A New Model for the Origin of Bipedality
Evelyn J. Bowers
Received: 11 April 2003 /Revised: 10 April 2004 /
Accepted: 4 January 2005 / Published online: 9 January 2007
# Springer Science + Business Media B.V. 2007
Abstract Scholars have long thought that bipedality evolved gradually in response to the
opening of the savanna. Recently, both parts of this concept have come into question. A
variety of benefits of bipedality have been posited as responsible, but a trait can not evolve
unless a useful mutation appears. Perhaps we need to stop wondering about selective
pressures and consider what kind of mutation might be involved in forming a bipedal pelvis.
Work on the evolution of development has shown that there are segmental control genes,
alterations in which have large effects. These include the hox genes, of which there are four
sets in humans, referred to as the HOX A, B, C, and D sequences. Changes in their activation
in embryogenesis alter the identity of vertebrae and limb structure. An alteration in the
control region of certain of the distal HOX D genes may well be responsible for the sudden
appearance of bipedality by moving the boundary between the lumbar and sacral vertebrae,
and so moving the position of the pelvis and lower limb origin. Pongids usually have three
lumbar vertebrae; early hominids, 6. Pongids also have 48 chromosomes while we have 46.
HOX D is located on our 2nd chromosome, the one that is a fusion of two pongid
chromosomes. If that fusion altered the onset of perhaps HOX D 10, so that it switched on a
couple of segments later, then the sacrum would form further down the vertebral column and
might be shorter. In this paper I look at the chromosomal location of HOXD and examine the
likelihood that the fusion of two panid chromosomes could have given rise to alterations in its
control resulting in the abrupt appearance of bipedality and accompanying changes in the
limbs and in the chela in which the HOX sequences are reused.
Keywords human evolution . developmental processes . chromosome rearrangements .
Hox genes
Human Evolution (2006) 21:241–250
DOI 10.1007/s11598-006-9021-x
E. J. Bowers (*)
Anthropology Department, Ball State University, Muncie, IN 47306, USA
e-mail: ejbowers@bsu.edu

Introduction
Scholars have long thought that bipedality evolved gradually in response to the opening of the
savanna [26–28]. The discovery of an apparent biped, Ardipithecus ramidus [17, 59] from a
wooded environment brings this model into question as do the discoveries of Sahelanthropus
tchadensis [5] and Orrorin tugenensis [52], probable bipeds with dates that may precede the
expansion of the savanna [44]. Moreover, there is little evidence for the gradual appearance of
bipedality in our ancestors. With the exception of Lucy, most if not all, of the
australopithecine pelves studied have been immature [1, 2] and the inference of adult shape
from that of a juvenile is problematical. The last phase of maturation, subsequent to
achievement of full height growth, involves widening of the pelvis to adult size [3]. It appears
that ontogenetic change has been taken for phylogenetic. In this paper, I present a mechanism
of chromosomal fusion which may account for both the abrupt origin of bipedality and the
rapid separation of the line leading to ourselves from that of the African Apes.
Synthetic Theory defined evolution as change in gene frequency in a population, and was
able to retain that definition when a gene expanded from an inference derived from
inheritance patterns to a molecular entity. It has been difficult for human paleontologists to
utilize this definition, as morphological genetics has, until recently, had very little to say about
anthroposcopically observable skeletal variation [4, 9]. With the burgeoning of studies on
the evolution of development in the past decade [16, 42], this deficit is starting to be filled.
Mark Stoneking of the Max Plank Institute for Evolutionary Biology wrote in the Human
Genome issue of Nature, “[An] area of increasing interest is identifying the molecular basis
of ‘normal’ phenotypic variation ... that is, variation of the old-fashioned morphological
kind, which is the traditional concern of anthropology. ... With the advent of the human
genome sequence and the S[ingle] N[ucleotide] P[olymorphism] database, ..., we are
ironically now posed to focus on phenotypes and what their diversity tells us about human
evolution-thereby bringing the anthropology back into molecular anthropology” [54].
Importance of Ontogeny for Paleontology
It is, I think, unfortunate that the study of growth and development has usually been
subsumed under biological anthropology (human biology), and so sometimes neglected in the
preparation of human paleontologists. There are a few scholars who are important exceptions
such as Nancy Minugh-Purvis [33–36], whose work on Neanderthal ontogeny has an
appreciable history, but they are rare. However, ontogeny is important for all mammals, living
and extinct. Melanie McCollum [31] called attention to the explanatory power of considering
the processes of development for understanding the phenotypic appearances seen in the fossil
record when she pointed out that the thick palate seen in robust Australopithecines is an effect
of the timing and duration of growth of their heads and faces. Dismorphologist Peter
Thorogood pointed out that, “[A] unified model of craniofacial development ... should be able
to explain not only the normal development of the head and face, but also the range of
craniofacial phenotypes seen phylogenetically and the dismorphologies seen by the clinician”
[55]. In a review of a Cold Spring Harbor Symposium volume titled Pattern Formation
During Development, David Leaf [24] pointed out that the analysis of development will
eventually “provide the proximal explanations of human evolution.”
242 Human Evolution (2006) 21:241–250