Major transitions in human evolution

If we restrict the use of Homo sapiens in the fossil record to specimens which share a significant number of synapomorphies in the skeleton with extant Homo sapiens, the origin of our species would be placed in the African late Middle Pleistocene, based on fossils such as Omo Kibish 1, Herto 1 and 2, and the Levantine material from Skhul and Qafzeh. However, genetic data suggest that we and our sister species Homo neanderthalensis shared a last common ancestor in the middle part of the Middle Pleistocene ~400ka, which is some 200,000 years earlier than the species origin indicated from the fossils already mentioned. Thus it is likely that the African fossil record will document early members of the sapiens lineage showing only some of the synapomorphies of late members of the lineage.

On that basis, I argue that human fossils such as those from Jebel Irhoud, Florisbad, Eliye Springs and Omo Kibish 2 do represent early members of the species, but variation across the African later Middle Pleistocene/early Middle Stone Age fossils shows that there was not a simple linear progression towards later sapiens morphology, and there was chronological overlap between different ‘archaic’ and ‘modern’ morphs. Even in the late Pleistocene of Africa and beyond we can find specimens which are clearly outside the range of Holocene Homo sapiens, showing the complexity of recent human evolution.

Professor Chris Stringer FRS, Natural History Museum, UK

Professor Chris Stringer FRS, Natural History Museum, UK

Professor Chris Stringer has worked at the Natural History Museum since 1973, and is now Research Leader in Human Origins and a Fellow of the Royal Society. His early research concentrated on the relationship of Neanderthals and early modern humans in Europe, but through his work on the 'Recent African Origin' model, he now collaborates with archaeologists, dating specialists and geneticists in attempting to reconstruct the evolution of modern humans globally. He has excavated at sites in Britain and abroad, directed the Ancient Human Occupation of Britain project (AHOB) for 13 years, and now co-directs the Pathways to Ancient Britain project (PAB). His recent books include The Origin of our Species (published in the USA as Lone Survivors, 2013) and Britain: one million years of the human story (with Rob Dinnis, 2014).

How human diversity evolved remains a fascinating and dynamic research field. Different processes – cultural and biological - acted to shape the evolution of different populations in time and space, creating a complex palimpsest of similarities and differences. The outcome of this process, at times accelerated by sharp demographic and geographical fluctuations, is that the population ancestral to all humans did not look or behave like any alive today. This has generated major questions regarding the evolution of human universal characters, as well as the nature and timing of major evolutionary events in the history of the species. The paucity of African fossils remains a major stumbling block for exploring some of these issues. However, major breakthroughs from genomics and palaeogenomics have revealed aspects of the demography of late Quaternary hominin groups and their interactions, as well as those between foragers and farmers. The latter have been critical in shaping the last 10,000 years, in what I have named the Holocene Filter, the mechanism by which human diversity throughout the world decreased as a consequence of the differential expansion of a few populations. However, there are other key moments in the history of our species –  ‘filters’, during which extinction would have been a major factor, and ‘transitions’, during which population growth enhances and exports evolutionary novelties, creating novel conditions for human biological adaptation and  accumulated culture. This paper explores the nature and timing of these key moments in evolution of human diversity.

Dr Marta Mirazón Lahr, University of Cambridge, UK

Dr Marta Mirazón Lahr, University of Cambridge, UK

Marta Mirazon Lahr, a Fellow of Clare College, is a Reader in Human Evolutionary Biology and Director of the Duckworth Collection at the University of Cambridge, where she and Robert Foley founded the Leverhulme Centre for Human Evolutionary Studies. The focus of her research is the evolution and diversity of our species, Homo sapiens. Her work involves a range of disciplines - human palaeontology, evolutionary genetics, behavioural ecology, linguistics and prehistoric archaeology. She has carried out fieldwork in the Amazon, the South Pacific, India, Oman, Libya and Kenya. She is the director of the IN-AFRICA Project in Kenya, and a co-investigator in the Trans-Sahara Project, and the African Genomes Project with the GeoGenetics Centre of Copenhagen.

Ancient DNA research on human remains have recently moved from the sequencing of short fragments of mitochondrial DNA mired with contamination to that of full genomes. This transition means that contamination levels can be probably quantified and that it has become possible testing complex scenarios of human population histories not revivable from uni-parentally inherited markers alone. Recent discoveries includes that Native Americans thought to be of east Asian origin also shares recent ancestry with western Eurasians through a Upper Palaeolithic population in central Asia, that populations of early anatomically modern humans in Europe were structured, contained longer tracks of neanderthal DNA and diversified from east Asians more than 37 thousand years ago. Ancient human genomics have also revealed that Aboriginal Australian ancestors diversified some 20-30 thousand years earlier from the African stuck than did eurasians. Ancient human genomics have also showed that the New World Arctic was populated twice and that significant cultural changes not always is associated with population movement, but can happen simply from the spread of ideas within a population and that peoples of the Clovis culture in North America are the direct ancestors of many contemporary Native Americans and are not closer related to Europeans or Asians. As such past genomics is transforming our view of human history and is likely to do so for the years to come.

Professor Eske Willerslev, University of Copenhagen, Denmark

Professor Eske Willerslev, University of Copenhagen, Denmark

Eske Willerslev is a Lundbeck Foundation Professor at University of Copenhagen and is The Prince Philip Professorship of Ecology and Evolutionary Biology elect at University of Cambridge. He is director of the Centre of Excellence at Centre for GeoGenetics and the Danish National CryoBank and Sequencing Facility. Willerslev is an evolutionary geneticist recognized for his studies on human dispersal, microbial long-term survival and evolution, megafaunal extinctions, DNA degradation, and environmental DNA. He is particularly known for sequencing the first ancient human genome, conducting the first large-scale multi-species ancient population genetic study, and establishing the field of environmental DNA, where modern and ancient DNA from organisms such as higher plants and animals are obtained directly from environmental samples such as sediments, ice and water. Willerslev has been visiting researcher at the MD Anderson Cancer Research Centre in Austin, Texas, and independent Welcome Trust Fellow at Oxford. He is Honorary Doctor at University of Oslo and has been Visiting Professor at Oxford University and Visiting Miller Professor at UC Berkeley.

The appearance of Later Stone Age (LSA) technologies in Africa represents a major change in the archaeological record, in which technologies, subsistence strategies, and patterns of socio-territorial organization closely resemble those seen in recent and historical foraging societies.  There are sharp discontinuities between the earliest LSA and latest Middle Stone Age (MSA) strata at many sites in southern, western, and northern Africa.  This pattern may simply be an artifact of the nature of the record of sediment preservation, or a signal of the periodic abandonment of these regions by early modern human populations.  In contrast, East Africa preserves a number of more-or-less continuous sequences across the MSA-LSA transition, and these preserve evidence for a complex and perhaps prolonged process of change, a pattern that might reflect a more continuous record of occupation in the area throughout Late Pleistocene climatic variations.  In addition to examining temporal variation across the MSA-LSA transition in East Africa, we use lithic and palaeoenvironmental (faunal) data from a longitudinal transect of LSA sites from the Last Glacial Maximum and Late Glacial to explore relationships between the environment and human behaviour that likely structures the archaeological record and the nature of the appearance in LSA technologies. We observe longitudinal gradients in past faunal community composition that parallel contemporary environmental gradients in East Africa. This is associated with longitudinal variation in the frequency of microliths (the defining LSA artifact type) from archaeological sites, a pattern we interpret as a response to variable environmental risk across space. Spatial and temporal variability in Pleistocene habitats may therefore play a key role in structuring the complex appearance of LSA technologies across Africa.

Dr Christian Tryon, Harvard University, USA

Dr Christian Tryon, Harvard University, USA

Dr Tryon is a Paleolithic archaeologist who focuses on the origin, diversity, and dispersal of modern humans in Middle and Late Pleistocene Africa, with an emphasis on lithic technology, chronology, and paleoenvironmental context.  He received his Ph.D. in 2003 from the University of Connecticut, has held postdoctoral positions through the Fyssen Foundation and the Smithsonian Institution’s National Museum of Natural History, and taught at the George Washington University and New York University before joining the faculty at Harvard in 2013.  He co-directs a number of field- and museum-based projects in Kenya and Tanzania, and has previously worked on sites in Turkey, France, and the US.  He lives in Cambridge, Massachusetts with his wife Rhonda and daughter Violet.

The three key evolved features of human uniqueness are a highly advanced cognition, hyper-prosocial proclivities, and a dependence on social learning.  These provide the capacity for cumulative culture that is so fundamental to the modern human niche.  Hyper-prosociality is the tendency for regular cooperation with un-related individuals without the expectation of immediate pay-offs, and can entail a cost to the cooperator.  The regularity and extent of this cooperation among humans is unique among living animals, and thus it is crucial to identify when this feature evolved in the hominin lineage.  Explaining the evolution of hyper-prosociality is challenging since Neo-Darwinian theory fails at the task.  Models that rest on multi-level selection in a cultural species appear more powerful, and those that identify inter-group conflict as providing the key selective context are persuasive.  Here it is hypothesized that the conditions for inter-cultural-group conflict developed in the Middle Stone Age among early modern humans and provided the selective context for hyper-prosociality.  This occurred when early modern humans first began exploiting dense and predictable food resources, escaping a dietary pattern of foraging for sparse, unpredictable resources exploited with a high mobility strategy.  The first evidence for this transition, dating to ~160,000 years ago at Pinnacle Point, is in a coastal inter-tidal context in southern Africa.  When these coastal adaptations matured, territorial defence was triggered, as predicted by the theory of economic defendability.  This conflict may have led to our highly cooperative species and prepared it to colonize the world and displace all other hominin species.

Professor Curtis Marean, Arizona State University, USA

Professor Curtis Marean, Arizona State University, USA

Curtis W. Marean (Ph.D. University of California at Berkeley 1990; Professor in the  School of Human Evolution and Social Change, Arizona State University and associate director of the Institute of Human Origins) has research interests on the origins of modern humans, the prehistory of Africa, paleoclimates and paleoenvironments, and the study of animal bones from archaeological sites. He is particularly interested in human occupation of coastal and grassland ecosystems.  He has conducted research in Kenya, Tanzania, and Somalia, and since 1991 has been conducting field research in coastal South Africa. He is the principal investigator for the South African Coast Paleoclimate, Paleoenvironment, Paleoecology, Paleoanthropology (SACP4) project based near Mossel Bay.

Terrestrial and marine sediment archives in and around East Africa place new constraints on the timing and magnitude of regional climate changes during the late Neogene. These records collectively document a progressive increase in C4 vegetation over the last 3-4 Ma suggesting a shift to more open conditions. They also document the importance of orbital-scale palaeohydrology and vegetation changes paced by precessional forcing of monsoonal circulation. Syntheses of terrestrial soil carbonate d13C records have shown strong regional differences in vegetation change.

Here, we compile marine and terrestrial stable isotopic records to explore the relative amplitudes of orbital-scale versus longer-term secular changes in East African climate and vegetation over the late Neogene. This approach exploits the proxy-specific recording biases inherent to soil carbonate, tooth enamel, and molecular biomarker records and the sediment archive. We attempt test the hypothesis that orbital-scale palaeohydrological and vegetation shifts were as large, or larger, than the long term secular trends. This study strives to understand the magnitudes of rainfall and vegetation variability ‘on the ground’ that occurred within and across individual hominid linage ranges.

Professor Peter de Menocal, Lamont-Doherty Earth Observatory of Columbia University, USA

Professor Peter de Menocal, Lamont-Doherty Earth Observatory of Columbia University, USA

Peter B. de Menocal is a Professor and Vice Chair in the Department of Earth and Environmental Sciences at Columbia University. At Lamont-Doherty Earth Observatory, he uses geochemical analyses of ocean sediments to understand past changes in tropical ocean temperatures and terrestrial climate with a special interest in African paleoclimate change and its influence on human evolution and culture. He was awarded the Lenfest Columbia Distinguished Faculty award in 2008, the Distinguished Brooksian award in 2013, and is a Fellow of the American Geophysical Union. He was presented with an honorary D. Sci. from St. Lawrence University (2009). His currently leading the new Center for Climate and Life that seeks to understand how climate change impacts the security of food, water, shelter, and to explore sustainable energy solutions.

Over the last 25 years the study of the evolution of hominin culture has been transformed by three developments: 1) a dramatic increase in archaeological fieldwork, especially in Africa, 2) a surge of experimental and observational research on cultural behaviour in nonhuman animals and, 3) the development of a body of theory and supporting empirical work in which culture is conceptualised as an evolutionary system that interacts with, but is distinct from, the genetic system. In this paper, I will review these developments and discuss their implications for our understanding of the evolution of cultural behaviour in the hominin lineage, paying particular attention to the major transitions identified by previous generations of researchers. I will show that there is still evidence that a number of major transitions occurred in the course of hominin cultural evolution, but they differ substantially from those recognized in the past in terms of their nature and/or their timing. I will finish by highlighting some of the questions about the evolution of hominin culture that I think need to be addressed as a matter of priority.

Professor Mark Collard, Simon Fraser University, Canada

Professor Mark Collard, Simon Fraser University, Canada

Mark Collard is Canada Research Chair in Human Evolutionary Studies, Professor of Archaeology, and director of the Human Evolutionary Studies Program at Simon Fraser University in British Columbia, Canada. He also holds a personal chair in the Department of Archaeology at the University of Aberdeen. Professor Collard earned a BA in Archaeology and Prehistory from the University of Sheffield, and a PhD in Hominid Palaeontology from the University of Liverpool. After his PhD, Professor Collard held a Wellcome Trust Postdoctoral Fellowship in University College London’s Department of Anthropology. This was followed by faculty positions at University College London, Washington State University, and the University of British Columbia. He has been at Simon Fraser University since 2007. Professor Collard has published on a wide range of topics in Archaeology and Anthropology, but much of his work in recent years has involved the application of methods and theory from evolutionary biology to cultural data.

Biologists have long recognised that variation in the rates of evolution provides significant insights into the evolutionary process, both generally and for particular lineages. While the heat has gone out of the debate over two extremes, of gradualism and punctuated equilibrium, it remains the case that change is not spread evenly across the evolution of any lineage. The same is true for human evolution. However, for human evolution, not only do we have a solid fossil record, there is also direct and temporally-spatially fixed behavioural information from the archaeological record, and increasingly evidence from aDNA on the more recent phases. This allows us – uniquely for any lineage – to look at behaviour, phenotype and genetics to explore the relative positioning and significance of changes in any transitions that may have occurred. This paper explores this diversity of evidence to test the hypotheses that there were major evolutionary transitions in human evolution, when they occurred and the processes involved, and the relative roles of climate, environment and competition. Analytical units and events are: the radiation of the australopithecines in the Plio-Pleistocene, evolution of the genus Homo (3 – 2 Ma), the development of Homo erectus and allies (2 – 1 Ma), and the evolution of Homo sapiens (0.5 - 0 Ma). The conclusions will be used to consider a) the nature of speciation, extinction and diversity in hominin evolution; b) the role of behaviour in the evolutionary process.

Professor Robert Foley FBA, University of Cambridge, UK

Professor Robert Foley FBA, University of Cambridge, UK

Robert Foley is the Leverhulme Professor of Human Evolution at the University of Cambridge, a Fellow of King’s College, and a Fellow of the British Academy. He is a co-founder, with Marta Mirazón Lahr, of the Leverhulme Centre of Human Evolutionary Studies at Cambridge, an inter-disciplinary research centre. His research has focused on the ecological basis of human evolution and behaviour, and in particular the application of Darwinian models to patterns and processes of human evolution, ranging from the early hominins to recent human populations. He has worked extensively in East Africa, and carried out fieldwork in other parts of Africa and Melanesia.