Growing up Neandertal
Parsing the Evolution of Homo sapiens
Certainly, the crania of both species had to grow during the course of development, although the adult skulls of Neandertals and modern humans are quite different. Perhaps Neandertals were born with skulls much like those of modern humans, but these skulls grew differently—earlier, faster or longer—to produce the distinctive shape of an adult Neandertal skull. Were this hypothesis true, it might suggest that modern humans descended from Neandertal ancestors by mutations that altered the timing or duration of cranial growth. For example, hulking Neandertal browridges might have evolved into the more modest browridges of modern, adult humans through slower growth of that region. In that case, an adult human would resemble an immature Neandertal, a condition called neoteny.
Frank L'Engle Williams of Georgia State University and Laurie Godfrey and Mike Sutherland of the University of Massachusetts described a test of the hypothesis that humans resembled neotenous Neandertals in a chapter of the recent book Patterns of Growth and Development in the Genus Homo (Cambridge University Press, 2003). The team measured 24 linear features on skulls from 41 Neandertals and 294 modern humans that represented every stage of the life cycle. They programmed these data into a computer model that tried to simulate the "evolution" of modern humans from adult or juvenile Neandertals by varying the rate of change for shape, size or a combination of both.
The attempt failed. Although Neandertal skulls are larger in all dimensions than human ones, simply making a Neandertal skull smaller does not produce something that looks like an adult human skull. The differences lie deeper than mere size. Using juvenile or immature Neandertals as a starting point worked no better: Adult human skulls were as different from those of immature Neandertals as the human specimens were from mature fossils. The authors' conclusion was that "humans make poor neotenous Neandertals," according to Williams.
If neoteny cannot explain the differences between human and Neandertal skulls, two possibilities remain: Either Neandertal newborns started with a different template and then grew like humans, or they started differently and grew differently as well.
Gail Krovitz of the University of Colorado at Denver addressed these competing hypotheses in a contribution to the same book (which she also helped edit). Unable to follow living Neandertals as they grew, she instead used a cross-sectional sample of Neandertal and human remains divided into five age classes on the basis of dental maturity, using human standards: 0-3 years; 3-6 years; 6-9 years; 9-13.5 years and greater than 13.5 years.
In this study, Krovitz recorded the three-dimensional coordinates of 39 anatomical landmarks on the crania of 5 immature and 18 adult Neandertals. The small sample size reflects the scarcity of the accessible, virtually complete specimens that are needed for this kind of study. Krovitz also took similar measurements on 230 immature and 142 adult human crania. After calculating averages for the landmark positions within each age class, she analyzed the distance increase between all possible pairs of landmarks in successive age classes. This procedure revealed the developmental stage and extent of cranial growth, by region, for each species.
In fact, there was scant variation over time in the well-known differences between Neandertal and human crania—the same distinctions held true across the board. In every age group, Neandertal fossils were longer in the lower face, wider across the orbits, longer front to back, and lower from the top of the braincase to the cranial base than human specimens, implying that some or all of the distinguishing features of Neandertal crania formed prior to birth. Yet in the midst of this uniformity, the patterns of growth were different. In general, between birth and age 9, Neandertal faces grew longer below the browridges and became more prognathic, while human faces widened across the orbits. From 9 to 13.5 years, Neandertal faces broadened across the orbits and elongated below the nasal aperture. In humans, the entire face lengthened. After 13.5 years of age, Neandertal crania continued to elongate and widen more than human crania, further increasing their prognathism.
These dissimilarities in growth are statistically significant and Krovitz believes they reflect a fundamentally different developmental pattern acting upon a different cranial shape found at or perhaps before birth. "Neandertals were born as baby Neandertals," she explains, "and throughout their life, the shape differences in their heads were accentuated by different patterns of growth."
Analyzing growth and development in fossil species is fraught with subtle pitfalls and plagued by small sample sizes. Yet independent analyses of dental and skeletal markers tell the same story, one of rapid growth and a nonhuman pattern of shape change throughout Neandertal childhood. If Neandertals were not "us" genetically, as most paleoanthropologists now believe, neither were they "us" in terms of development. Perhaps a suitable epitaph for Neandertals is this:
Thickened, heavy skulls long gone,
They grew up strange, lived fast, died young.