An Evolutionary Anthropological Perspective on Modern Human Origins Curtis W. Marean, Institute of Human Origins, School of Human Evolution and Social Change, PO Box 872402, Arizona State University, Tempe, AZ 85287-2402 USA, e-mail [email protected], Office Phone: 480-965-7796 And Faculty of Science, Nelson Mandela Metropolitan University, Port Elizabeth, Eastern Cape 6031, South Africa

Keywords Evolutionary anthropology, Neanderthals, paleoanthropology, human origins, evolutionary genetics

Abstract Modern humans are a spectacular species anomaly among all fossil and living animals. The final evolutionary steps that produced this anomaly occurred late in its evolution during the origins of modern humans. Ultimate explanations for this evolutionary trajectory are best attained through synthetic studies that integrate genetics, biological anthropology, and archaeology all resting firmly in the field of evolutionary anthropology, yet these fields of endeavor typically operate in relative isolation. This paper seeks a synthetic overview of the origins of modern humans and identifies the three pillars of this human uniqueness as an evolved advanced cognition, hyperprosociality, and psychology for social learning, all which are the foundations for cumulative culture that is the dominant adaptation of our species. While all members of the Homo line evolved slowly in the direction of advancing cognition, the evidence reviewed here shows that only modern humans evolved extreme levels of prosociality and social learning, and an explanation for why this was is offered. These three traits were in place sometime around 200100 ka, producing a creature capable of extra-ordinary social and technological structures, but also empowered to make war as large groups with advanced weapons. The advance out of Africa and the annihilation of other hominin taxa, and many unprepared megafauna, was assured. INTRODUCTION One of the great contributions of paleoanthropology to science is the scientific consensus that modern humans evolved in Africa and spread across the globe from there. Not so long ago the field was divided and in battle – the seminal 1989 collection of papers from a Cambridge conference (Mellars & Stringer 1989) displays two camps vigorously supporting two competing models for the origins of modern humans (the “single origins model” and “multi-regional continuity model”). A follow-up conference published in 2007 tells a different story (Mellars et al 2007) – multi-regional continuity has expired and the single origins model remains. This consensus leads us to new exciting questions about our evolutionary path to being human and the planet’s sole human species. When and where did the modern human lineage evolve in Africa? Was this an abrupt event with a single geographically isolated population or a gradual event across Africa? What do we know about the formation of structure (ethno-linguistic groups) to the populations of these early modern humans? Did modern humans exit Africa through the Levant or across Arabia? As modern humans left Africa, what was the nature of the interaction 1

with the resident hominins such as Neanderthals, and how fast were our sister species replaced? Was there transmission of genes and culture between these populations, and were there differences in behavior and abilities that resulted in advantages to the invading modern humans? Many fields of enquiry contributed to this consensus. Evolutionary genetics had a decisive impact and is one of the most dynamic fields of human origins enquiry. Biological anthropology is making outstanding strides revealing patterns in growth and development that illuminate life history, and bio-mechanical and functional morphological studies are revealing ancient activity patterns. New field and laboratory techniques and advances in geochronology are solving our need for chronology. Archaeology continues to discover when, where, and in what context behavioral and cultural milestones occurred. Despite this dynamism in all the involved fields, the disciplines of genetics, biological anthropology and archaeology continue to struggle to attain synthetic and ultimate explanations for modern human origins. A central theme of this review is that the best way to approach the origins of modern humans is through the principles of evolutionary anthropology, where social and biological science is joined to create an integrated explanatory framework. In 2003, Henshilwood and Marean lamented the lack of general theory for understanding the origins of behavioral modernity – that theory is now taking shape through the contributions of a wide number of disciplines and anthropological sub-disciplines seeking to understand the origins of human uniqueness. This review will synthesize those results as a pathway to probe the archaeological, biological, and genetic record for the origins of Homo sapiens. THE BIOLOGICAL BACKGROUND The genetic and fossil records (Figure 1) indicate that the lineage that leads to extant modern humans appeared between ~300-100 thousand years ago (ka) (Endicott et al 2010, Scally & Durbin 2012, Underhill & Kivisild 2007), and that the time to the most recent common ancestor (TMRCA) for mitochondrial and Y-chromosome DNA are now shown to be concordant (Poznik et al 2013). A recent analysis of the Y chromosome provides a very ancient range with a 95% confidence interval of 581-237 ka (Mendez et al 2013). Under the traditional mutation rate, the lineage likely evolved during the long cold glacial Marine Isotope Stage 6 (MIS6), began expanding during the warm interglacial MIS5e commencing ~130 ka, and expanded its range to most of the Old World by at least 40 ka (Henn et al 2012), though a critique of this overall approach to the genetic data can be found in (Pickrell & Reich 2014). I will refer to the 600-40 ka unit of time as the modern human origins timespan (MHOTS). There is a backstory, and I will use 800 ka as a convenient starting point. Interpretations of the current genetic and fossil records suggest the world had a diverse set of hominin lineages between ~800 and 40 ka. There was a modern human lineage in Africa, at least one archaic African lineage (Hammer et al 2011, Lachance et al 2012, Mendez et al 2013) that may also have a fossil example (Harvati et al 2011), two archaic Eurasian lineages (Neanderthals and Denisovans) (Meyer et al 2014), and an unknown archaic Eurasian that may have been Homo erectus (Prufer et al 2013). There is also the very archaic Flores lineage, but since it is unrelated to modern human origins (Argue et al 2009) it will not be discussed here. There may be other lineages that are not yet discovered. The genetic record suggests that from ~800-500 ka there is gene flow between the African and Eurasian populations, and the fossils display anatomical similarity (Stringer 2012), and there are also broad similarities in the stone tool technologies between Africa and western Eurasia. The hominin fossils from this time are often assigned to Homo heidelbergensis, but there is debate over the taxonomic specifics 2

(Stringer 2012). The record from eastern Eurasia is still unclear and poorly dated. The genetic, fossil, and archaeological records imply that the pathway out of Africa is at least sometimes open between ~800-400 ka so that populations and genes and ideas are being transferred between and across these landmasses. The genetic evidence suggests that the split time between modern humans in Africa and Neanderthals/Denisovans is between ~600-300 ka (Meyer et al 2014, Prufer et al 2013) and genetic isolation begins the process of divergence between African and Eurasian lineages. The fossil record between 500-200 ka is sparse and most numeric age estimates are uncertain, particularly in Africa, and there is little from eastern Eurasia that is numerically dated. The fossil hominins from Sima de los Huesos (Atapuerca, Spain) have morphological affinities to Neanderthals (Stringer 2012) but genetically group closer with Denisovans (Meyer et al 2014) and are imprecisely dated from ~600-300 ka. There is widespread agreement that Neanderthals have appeared as a recognizable species by ~200 ka, and their unique characteristics begin to accrete far earlier (Hublin 2009). The most recent estimate for the split between Denisovans and Neanderthals is ~470-190 ka (Prufer et al 2013). By ~196 ka the fossil record shows that Homo sapiens is present in Africa. The earliest fossils include Herto (Clark et al 2003, White et al 2003), Omo Kibish (McDougall et al 2005), and Jebel Irhoud (Hublin 1992), all of which are on the outer range of variation for modern humans. Early in the Late Pleistocene there are fossil hominins from North and South Africa that are anatomically modern humans (Hublin 1992, Rightmire 1979), and there are anatomically modern fossils in the Levant that date to MIS5 (Valladas et al 1988) that are likely north-east extensions of this African modern human population. The desert corridor between Egypt and the Levant was wetter and more easily crossed in MIS5 (Vaks et al 2007). Sometime between ~100-50 ka a founder population of modern humans leaves Africa and is the source for all Eurasian-derived populations (Henn et al 2012). The dispersing modern humans came into contact with resident archaic lineages and there was limited inter-breeding. Archaic lineages that persisted in Africa inter-bred with modern humans as late as ~35-10 ka (Hammer et al 2011, Harvati et al 2011) and hybridization occurred between the early founder group of modern humans and Neanderthals (Green et al 2010) between ~58-52 ka (Fu et al 2014), as well as between an eastern-moving sub-population of modern humans and Denisovans (Reich et al 2011). The genetic evidence shows that modern human-Neanderthal hybridization events were rarely successful and most introgressed alleles were the focus of negative selection. Some alleles conferred positive results and were positively selected (Ding et al 2013), associated with skin and hair color from Neanderthals (Sankararaman et al 2014) and high altitude adaptation from Denisovans (Huerta-Sanchez et al 2014). This genetic introgression has led some to conclude that the new evidence “blurs previously accepted taxonomic boundaries between Upper Pleistocene hominins” (d’Errico & Banks 2013): S371). This is a misinterpretation – genetic introgression across taxonomic lines is common and happens in all major higher organisms (Barton & Hewitt 1989). The Quaternary, with its pattern of extreme climatic flux and species range changes, is a particularly fertile time for introgression and the formation of hybrid zones (Hewitt 2011). There is now debate over whether the original lineage for modern humans evolved in one small region of Africa, what can be called the “strong single lineage model”, or was a more continent-wide process. If the former is correct, it also then raises the question “what region”? The strong single lineage model gains traction if the origin of the lineage occurs in a glacial phase, when it is thought that much of Africa was arid and would have isolated refuge zones 3

(Lahr & Foley 1998). The extreme rarity of sites dating to MIS6 and earlier glacials, suggesting a small human footprint, joined to genetic evidence for very small populations has also added impetus to this argument (Fagundes et al 2007). The Cape Floral Region of South Africa has been argued to have had the right environmental conditions for a refuge during glacials (Marean 2010, Marean et al 2014), and the antiquity of the indigenous surviving lineages in the southern African sub-region provides some support for southern Africa being a favored location (Henn et al 2011, Pickrell et al 2012). Our consensus model of modern human origins has gone from multi-regional continuity, to a hard and simple single origins model, and now to a single origins model with genetic introgression from archaic and extinct lineages into the modern human genome. The rapid advances in evolutionary genetics assure us that more surprises are on the way. A snap-shot of the world at 140 ka would have a variety of hominin lineages with cultural capacities in Africa and Eurasia; at 50 ka it would look much the same; but a snapshot at 30 ka would have a single modern human lineage throughout the Old World, with all the others extinct except for tantalizing snippets of their DNA embedded in the survivor. For some time there has been debate around the timing of modern human arrivals into Eurasia and Sahul. Both debates center on whether the arrival is before or after ~45 ka, and of course this is near the maximum age of reliable radiocarbon dating. In my opinion it is clear that we are beginning a major revision on the timing of the arrival of modern humans into extra-African regions. The application of more refined radiocarbon techniques (Higham 2011, Higham et al 2014) as well as dating techniques that lack a ~45 ka ceiling (Jacobs & Roberts 2007) is driving this revision. But we can say that at least by ~40 ka all the major landmasses of the Old World are occupied by populations of modern humans and soon all sister taxa would be extinct. It is unclear how long this extinction process lasted, but the result was our lonely species, and a planet-wide xenocide seems uncomfortably plausible. FROM BIOLOGICAL CONSENSUS TO BEHAVIORAL CONTENTION The biological record for modern human origins has an African origins consensus but the behavioral side of the story is still debated. When the multi-regional model still held sway, and it was thought that the modern human lineage did not appear until roughly ~45 ka, the “Human Revolution” model for behavioral evolution predominated. This idea posited that “modern human behavior” appeared rapidly and was indicated by the sudden expression of complex material culture that we see in Europe with the turnover between the Middle and Upper Paleolithic (Gamble 1994, Klein 2000). Prior to ~40 ka there was precious little evidence for long distance exchange, almost no evidence for symbolic behavior, and technology was thought to be simple and lacking bone tools, microliths, and other more complex-recipe technologies. In Europe these appeared as a “revolution” with the Upper Paleolithic, and this pattern was extended to the lesser-known African continent. The paleoanthropological community used a series of traits as markers for the appearance of modern behavior – if these traits were present, then modern behavior was as well. This argument was eventually critiqued on both an empirical and theoretical basis. Many traits were shown to appear earlier than 40 ka in Africa (McBrearty & Brooks 2000). The “trait list approach” was also critiqued on theoretical grounds (Henshilwood & Marean 2003, Wadley 2003) – it had arisen empirically in the European record and was not directly applicable to the African record, many of the patterns could be explained as resulting from other processes such as differing adaptations to the tropics and temperate climates, and taphonomic and sample size 4

effects. Africanists in particular called for a more theory driven approach, and some argued that the core feature of “behavioral modernity” was a complex cognition that produced a symbolically mediated world view and allowed language, and that clear proxies for symbolic behavior were needed to search for the origins of modern human behavior. The trait list developed inductively from observations of the European record without the needed theoretical justification, so where does it stand today? There is consensus that the “human revolution” pattern as originally formulated is now falsified - Africa does not show a sudden appearance of complexity at ~40 ka – many of the traits appear in Africa far earlier. While the trait list approach for recognizing modern human behavior needs to be abandoned, the continued construction of an understanding of the timing and appearance of important “milestones” remains a key part of the human origins research strategy. The first and most important job of the field paleoanthropologist is to discover the evidence, put it on a precise and accurate timeline, and enrich our understanding of the context. Paleoanthropologists have done an outstanding job at this, but we have been less effective at generating theory, but that theory is taking shape as a result of research in other disciplines. A Theoretical Grounding to the Research Problem Humans like all species have a suite of features (biological and behavioral) that when compiled form a unique whole. It is considered in biological theory unlikely for two species that have the same niche to overlap geographically for a prolonged period of time. The result is that we can write an “abstract” for each species that describes it rather well in both biological and behavioral characters and in a unique fashion. Once we have that abstract for a species, then we can tackle the task of proximate and ultimate explanations for the evolution of that organism’s adaptive system. Paleoanthropologists, and particularly archaeologists, for years have used the term “behavioral modernity” or “modern human behavior” as a catch-all for the package of behavioral traits that are unique to modern humans. Archaeologist have been remiss at defining “behavioral modernity”, or the behavioral portion of our abstract of modern humans, creating confusion and consternation. Nowell (Nowell 2010) provides a useful review of the various ways archaeologists have defined behavioral modernity. Shea (Shea 2011) critiqued the use of the term “behavioral modernity”, arguing for replacement with a focus on “behavioral variability”. The unique character of modern humans, according to Shea, is their extreme behavioral flexibility in the face of variable external stimuli, and archaeologists should focus on quantitative assessments of this variability. But replacing the study of the package referred to as “behavioral modernity” with quantitative assessments of behavioral variability only replaces an imprecisely written species abstract with one sentence of that abstract, albeit an important one. Variability and flexibility are proximate expressions made possible by the presence of proclivities and traits that are almost certainly embedded in the genome. These should be the focus for evolutionary (ultimate, sensu (Smith 2013)) explanations. Some (Ames et al 2013, d’Errico & Banks 2014) take the view that we should approach the archaeological and biological records separately. Among these authors there is dismay that archaeologists are hamstrung by the taxonomic distinction between modern humans and Neanderthals. They argue that our comparative studies of Neanderthals and modern humans should reject biological conclusions and see the differences in the archaeological records as arising from mechanisms that lead societies to develop different adaptations (d’Errico & Banks 2014). We may discover that Neanderthals and modern humans were indistinguishable in 5

regards to their genetically based adaptations, and that differences between them expressed in the archaeological record may be due to varying cultural adaptations, societal evolution, or demography. But that is a hypothesis to test, not an assumption that forms the basis of a research strategy. The three lineages (modern humans, Neanderthals, and Denisovans) that we know of now were reproductively isolated for long periods of time, evolved in separate landmasses and environments, and only one remains. We must scientifically test the possibility that these separate lineages had different adaptations that were molded through differing histories of biological and gene-culture evolution. To do that, we need to start with the species we know best and define in a clear way what is unique about modern humans. What is our species abstract for Homo sapiens? Human Uniqueness Theory and an Evolutionary Anthropological Approach A surge in research in many disciplines has defined the similarities and differences between modern humans and other extant species. From it is arising the beginnings of a general theory for human uniqueness (Hill et al 2009). This theory is evolutionary anthropological (sensu (Smith 2013)) in that it is grounded in neo-Darwinian theory and seeks ultimate explanations for adaptations, it employs theory developed for understanding cultural transmission (Henrich & McElreath 2003), and integrates gene-culture coevolution (Richerson & Boyd 2005). It combines biological, behavioral, and cultural perspectives with the goal of developing a synthetic evolutionary explanation for the spectacular uniqueness our species. The study of modern human origins can be comfortably subsumed within this broader research agenda. Paleoanthropologists regularly seek to discover when, where, and under what contexts various important anatomical (such as bipedalism or canine reduction) characters appear, and seek to explain this appearance. Human Uniqueness Theory (HUT) seeks to do the same for behavioral and cultural traits in a way that integrates biology. Archaeologists have focused much of their attention on the evolution of a complex cognition (Coolidge & Wynn 2009, Henshilwood & Dubreuil 2011, Henshilwood & Marean 2003), and certainly it is central to the definition of human uniqueness. But equally important are hyper-prosociality and a psychology for social learning, and a proclivity to both is likely evolved and genetically based. By hyper-prosociality I mean the tendency for regular (everyday) cooperation, that can entail a cost to the cooperator, with un-related individuals without the expectation of immediate pay-offs. The regularity and extent of this cooperation among humans is unique among living animals, though there examples of it at lower levels in other animals (Clutton-Brock 2002). Highly advanced cognition operating in a symbolic framework, hyperprosocial proclivities, and a dependence on social learning underlie the more proximate behavioral and cultural expressions that researchers have imprecisely referred to as “behavioral modernity”, and provide the capacity for cumulative culture so fundamental to the modern human niche. This results in the high cultural and behavioral variability pointed to by Shea (2011) as the three operate together in reaction to regional (biological and social environmental) context. No other living animal has this combination of hyper-prosociality, heighted social learning, and advanced cognition. They are the unique descriptors in our species abstract and, in my opinion, are what archaeologists were chasing with their fuzzy concept of “modern human behavior”. They are the three pillars of human uniqueness and the ultimate foundation of our adaptation.

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THE EVIDENCE FOR THE EVOLUTIONARY HISTORY OF THE UNIQUE MODERN HUMAN ADAPTATION Behavioral traits that have a genetic component are often parts of anatomical and physiological systems and subject to interactions between genetics and culture. For example, the human dependence on learning is linked to prolonged juvenile dependence, and this expresses itself in growth patterns, which have an anatomical expression in teeth and the pelvis, all of which underlies an enlarged brain and complex cognition. To study the evolutionary history of these traits we need proxies for each and they need to be studied synthetically in a way that integrates the biological and material cultural records. The first problem we face is that the expression of these traits is not binary. For example, hyper-prosociality and advanced cognition is not a yes or no proposition – we can expect that different hominin taxa could have differing levels of prosociality and cognitive complexity and social learning psychology varying over time within a lineage. In the discussion below I will examine the evidence for the presence and character of expression of these three components of human uniqueness in the paleoanthropological records of the various lineages across the MHOTS, excluding Denisovans since at this time we simply cannot assign a fossil and archaeological sample to them with any confidence. Importantly, I filter this discussion down to evidence that must meet two criteria. First, the evidence must be derived from very secure archaeological and geological contexts. In many instances this criteria excludes finds that are derived from old excavations where the contextual details were not recorded. Second, the evidence must be published in commendable detail so that it can be carefully evaluated. The Evidence for an Advanced Cognition Humans have a complex cognition that allows a symbolically mediated world-view, and its most complex symbolic system, language, is an essential component of the modern human adaptation (Pinker 2010, Tomasello 1999, Tomasello 2008). Language capacity has cognitive and anatomical requirements, and symbolic capacity expresses itself in material culture, and these anatomical and material expressions form the basis of our proxies for documenting their origin and character. Modern humans are highly creative (Andreasen 2006, Pinker 2010), recognizing out of the box solutions and unlikely relations, and innovate at very high rates, and these result in inventions that can be the source of both minor and major adaptive advances . The modern human technological pattern is replete with innovation and this should be detectable in the archaeological record, and it drives high levels of technological variation in reaction to varying external stimuli such as differing environments. The most direct evidence for a complex cognition is brain size and organization. Neanderthals were at least as encephalized as modern humans (Holloway et al 2004). However, when corrected for the larger size of their bodies and visual systems, Neanderthals had smaller brains than modern humans (Pearce et al 2013). The brain shapes of Neanderthals and modern humans differ significantly with modern human brains having a uniquely globular shape and enlarged parietal lobes (Bruner 2010) as a derived feature while Neanderthal brain shape resembles the primitive condition in Asian H. erectus (Balzeau et al 2012). The anatomical brain data leads us to anticipate cognitive differences between modern humans and Neanderthals. A brain capacity large enough for modern human cognition is in place by ~196 ka (Day 1969, McDougall et al 2008). The origins of symbolic thought and language has been approached through the study of archaeological artifacts, anatomy, genetics, and statistical analysis of languages. Debate exists 7

over archaeological proxies for the presence of language (Botha 2010, Henshilwood & Dubreuil 2009). Similarly, there is little consensus on the fossil anatomical proxies of language and when they appear, with some arguing for an appearance at ~50 ka (Lieberman 2007) or ~100 ka (Shultz et al 2012). There was hope that the FOXP2 gene would provide some resolution, but the issue is more complicated than originally thought (Fisher & Scharff 2009). Since the KhoiSan lineage separates at >100 ka and is quite isolated until recently (Henn et al 2011, Schlebusch et al 2012), the most parsimonious explanation is that the shared common ancestor dating prior to this had full language capacity. An estimate of the origins of language based on modern phonemic diversity suggests an age, depending on the assumptions, between ~300-100 ka (Perreault & Mathew 2012). It is unclear if the shared common ancestor of the African and Eurasian lineages had language capacity and at what level it existed, but the phonemic diversity analysis overall suggests an origin that postdates the split between Neanderthals and modern humans. There is consensus that language requires a complex cognition and symbolic capacity, so our next question must focus on material proxies for these. Archaeologists have approached symbolic capacity by searching for material expressions of symbolic behavior including the creation of symbols and symbolic artifacts (such as jewelry), the use of pigments signaling color symbolism (painting and drawing), and the collection of what I will call “special items” – crystals, sea shells, carnivore teeth, and other special things collected in a non-functional context. Special items such as sea shells appear in South Africa ~110 ka (Jerardino & Marean 2010, Singer & Wymer 1982). Shells were collected and transported ~35 km by early modern humans at Qafzeh in the Levant (Bar-Yosef Mayer et al 2009), and ~60 km inland in Spain by Neanderthals (Zilhão et al 2010). Pigments found in archaeological sites may not always have been used for symbolic representation since they might also have been used for other things like camouflage, skin protection, or functional tasks (Wadley et al 2004). The pigments that are most commonly studied as the best indicators of colorants for symbolic expression are red ochres (iron oxides). The site of Maastricht-Belvédère in the Netherlands dated to ~250-200 ka has reddish stains that have been interpreted as hematites that were purposefully brought in (Roebroeks et al 2012), but the lack of true worked ochre pieces and the uniqueness of this pattern raises concern. Well described and clearly utilized ochre is present at ~162 at PP13B in South Africa (Marean et al 2007, Watts 2010), and pigments are reported from Twin Rivers in Zambia (Barham 1998, Barham 2002) but the age is contested (Barham 2012, Herries 2011). Starting at ~110 ka there are many sites in South Africa with large quantities of utilized and un-utilized ochre (Henshilwood et al 2009, Watts 2002, Watts 2010) and there is often a strong bias for red ochre (Watts 2010). Ochre processing kits have been found dating to ~100 ka at Blombos Cave in South Africa (Henshilwood et al 2011). Early modern humans dated to ~92 ka in the Levant are associated with large quantities of ochre while it is lacking from Levantine sites assigned to Neanderthals (Hovers et al 2003). Ochre pieces are uncommon on European sites associated with Neanderthals but rise in abundance near the MP-UP transition. Manganese is abundant at Pech-de-l’Aze´ I dated to near the MP-UP boundary (D'Errico 2003, Soressi et al 2008). There is a large quantity of pigment in layers assigned to the Châtelperronian at Grotte du Renne in France at the MP-UP boundary (Caron et al 2011), but the authorship of the Châtelperronian is disputed (Bar-Yosef & Bordes 2010). Red stains on shells at a MP site in Spain is interpreted as pigment (Zilhão et al 2010). There is evidence for the creation of symbols in the form of abstract designs among early modern humans in Africa. At ~100-60 ka there are cross-hatched designs on bone (d'Errico & 8

Henshilwood 2007), ochre (Henshilwood et al 2009, Henshilwood et al 2002, Mackay & Welz 2008), and ostrich eggshell in South Africa (Texier et al 2010). Beads are present in South Africa at ~70 ka (Henshilwood et al 2004), North Africa at ~80 ka (Bouzouggar et al 2007), and possibly the Levant with early modern humans at ~92 ka (Bar-Yosef Mayer et al 2009). There are ornaments in Châtelperronian contexts in Europe (Caron et al 2011). The archaeological record has some proxies to study the capacity for creativity and innovation. Both Neanderthals and early modern humans had the capacity to build fires for cooking and heat, and this seems to be a shared derived capacity having its origins back to ~790 ka (Goren-Inbar et al 2004). Pyrotechnologies are innovations that build on the basic use of fire for cooking, heat, and defense from predators. The earliest known pyrotechnology is stone tool heat treatment. Heat treatment in South Africa differs substantially from that in Eurasia in that the stone, silcrete, in its raw form is poor for stone tool manufacture (Brown et al 2009) while flint in its raw state is a high quality raw material (Crabtree & Butler 1964). Heat treatment of silcrete requires an unlikely mental association between a stone of poor quality for manufacture to one that is completely transformed to a high quality raw material. Heat treatment of flint is a minor adjustment, but even so was never employed by Neanderthals. Modern humans in Africa (Watts 2010) and the Levant (Godfrey-Smith & Ilani 2004, Salomon et al 2012) also recognized that heat could be used to transform yellow ochre to red. A crucial innovation that is documented in South Africa by about ~65 ka is the modification of ostrich eggs into flasks for carrying water (Texier et al 2010). This breakthrough technology allows caching and carrying water, which opens up new habitats to cross and exploit. Early modern humans in Africa innovated in the development of microlithic technology ~71 ka (Brown et al 2012) and ballistically sophisticated bone projectile points (Henshilwood et al 2001). While blade technology is present among Neanderthals it is uncommon and less formal than in Africa, and there is no evidence that Neanderthals ever used microlithic technology and bone projectile points, though both characterized early modern humans in Europe. This may explain the common trauma experienced by Neanderthals and their relatively short life spans (Berger & Trinkaus 1995, Trinkaus 1995) – the advanced projectile weapons that put a safe distance between hunter and prey were not in the Neanderthal arsenal. In summary, the record as it stands now shows that the African lineage had a brain similar in size to modern humans by 195 ka, symbolic behavior indicated by use of pigments appears by ~160 ka, and then there is regular evidence for symbolic behavior through the presence of ornaments and abstract designs starting at ~110 ka. Alongside this symbolic capacity is evidence for innovations and novel associations in the form of heat treatment by 160 ka, and other complex technologies by 110 ka and later. The record for the Neanderthal lineage is decidedly different. The large brain size is likely to be proportionally smaller when corrected by other factors, and the expressions of symbolic behavior, novel associations, and innovation and creativity is less compelling and when present, typically temporally late and near the time of entry by modern humans. It is also patchy despite the robust sample of sites in Western Eurasia, and the environmental conditions are such that we would expect a greater deal of technological complexity. The African lineage shows good evidence for a fully modern human complex cognition by ~200-160 ka, while the Neanderthal lineage lacks such clear evidence. The Evidence for a Psychology of Social Learning Humans excel at high fidelity transfer of information to offspring and group members, by relying intensely on social learning through imitation with a focus on process (McGuigan et al 9

2011, Nielsen et al 2014), suggesting an innate social learning psychology (Tomasello 1999, Tomasello et al 1993). Humans have life history adaptations to facilitate this, and high fidelity transfer results in complex recipe technologies, and these form the basis for proxies for recognizing the origins of modern human levels of social learning in the past. Careful modification of learned information regularly occurs, these modifications can be transmitted, and this process is essential for cumulative culture (Boyd et al 2011), and the build-up in the complexity of information, behaviors, cultural characteristics, and technologies (the ratchet) (Tennie et al 2009). This social learning psychology allows cumulative culture to evolve over time and respond quickly to external stimuli like environmental change (Boyd et al 2011, Richerson & Boyd 2000). The modern human social learning machinery results in geographically patterned and sometimes rapid material cultural change in reaction to environmental change. High fidelity transfer of information and social learning is facilitated by a complex cognition and extended juvenile learning period with slow brain and body growth during which time human children are nearly totally dependent on parents and allo-parents. During this prolonged period of juvenile dependency modern human juveniles focus intently on observation and imitation for acquisition of information and skills such as language (Tomasello 1999). The biological record preserves evidence for the speed and mode of this period in the dental and boney tissues of the body. Modern human juvenile dental immaturity is particularly prolonged, consistent with their period of dependency, while that of Neanderthals was faster (Smith et al 2010). An early member of H. sapiens from Jebel Irhoud in North Africa dating to at least ~160 ka had a fully modern pattern of tooth eruption and dental development as modern humans (Smith et al 2007). Neanderthal brains grew quicker during infancy than modern human brains though the cognitive implications of this are unclear (Ponce de León et al 2008). The consensus is that Neanderthal juvenile growth was accelerated (Schwartz 2012), which likely placed limits on the quality of information transfer and retention. Archaeologists study the complexity of multi-step technologies with the assumption that complex recipe tasks require high fidelity transfer of information and social leaning. The investigation of this issue is still in its infancy in paleoanthropology – we need experimental studies of Stone Age technology to better understand the social learning requirements of these technological systems. Some technologies have been suggested as being of sufficient complexity as to require high fidelity social learning, while at the same time having profound adaptive significance. I review some of these and their evidence of appearance as an archaeological proxy for social learning. Advanced projectile technologies such as atlatl and bow and arrow and the sometimes associated microlithic technology (Brown et al 2012, Lombard & Haidle 2012) have been argued to be dependent on strong social learning systems. The technological accoutrements of advanced projectile weapons like microlithic technology and bone projectile points appear ~71 ka in South Africa (Brown et al 2012, Henshilwood et al 2001) but never appeared outside Africa until modern humans arrived. The stone tool evidence suggests that Neanderthals lacked advanced projectile weapons such as atlatls and bows and arrows (Shea 2006), and anatomical evidence suggests that they lacked the morphologies associated with this ability (Churchill et al 2009, Rhodes & Churchill 2009). Systematic coastal foraging in tidally strong coastlines requires an understanding of the relation between the lunar driven tidal systems and the varying return rates and safety that are a function of these systems (Marean 2010, Marean 2011). This requires a lunar calendar that 10

could only be passed on in a social learning context. The impacts of systematic coastal foraging are enormous - coastal adaptations result in the most complex hunter-gatherer systems documented. True coastal adaptations appear in South Africa by ~110 ka (Marean 2011, Marean 2014). While the evidence from coastal MP Eurasian sites documents rare and very low level coastal foraging (Cortés-Sánchez et al 2011, Stringer et al 2008), these sites fail to provide evidence for systematic coastal foraging or true coastal adaptations (Marean 2014). Other “complex recipe” technologies reliant on transmission through social learning include the use of poisons and advanced pyrotechnologies (using heat to modify materials) (Brown et al 2009), the production of mastics and glues (Charrié-Duhaut et al 2013), and hot rock technology (Marean & Assefa 2005). Many pyrotechnologies demand understandings of the relationship between fire, heat, temperature, and then high fidelity transfer of the details and technological steps. Modern humans in Africa developed heat treatment technology at ~160 ka while it did not appear in Europe until ~20 ka (Brown et al 2009). Glues and mastics often require pyrotechnology, and these appear in South Africa by ~65 ka (Charrié-Duhaut et al 2013) while hominins in Eurasia during the MP developed birch-tar as a glue (Koller et al 2001, Mazza et al 2006). When modern humans entered Europe they brought with them hot rock technology which Neanderthals appear to have lacked (Marean & Assefa 1999). In summary, the available record documents that the African lineage had a life history pattern conducive to advanced social learning by at least ~160 ka, while the Neanderthal lineage retained the primitive condition. The archaeological record of technologies that require advanced social learning is relatively consistent with the biological record. In Africa we have several good proxies for advanced social learning appearing between 160-65 ka, while in western Eurasia the record for advanced technologies requiring social learning is much less compelling for times associated with Neanderthal occupation. The Evidence for Hyper-Prosociality Modern humans cooperate at high levels with non-kin and have deep-seated non-selfish preferences that are unique among living animals (Bowles & Gintis 2011, Fehr & Fischbacher 2003, Fehr & Gächter 2002, Henrich 2004, Henrich et al 2001, Henrich & Henrich 2006) while varying by cultural context (Henrich et al 2010, Henrich & Henrich 2006, House et al 2013). It has been argued that these high levels of cooperation could have evolved in the contexts of hunting and food sharing (Hill 2002) or warfare (Bowles 2009, Bowles & Gintis 2011). Modern humans rely on cooperative breeding (Hill & Hurtado 2009, Hrdy 2011), with individuals other than genetic parents engage in behaviors that increase juvenile survival and/or the fertility and survival of reproductive adults at a cost to themselves. This hyper-prosociality allows an extraordinary web of social relationships within groups and between groups made possible by our ability to accept non-kin as potentially cooperative individuals, our ability to keep track of complex relationships through language and memory, and the ability to tie groups together with formal mechanisms. When studying the origins of hyper-prosociality researchers can look at various levels of social interactions to probe its origins. The lowest level is the relation between individuals in groups. Modern humans share food between unrelated individuals within groups, and will feed and nurse injured individuals back to health, so that injuries that temporally incapacitate are no longer lethal events, so mortality is lowered and lifespan increased (Kaplan et al 2000). Severe injuries that otherwise could be fatal are survived and can leave markers on the skeleton of these events. The well-known badly injured Shanidar Neanderthal (Trinkaus 1983) is sometimes 11

pointed to as indicating care for group members, but it is important to note that he may have lived among kin and thus this is not a clear indicator of hyper-prosociality. One way to address this issue is at the population level - did people on average live beyond their reproductive value? Neanderthals lived short lives relative to modern humans (Trinkaus 1995), perhaps suggesting lower levels of prosocial maintenance. Another marker of hyper-prosociality is the organization of bands of unrelated individuals into higher level cooperative structures (ethno-linguistic groups), and the building of network connections between bands and even ethno-linguistic groups, often for the purpose of mate exchange and risk reduction. The formation of genetic structure that is unexplainable by biogeographic isolation can be a signal of the formation of ethno-linguistic groups. One potential proxy is the use of strontium isotopes to determine individual movement patterns, and if this differs substantially from the place of death, it could suggest out-group marriage. The movements of goods as gifts and trade items is another potential proxy. Geographically structured stylistic attributes can provide evidence for intergroup signaling and ethno-linguistic group formation (Larick 1985, Wiessner 1983). The genetic record in Africa has detected the formation of structure dating back to ~200100 ka which suggests the formation of Khoi-San ethno-linguistic groups (Kim et al 2014, Schlebusch et al 2013, Schlebusch et al 2012). Other ancient lineages such as the Pygmies and Hadza are recognized to arise after this (Lachance et al 2012, Tishkoff et al 2009). African MSA stone tools show patterns that suggest regional structure in temporally restricted style including Stillbay bifaces, Howieson’s Poort crescents, Aterian bifaces and perhaps others that are not yet well dated indicating an emerging regional structure that is consistent with the genetic evidence for ethno-linguistic group formation (Clark 1988, Marean & Assefa 2005, McBrearty & Brooks 2000). MP assemblages in Eurasia show no evidence for intergroup signaling (Kuhn 2013) while this is common in the succeeding UP (Vanhaeren & d'Errico 2006). In archaeology the study of the chemistry of items and raw materials that could be moving between groups is a potential proxy for extended social networks. In East Africa the analysis of MSA obsidian documents some very distant transport >300 km, while some sites have obsidian from sources 55-45 km from site (Clark et al 1984, Merrick & Brown 1984, Merrick et al 1994, Negash et al 2011, Negash & Shackley 2006). Silcrete in Botswana traveled over 200 km (Nash et al 2013). The record for Neanderthals documents a pattern focused on the use of local raw materials while there is a significant expansion of use of raw materials from distant sources in the UP (Féblot-Augustins 1993, Kuhn 2013). In summary, the evidence for the appearance of the proclivity for hyper-prosociality is in its infancy and poses an exciting research agenda. In Africa we have some tantalizing evidence that ethno-linguistic structures formed by 200-100 ka, perhaps earlier, and that exchange networks were in place as well. The evidence in Eurasia for such structures and networks is only present after the dispersal of modern humans. A MODEL FOR THE FINAL ASSEMBLY OF THE UNIQUELY HUMAN CHARACTERISTICS Scientists have recognized several evolved traits unique to modern humans. I have emphasized an advanced cognition, hyper-prosociality, and a slavish reliance on social learning as being the key final set that evolved, uniquely, to high levels of refinement in the modern human lineage and in operation, together, allow the spectacular cumulative culture which is the foundation adaptation of modern humans. There are also a set of milestones that were crucial to 12

the evolution of these unique traits and may be key pre-adaptations that were required to be in place before these unique traits evolved, but are not unique to modern humans. These are also crucial targets of research by paleoanthropology, and include regular meat eating, bipedality, and tool use and tool making. By 800 ka there were hominin(s) that had crossed all these milestones and evolved low levels of the key three traits, yet clearly differed significantly from modern humans, the latter departing in a spectacular way from this Middle Pleistocene forbear. The challenge for the future is to build a precise and accurate timeline of the appearance of these milestones and unique traits, develop an understanding of the context of their appearance, and link them into a synthetic theory for human origins. While our immediate goal as scientists may be proximate explanations for these traits, the crucial and synthetic ultimate explanations provide the gravity for our efforts. In that spirit, I offer here a basic timeline for the latter three traits that I have emphasized, and some tentative causative (ultimate) explanations for their appearance. In the latter third of the Middle Pleistocene, Africa and Eurasia were populated by a hominin that in anatomy and brain size was essentially modern, but lacked the advanced cognition, hyper-prosocial and social learning features of modern humans. It lived at very low densities in highly dispersed groups, was highly mobile, but was not structured into ethnolinguistic groups as modern humans are – it had chimpanzee-like social structure. This hominin was reliant on technology, unlike other primates, and for this reason was subject to slow selection that gradually ratcheted up its cognition and dependence on social learning. This is why the archaeological record appears so temporally static through much of the Pleistocene. There was a co-evolution of technology, a reliance on it, and increasing cognitive complexity and social learning psychology that went on for nearly two million years, but it was slow. But none of this explains the evolution of hyper-prosociality, without which cumulative culture cannot fluoresce to its levels in H. sapiens. The increasingly strong glacial pulses of the Pleistocene isolated populations, drove some extinct, and pushed phenotypic variation to high levels. During the latter third of the Pleistocene during the glacial phase of MIS8 or 6, one lineage recognized and mastered the trick of productive coastal foraging and broke into the coastal foraging niche. Productive coastal foraging requires command of a lunar calendar, and the ability to create this symbolically complex system must have been outside hominin abilities until rather late; Neanderthals never got it despite being in productive coastal zones (Marean 2014). This tipping point in diet likely first happened on the southern African coast where inter-tidal resources are extremely rich and the lineage there found a productive environment where foraging returns were immune to the glacial pulses (Marean 2010, Marean 2011). Unlike all other food resources previously exploited by hominins, this one shared all the following features – it was patchily distributed, dense, high ranked, and available all year (unlike seasonally structured terrestrial resources like migratory animals, breeding birds, etc.). This reliance on these dense and predictable resources stimulated active boundary defense of those inter-tidal patches, and year-long residency (Marean 2014). This was the origins of the land tenure system so widely documented in hunter-gatherer ethnography, where groups exert exclusive land use rights to specific parcels of land and express ownership through cultural expressions such as boundary markers and totemic myths. Ever since that initial shift to dense and predictable resources, there has been a regular drift in the evolution of human diet toward harnessing resource density and predictability, with the obvious blow-back being increasing

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conflict, social and economic variation, and many impacts on nutrition and biology, all reaching their crescendo in food production. Triggering territoriality, conflict over resources set the conditions for strong selection for hyper-prosocial proclivities (Bowles 2009, Bowles & Gintis 2011). As this trait spread through this population, inter-band cooperation evolved into the first ethno-linguistic groups. When these were in place in Africa, and I think this occurred sometime around 200-100 ka, the three foundations of the modern human cumulative culture adaptation were in place, producing a creature capable of extra-ordinary social and technological structures. H. sapiens was also empowered with a new ability to make war as large groups, with advanced projectile weaponry, and the advance out of Africa and the annihilation of other hominin taxa, and many unprepared megafauna, was assured. CONCLUSIONS The goal of paleoanthropologists investigating modern human origins is to provide ultimate explanations for the origin of our species, its unique characteristics, and its final spot as the sole surviving hominin. A central theme of this review is that these ultimate explanations are best attained through synthetic studies that integrate genetics, biological anthropology, and archaeology all resting firmly in the field of evolutionary anthropology. Parochial scientific approaches blinkered by the walls of disciplinary boundaries can never attain such explanations. Evolutionary anthropology is grounded in neo-Darwinian theory and seeks ultimate explanations for adaptations, and it embraces theory developed for understanding cultural transmission and gene-culture coevolution in recognition of the specialness of culture and its relationship to evolutionary forces. A general theory for human uniqueness is emerging, and therein is found the path to material proxies for studying the evolution of Homo sapiens. The foundations of this human uniqueness are an advanced cognition, hyper-prosociality, and a psychology for social learning. The proxy records for an advanced cognition, hyper-prosociality, and a psychology for social learning differ between African and non-African lineages through the MHOTS (Figure 1). The evidence for these unique features is earlier, more recurrent, and more secure in Africa. This has been challenged by some who point to a ‘flickering’ pattern to the African evidence, likely in response to the potential for complicated technologies to be subject to loss due to changes in population size and network connections being reduced during glacial climate phases (D'Errico & Stringer 2011, Powell et al 2009). The empirical evidence for this flickering character does not hold (Brown et al 2012), and many of the technological advances occur in glacial phases (Figure 1) when that theory predicts them to suffer loss (Marean et al 2014). All modern humans are descended from a lineage of Homo sapiens that arose in Africa, probably in the latter third of the Pleistocene, during a long cold glacial phase. While it appears likely that there may have been several lineages of Homo sapiens, there is increasing evidence that one gave rise to all modern humans. It is unknown where this lineage resided, but several factors point to southern Africa as a strong candidate region. Biological anthropological and archaeological evidence are concordant in showing that proxies for advanced cognition, hyperprosociality, and a psychology for social learning were in place with the African lineage sometime between 200-150 ka, but in the Neanderthal lineage these adaptations had not diverged substantially from the primitive state shared from a common ancestor. This combination of new adaptations in the African lineage created a dominant species that is unique beyond the truism that “all species are unique” (Hill et al 2009). By 100-50 ka a founder population of modern 14

humans dispersed from Africa, and as it encountered archaic lineages it interbred with some, picking up alleles that are still being studied, but at least some appear to have fixed in our lineage due to advantages they conferred. By ~40 ka the modern human lineage was present in Africa, Eurasia, and Sahul and the other archaic lineages were either extinct or on their way. This coassociation of rapid dispersal, advanced features of technology and group organization that facilitate war and predation, and extinction of competitors makes a xenocide hypothesis reasonably parsimonious as an explanation for the current single hominin species. DISCLOSURE STATEMENT The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. ACKNOWLEDGMENTS This paper benefitted from the critical reading of Rob Boyd, Erich Fisher, Kim Hill, and Kevin Langergraber, and from the intellectual influence of the broader Institute of Human Origins group investigating the origins of human uniqueness.

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SIDEBAR All summaries of evolutionary history rely on age estimates, and the evolutionary histories currently being reconstructed rest on age estimates from two main approaches; geochronological dating of paleontological and archaeological materials, and genetic methods reliant on estimates of mutation rates. Geochronological methods are currently in a phase of development where the methods are being carefully refined to provide more precise and accurate age estimates. Evolutionary genetics has traditionally relied on mutation rate estimates from fossil taxa divergence age estimates, which are of course reliant on fossil data, their phylogenetic interpretation, and their associated geochronological age estimates. With new technologies it is now possible to make direct estimates of mutation rates, and these new rate estimates are surprisingly about half the speed of fossil-based estimates. Using these rates effectively doubles all the age estimates from the fossil-based estimates (Scally & Durbin 2012). In this paper I will provide estimates that span the minimum and maximum if we were to use both rates, and Figure 1 shows these two rates side-by-side. Clearly, there is great uncertainty in these age estimates.

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TERM DEFINITIONS Introgression - the spread of genes across a reproductive barrier Marine isotope stage (MIS) – a phase of climate measured in thousands of years recognized by proxies of global temperature Lineage – a sequence of species that form a line of descent Prosocial behavior – extremely cooperative behavior, often for the benefit of others or society without expectation of payoff Modern human behavior – a concept developed by archaeologists to encompass the suite of unique behavioral features of modern humans Evolutionary Anthropology - neo-Darwinian approach to biology, behavior, and culture seeking ultimate explanations, embraces theory developed for understanding cultural transmission and gene-culture coevolution

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FIGURE CAPTIONS Figure 1. A synthesis of the biological and behavioral record for modern human origins. In the center of the figure is the timespan for modern human origins with a temperature proxy (the δ deuterium record from the EPICA ice core) showing that for the majority of the modern human origins time span the world was in a cold phase, and climate was unstable and increasingly so. The biological record summarized in this paper is displayed across the top. It synthesizes both the fossil and genetic records, and also shows the different inferred evolutionary timescales as reconstructed from a mutation rate directly measured using newer sequencing technology observing changes in new mutations (above), and the more traditional rate estimated from fossil divergence estimates (below). The archaeological evidence as summarized in this paper for the origins and appearance of high levels of cognition, hyper-prosocial behavior, and refined social learning are shown below. Erich Fisher produced this figure.

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SUMMARY POINTS LIST There is consensus that all modern humans are descended from an African lineage and modern humans arose only in Africa and spread across the globe. As the modern human lineage spread through Africa and Eurasia, it encountered and interbred with archaic resident hominins such as Neanderthals. Such hybridization is common among all higher orders of animals, and does not suggest these were the same species. Within a short period of time, only one hominin species was left – modern humans – and all others were extinct. Small snippets of our sister taxa’s DNA is preserved in our genome as evidence of our interactions. The foundation of the modern human adaptation is cumulative culture, which allows a spectacular cultural complexity and variability to arise. The capacity for cumulative culture rests on an evolved advanced cognition, hyper-prosociality, and a psychology for social learning. Modern humans evolved an advanced cognition, hyper-prosociality, and a psychology for social learning at an extremely advanced and unique level of complexity earlier than all other hominin taxa. This allowed their rapid global spread and replacement of closely related taxa. The archaeological evidence demonstrates the early appearance of these three key traits in Africa relatively early while there is little evidence for their evolution to similar levels of complexity among non-modern human lineages. Modern humans brought this new adaptation with them as they left Africa. Evolutionary Anthropology provides a rigorous and sophisticated investigative framework for studying modern human origins and developing an understanding of how humans evolved into such a unique species. Its strength rests in its grounding in neo-Darwinian theory and eschewal of disciplinary boundaries.

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