Professor Mark Thomas
An African American Paternal Lineage Adds an Extremely Ancient Root to the Human Y Chromosome Phylogenetic Tree: What Difference Does it Make?
Mark G. Thomas¹, Krishna R. Veeramah¹’², and Michael F. Hammer¹
1 Research Department of Genetics, Evolution, and Environment, University College London, London WC1E 6BT, UK
2 Division of Biotechnology, Arizona Research Laboratories, University of Arizona, Tucson, AZ 85721, USA.
Since the seminal ‘Mitochondrial Eve’ study published by Cann, Stoneking and Wilson in 1987, estimates of the temporal and geographic location of sex-specific most recent common ancestors (MRCAs) have profoundly influenced our understanding of human evolution. We recently announced the discovery of a Y chromosome lineage – termed A00 – that carries the ancestral state for all SNPs that define the basal portion of the Y chromosome phylogenetic tree. This lineage was first identified in an African American using a large consumer database, but later found in a number of Mbo people from western Cameroon. Using ~240 kb of sequence, we estimate the time to the MRCA for the Y tree as 338 thousand years ago (kya) (95% CI. 237–581 kya). This is older than previous estimates of the mtDNA MRCA date, and considerably older than previous estimates of the Y chromosome MRCA date. It also predates the oldest anatomically modern human fossils. It may be tempting to interpret these new estimates of the depth of the Y chromosome tree, and the location and rarity of the A00 lineage, as indicating a forest, rather than savannah origin for AMH, or Africa Archaic admixture with AMH, or a greater time depth for AMH. However, these hypotheses are neither inconsistent nor strongly supported by our A00 data. The genealogical process, which determines the shape and depth of the Y chromosome tree, and is itself shaped by demographic history, is highly stochastic. This means that a very wide range of different population histories could explain observed patterns of Y chromosome diversity, and none should be particularly favoured based on our findings.
Dr. Peter Walsh, Department of Archaeology & Anthropology, University of Cambridge
Pathogen Rain, Spatial Structure and the Evolution of Human Genetic and Cultural Diversity
A defining characteristic of the Central African forest is the high density of biting arthropods, bats, and non-human primates that act as vectors or reservoirs of human disease. Here I use a suite of simulation models to illustrate how failure to account for spatial structure caused by “behavioural immune” responses to this heavy pathogen rain can cause substantive misinterpretation of important events and processes in human evolutionary history. I first present a simulation model examining how demography and social behaviour may evolve in response to variation in pathogen rain. This model predicts increases in longevity, age of natal dispersal, dispersal distance, social group size, and social network complexity that map to observed diversity in genetic, demographic, and social network diversity amongst humans, gorillas, and chimpanzees. I then use these results to explore how escape from forest pathogen rain may have affected spatial patterns of genetic and cultural diversity. I find that widespread disregard for how dispersal rate and social group size affect genetic drift causes coalescent models to substantially overestimate the antiquity of events such as the human chimpanzee split, modern human-Neanderthal divergence, and modern human departure from Africa. Failure to account for viscous spatial structure also likely inflates rates at which founder effects and natural selection are reported. I close with some results on how behavioural immune effects on social network structure may have influenced technological innovation in humans. I find that changes in network structure attendant with the escape from forest pathogen rain can result in abrupt phase transitions in technological complexity. Combined with a suite of new simulation and Bayesian inference tools, a spatially aware approach to data analysis has the potential to transform or understanding of human genetic and cultural evolution.
Dr. Thomas Currie, Department of Biosciences, College of Life and Environmental Sciences,
University of Exeter, Cornwall Campus, United Kingdom
Cultural Evolution: Opportunities and challenges for investigating a rainforest origin of modern Homo sapiens
Humans have dual inheritance system, i.e. in addition to genetic influences our phenotype is governed by culturally transmitted information. This has many consequences for human evolution, e.g. it provides humans with an enormous degree of behavioural flexibility allowing us to inhabit diverse ecological settings, and cultural changes can feed back and affect biological evolution. In recent years a formal body of theory has been developed that shows how culture changes in ways analogous to, yet sometimes different from, biological evolution. Ecological factors can affect patterns of cultural diversity in a number of ways e.g. through selection for certain technologies or social systems, the distribution of cultural groups in geographical space, or demographic effects on rates of cultural transmission. In this talk I will use insights from cultural evolutionary theory to examine rainforests and savannahs as likely cradles of modern Homo sapiens. Using known patterns of the relationships between cultural group diversity and ecological predictors I will investigate whether we can infer anything about the population structure in the past in Africa and how this can tie into results from genetic studies. I will also briefly discuss the possibility of using cultural data to infer the location of expansion and diversification of early human populations, and the challenges facing such a task.
Dr. Corey Fincher, Institute of Neuroscience and Psychology, University of Glasgow
Behavioral Immunity and the Parasite-driven-wedge Model of the Genesis of Cultures and Species
Regionally localized coevolutionary races between parasites and their hosts result in three anti-pathogen behaviors: preference for ingroup interaction, out-group avoidance (xenophobia), and limited dispersal (philopatry). All three behaviors comprise behavioral immunity. They become linked within individuals through genetic linkage disequilibrium. In the case of human cultural behavioral immunity, linkage of behavioral immunity traits within individuals results in cultural linkage disequilibrium. Linkage by either process also includes linkage with genetic immunity to local parasites. These events create a wedge that gives rise to intergroup boundaries that effectively fractionate, locally isolate, and diversify the original range of a culture or species, leading to the genesis of multiple discrete groups from one. This parasite-driven-wedge process of diversification should be predominant in regions of high parasite stress, leading to high diversity of species, subspecies and cultures in such regions. A variety of evidence supports this model including global patterns of species diversity, human language and religion diversity, and home range sizes of traditional human societies.