This morning's announcement of the first results from the Rising Star Expedition did not disappoint: a 35-page open access paper with 47 authors (Berger et al. 2015) describing fossil remains (Homo naledi) from at least 15 individuals recovered since 2013 from a cave in South Africa, and another paper by Dirks et al. (2015) describing the physical context of the fossils. I'm friends with several of the authors, and I am so happy for them both personally and professionally.  And I'm jealous. But we'll leave that aside for now.

There are so many things that are awesome about this project that it's hard to even know where to start.  If you don't follow paleoanthropology closely, you may not fully grasp how unusual this discovery is, how novel the approach to excavation and analysis was, and how @#!$&*% badass the results are. The story of the Rising Star Expedition is the Mad Max: Fury Road of paleoanthropology.  I hope we start seeing more movies like this.

This is the largest single hominin fossil assemblage yet discovered in Africa, it is remarkable well-preserved, and it was analyzed and reported upon in record time.  Two years from discovery to publication is blisteringly fast in the world of paleoanthropology (the publication of Ardipethecus ramidus, a 4.4 million-year-old putative hominin from Ethiopia, famously took over 15 years).  Speeding up initial analysis by inviting a crowd of young, hungry scholars to contribute to the work (hence the 47 authors on the first paper) was a masterful stroke by project leader Lee Berger.  The Rising Star Expedition has convincingly demonstrated the utility of a new paradigm that challenges traditional notions about how paleoanthropology should be done and what constitutes an appropriate pace for analysis and publication.  Bravo on that front: totally @#!$&*% badass.

The initial findings/interpretations/conclusions from Rising Star are also pretty @#!$&*% badass.  Here are a few that jump out to me as important during a first quick pass through the paper:

Mosaic of Primitive and Derived Features. According to the authors, the skeletons of Homo naledi preserve a mixture of human-like and australopithecine-like features unlike that seen in any other fossil remains:

H. naledi has humanlike manipulatory adaptations of the hand and wrist. It also exhibits a humanlike foot and lower limb.  These humanlike aspects are contrasted in the postcrania with a more primititive or australopith-like trunk, shoulder, pelvis and proximal femur (Berger et al. 2015:2).

The appearance of a different "mosaic" of primitive and derived features was also described by Lee Berger for the Australopithecus sediba fossils (a rather late gracile australopithecine from South Africa), highlighting the difficulty understanding extinct hominins based on single bones or very fragmentary skeletons.  The skeleton of sediba appeared to have such a strange mixture of features that some researchers suggested the remains were actually those of two different species mixed together. I think it will be pretty difficult to make that argument for the naledi remains because of the number of specimens and their apparent morphological homogeneity.  The implications of the mosaic aspects of the Rising Star fossils will have to be dealt with rather than dismissed. The features that we associate with being human apparently did not come together as a "package deal."

South, Not East?  Identifying naledi as a member of our genus means it is a potential human ancestor.  The center of gravity in the study of the emergence of the genus Homo has been in East Africa for many decades. But if naledi is ancestral to our lineage, what does that mean for the fossils of possible human ancestors found in East Africa?  Do they get pushed out of the lineage?  Do they have to leave the adult table at Thanksgiving and go sit with the little kids?  There is no possible fossil assemblage from East Africa of early Homo that compares with the one from Rising Star in terms of size and its potential to tell us about so many aspects of the skeleton. I think that means that, unless you want to just assume that East Africa is where everything important happened, naledi has to be dealt with in any serious discussion of the origins of Homo. And that brings us once again to the "species" issue.

What Does "Species" Mean?  I wrote a little bit about species concepts inthis post from May when a new species (Australopithecus  deyiremeda) was proposed for the fossil of another purported human ancestor. My point in that post was that if we're imagining that each "species" is a reproductively isolated population, naming a new species has a lot of implications that I think are tough to justify based on fragmentary fossils.  It's most problematic when species are named based on very little physical evidence.  There's a lot of material from Rising Star, however, and I would guess that many people are comfortable with creating a new taxon based on the amount of material that Berger et al. (2015) have analyzed. 

That's fine. But I'm still wondering what is meant by the term "species."  I didn't see a definition of "species" in the paper (it's possible I missed it), but I did see this passage in the National Geographic story about Rising Star that came out today:

"Berger himself thinks the right metaphor for human evolution, instead of a tree branching from a single root, is a braided stream: a river that divides into channels, only to merge again downstream. Similarly, the various hominin types that inhabited the landscapes of Africa must at some point have diverged from a common ancestor. But then farther down the river of time they may have coalesced again, so that we, at the river’s mouth, carry in us today a bit of East Africa, a bit of South Africa, and a whole lot of history we have no notion of whatsoever."

I like the metaphor of a braided stream, but it sounds a lot more like we're describing populations within a single biological species rather than species-level divergences.  Once the "species" streams diverge through reproductive isolation, how could they ever converge again?  Using a biological species concept, they can't.  We may need to give things names so we can talk about them and compare them, but it seems to me, again, that our taxonomic terminology and practices are ill-suited and maybe counter-productive for actually describing and understanding the patterns and processes that we're interested in.

But There's No Date! There are no dates associated with the Rising Star fossils. That means we really have no independent handle on where in time they go, which means it's hard to use them to test specific hypotheses about the patterns, processes, and history of human evolution. Without dates it will really tough to understand how they might fit in with the East African materials, or sediba, or the recently-announced 3.3 million-year-old stone tools from Kenya. Wherever these fossils "go" in time they'll make a splash, but we don't know right now where the ripples will be. I hope we get some information about that soon.

Was naledi Burying Its Dead?  Surely one of the most controversial interpretations of the Rising Star assemblage will be that it accumulated through intentional disposal of the dead.  That's the conclusion of this paper by Dirks et al. (2015):

"Preliminary evidence is consistent with deliberate body disposal in a single location, by a hominin species other than Homo sapiens, at an as-yet unknown date."

That lack of a date is especially painful here. The earliest claims that I know of for regularized treatment of the dead come from the site of Sima de los Huesos (Atapuerca), where the bodies of 28 individuals were thrown into a cave in Spain around 400-500 thousand years ago. Evidence for the patterned, ritualized treatment of the dead at Atapuerca fits well with other possible signs of a cognitive emergence about half a million years ago, including the shell of similar age announced last year that was apparently carved by Homo erectus and evidence of the removal of flesh from the Bodo cranium at about 600,000 years ago. 

If the Rising Star fossils date to the origins of Homo, as the researchers suggest, and if the assemblage accumulated through intentional cultural behavior, it will push ritual treatment of the dead much farther back in time than we have ever considered. Two million years ago?  I bet there are probably people out there who still don't even think Neanderthals were burying their dead. That's sure to spark some arguments. 

Jealousy aside, it's going to be great to watch what happens next. I'm sure there will be more results from Rising Star (hopefully soon), and I'm sure there will be a lot of reaction to what came out today.  It's a totally @#!$&*% badass project with totally @#!$&*% badass results.  This is a great time to be paying attention to human evolution - maybe the best ever.  Fingers crossed we get an announcement of australopithecine DNA soon.

Last week I wrote a post about the 3.3-million-year-old pre-Oldowan stone tool assemblage reported from the Lomekwi 3 (LOM3) site in Kenya by Harmand et al. (2015).  As I was writing that, I remembered a 2004 paper by Sophie A. de Beaune titled "The Invention of Technology" (Current Anthropology 45(2):139-162) that I had read in grad school.  That paper takes a long-term view of the evolution of technology focusing on the development and proliferation of different kinds of percussion.  Now that we have direct evidence of what kinds of stone tool technologies preceded Oldowan, I wanted to take another look at de Beaune's work.

Her basic premise, if I understand it, is that one can create a "phylotechnical tree"  of actions associated with different kinds of percussion.  Following Leroi-Gourhan (1971), her use of the term "percussion" includes actions such as sawing, chopping, cutting, and puncturing.  All of these different actions would ultimately have had a common origin in what de Beanue calls "thrusting percussion" (using one object to forcefully strike another with the intent of cracking or smashing it). The primacy of thrusting percussion is supported by its ethnographically-observed use among chimpanzees: some chimps crack hard fruits by smashing them between a hammer and an anvil.  Thus, de Beaune argues, thrusting percussion would have been utilized by the earliest hominids and preceded the more formalized stone tool technologies we can recognize in Oldowan.

How, why, and when did thrusting percussion, perhaps first used solely as an action employed to crack animal or vegetable materials, begin to be used to used to crack stone?  Those are the questions that can potentially be addressed directly by the assemblage reported from LOM3 (and hopefully more to be found in the future). 

To the "when" question, LOM3 answers "by at least 3.3 million years ago."  It's hard to imagine that the earliest identified example of something actually marks its earliest occurrence, so it's probably safe to presume that the behaviors that created LOM3 were present sometime prior to 3.3 MYA.

The first publication on 149 pieces of worked stone from LOM3 also gives us some insight into the "how" question.  According to the authors (pp. 311-312), the assemblage contains 83 cores (pieces of stone used for the removal of flakes) and 35 flakes.  The remainder of the stone pieces are interpreted as "potential anvils" (n=7), "percussors" (n=7), "worked cobbles" (n=3), "split cobbles" (n=2), and indeterminate fragments (n=12). You can see 3D digital models of some of the artifacts here.  

The LOM3 cores are not small.  The mean mass is 3.1 kg (6.8 pounds): that's heavier than a standard brick but lighter than your average bowling ball.  The flakes, anvils, and percussors are large, also, compared to those from later Oldowan sites and from those in assemblages produced by wild chimpanzees (p. 313). Although some artifacts have a series of flakes detached, patterns of fracture and flake removal suggest to the authors that the "precision of the percussive motion was also also occasionally poorly controlled" (p. 313):

"The dimensions and the percussive-related features visible on the artefacts suggest the LOM3 hominins were combining core reduction and battering activities and may have used artefacts variously: as anvils, cores to produce flakes, and/or as pounding tools. . . . The arm and hand motions entailed in the two main modes of knapping suggested for the LOM3 assemblage, passive hammer and bipolar, are arguably more similar to those involved in the hammer-on-anvil technique chimpanzees and other primates use when engaged in nut cracking than to the direct freehand percussion evident in Oldowan assemblages." (p. 313)

That sounds to me like a description that's pretty consistent with a manufacturing strategy based largely on chimp-like "thrusting percussion," and perhaps exactly what one would expect to precede Oldowan based on de Beaune's analysis.

What about the "why" question? What caused hominids to start using thrusting percussion to produce tools?  Answering that question is tougher than addressing the "when" and "how" questions. 

I don't think it has much to do with a change in physical anatomy -- specifically that of the hand -- for three inter-related reasons.  First, as I discussed before, I think there's a lot of evidence that suggests that hands with the capacity for human-like precision gripping were widespread among early hominids, including the australopithecines of around 3.3 MYA.  (See also this comment on australopithecine hands that just came out in Science today.)  Second, as discussed by de Beaune (p. 141-142), the physical actions required to smash one rock with another are not all that different than the actions required to smash a piece of fruit on an anvil: no new anatomy was even required to shift the "target" of the percussion to stone.  Third, even with the limitations imposed by their hand anatomy, chimpanzees can be taught to use freehand percussion to make stone tools (see this video of Kanzi, for example).

If the "invention of technology" (meaning, in this case, chipped stone technology) wasn't dependent upon a change in anatomy, what about a change in cognition? Again following Leroi-Gourhan, de Beaune (2004:142) discusses the nature of the distinction between using a hammerstone to smash something to process food and hitting a stone with another stone to produce a cutting edge:

"While these activities involved related movements, that of intentionally splitting a cobble to produce a cutting tools, although "exceedingly simple," was in [Leroi-Gourhan's] view eminently human in that it "implied a real state of technical consciousness.""

Maybe there does have to be a cognitive change to explain the shift to producing and using stone tools.  But, as we know from the Kanzi example, there's nothing lacking in the chimp brain that prevents them from making and use simple chipped stone tools when they're taught.  But, as far as we know, they have to be taught (the last time I checked, though, humans also need to be taught to do it).

Surely an important thing to understand about the shift to using stone-on-stone percussion to make stone tools is what that shift gets you: a tool with a cutting edge unlike anything that exists in nature.  A sharp-edged flake can be used for what de Beaune calls "linear resting percussion"  (cutting and chopping).  You can do a lot of things with an edged tool that you can't do with a blunt one (and that you can't do with your teeth if, like australopithecines, you lack the large canines of chimps and many other non-human primates).  You can sharpen a stick. You can grate and slice plants. And you can cut meat from bones and disarticulate an animal carcass by severing ligaments.  We have some direct evidence of this last activity in the form of the 3.4-million-year-old cutmarked bones reported from Dikkika, Ethiopia, in 2010.  Maybe the battlefield of the hunter-scavenger debate, now several decades old, will be reinvigorated by a transplantation from the Pleistocene to the Pliocene.

Does the emergence of chipped stone technologies during the Pliocene signal an adaptive shift, a cognitive shift, or both?  With the publication of the LOM3 tools and the announcement last week of a new fossil australopithecine from about the same time period and neighborhood, East Africa 3.3 million-years-ago sounds like a pretty interesting place to be.  If, as suggested by ethnographic data from chimps, gorillas, and orangutans, the capacity to use tools is really a homology that extends deep into the Great Ape lineage, it's probably not fair to refer to the production of chipped stone tools as the "invention of technology."  But it is a watershed nonetheless.  The shift to using one set of tools (hammers and anvils) specifically to make other, qualitatively different tools (cutting implements) that potentially open up new subsistence niches and eventually (possibly) become involved in the feedbacks between biology, technology, and culture which are entangled in the emergence of our genus is something worth knowing about:  who did it?  why? what were the tools used for? what changed as a result? 

The assemblage from LOM3 opens up a tantalizing window on those questions.  In those 149 pieces of stone, we have evidence of a stone tool production strategy that used "passive hammer" techniques to produce cutting tools, somewhere in time much closer to the dawn of stone tool production than anything called Oldowan.  Judging by the size of the cores and flakes, the technique appears to have been more dependent on brute force than finesse.  The results, however -- the creation of cutting tools from a natural setting that provided none -- may have been transformational.  I look forward to seeing how the data from the small LOM3 assemblage get incorporated into models of human evolution, and I hope that people working in East Africa are already busy finding more sites.  And I hope that people working outside of East Africa are actively searching for stone tools in Pliocene deposits.  It's a great time to be following paleoanthropology.

Leroi-Gourhan, A. (1971). L'Homme et la Matiere. Paris: Albin Michel.

The naming of a new species of hominid -- Australopithecus  deyiremeda -- made a lot of news this week.  The purpose of this post is not to worry over the details of the fossils that were used to construct this new taxon, but to ask for some clarification about what is actually meant by the term "species" as paleoanthropologists use it.  I'm going to tell you what I think it means, then I'm going to complain about it a little bit, then I'm going to ask you to tell me what you think it means. (Full disclosure: I fall at the lumper end of the lumper-splitter spectrum, and I think there are too many named "species" in our family tree.) 

In their paper, titled "New Species from Ethiopia Further Expands Middle Pliocene Hominin Diversity" (Nature 521:483-488), Yohannes Halie-Selassie and colleagues use the word "species" 17 times but provide no explicit definition of the term.  What is a "species"?  What are the implications of defining a "new species" of hominid?

Like so many other things, it depends. There exists a smorgasbord of different species concepts to choose from. A "typological species," for example, is a classification based on the co-occurrence of shared features, while an "evolutionary species" is defined based on the integrity of an ancestral lineage (without branching, there is no new species).  A "biological species" is generally defined as a group of organisms that can breed with one another but not with other groups of organisms.  In other words, a biological species is reproductively isolated from all other biological species.  The biological species concept is perhaps the one most frequently applied in biology, especially to living populations of plants and animals. 

I think a "biological species" is also what most people, paleoanthropologists included, mean when they talk about "species" of hominids. 

The distinctions among the various species concepts are not just academic when they're applied to fossil hominids.  They have implications for our notions about what variability means in the fossil record and how we interpret that variability in terms of the patterns and processes of human evolution.  Because reproductive isolation is the entire basis of the biological species concept, individuals in a biological species (by definition) could not and did not interbreed with any of their contemporaries outside their own species.  There cannot be multiple, co-existing species of human ancestors: a "new species" is either a human ancestor or somewhere off on a side-branch of our evolutionary family tree.  The discovery of a human ancestor that pushes someone else off onto a side branch is much more exciting that the discovery of another non-contender. You can see that in the enthusiasm of headlines about Australopithecus deyiremeda such as "Doubt cast on Lucy's place in human evolution" and "New hominid discovery older than Lucy raises more questions on human ancestry."  They might as well read "Don't let the door hit you in the ass on way out, Lucy."

I'm not an expert on paleoanthropology, and I've never directly analyzed or attempted to describe or classify the remains of a fossil hominid.  But I am someone who regularly attempts to describe variability (mostly in lithic artifacts) and make sound interpretations about what that variability means.  In any case, when you're looking at continuous variability, you can split all you want. There is easily detectable variability in just about everything not produced by a machine, so ultimately it's no great feat to break continuous variability down into as small of groups as you want (groups of one, if that makes you happy).  But what do those groups mean?  That's a hard question to answer without having a sample of a decent size that let's you investigate how the variability you're looking at is structured. That's why I'm a fan of trying to understand how variability is structured before trying to create groupings that have some analytical value. 

The pitfall of aggressive splitting of the fossil record is that the groupings you produce (the biological species) are tied to important assumptions about the processes that produced those groupings.  While I think the biological species concept is one that is clear and makes ecological and evolutionary sense, I also think its uncritical application to the fossil record is less than useful.  First, I think it's impossible to operationalize consistently and objectively (how can you determine if the populations represented by fossil individuals were capable of inter-breeding?).  So I don't trust that "species" that are equivalent in taxonomic terms reflect populations that are equivalent in evolutionary terms.  Second, I think naming lots of "species" short-circuits our study of what variability in the fossil record means by erring on the side of attributing it to species-level differences.  Under the biological species concept, species are non-overlapping, nominal categories, and speciation is a one-way street. By calling a fossil hominid a new biological species, you are making a statement about the nature of the relationship between that fossil and all other fossil hominids.  Given the sparse nature of the fossil record, I don't think that we can really make those kinds of statements with much confidence.  Using the scalpel of the biological species concept ties us to those assumptions, however.

So I'm very skeptical of the reality of the number of named species that currently inhabit the hominid family tree. How many are there now?  Twenty?  Thirty?  More?  I wonder what would happen if we started fresh and re-analyzed all the Pliocene and Pleistocene hominid fossils discovered over the last 120 years.  What would the structure of variability look like, and how would we interpret that variability if we erased all the existing species names and the historical legacies of discovery that accompanied them and based our groupings on patterns of variability across time and space? Who knows. I also wonder how many "species" of domestic dogs paleoanthropologists would define given a sampling of their bones.

Anyway, splitters be splittin,' and there's not much I can do about it.  When I taught my 200-level Human Origins class last year, I made the decision to focus not on the minutiae of "species" in the fossil record, but on what various lines of evidence could tell us about the timing, processes, causes, and effects of changes in human anatomy and behavior over evolutionary time.  We talked about species concepts and why they matter, and I gave my class my opinion that we're too quick to name new species and perhaps too reluctant to first look at what variability might mean outside the constraints of a species-level classification.  Is Homo antecessor a legitimate "biological species"?  Is the Homo erectus/ergaster division useful? If we know that Neanderthals and Homo sapiens exchanged DNA, why are we still calling them separate species?

Maybe I've wrongly identified the dominant species concept that is active in the background of paleoanthropological thought.  Maybe a new name isn't meant to assume reproductive isolation and all that that implies. If I've gotten it wrong, please correct me.  Maybe I missed something somewhere.  In my own published work, I've been asked to provide clarifying definitions for such controversial terms such as "household," "process," "model," and "projectile point."  President Clinton famously debated the meaning of the word "is." Is it too much to ask for a clarifying definition of "species" when we define a new one? 

Update (6/5/2015): This post was discussed in Barbara King's blog post for NPR titled "Declaring The Discovery Of A New Species Can Get Tricky."

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? 

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.

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).

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.

Last week I posted a short piece wondering aloud if we are safe in assuming that female australopithecines, rather than males, were the ones giving assistance to other australopithecines during birth.  The response of one of my friends on Twitter was that "Sexual selection says they should've been too busy getting busy to care." Like the ambiguity in the student paper that prompted me to ask the male/female birth assistance question in the first place, I'm not exactly sure of the intent of the response.  Was it to use the notion of sexual selection to dismiss the idea that males could have played a beneficial role in australopithecine births? Or was it to poke fun at how much weight we give sexual selection?

Sexual selection is selection (differential reproduction) that occurs when some individuals reproduce more than others because they are better at securing mates, rather than because of interaction with the environment (as in natural selection. Charles Darwin coined the term and explained in On the Origin of Species (1859:88).  Sexual selection, he wrote,

"depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.  Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny.  But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. . . . The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons." 

How important was sexual selection among australopithecines? We have traditionally looked at two aspects of sexual dimorphism (differences between males and females) to evaluate the degree of male-male competition in primates: canine size and body size.  If sexual selection was important among australopithecines (as it is chimpanzees and gorillas, our two closest living relatives) we would expect to see males with significantly larger canines and of significantly greater body size than females.  The fossil record isn't as clear on these things as you might think.

Canine Size

The drawing below shows maxillary dentitions from a chimpanzee, an Australopithecus afarensis (ca. 3.9-2.9 MYA), and a modern human.  I think the original drawing is from the (1981) book Lucy: The Beginnings of Humankind by Donald Johanson and Maitland Edey.  The difference in the relative sizes of the canines (indicated with red arrows) is obvious.  Chimpanzees (especially males) have large canines.  The space between the canine and the incisors (called a diastema) is there to accommodate the opposing canine when the jaws are closed.  The canines of modern humans barely protrude at all, and there is no diastema.  The canines of australopithecines are larger than those of modern humans, but smaller than those of chimps.  They protrude slightly, and there is a small diastema. 

The australopithecine specimen in the drawing is a reconstruction of the palate of Lucy (AL 200-1), traditionally interpreted as the remains of a female (but see below).  So that single specimen alone tells us nothing about male canine size and nothing about sexual dimorphism in canine size. Studies looking at samples of australopithecines have come to different conclusions about the degree of sexual dimorphism in the canines.  A 1997 paper by J.Michael Plavcana and Carel P. van Schaik  concluded that:

"Estimates of canine dimorphism, relative canine size, and body weight dimorphism in australopithecines provide little definitive information about male–male competition or mating systems. Dimorphism of Australopithecus africanus and Australopithecus robustus can be reconciled with a mating system characterized by low-intensity male–male competition. The pattern of dimorphism and relative canine size in Australopithecus afarensis and A. robustus provides contradictory evidence about mating systems and male–male competition."

This 2005 paper by Sang-Hee Lee paper concluded (based on a resampling method that wasn't dependent on sex estimates) that canine sexual dimorphism in Australopithecus afarensis was comparable to that seen in chimpanzees.

The canines of Ardipithecus ramidus (ca. 4.4 MYA) are smaller than those of chimpanzees but larger than those of modern humans.  The image below shows a comparison of a modern human (left), Ardipithecus (middle), and chimpanzee (right) (source).  The teeth in the image belong to "Ardi," the relatively complete skeleton of Ardipithecus ramidus discovered in 1994 and published in 2009. That skeleton has been interpreted as the remains of a female.  An analysis of other Aridipithecus ramidus teeth (Suwa et al. 2009) concluded that male and female canine teeth were of similar size.

If Ardipithecus is a hominid and the dental remains have been interpreted correctly, it suggests that males early in our lineage did not have large canines, presumably indicating that they were not vigorously competing with one another for mates (i.e., sexual selection was relatively unimportant).  It would also suggests that the LCA was unlike chimpanzees in many ways (e.g., perhaps an arboreal biped with a flexible back, mobile wrist, and generalized dentition rather than a knuckle-walking quadruped), making the chimpanzee overall a poor model of the LCA and therefore a poor model upon which to base explanations of evolutionary change in our lineage.

The single published skull from Sahelanthropus, a possible hominid from about 7 MYA, has been interpreted as that of a male with a relatively small canine.  I don't claim to have read the Sahelanthropus studies in detail, but I'm dubious that you can accurately estimate sex for a single skull from species about which so little else is known.  If the skull is that of a male, whether or not it's a hominid, it would be consistent with a low level of sexual selection in late Miocene apes. If it's a female it doesn't tell us much about competition between males. 

So the canine picture is a confusing one.  The canines of australopithecines sure don't look large to me (compared to chimpanzees), but at least some studies suggest a relatively high degree of dimorphism among some australopithecines.  If either Ardipithecus or Sahelanthropus was a hominid with a low degree of sexual dimorphism of the canines (suggesting low male-male competition), greater canine sexual dimorphism among australopithecines would  suggest an increase in sexual selection during the Pliocene.  If the LCA was more like a chimpanzee, however, sexual selection may have decreased during the Pliocene.

Body Size

The body size picture is also confusing. You can find a paper to support whatever you want, from a high degree of sexual dimorphism in body size (like gorillas) to a low degree of sexual dimorphism (like modern humans).  When I talk to my Human Origins class about it, I just tell them what sexual dimorphism is and why we would like to know about it, and then confess that I can't make up my mind what the best answer is at the moment.

A paper published last week came down on the side of low sexual dimorphism in body size.  The authors, Philip Reno and Owen Lovejoy, conclude that:

"The relatively stable size patterns observed between Ardipithecus and Australopithecus suggest there was not strong selection for greater male body size that would result from a reproductive strategy arising from increased individual male reproductive success via inter-individual aggression. In fact, the reduction in canine dimorphism with feminization in the male would argue for reduced “agonistic” behaviors (Lovejoy, 2009). This is particularly so given the strong association between canine dimorphism and reproductive behavior in anthropoids (Plavcan, 2012b) and the lack of a dramatic dietary shift associated with canine modification in early hominids (Suwa et al., 2009)."

This is not a new argument from Lovejoy, who has been hypothesizing the presence of monogamous reproductive strategies among early hominids for decades.

What's Love Got to Do With It?

We've got a lot to learn about australopithecine social organization and the lack of clarity about sexual dimorphism does not help.  In my post about whether birth assistance was a gendered activity, I reasoned that it would be more likely that males would be involved in birth assistance if males and females were pair-bonded. In that circumstance, paternity would be more-or-less certain, and male behavior that increased the success of reproduction would be selected for.  Overall, I like the anatomical evidence for a relatively low level of sexual selection among australopithecines, consistent with low levels of male-male competition.  Without being able to accurately determine the sex of australopithecine fossils, however, its hard to have a lot of confidence. If Lovejoy is right about Ardipithecus, male-female pair-bonding was already present in the ancestors of australopithecines (could it even have been typical of many apes in the late Miocene?). If the LCA was more like a chimpanzee, however, sexual selection may have been strong at the time of the divergence of the chimpanzee and human lineages.

Even if australopithecines had a monogamous, pair-bonded mating system, however, that doesn't mean there was anything like culture attached to it.  It may have just been part of a hard-wired biological adaptation, one that emerged along with bipedalism because it made evolutionary sense. Along with certainty of paternity would come greater paternal investment in offspring, presumably resulting in a higher survival rate (hence being selected for). The low/moderate amounts of sexual dimorphism in body size could be accounted for by the positive relationship between body size and energy efficiency during bipedal locomotion: males provisioning females and their offspring would have to travel longer distances than females, selecting for larger body sizes among males (but not larger canines) (see Daniel Lieberman's book The Story of the Human Body).  In this scenario, "love" would be related to sexual dimorphism not because of male-male competition but because bigger males would be better providers.

Convinced?  I'm not (either way). But it's worth thinking about.

I've always like Karen Rosenberg and Wenda Trevathan's (2002) paper "Birth, Obstetrics, and Human Evolution" (available here).  It does a nice job of exploring the social implications of anatomical changes related to the emergence of bipedalism, focusing on how the shape of the pelvis in australopithecines would have complicated birth and transformed delivery from a solitary experience into one that often required assistance.  I used a condensed version of the article (Scientific American 13:80-85, 2003) in my 200-level Human Origins class: it really helps students see how you can make connections between fossils and behaviors and how one aspect of human evolution can have implications for others.

While I was reading one of the student papers from that class this semester, I started to question the model I had in my head of australopithecine females assisting each other in birth.  The student wrote something that seemed to imply (intentionally or not - I'm not sure) that males were the ones giving assistance at birth. It struck me as odd, which made me ask myself why it should strike me as odd: why did I assume females and not males?

I don't think Rosenberg and Trevathan ever specify that females would have been the ones providing assistance to other females, but they use the term "midwifery" several times in their papers.  Though not technically defined today as excluding males, the term "midwife" carries a lot of history that associates it very closely with females.  This is the etymology as provided by Wikipedia:

"The term midwife is derived from Middle English: midwyf literally "with-woman", i.e. "the woman with (the mother at birth), the woman assisting" (in Middle English and Old English, mid = "with", wīf = "woman")."

Based on a quick perusal of some of the web resources that pop up first (e.g.,this, this, and this) the idea of the ancient origins of the association between females and birth assistance is widespread.  I have no reason to doubt that this is correct: I would presume a strong female bias in birth assistance could be amply demonstrated both historically and ethnographically. 

So should we assume that females were also providing birth assistance among early hominids?

Maybe not.

It seems clear that our modern conceptions of who should provide assistance at birth are culture-bound.  By "culture-bound" I mean connected to other shared, leaned aspects of human societies, behaviors, and symbolic systems.  I don't think I'll get an argument if I state that there is zero evidence for anything like human culture among australopithecines. So what happens to our gendered conceptions of birth assistance if you remove all or most of their cultural underpinnings? Would birth assistance still be performed primarily by females?

For the sake of argument, consider how selection might act to reinforce the behavior of males assisting females during birth.  If labor and delivery are dangerous for both the female and the neonate, any assistance a male mate can provide increases the chances his genes will be passed on.  This would be true in a situation of stable male-female pair bonding, as pair-bonding decreases the uncertainty of paternity.  The inclination of males to provide effective assistance during birth would be selected for if pair-bonding was present and birth was dangerous.

If we assume that pair-bonding among australopithecines would not have been the same thing as "marriage" (a human cultural behavior), why do we assume that australopithecine birth assistance would be the same thing as "midwifery"? Our historical and ethnographic record of what humans do now really only gets us so far in addressing questions like this: the universality of a cultural practice among modern humans does not necessarily mean it always existed in the same form deep into the past.

Update (5/3/2015):  The response of one of my friends on Twitter to this post was that "Sexual selection says they should've been too busy getting busy to care." I wrote this quick post taking an informal look at ideas about sexual dimorphism and sexual selection among australopithecines.  It looks to me like the jury is still out, as there's a lot of conflicting information about the degree of sexual dimorphism in the two criteria we look at most often in primates: canine size and body size.