I don't really have a strong opinion.  I'll just say that finding a stone tool assemblage in east Africa that dates to the same time period as the purported cutmarks mitigates the "but where are the tools?" question for me.

Second, the idea that only humans use tools (and therefore evidence of tool use should only be associated with the genus Homo) is an antiquated one that has been solidly falsified by studying living, non-human primates.  The use of tools has been widely observed among wild chimpanzees, our closest living relative (and also among more distant relatives such as orangutans and gorillas).  The most parsimonious explanation for the presence of tool-using behaviors in chimpanzees and humans is that those behaviors were also present in the Last Common Ancestor (LCA).  If correct, that means that all hominids/hominins (as well as all members of the lineage leading to chimpanzees) had some capacity to make and use tools. If not correct, we need to explain the independent emergence of tool use in both lineages.  I think the first possibility (that the capacity to use tools is a homology) is more likely, and makes it much easier to explain the widespread use of tools among great apes and some other primates. The LOM3 assemblage pushes our understanding of a particular kind of tool use (stone tool use) back in time, but it is by no means at odds with the general idea that all hominids had the capacity to use tools.  It provides direct evidence, rather, to help evaluate hypotheses about the timing and nature of the evolution of tool-using behaviors that are peculiar to humans.

The presence of tool-using behaviors among several of our closest relatives suggests that the cognitive hardware required for tool use was present deep in the Great Ape lineage: it doesn't take a big, human-like brain to make and use simple tools. But what about other parts of our anatomy? 


Picture
Comparison of human and chimpanzee hands.
Picture
Comparison of distal phalanges (bones at the end of the thumb) in chimps (Pan), gorillas, Orrorin, modern humans (Homo) and Homo habilis (OH 7) (source: Almécija et al. 2010).
Human hands and chimpanzee hands -- both of which are capable of making and using tools -- differ significantly in several ways. Walking on two legs has removed selection related to locomotion from affecting the human hand, allowing our hands to be more-or-less optimized for manipulating objects (e.g., making and using tools).  As quadrupeds, chimpanzees operate under a different set of restraints.  A chimpanzee's hand anatomy reflects compromises between an appendage that can be used to manipulate objects and one that has to function for both arboreal and terrestrial locomotion.  Those demands of locomotion have produced a hand with long fingers and a stiff wrist:  long fingers are useful for grasping branches while a chimpanzee is in the trees; a stiff wrist serves to accommodate the forces that are transferred through a chimp's hand while it is walking on its knuckles. 

The features of a chimp's hand make it harder for a chimpanzee to exert precise control over objects.  The long fingers make a human-like "precision grip" (where the pad of the thumb is opposed directly against the pad of the index finger, as when you hold a key) impossible.  The stiff wrist places limitations on the range of mobility.   Although chimps can be taught to make and use simple stone tools (e.g., Kanzi), their hand anatomy works against them.

One of the features of a human hand is the broad, flat distal phalanx of the thumb.  Because of our precision grip (enabled by our relatively short fingers), we are able to exert a lot of force between our thumb and forefinger. The broad bones at the ends of our thumbs reflect those strong forces.  The shape of the distal thumb bone of OH 7 was one of the criteria used to define Homo habilis in the original 1964 paper by Louis Leakey, Philip Tobias and J. R. Napier:

". . . the hand bones resemble those of Homo sapiens sapiens in the presence of broad, stout, terminal phalanges on fingers and thumb . . ." (Leakey et al. 1964:8).

As more fossil hands have been discovered in the decades that followed, it has become apparent that many hominids had "broad, stout, terminal phalanges" in their thumbs.  The illustration above (from
Almécija et al. 2010) shows the OH 7 thumb bone compared to the thumb of Orrorin tugenensis (a possible hominid from around 6 MYA), a modern human, a chimpanzee, and a gorilla. Orrorin had a broad thumb.  What about robust australopithecines?  Yep. Australopithecus sediba?  Yep.  It looks like there were a lot of hominids that may have had good features for tool-using hands. If Ardipithecus ramidus (4.4 MYA) was a hominid, it suggests that a chimpanzee's hand is in fact more derived from the ancestral condition than a human hand:  the LCA's hand may have been "pre-adapted" for tool use with a pliable, mobile wrist.  All that was needed to make the transition to a human-like hand was to shorten the long fingers (which could have happened in the process of shifting to a fully terrestrial adaptation) and broaden the thumb as a precision grip became possible. If tool use was at all important to our pre-Homo ancestors, the selective pressure to shorten the fingers would have been present all along, just less constrained once long fingers were no longer needed for a partially arboreal adaptation.

So it looks like the cognitive capacity for tool use among our ancestors was probably present by at least the end of the Miocene (in the LCA), and the changes to hand anatomy that allowed human-like grasping were well underway during the Pliocene (ca. 5.3-2.6 MYA).  The discovery of stone tools dating to 3.3 MYA doesn't conflict with any lines of evidence that I know of suggesting when we could see the earliest stone tools.  The interesting questions that we can start address with the publication of the information from Lomekwi, really, are the "who" and the "why" questions: Why did hominids start making and using stone tools?  Which hominids were making these tools?  And what did tool use have to do with other aspects of human and hominid evolution?

Harmand et al. (2015:314) find differences between the lithic materials from LOM3 and Oldowan, and propose that the technology be given a new name: Lomekwian. 

"The LOM3 knapper's understanding of stone fracture mechanics and grammars of action is clearly less developed than that reflected in early Oldowan assemblages and neither were they predominantly using free-hand techniques. The LOM3 assemblage could represent a technological stage between a hypothetical pounding-oriented stone tool use by an earlier hominin and the flaking-oriented knapping behavior of later, Oldowan toolmakers."

The identification of "Lomekwian" tools is going to open up some new thinking about the roles of tool use in general (and stone tools in particular) in human and hominid evolution, not because stone tools at 3.3 MYA were unexpected, but because now we have some hard evidence of what those technologies might have been like. I don't work in Africa, but I'm probably not going too far out on a limb to suggest that there are plenty of places with mid- to late-Pliocene deposits that might be fertile ground for finding more direct evidence of these pre-Oldowan stone tool technologies.  It's going to be great to watch that story emerge.


ResearchBlogging.org
Harmand S, Lewis JE, Feibel CS, Lepre CJ, Prat S, Lenoble A, Boës X, Quinn RL, Brenet M, Arroyo A, Taylor N, Clément S, Daver G, Brugal JP, Leakey L, Mortlock RA, Wright JD, Lokorodi S, Kirwa C, Kent DV, & Roche H (2015). 3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya. Nature, 521 (7552), 310-5 PMID: 25993961
4 Comments

The Dependency Ratio in Human Evolution

5/15/2015

3 Comments

 
As far as I know, humans are unique among animals in having an extended period between weaning and being able to subsist on their own.  We call this “childhood.”  The long period of post-weaning dependence provides our large brains with a lot of time to mature.  It also requires a lot of parental investment (in terms of time, energy, calories, etc.) and means that we would have to wait a long time between offspring if each one had to independent before the mother could have another.   We don’t do that, tending to have a shorter interval between subsequent births (the inter-birth interval, or IBI) than other great apes.  The long period of childhood dependence and the short IBI mean that, as a species, humans tend to have multiple, dependent offspring of different ages at the same time.  Speaking as a parent of multiple, dependent offspring of different ages, I can tell you that this is often no walk in the park.  This peculiar human strategy has a lot of costs.

Understanding when, how, and why this distinctly human reproductive strategy developed is a great evolutionary question.  Reducing the IBI increases the potential fertility of human populations, but also creates new demands on the energies of parents and families.  Human families today often offset those extra energy demands by getting help (evolutionary anthropologists call it “cooperative breeding”).  A new paper in the Journal of Human Evolution titled “When Mothers Need Others” by Karen Kramer and Erik
Otárola-Castillo tries to further our understanding of where cooperative breeding comes from, using an “exploratory model” to try to understand the selective pressures associated with the evolution of human-like patterns of reproduction and child-rearing.  The goal of the paper “is to develop a model to predict those life history transitions where selective pressure would have been strongest for cooperative childrearing” (pg. 5). 

Kramer and
Otárola-Castillo call their model the “Force of Dependence Model.”  Their model is a simple one, calculating “the net cost of offspring as a function of dispersal age, birth intervals and juvenile dependence” as a 3-dimensional surface (Supplementary Online Material  from Kramer and Otárola-Castillo 2015).  The authors use several different combinations of settings to represent a range of conditions from “ancestral” (juvenile independence at age 10, IBI of 6 years, and a dispersal age of 14) to “most derived” (juvenile independence at age 20, IBI of 3 years, and a dispersal age of 20).  Their graphs show that “net costs” within a domestic group (a mother and her offspring) are lower when offspring are spaced further apart and become independent at a younger age.  When offspring hang around past the age of juvenile independence, there is a net benefit to the domestic group as their productive capacities can be used to offset the drain of their younger siblings. The authors find that the strain points – where selective pressures for assistance would be greatest – occur in domestic groups with the most derived set of characteristics: late juvenile independence and a low IBI (lots of children who remain dependent for a long time).   

As I understand it, the “net cost” in this model more-or-less mirrors the dependency ratio (the ratio of consumers to producers) of a domestic group or family, something anthropologists have been interested in understanding for a long time.  The higher the ratio of consumers to producers, the higher the dependency ratio, and the higher the “cost” to each producer supporting the family.  The dependency ratio changes through the lifespan of a family in a patterned way: every domestic group that has children goes through a “pinch” period when the dependency ratio is highest, and the pinch period logically corresponds to the time when there are a lot of dependent offspring.  As I wrote in my 2013 paper in the Journal of Anthropological Archaeology (“Subsistence Economics, Family Size, and the Emergence of Social Complexity in Hunter-Gatherer Systems in Eastern North America,” available here):

“the duration and amplitude of the ‘pinch’ is affected by the rapidity of the addition of offspring and how quickly those offspring turn from consumers into producers.  The rapidity of addition of offspring will depend on factors such as fertility, infant and childhood mortality, and the number of wives. The productive potential of children will be affected by the presence and distribution of resources that can be procured by children and the foraging strategies that are employed to exploit those resources” (White 2013:128).

The main part of that paper used an agent-based model (ABM) to try to understand how the distribution of family size changes when the age at which children become producers (the “age of juvenile independence” in Kramer and
Otárola-Castillo’s model) decreases and there is an incentive for polygynous marriage.  In addition to the ABM, I used a simple spreadsheet model to show how the dependency ratio changed through the course of the developmental cycle of an individual family in cases where the age at production was low (8 years old) and where it was high (14 years old).  In this simple model, I used an IBI of 3, a dispersal age of 16 for females and 20 for males, and a female reproductive period spanning ages 20-35 years (giving a total fertility of 6 offspring).

The figure below compares Kramer and
Otárola-Castillo’s graphs from their cases with early and late juvenile independence (holding IBI at 3 and dispersal age at 14) with my data on changes in dependency ratio through the developmental cycle in cases with a single reproducing female and an age at production of 8 (top) and 14 (bottom).   My model data are the same as in my 2013 paper (Figure 5), but I have re-graphed them to make comparison with Kramer and Otárola-Castillo’s figure easier.  I have redrawn the graphs from Kramer and Otárola’s Figure 1 (third graphs from the left, top and bottom rows).   The dotted lines on the graphs of my results indicate a dependency ratio of 1.75, which is what I have generally used in my modeling efforts as a “typical” dependency ratio among hunter-gatherers (following Binford 2001:230).
Picture

My results showed the same pattern as Kramer and Otárola-Castillo’s:  the peak of the “pinch” comes earlier and is less severe when children become producers at an earlier age.  Even though our models have some differences (and some of the values of the parameters were different), the correspondence in results is notable. Compare, for example, when the amplitude of the “pinch” (peak dependency ratio in my results, greatest net cost in Kramer and Otárola-Castillo’s results; marked by stars) is greatest and the differences in amplitude between the early and late ages of juvenile contributions to subsistence.

The correspondence between my results and Kramer and
Otárola-Castillo’s is unsurprising.  The idea that the dependency ratio of a domestic group changes through the course of its developmental cycle in a somewhat predictable way is not new (and the idea that the “pinch” comes when you have lots of little kids running around at the same time won’t come as a revelation to anyone who has multiple children).   This is a phenomenon that has been studied for decades (e.g., Chayanov 1966; Donham 1999; Fortes 1958; Goody 1958) and recognized as a key aspect of how hunter-gatherers organize themselves (Binford 2001:229).

So where does this kind of work put us in terms of understanding the evolution of human reproduction, society, and family life?  I think it primarily puts us in a spot where we’re asking some good questions.  Going back to the issue of the origins of monogamous pair-bonding (which I touched on briefly in this post about birth assistance and this post about australopithecine sexual dimorphism), having a two person (male-female) unit forming the core of a domestic group would have a mitigating effect on the strain caused by a decrease in IBI (i.e., you’d be adding another producer into the equation).  If the appearance of male-female pair-bonding was associated with a sexual division of labor (which is I think what most of us would hypothesize), males and females would presumably be focused on procuring somewhat different sets of subsistence resources.  Offspring could be largely “independent” with regards to some of those resources but not to others – think about the difference between collecting berries and running down large game.  A sexual division of labor and an environment where relatively young children could make some contribution to their own subsistence (even if that contribution does not include the full range of resources that are exploited) would go a long way toward easing the “pinch” that comes from having more children spaced closer together.

When does this happen in human evolution?  Of course that’s a tough thing to get at directly.  I think if you took a poll, the winner would probably be “around the time our genus emerges” or “with Homo erectus.”  An increase in total fertility (coincident with a lowering of the IBI) would help explain the population growth that must have been part of the dispersal of our species out of Africa prior to 1.8 million years ago.  It would also fit nicely with the evidence for an increased exploitation of animal resources around that same time.  Maybe Glynn Isaac was right all along to propose the emergence of human-like central place foraging with home bases and a sexual division of labor at the beginning of the Lower Paleolithic?

But what if monogamous pair-bonding and a sexual division of labor appeared much earlier – with australopithecines or even some pre-australopithecine like Ardipithecus?  If those things came along with bipedal locomotion, would a decreased IBI and increased fertility have followed automatically?  Maybe not.  Perhaps those earlier hominids just didn’t have the wherewithal to exploit their environments like later hominids did – perhaps the diversity of the resource base they could exploit wasn’t great enough to really leverage a sexual division of labor until animal products became readily attainable.  That may have required a suite of anatomical adaptations for daytime exhaustion hunting (loss of body hair, skin pigmentation, greater body size, stiffer foot) and cognitive/behavioral adaptations for making and using stone tools to process carcasses.  The date of the “earliest” proposed use of stone tools continues to be pushed  back (now it’s at 3.3. million years ago), but as far as I know the density of stone tools and butchered animal bones that appears at about 1.8 million years ago is unlike anything that precedes it.

More modeling work will be required to really understand how changes in the dependency ratio might have articulated with changes in reproductive, social, and technological behaviors deep in human prehistory.  In order to understand what changes in reproduction might have meant in terms of social interactions, however, we’ll need a different grade of model than that used by Kramer and
Otárola-Castillo.  Of course I’m going to say that complex systems modeling is the way to go on this:  it will let us get past the limitations of deterministic inputs and help us understand how constraints, costs, and interactions would have played out within a society.   In order for “others” to help with raising and provisioning multiple dependents, those others had to have existed within these small-scale hominid societies and (again, speaking as someone involved in raising multiple small kids) there wouldn't have been some inexhaustible Plio-Pleistocene babysitting pool of “others” out there just waiting to step in and provide extra calories for a few years.  A different kind of modeling effort with broader scope will let us get at the group- and society-level contexts in which family-level changes in child-bearing and child-rearing would have played out. Stay tuned.
References

Binford, Lewis R.  2001. Constructing Frames of Reference: An Analytical Method for Archaeological Theory Building Using Hunter-Gatherer and Environmental Data Sets.  University of California Press, Berkeley.

Chayanov, A. V.  1966.  A. V. Chayanov on the Theory of Peasant Economy.  University of Wisconsin Press, Madison.

Donham, Donald L. 1999.  History, power, ideology: Central issues in Marxism and anthropology.  University of California Press, Berkeley.

Fortes, Meyer.  1958.  Introduction.  In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 1-14.  Cambridge Papers in Social Anthropology.  Cambridge University Press, London.

Goody, Jack.  1958.  The Fission of Domestic Groups among the LoDagaba.  In The Developmental Cycle in Domestic Groups, edited by Jack Goody, pp. 53-91.  Cambridge Papers in Social Anthropology.  Cambridge University Press, London.
ResearchBlogging.org
Kramer, K., & Otárola-Castillo, E. (2015). When mothers need others: The impact of hominin life history evolution on cooperative breeding Journal of Human Evolution DOI: 10.1016/j.jhevol.2015.01.009
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