Welcome by CARTA Co-Director and Salk Institute President, Gerald Joyce. Opening Remarks by CARTA Executive Co-Director and event co-chair, Pascal Gagneux.
Comparative Anthropogeny and Other Approaches to Human Origins
Friday, November 03, 2023Abstracts
Abstracts
Despite broad agreement that Homo sapiens originated in Africa, considerable uncertainty surrounds specific models of divergence and migration across the continent. Progress is hampered by a shortage of fossil and genomic data, as well as variability in previous estimates of divergence times. I will give an overview of popular models for human origins and then discuss how newly sequenced whole genomes from Khoe-San individuals in southern Africa help test different hypotheses. In our research, we infer a reticulated African population history in which present-day population structure dates back to Marine Isotope Stage 5. The earliest population divergence among contemporary populations occurred 120,000 to 135,000 years ago and was preceded by links between two or more weakly differentiated ancestral Homo populations connected by gene flow over hundreds of thousands of years. Such “weakly structured stem” models explain patterns of polymorphism that had previously been attributed to contributions from archaic hominins in Africa. In contrast to models with archaic introgression, we predict that fossil remains from coexisting ancestral populations should be genetically and morphologically similar. I emphasize how “model misspecification” explains the variation in previous estimates of divergence times, and argue that studying a range of models is key to making robust inferences about deep history.
Since the split of Homo sapiens from the last common nonhuman primate (NHP) ancestor, the human brain has substantially altered its size, structure, and connectivity. The human brain has a larger mass with respect to body weight, increased cortical neurons with respect to size, an expanded proliferative zone, and unique connectivity patterns. Human neurons exhibit an exceptionally delayed time course, exhibiting more mature characteristics after a prolonged time compared to other primates, a characteristic termed neoteny. It is hypothesized that this longer developmental period plays a role in the aforementioned structural and connectivity differences. It has long been proposed that the phenotypic differences between closely related species may be driven, in part, by divergent transcriptional regulation rather than by novel protein-coding sequence, given the extensive conservation of the protein coding DNA sequences in the primate lineage. However, how these regulatory mechanisms play a role in the protracted maturation process in human neurons remains largely unknown. In this lecture, we will discuss molecular factors that can contribute to the prolonged maturation of human neurons compared to other primates and the impact on human development and neurodevelopmental diseases.
Ethnology is the practice of comparing and contrasting the features of multiple ethnohistorically-documented human societies. Also known as ‘cross-cultural analysis’ or ‘comparative anthropology,’ it has a long association with archaeology. For example, the pioneering archaeologist Augustus Pitt Rivers was also an exponent of ethnology. Similarly, the career-capping book of the most influential archaeologist of the second half of the 20th century, Lewis Binford, is a work of ethnology. However, ethnology has never been considered a key archaeological tool. In this talk, I will argue that it should be. Drawing on my own work and that of colleagues, I will show that there are both theoretical and practical reasons for archaeologists to enthusiastically embrace ethnology. Adding it to the suite of techniques that we expect archaeology undergraduates and graduate students to be familiar with will enable the discipline to make faster progress with the task of understanding the patterns in the archaeological record.
The most complex organizations in the living world beside those of humans are the colonies of ants. I will argue that points of comparison between sharply different organisms like ants and humans are exceptionally valuable to science, and indeed that modern humans are in many ways much more like certain ants than we are to our nearest relatives, the chimpanzees. I consider such issues as the role of individuality and group identity in ant societies; the advantages to ants of flat organizations without leaders or hierarchies; and what we can learn from ants with respect to direct and indirect communication, self-organization, job specialization, labor coordination, and global domination.
The human genome contains segments of DNA with non-human origins. This introgressed genetic material is remnants of mating events between early modern humans and their archaic contemporaries (e.g., Neanderthals and Denisovans). In this talk, I will review the evidence for such genetic material, its consequences on phenotypic diversity in modern humans, and discuss if this process - archaic introgression - is typical among other great ape species.
High-altitude adaptation stands out as one of the most notable examples of evolution within our species. Despite similar challenges of decreased oxygen availability, human groups on different continents have followed unique evolutionary trajectories. I will discuss how genomic, molecular, and physiological discoveries reveal key insights into human-specific evolutionary changes, examine comparative findings and limitations, and consider alternative approaches for understanding distinct facets of this extraordinary human phenomenon.
Humans do not exhibit visible manifestations of ovulation. In contrast, chimpanzee and bonobo females exhibit conspicuous swellings of their genital area during their fertile (periovulatory) period. This observation has led to the notion of “concealed ovulation” and it has been argued that this represents a distinctly human adaptation. Adaptationist scenarios proposed include promotion of paternal investment, confusing paternity to reduce the risk of infanticide, facilitating clandestine mating and female choice, and reducing female rivalry. Ovulation is unsignaled in many non-human primates, including the other great apes, gorillas, and orangutans. Thus, the null hypothesis that humans simply retain ancestral unsignaled ovulation remains viable, especially in light of the fact that self-reported patterns of mating behavior in humans do not seem to correlate with periovulatory periods. There is, however, a large and contentious literature on the potential existence of subtle ways in which women’s reproductive cycles influence behavior in both sexes. Of course, many human cultures across the globe have repeatedly evolved strong menstrual taboos, which could be considered cultural means of forcing women to declare their fertility status.
Life history theory predicts that inter-birth intervals (IBIs) will depend on a trade-off between maternal investment in current and future offspring/reproduction, mediated by somatic maintenance of the mother. IBIs are therefore influenced by the amount of energy available to the mother and the infant. Usually, IBI scales with the body size of the mother and the infant, with a larger relative body size of infant to mother correlating to slower breeding. Human IBIs, however, are much shorter relative to body size than other apes. One of the suggested reasons for our species’ relatively shorter IBI is the presence of cooperative breeding: help from other member(s) of the social group in provisioning the mother and/or the infant alleviates energetic constraints on the mother. Other cooperatively breeding species, like the South American callitrichids, and even ones that have little help from other members of their group other than carrying infants, show shorter IBIs than expected. Another potential factor alleviating energetic constraints on mothers in humans might have been the advent of meat-eating, which made early weaning possible starting ~2.5 mya with the Homo lineage.
Human language is a strong contender for the title of most often named species-specific feature in the literature. But why is that? In this talk, I explore what we could mean by "human language", and how different conceptions of language inevitably lead to different answers about whether it is species-specific. While syntax is a central feature, it is only one of several, and the uniqueness of human language is that it arose from a combination of, perhaps, ordinary ingredients.
Wrap-up by CARTA internal advisor and event co-chair, Carol Marchetto. Q&A with all speakers. Closing Remarks by CARTA Co-Director and Salk Institute President, Gerald Joyce.
