A little human with very big feet

hobbit feet
A recent conference and a handful of publications on the diminutive hominin Homo floresiensis (a.k.a. the “hobbit”) have brought to light the difficulty of placing this fossil neatly into the human family tree. A recent study of the 18,000 year old fossil’s feet has raised many intriguing questions. The difficulty in situating this species is that it has an complex mix of both ancestral and derived traits.

The pelvis and legs all clearly demonstrate that
Homo floresiensis was bipedal. However, the hominin’s feet are unusually long compared to the leg. This combination of a long foot and a relatively short leg is seen in some apes but not in hominins. The navicular acts like the keystone in the arch of the human foot and is elevated from the ground except in people who have fallen arches. In Homo floresiensis this bone has a well developed tuberosity meaning that it was in contact with the ground, like in other flat-footed great apes and early hominins. The overall shape of the foot means that this hominin would not have been able to run long distances very efficiently — a distinguishing feature of later Homo. While the hominin had a short chimp-like big toe, it was not opposable like in other hominins. The long, curved lateral toes resemble a chimpanzee’s, rather than those of a human which are short and straight. The distal first metatarsal is squared off like in modern humans but this feature is not found in other early hominins, such as Australopithecus afarensis, Paranthropus robustus or the early Homo remains from the Georgian site of Dmanisi.

It is generally thought that
Homo erectus was the first hominin to leave Africa, soon after their first appearance in the archaeological record around 1.9 million years ago. Based solely on archaeological data Homo erectus seems like the best ancestor for Homo floresiensis. Homo erectus has been show to be a highly variable species. Homo floresiensis could represent a descendent of Homo erectus that adapted to island life through a dramatic decrease in size. While some of the plesiomorphic traits of Homo floresiensis may be explained through evolutionary reversals, it is unlikely to account for all of the primitive traits in the skeleton as a whole. It has been suggested that the ancestor of Homo floresiensis was not Homo erectus but rather a more primitive hominin. The 1.8 million year old skeletal Homo remains from Dmanisi are relatively primitive. At first glance, this species might seem like a good ancestral candidate for Homo floresiensis. However, unlike Homo floresiensis, the Dmanisi specimens have quite modern limb proportions. Homo habilis has also been forwarded as a possible candidate although there is little archaeological evidence to suggest that that species ever left Africa. Until further evidence comes along, the jury is out on this miniature human.

Reference cited
Jungers et al. The foot of Homo floresiensis. Nature 459, 81-84 (7 May 2009) | doi:10.1038/nature07989

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Above photo modified from original by Mamoritai under creative commons license.
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Is craniometry scientific?

Craniometry is the measurement of the bones of the skull. The technique is primarily used to determine ancestry, population distances and sex. It differs greatly from the psuedoscientific fields of phrenology and physiognomy, although in the general populace the distinction is not so clear. The book Postmodernism and Race has the following definition: "Craniometry is the measurement of human skulls as an indication of intelligence", while a recent blog article on craniometry states that it "claims to be able to predict the intelligence and nature of human being." Craniometry says nothing about intelligence or human nature.

Craniometric studies were, to a large degree, racially motivated in the early decades of the twentieth century, with anthropologists trying to validate their preconceived racial categories. In 1912, Franz Boas published a study challenging the prevailing notion that certain cranial measurements were under ironclad genetic control. He studied the head form of some 13,000 European immigrants and their American-born children. He found significant differences in the shape of the heads between parents and their children, which he interpreted as evidence for cranial plasticity. In other words, environment, not genetics, shapes cranial morphology. During the subsequent decades, Boas' results came to be largely accepted by the anthropological community, with students of the Boasian school disregarding craniometric studies as an ill-fated enterprise. For many years, anthropologists steered clear of craniometry, instead focusing their expertise in other less stigmatised areas of physical anthropology such as palaeopathology.

Quite recently papers by Sparks
et al (2002) and Gravlee et al (2003) have re-examined Boas' original data. However, those expecting the final word on cranial plasticity were to be disappointed. The anthropologist Milford Wolpoff is quoted as saying in 1975 "The data do not speak for themselves. I have been in rooms with data and listened very carefully. They never said a word." This is particularly true of these two papers, which use the very same data to come to divergent conclusion. While Gravlee et al believe Boas to be essential correct, Sparks et al came down firmly on the other side. The real answer, I believe, is to be found between the lines.

Boas did, in fact, find a statistically significant environmental effect in his study but this begs the question of whether it is a meaningful effect. Sparks suggests that while the effect is real, it only constitutes a tiny proportion of variation. In fact, considering the size of Boas' sample (~13,000) it is almost impossible not to find statistically significant results; biology is, after all, intrinsically variable. It may be the case that Boas played up the importance of the environmental effects as a reaction to the racial thinking that was prevalent at that time.

Perhaps, the biggest problem with Boas' methodology was his reliance on only a handful of measurements and particularly the use of the cephalic index (ratio of head breadth to head length). Anders Retzius introduced the cephalic index as way of classifying skulls based on their overall shape. He defined three main categories: dolichocephalic (long headed), brachycephalic (broad headed) and mesocephalic (intermediate headed).

Most modern biological anthropologists are of the opinion that the use of a couple of measurements to describe a multi-complex structure such as the skull is absurd. Today, biological anthropologist will take dozens of measurements of the skull. W. W. Howells, who measured thousands of skulls from all over the world, had the following to say about the cephalc index: "When Anders Retzius, a century and half ago, invented the cranial index, he gave us an answer for which there was no question." Even Boas himself wrote the following in 1940: "Measurements should always have a biological significance. As soon as they lose their significance they lose also their descriptive value."

Craniometry is used today in biological anthropology as a means of determining the relationships of peoples through their phenotype. A phenotype is the visible manifestation of a genotype. Since there is rarely a one to one relationship between the genotype and phenotype we must first demonstrate that the phenotype is an accurate reflection of the genotype. If this is not the case craniometry would be no more scientific than phrenology. Narrow-sense heritability is the proportion of phenotypic variation that arises from only the additive genetic differences among individuals and is expressed as h
2 = VA/VP. Heribtaility is measured on a scale of 0 (no heritable variation) to 1 (all phenotypic variation is due to additive genetic effects). The average cranial h2 has been estimated at around 0.55 (Relethford 1994; Devor 1987). A heritability greater than 0.5 indicates that most phenotypic variation is the examined traits are attributable to genetic factors. Thus the proportionality of genotypic to phenotypic variance is a reasonable assumption. The true litmus test of any hypothesis is its predictive power. Craniometric data is used with surprising accuracy by forensic anthropologists to determine likely ancestry of unknown individual and by palaeoanthropologists to determine our relationship to other hominins. The modern scientific practice of craniometry distinguishes itself from psuedosciences like phrenology and physiognomy in that it is based on sound biological theory, it is testable, it is predictive and objective.

References cited
Boas F (1912) Changes in the bodily form of descendants of immigrants. American Anthropologist 14: 530-562.

Sparks CS, Jantz RL (2002) A reassessment of human cranial plasticity: Boas revisited. Proc. Natl. Acad. Sci. USA 99: 14636-14639.

Gravlee CC, Bernard HR, Leonard WR (2003) Heredity, Environment, and Cranial Form: A Reanalysis of Boas's Immigrant Data. American Anthropologist.

Devor EJ (1987) Transmission of human craniofacial dimensions. J Craniofac Genet Dev Biol 7: 95-106.

Relethford JH (1994) Craniometric variation among modern human populations. Am J Phys Anthropol 95: 53-62.

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Ad Hominin. What's in a name?

Until quite recently humans and their ancestors were referred to as hominids. This was a reflection of taxonomy of the time: the family Hominidae consisted of humans, while the family Pongidae consisted of orangutans, chimpanzees, gorillas and bonobos. However, molecular studies showed humans, chimpanzees and bonobos to be more closely related than previously thought. The current consensus taxonomy for the family Hominidae consists of the following subfamilies: Ponginae (orangutans), Gorillinae (gorillas) and Homininae (chimpanzees, bonobos, humans). Strictly speaking chimps, bonobos and humans are all hominids. However, we still need a way to talk about ourselves and our ancestors. The Homininae subfamily is divided into two tribes: Panini (chimpanzee, bonobo) and Hominini (humans). Thus, in recent years there has been a growing trend of referring to humans and their ancestors as hominins.The Latin prefix ad means "towards", so ad hominin can be translated as "towards humankind". Many of you will undoubtedly notice the allusion to argumentum ad hominem - a type of argument where one attacks the character of the opponent rather than their arguments. This pun is intended and reflects my interest in critical thinking and logic.Many people lack the cognitive toolkit to critically examine such psuedoscientific claims as homeopathy, psychokinesis or acupuncture. As a physical anthropologist I am well aware that evolution is not impervious to wild misconceptions. It is my hope to give people the tools and knowledge necessary to recognise fallacious arguments and critically examine extraordinary claims.

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