The anatomical, genetic and behavioural origin of modern humans
Fossil and genetic evidence indicates that modern humans evolved in Africa, probably around 250,000 years ago, which corresponds to the end of the period known as the Middle Pleistocene. The oldest known fossils of Homo sapiens – found in the Omo Kibish Formation by Richard Leakey in 19671,2, are located just at the northern edge of the Turkana Basin in southern Ethiopia. These fossils, and their associated stone tools, have been dated to ca. 195,000 years2,3, or early Marine Isotope Stage (MIS) 6. Slightly younger at ca. 160,000 years, the fossil remains from Herto provide a further glimpse into the morphological diversity and behavioural complexity of early humans4,5, including evidence for defleshing (in the form of cut-marks) and carrying/keeping (in the form of polishing) on a child’s cranium. An expansion of the human behavioural repertoire at this time is also revealed by the evidence for the earliest marine resource exploitation and changes in lithic technology at Pinnacle Point, South Africa6,7.
However, the cognitive and behavioural record of our origins is fascinatingly complex. Early modern human fossils (dating from MIS6 to MIS3) in Africa are associated with Middle Stone Age (MSA) stone tools, part of the Mode 3 industries as defined by G. Clark8,9. However, they are not the only hominins to make and use MSA artefacts. The earliest MSA sites date to ~300,000 years ago (at sites such as Kapthurin10 and Malewa Gorge11 in Kenya, Twin Rivers12 in Zambia, Gademotta and Kulkuletti13 in Ethiopia, and Kathu Pan14 and Florisbad15,16 in South Africa), thus pre-dating significantly the first evidence for a modern human anatomy. Although the number of MSA sites dated to MIS7 and MIS6 is small, present knowledge suggests that the evolution of a modern human morphology is not associated with a significant change in behaviour – the way of making stone tools or range of tools used does not change substantially, terrestrial hunting strategies do not change significantly (although the addition of fish to the diet as seen at Pinnacle Point was probably of great importance), nor does the procurement of raw materials or the material expression of identity. The archaeological signature of behaviours that reflect the inventive, social and dynamic cognition of the modern mind appear afterwards, as individual additions to the behavioural repertoire of different modern human populations. While this asynchrony between the first evidence for a modern human anatomy and the earliest expressions of what has been called ‘behavioural modernity’17 could represent a true evolutionary diachronic process, it could also be argued that new cognitive capacities evolved in tandem with the main biological changes in our species, but their expression in terms of changes in subsistence behaviours, symbolism, social expressions of cultural norms, etc., would be contingent on economic and demographic circumstances18,19.
This date range for the earliest modern human fossils (200,000-160,000 years) is consistent with the coalescence (the point in time before genetic lineages differentiate) estimate for all existing mitochondrial DNA (mtDNA) diversity in living humans20. Although new genomic data have questioned many of the historical reconstructions made earlier on the basis of mtDNA, the coalescence of living mtDNA and Y chromosome genetic lineages remains an important chronological marker for identifying when the last common ancestor of those lineages lived. Genetic evidence has also thrown light on prehistoric demography – the genetic diversity of humans today indicates that the ancestral population (estimated in genetics as the effective population size) of all living people was very small, probably in the order of a few thousand individuals21, and that such small population sizes, interspersed with moments of expansion and subsequent contraction, were characteristic of our lineage (including our sister lineage, the Neanderthals) in the long-term.
Such small populations would have had bounded geographic ranges – a fact central to this project. At present, we do not know where these ranges were; the fossil evidence might point to East Africa, but sites such as Pinnacle Point suggest a more complex spatial scenario. Indeed, the coalescent of the two basal human mtDNA clades (L0 and L1-6) suggests that the ancestral human population became fragmented during the glacial period known as Marine Isotope Stage 6 (MIS6, 190,00-130,000 years ago), and that some of the genetic differences observed among African groups today – such as between Khoisan and Bantu speaking groups – have probably been accumulating since then22.