Aug 2009

Measuring cranial variation using geography as a proxy for neutral genetic distances

world globe flushing
Certain anatomical features of the human skeleton are known to vary with geography and climate. To what extent each variable contributes to our physical makeup is less clear. The problem is that populations with similar climate are geographically close to one another. Even if we find shared traits among populations from similar climates it may be just as a result of geographic proximity (and thus clinal gene flow), rather than shared common ancestry.

As I mentioned in my previous post, anthropologists often compared cranial data to matched microsatellite datasets. However, it is rarely possible to get an exact match between the cranial and microsatellite populations. The anthropologist will instead use populations that are genetically similar and which may or may not be representative of the target population. Another option is to substitute microsatellite data with geographic distances, since studies have found a strong correlation between genetic distance and geographic distance (Manica et al. 2005; Ramachandran et al. 2005; Romero et al. 2008). This allows us to get around the need to match phenotypic data with genetic datasets. Read More...

Examining cranial robusticity

robust hominin
Palaeoanthropologist Darren Curnoe (2009) gives the following biological definition of the term ‘robust’:

…a descriptive anatomical term referring to individuals, complexes, organs, structures or traits which are heavily built, rugged, well defined or corpulent.

Bones tend to more robust where muscles, tendons or ligaments insert into the periosteum. When these insertion sites are subjected to stress, blood flow increases. This in turn stimulates the production of osteoblasts, which lay down extra bone. With respect to the skull the term robust is generally used to refer to so-called superstructures, such as the supraorbital ridges, occipital crests or zygomaxillary tuberosities. Anthropologists often classify robusticity based on the relative expression of a particular trait, or indeed its absence. Given that robusticity is related to physical stress, traits tend to be more pronounced in males and in certain populations (e.g. Aboriginal Australians and Fuegians).

The retention of robust features in certain populations, particularly Aboriginal Australians, has been used to support the multiregional hypothesis of human origins (e.g. Wolpoff et al. 2001; Frayer et al., 1993). On the other hand, proponents of a replacement model see robust traits (e.g. in Australian Aboriginal populations) as retained plesiomorphies and argue that these traits cannot be used to show continuity (Lieberman 2000). In response, many multiregionalists have revised their position to suggest that the reduction of the browridge in later Neandertals, such as St Césaire and Vindija, represents a synapomorphy between Neandertals and modern humans, likely due to interbreeding. The underlying assumption here is that these robust traits have a strong genetic component. Furthermore, there is a notable decrease in cranial robusticity from the early Upper Palaeolithic to late Upper Palaeolithic. It has been suggested that this may reflect changes in diet. Transition from hunter-gather to agricultural lifestyle is associated with a reduction in cranial robusticity, although correlation does not necessarily prove causation. However, not all hunter-gather groups are universally more robust than argriculturalists, which might suggest some other factors at play. Read More...

Why the Aquatic Ape Hypothesis doesn't hold water

Among this week’s new videos from TED, was a talk given by Elaine Morgan – the chief promoter of the Aquatic Ape Hypothesis (AAH). The AAH was first formulated by Alister Hardy and is the idea that human evolution went through an aquatic stage, which in turn explains many of the features of the human physiology. For anybody with a poor understanding of evolutionary biology the AAH arguments can seem quite compelling. Instead of repeating the numerous reasons why the AAH fails (Jim Moore has an entire website dedicated to this), I wish to address some of the specific arguments made in this video.

Morgan starts off my stating that "… there's one aspect of this story which they [evolutionists] have thrown no light on and they seem anxious to skirt around and step over it and talk about something else. So I'm going to talk about it. It's the question of why are we so different from the chimpanzees?". Either Morgan has not been reading the hundreds of research papers that have addressed these important questions or she is trying to hoodwink her audience. Palaeoanthropologists and primatologists have long recognised the value of studying human and chimp differences in order to understand our shared evolutionary history. In fact, it is impossible to talk about functional anatomy and phylogenetic history in humans without reference to our closest extinct and living hominin relatives. Read More...