reflections on women in evolution: looking back and going forward

Voices Vol. 9, No. 1 Winter, 2009d18d

Reflections on Women in Evolution: Looking Back and Going ForwardAdrienne Zihlman, University of California Santa Cruz

New Information from the 1960s and 1970s When my late colleague Nancy Tanner and I began writing about the role of women in evolution, they were invisible in the evolutionary story, and women in the discipline were struggling to have their voices heard. Our students at the University of California Santa Cruz, were aware of the changes taking place in the wider society, notably the women’s movement. At our students urging, Nancy and I developed a seminar entitled “The biological and cultural bases of sex roles” and first taught it in 1971. “Gender” was not yet in the lexicon. Nancy and I were inspired by the class discussions and began writing down our ideas. We made our first presentation at the American Anthropological Association meetings in 1974 “Putting women in evolution” and followed this with several publications in the ensuing years. In setting up our class, few publications on women were available, and several of our assigned readings were mimeographed articles then circulating through informal networks. Notable was a paper “Woman the Gatherer: male bias in anthropology” by Sally Linton based on a 1970 AAA presentation. In the realm of human evolution, popular reconstructions centered on some version of hunting, for example, Robert Ardrey’s “man the killer ape” or Lionel Tiger’s “men in groups.” It was taken for granted that men’s activities were central in the evolution of human behavior. The “hunting package” emphasized the discovery of and taste for meat, that this new food source separated our ancient ancestors from their vegetarian ape relatives. Men and women were viewed as pair-bonded, and behaviors and expectations were rigidly determined by sex; only men hunted and shared the spoils with women – only men hunted and shared food, while presumably women busied themselves around the presumed campfire awaiting the men’s return. Hunting behavior and eating meat were given credit for the invention of language, male cooperation, and the large human brain. As our class progressed, students raised inevitable questions: where were the women, and what was their role during evolution? Parallel questions were emerging from several areas in anthropology as well as in other disciplines. Nancy and I realized that much new research from the 1960s and early 1970s served as evidence for reconstructing a different view and speculating about the role of females in the origin and early evolution of the human lineage; we maintained that a focus on women and children, would give

a more complete picture of the past. We approached our task convinced, perhaps naively, that new evidence would provide a well-supported counter argument that gave women visibility. We drew on 4 areas of research from 1960s and 1970s and from new discussions of sexual selection. Molecular evidence. This path-breaking area indirectly measured the DNA and therefore evolutionary relationships among living species and placed humans closest to the African apes, the chimpanzees and gorillas. The molecular similarities were so pronounced and differences so few that researchers estimated a divergence time between human and ape lineages as recent as 5 million years ago. The findings of Sarich and Wilson were very controversial and were not accepted for some time. Chimpanzees. In light of this genetic closeness Nancy and I maintained that reliance on chimpanzee anatomy and behavior to represent the common ancestor was supportable and more compelling than baboon or carnivore behavior, species that were frequently employed as models for early human behavior. Anatomically humans are most similar to chimpanzees in overall size, and in details of the skeleton, dentition, and muscle attachments, similarities long appreciated by Darwin, Huxley, Gregory and Washburn who attributed the anatomical differences in the pelvis, legs and feet to the unique human locomotor adaptation -- habitual walking on two, rather than four limbs. In addition, field research on chimpanzee behavior produced many surprises that emphasized their “humanness” and took anthropology by storm. I remember well in 1968 when Jane Goodall, for the first time speaking before a large anthropology audience, narrated unedited film (with no dramatic musical background!) of Gombe chimpanzee behavior at the annual AAA meetings in Washington D.C. The audience sat in awe as they observed tool using, maternal care of infants, greeting behaviors, and gestures of reassurance like hugging and kissing, all so eerily human. Nancy and I drew on this wealth of research from Gombe chimpanzees in visualizing human origins. Diet. Goodall and other researchers documented diversity in chimpanzee diets. Not only did they eat a variety of plant foods as expected, they also ate insects and preyed on small animals. Rather than being strictly vegetarians, chimpanzees are class omnivores. The vegetarian label did not apply. The evidence demonstrated that the dietary boundaries between chimpanzees and humans as proposed in the hunting scenarios were artificial.


Tools. That chimpanzees used tools came as a startling surprise. Female and male chimpanzees altered materials and used them in a variety of ways, and it appeared that adult females used tools more frequently and for longer periods during tasks such as fishing for termites – with their infants and juveniles learning and practicing by their side. Hunting behavior. Another surprise was that chimpanzees not only eat meat, but they are opportunistic predators and at times seemed to hunt deliberately. Most often males were the predators, though females had been observed with their own prey items. Social relations. Goodall’s study would turn out to continue for decades. Her long-term observations documented the lifelong associations between a mother and her offspring and among siblings. From her focus on individuals, we learned of their growth and development, personalities and behavioral variations. Females usually leave their natal group as adolescents whereas males stay in their natal groups. Their elaborate communication system of gestures, facial expressions and vocalizations convey aggression as well as friendly intent, and films of sexual behavior showed that females exerted at least some choice of mates. The burgeoning field research on chimpanzee behavior in other populations as well as on behavior of other primate species such as baboons and macaques provided rich detail for grasping some understanding of social behavior in other primate species. At the time Nancy and I began writing, hardly any field data were available on Pan paniscus (pygmy chimpanzees or bonobos), and I was just beginning my anatomical research on this species. This species has turned out to have behaviors we hypothesized for early hominids. Hunter-gatherers. A third line of evidence focused on the Bushmen hunter-gatherers of the Kalahari, people living in the savanna-mosaic of southern Africa, based on research undertaken by Richard Lee, Patricia Draper and their colleagues. The findings revealed a nomadic way of life that Nancy and I thought might be reminiscent of our early ancestors in that women were anything but weak, passive, or invisible. To the contrary, women were physically strong and were central to every aspect of group life. They walked long distances and collected widely dispersed foods, that they carried back to camp, along with infants and tools, burdens that could approach more than half a woman’s body weight. They fashioned skin karosses for carrying their nursing children, food items and tools. Women carried their infants 3 to 4 years. Richard Lee estimated the distance about 1500 miles a year; consequently, he suggested that a carrying device must have been an early invention in human evolution. Women fashioned, transported, and used the all-purpose wooden digging sticks, with a fire-hardened point, along with a stone handy for sharpening them. The sticks were effective

for digging up underground roots and tubers hidden from view and deeply buried 2-3 feet. Finding and unearthing them required profound knowledge, skill, and strength for some serious digging! Paleontology and archaeology. Fossil discoveries from sites in the savanna regions of eastern and southern Africa highlighted anatomical evidence of pelvic, leg and foot bones for interpreting locomotion and foraging, and dentition for interpreting diet. In the 1960s radiometric dating revealed that hominid fossil sites were much older than previously imaged. In the 1970s, the fossil footprints in volcanic ash from Laetoli, Tanzania, and the famous “Lucy” skeleton from Ethiopia, with pelvis and limb bones sufficiently complete to see body proportions indicated that habitual bipedal locomotion developed nearly 4 million years ago (Reader, 1981; Johanson and Edey, 1981). These early hominid fossils were grouped as the australopithecines, and comprised several species. Anatomically they all had large and well-worn molar and premolar teeth, and associated robust facial and cranial bones, unlike other mammals that rely on meat as a main dietary item, and their small canine teeth were unlike apes and other primates. The large molar teeth suggested to us that they ate hard or gritty plant foods that required substantial chewing, and their small canines indicated less aggressive behavior. When hominid remains were found in association with animal bones, it was always assumed that the animals represented the leftovers from hominid meals. However, systematic study of bone accumulations in South African cave sites pointed to carnivores as the agents responsible for accumulating the bones; early humans were a prey item rather than a meat-eating predator (Brain, 1970). Meanwhile, archaeological research showed that the earliest stone tools associated with hominids dated to about 2 million years ago, nearly two million years after the earliest appearance of physical remains of the hominids themselves. Nancy and I argued that none of these simple pebble choppers or flakes could be effective for killing prey. The absence of stone tools at 3 to 4 million years ago did not rule out the possibility that hominids made tools of organic material that would not easily fossilize, a point Richard Lee had made from his observations on Bushman technology. In sum, the evidence from the 1960s and 1970s did not, in our view, support the man the hunter scenario.

Reconstructing the Role of Women in Evolution. Our approach was based on several tenets: that an evolutionary perspective should reflect genetic and behavioral continuity as well as behavioral innovation; that natural selection and adaptation should be considered; and that population survival and increase depended on women’s reproductive outcome. Nancy and I used the new


information to construct an argument of evolutionary continuity, from a chimpanzee-like ancestor to earliest human ancestors at 4 million years ago, that required fewer leaps of faith than did baboons or carnivores as a ‘beginning point.’ We reasoned that physical structures and locomotor potential, as well as social tendencies, emotions and communication must have continuity between chimpanzees and humans. In our view early ancestral women not only produced and cared for their offspring; they also made and used tools and played a central role in subsistence and in social life through collecting and sharing food: our image of competent and resourceful women. In our attempt to explain the transition from ape to human, we specified how ancestral women might have responded to the challenges of a new environment - that of the savanna mosaic of eastern and southern Africa. We emphasized women’s contribution to a new way of life through their invention of carrying devices and organic digging implements, tools for collecting and processing a wide range of mainly plant foods, especially underground roots and tubers. Walking long distances carrying children, tools, and food for sharing defined active, not passive, women and little evidence of these activities would survive in a fossil record. We did not discount meat as a dietary item but downplayed its importance and instead emphasized an omnivorous diet. We argued that opportunistic predation on small animal prey persisted and may have expanded in this early stage of evolution. In addressing the origin of hunting, we acknowledged problems of definition, that is, hunting might stand for anything from catching prey with the hands, to butchering scavenged carcasses, to elaborate methods of pursuit and advanced technologies such as stone tools with points. Rather than an evolutionary innovation, we viewed hunting with tools as developing in stages over time, building on gathering and simpler tools. Geza Teleki presented a similar, well documented argument. We rejected a rigid sexual division of labor, though did acknowledge differences in frequency of activities, in that males had more energy to range widely and to pursue prey more often, whereas females carrying children were more restricted and could not engage in activities that required such energy output. We emphasized female choice of mates based on a renewed focus on Darwin’s concept of sexual selection (Campbell, 1972). We maintained that females were choosing friendly, helpful males. Thus we downplayed male aggression and suggested that males were incorporated into social groups through bonds they maintained with their mothers. There were many reactions to these ideas (reviewed in Zihlman, 1987 and 1997). One reaction was to ignore the evidence we presented. One, perhaps extreme example suggested that we focused on chimpanzees rather than baboons to model prehuman ancestors because “male

dominance is less popular as a research perspective than the putatively more peaceful chimpanzees,” and that “the hunting hypothesis has fallen from favor because of feminist revision,” (Tooby and DeVore, 1987: 222). This reaction suggested two things: that male dominance and hunting should not/ cannot be questioned; and that information from several lines of evidence is irrelevant to a counter argument. Rather than a serious discussion of evidence, a dismissal of our hypothesis as “feminist revision” frames the discussion in ideological or political terms, and places the study of human evolution out of the realm of science, certainly not the position of credible physical anthropologists.

New Information in the 21st century.Nancy and I argued then, as I do now, that hypotheses based on data and a solid evolutionary theoretical framework can be predictive. Looking back, I see that a number of our hypotheses have been supported by new data. During the 1980s and 1990s, women primatologists and anthropologists through their data-oriented and theoretical publications provided important insights on female reproduction, social relationships, and on human evolution (e.g.Altmann, 1980; Dahlberg, 1981; Fedigan, 1979, 1986; Smuts, 1985; Strum, 1987; McFarland, 1997; Morbeck, et al. 1997). Research on chimpanzees have yielded information on behavior of both species, Pan troglodytes (common chimpanzee), and the less well known Pan paniscus that fit with our earlier thinking. Molecular evidence. The evidence today overwhelmingly documents the close relationship between humans and chimpanzees and a recent divergence between the two lineages at 5-6 million years ago, a finding shown to be counter to what used to be a more “intuitive” conclusion that chimpanzees and gorillas were the closest pair. With the sequencing of genomes of chimpanzees and humans, and the Neanderthal genome in progress, chimpanzees represent the baseline against which both Neanderthal and Homo sapiens are compared. The studies of nuclear DNA, mitochrondrial DNA and the Y chromosome also reveal the origin of Homo sapiens occurred in Africa some 200,000 years ago. And, it came as a surprise that the Bushmen, once more widespread, have their roots in this ancient population (Olson 2002; Wells, 2002). In a real sense, the Bushmen represent “the first people” living “the old way” (Thomas, 2006). Chimpanzee research. Field studies of Pan troglodytes and Pan paniscus have revealed new information about chimpanzees in general as well as specifics of female and male behavior; the findings underscore the relevance of chimpanzees as a model for our early ancestors: 1. Chimpanzees are remarkably adaptable, able to survive in diverse habitats, even in savanna regions similar to our early ancestors; their skill and range of tool using continues to amaze us (Sanz and Morgan, 2009; Boesch, Head, and Robbins, 2009).


2. That female chimpanzees are more highly skilled than males in several aspects of tool using is now well documented (e.g. Boesch and Boesch-Achermann , 2000). The sex differences appear by 5 years of age; females watch their mothers closely and have a longer attention span than young males (Lonsdorf, Eberly and Pusey, 2004). Female chimpanzees rather than males leave their natal groups and so spread traditions such as tool-using across space as they move into new groups as well as through time by socializing the next generation and (Matsuzawa, 1989). 3. At most Pan troglodytes field sites males engage in predatory behavior more often than females and the techniques vary in different populations; in some populations male cooperate, possibly for male bonding (Mitani and Watts, 2001). Yet females do use tools in predation (Huffman and Kalunde 1993) and may even elaborately fashion twigs into little spears with sharp point to stab small bush babies that sleep in hollows of trees (Pruetz and Bertolani, 2007). Pan paniscus also engage in catching and eating monkeys and other prey items, and – not surprising to me -- females are active and successful hunters (Hohmann and Fruth, 2008; Surbeck and Hohmann, 2008). 4. A surprise from the literature is the very different behavior of males in the two chimpanzee species. Adult male Pan paniscus are less aggressive and bond more closely with their mothers than with other males, a contrast with Pan troglodytes (Kano, 1992). This expands possibilities for visualizing the role of males among early hominid societies; it is possible to emphasize the more sociable Pan paniscus as an alternative to more aggressive Pan troglodytes males. The increased information on both species presents a broader behavioral base for envisioning the transformation to early hominids (Zihlman, 1996). Paleontological and archaeological records. In recent years, a plethora of African hominid fossils older than 3 million years confirm that there were a variety of australopithecine species who shared bipedal locomotion, large, well worn molars, and small brains the range of extant chimpanzees. They are sometimes referred to as “bipedal chimpanzees,” a recognition of the evolutionary closeness between early hominids and chimpanzees. Evidence for hunting as opposed to butchering is not convincing before 500, 000 years ago, as Nancy and I concluded. Perhaps more efficient tools contributed to advanced hunting methods after 200,000 years ago and the appearance of Homo sapiens (Webb and Domanski, 2009; Brown, et al., 2009). There is still much to learn about the subsistence patterns, technology, and hunting methods in these later hominids. This is an active area of current research!

Going Forward A sampling of recent ideas shows a range of approaches to early hominid behavior, but as we move forward with more inclusive frameworks hunting and meat-eating remain prominent. The function of language facilitates planning and coordinating hunting groups, pair bonding (“staying faithful to one sexual partner”) and sharing food (Szathmary and Szamado, 2008). The “expensive tissue hypothesis” explains the emergence of a larger brain size in the genus Homo, compared to the earlier australopithecines, dependent upon a dietary shift to higher quality food, that is, meat presumably obtained from hunting (Aiello and Wheeler, 1995). The invisibility of women in evolutionary studies is unfortunately often replicated even in the critique of these approaches. In a recent text dedicated to recovering the invisible women of prehistory, the authors neglect the research done in the seventies and early eighties by women on this very topic (Adovasio, Soffer, and Page, 2007). Barbara King’s review of the book highlights this irony. Unfortunately, it is the case that women and their perspective continue to be invisible or ignored in many reconstructions of early hominid behavior. A cover article published in Nature is a case in point. The argument focused on the evolution of human locomotion and attempted to demonstrated that natural selection for endurance running – not walking! – shaped the body form of the genus Homo (Bramble and Lieberman, 2004). This idea is becoming quite popular and has been reproduced in collections for introductory students. There are several problems here. For one, elite women endurance runners, like their male counterparts, have a narrow pelvis and low levels of body fat. This low level of fat may be adequate for men, but reproductive women need a broad pelvis and sufficient body fat to see them through all phases of reproduction from ovulation, conception, pregnancy to lactation. A body form evolved for endurance running would not accommodate the demands of pregnancy and birth in a bipedal female. What is optimal for female endurance runners may be catastrophic for reproductive females. Furthermore, the proposal ignores data from both knuckle-walking apes and nomadic foragers where running long distances is not a major activity. Women carrying infants and food cannot engage in this locomotor behavior. Hence, the emphasis on endurance running is necessarily restricted to men’s evolving bodies! However, there are direct challenges to the hunting hypothesis. In Hart and Sussman’s Man the Hunted. Primates, Predators, and Human Evolution (2009) the authors approach the subject from the point of view of predators and amass a range of evidence to argue that early humans were more frequently the prey rather than the predator. In addition, a number of new studies on foraging, food selection and


processing have broadened our perspective beyond man the hunter. There is renewed emphasis on dietary flexibility of our ancient ancestors, with detailed study of cranial and dental remains of australopithecines (e.g. Teaford and Ungar, 2000), re-stressing once again the dietary flexibility of omnivorous chimpanzees, humans ,and fossil hominids (Leonard, 2002), and re-discovering the importance of roots and tubers in australopithecine origins (now given the acronym USO for Underground Storage Organs!) (Laden and Wrangham, 2005). Novel experiments emphasize women’s foraging abilities. In a large farmers’ market women’s spatial memory demonstrated their skill for locating food items with higher nutritional quality. The authors suggest this ability is a result of natural selection that enhanced women’s gathering activities in the past (New et al., 2007). A recent discovery parallels the speculations that Nancy Tanner and I imagined for the earliest hominids, that is, chimpanzees in a savanna region of Tanzania using tools to dig up roots -- to me, it is as if the australopithecines are alive and well in Africa. The authors apparently had the same reaction, as they suggested the behavior demonstrates that exploitation of similar resources could have been within the abilities of our earliest ancestors (Hernandez-Aguilar, Moore, and Pickering, 2007). Before this discovery, Elizabeth Marshall Thomas as early observer of the Bushman, re-emphasized the importance of the digging stick, its antiquity, lightweight, ease in carrying, and its multipurpose functions. And yet its contribution to subsistence by women’s activities is easily lost because organic materials rarely fossilize. In several realms, our theoretical framework is becoming more inclusive through addressing dimensions of female survival and reproduction, but taking into account other ages as well. Fossil discoveries of juveniles and infants, such as “Turkana boy” in Kenya or Dikika in Ethiopia (Walker and Shipman, 1996; Johanson and Wong, 2009) have encouraged comparative studies of growth and development (e.g. Bolter and Zihlman 2006), consideration of a childhood stage in evolution (Bogin, 2006), and of mother-child interactions in the evolution of language (Falk, 2009). Recognition of grandmothers brings attention to older women and their evolutionary contribution to the care of the younger generation (Hawkes et al 1998; Hrdy, 2009). An early hominid adaptation may have been to expand childcare to involve many members of the social group. Our students in the 1970s were part of a movement toward understanding the centrality of women in society. Now the role of women is obvious, underscored by worldwide efforts of governments and other institutions to combat poverty by focusing on women’s businesses and work efforts, through microcredit programs that support female economies, through educating girls and maintaining

women’s health. These programs signal an acknowledgement that women are the carriers of culture through generations. The widespread recognition that women are critical to the fabric of society is exemplified by the popular appeal of Greg Mortenson’s best-selling Three Cups of Tea about building schools to educate girls in Pakistan. Whatever the ideas or reactions within physical anthropology, there is ample evidence that the wider society understands that women – wherever they live around the globe–hold the key to a healthy and prosperous society.

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