Despite much data, there is no unanimity over how to define Homo sapiens in the fossil record. Here, we examine cranial variation among Pleistocene and recent human fossils by using a model of cranial growth to identify unique derived features (autapomorphies) that reliably distinguish fossils attributed to “anatomically modern” H. sapiens (AMHS) from those attributed to various taxa of “archaic” Homo spp. (AH) and to test hypotheses about the changes in cranial development that underlie the origin of modern human cranial form. In terms of pattern, AMHS crania are uniquely characterized by two general structural autapomorphies: facial retraction and neurocranial globularity. Morphometric analysis of the ontogeny of these autapomorphies indicates that the developmental changes that led to modern human cranial form derive from a combination of shifts in cranial base angle, cranial fossae length and width, and facial length. These morphological changes, some of which may have occurred because of relative size increases in the temporal and possibly the frontal lobes, occur early in ontogeny, and their effects on facial retraction and neurocranial globularity discriminate AMHS from AH crania. The existence of these autapomorphies supports the hypothesis that AMHS is a distinct species from taxa of “archaic” Homo (e.g., Homo neanderthalensis).
Paradoxically, our own species, Homo sapiens, is one of the most poorly defined species of hominids. The recent human fossil record has a confusing pattern of variation, with numerous vaguely defined taxa (e.g., “archaic” H. sapiens, “modern” H. sapiens, Homo heidelbergensis, Homo helmei, Homo rhodesiensis), most of which are not widely accepted. A major source of this confusion is the lack of established unique derived features (autapomorphies) of “anatomically modern” H. sapiens (AMHS). The most frequently used diagnosis for AMHS is Day and Stringer’s (1), which is based solely on cranial features (listed in Table 1), and which has since been expanded and scrutinized (2–6). However, there are at least two major problems with the diagnostic features in Table 1. First, most of the features are difficult to use as phylogenetic characters because they describe cranial vault globularity, and are thus not structurally or developmentally independent. A second, more fundamental problem is their failure to discriminate reliably between “archaic” Homo spp. (AH) and AMHS. Many recent human crania fall outside the supposed range of AMHS variation for some features, and a few skulls generally attributed to AH fall within the range of AMHS variation (7, 8). Many researchers (e.g., ref. 9) thus consider H. sapiens to be a morphologically diverse species with archaic and anatomically modern grades.
Although H. sapiens may include anatomically modern and archaic variants, an increasingly popular view is that AMHS is a distinct species. The best support for this hypothesis comes from genetic evidence for an African origin of extant human populations between 100,000 and 200,000 years ago, and for divergence between humans and Neanderthals about 500,000–600,000 years ago (10–12). Testing this hypothesis by using cranial features, however, is a challenge because of the substantial integration that occurs among the various semi-independent units of the cranium (13, 14). Recent evolutionary developmental studies show that major changes in form associated with speciation typically result from ontogenetically early alterations in the regulation of growth, leading to multiple correlated phenotypic novelties (15, 16). Thus, interactions at multiple hierarchical levels of development—from individual genes to structural modules (integrated suites of characters that grow as a unit)—confound efforts to define basic independent characters. Yet such autapomorphies are predicted to exist if AMHS evolved as a separate lineage from AH.
We test here the hypothesis that AMHS is a distinct species in a phylogenetic sense, recognizable on the basis of one or more autapomorphies, against the null hypothesis that AMHS has no autapomorphies, indicating inclusion in a separate lineage. To this end, we report three analyses that examine cranial variation in recent Homo by using a developmental model of cranial evolution. First, we use factor analysis to identify structurally important combinations of variables that covary among AMHS crania. Second, we use ANOVA and comparisons of sample ranges to test whether these structural differences discriminate reliably between AMHS and AH. Finally, we combine two morphometric analyses to investigate hypotheses about the developmental shifts that influence the major structural differences between AH and AMHS cranial form. First, by comparing the pattern of three-dimensional cranial shape in adult AH and AMHS by using landmarks that include major loci of cranial growth, we identify cranial regions that appear to contribute to shape differences between the taxa. Second, we test whether variables that quantify the same shape differences between AH and AMHS contribute during ontogeny to the major cranial differences between humans and our closest extant relatives, chimpanzees.