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.

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.

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.

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.

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.

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.

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.

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.