A computer simulation of hominid dispersal from Africa

 

The default simulation

To illustrate STEPPINGOUT we begin by describing a single run which uses what we term the default parameter settings. Other than ensuring that the defined colonisation and extinction rates provide an appropriate arrival date for Dmanisi, these parameter values are largely arbitrarily defined. They serve to provide a base-line simulation against which other simulations can be compared by a process of experimentation with parameter values.

Having began with a population cluster in East Africa  the population disperses throughout Africa and into Eurasia as illustrated by the three maps showing hominid distributions at 1.7, 1.5 and 0.5 mya, illustrating the whereabouts of the three categories warm (red), generalist (green) and cold adapted (blue) hominid types. The growth and fluctuations of the these three dietary categories of hominid, showing the impact of both dispersal into high latitudes and the climatic cycles are shown here, and for all 6 populations, here.

Simulated dates for arrival at our six selected localities can be compared with the archaeological record. The Dmanisi date of 1.78 is a realistic estimate for that site and, as described above, is simply a construct of the parameter settings. None of the other dates have been pre-determined in this fashion and they show a marked discrepancy with those currently available from the archaeological and fossil records – with arrival at Boxgrove at 1.65 mya, and Java at 1.18 mya.

The history of occupation at these localities – that being the specific triangular grid cell within which the site is located is shown here. This shows that once hominids have arrived at Boxgrove or Atapuerca they are effectively permanently present from that date. In contrast occupation at the other localities is more sporadic, especially at ‘Ubediya. The European sites also have a relatively stable type of hominid present; the Asia sites are quite variable, especially that of Dmanisi which has no less than five different types present at some time during its occupation history.

The distribution of hominids with regard to environmental zones for the whole course of the simulation is illustrated here. Each pie chart summarises three different categories of zones: (1) those for which cold adapted hominids are favoured over others at all stages of the climatic cycle; (2) zones in which the most favoured hominid varies with the stages of the climatic cycle; (3) zones in which warm adapted hominids are always favoured. As the pie charts illustrate, the non-favoured hominids always have a presence within each of these three groups of environmental zones. Even in the environments which range from subarctic to temperate at a full interglacial 5% of the total number of hominid occupations is by warm adapted types.

The results shown are from one single run of STEPPINGOUT; as a stochastic element is involved a second or third run with the same parameter settings will produce different results. To explore what range of variability arises 30 runs were made with the default parameter settings. In seven (23%) of those runs, hominids remained in Africa for the whole duration of the simulation. The reason is that there are relatively short windows of time when the environmental conditions at the two exit routes enable hominid survival and hence dispersal from Africa. Once out of Africa, the six localities are almost always reached during the duration of the simulation run. The results with regard to the arrival dates at the six localities for the remaining 23 runs are illustrated here. That for Dmanisi is normally distributed around 1.7mya, as expected in light of the model design. Those for Java range from 1.5 to 0.8 mya, while hominids have always reached Atapuerca by 1.3mya and there is only three runs in which they arrive at Boxgrove after this date. The shape of the distributions vary between the localities: that of Java is notably broad reflecting its distance from Africa and that historical chance is playing a greater role as to when hominids arrive than it is for those sites close to Africa.