Some of my former students have seemed to a mental block when it came to the possibility of one species evolving into another, humans from chimplike ancestors, for example, because they assume that “humanness” is a fixed quality and “chimpness” is a fixed quality and one could not change into another.  After all, in comparing humans and chimps, there are skeletal differences, muscular differences, neural differences, etc.  What is often unappreciated is the degree to which “humanness” can vary.  In teaching human anatomy, an instructor must lie to their students to some degree.  The instructor must say that in humans, there are so many muscles in the forearm flexor compartment, often knowing full well that at least some of the students in the class might have a different number.  The instructor must describe the bones of the human skeleton although some of the students may have different numbers of bones other than what is described as being the human condition.  The instructor must describe the cardiovascular system including the great vessels which leave the heart or the circle of Willis in the brain knowing that it is not uncommon for students in the class to have alternate arrangements.

     To the best of my knowledge, there is not one aspect of human anatomy and physiology which does not vary.  Some variations are common and are not of clinical significance.  Some variations are potentially detrimental to a person’s health.  Some variations are serious deficits that prevent fetuses from developing to term.  To the best of my knowledge, it is not possible to name a specific single anatomical trait which is identical in all humans (although some of the humans who possessed a variation may have died as fetuses because of it).   When considering whether one species can evolve into another, it is relevant to consider that the differences between average members of two species with reference to many traits are actually less that the differences which can occur among the populations of each of the two species.  



     Human skeletons differ.  The overall shape of bones can differ, as is evident in the following three images of skulls (note the shape of the braincases and the relative positions of the eyes, for example).

skulls skulls
The mandibular notch of lower jaws can vary.
lower jaws lower jaws
The sternum can vary between individuals.
The size of the cribiform plate in the cranium can vary between individuals.
cribiform plate cribiform plate
The relative positions of the acromion, coracoid process, and glenoid cavity can vary in the shoulder joint.
The size of certain processes can vary.  In the following skull, the styloid process is smaller than usual.

       A number of bones are actually composite structures which result from the fusion of multiple fetal bones.  (Often these fetal structures represent the homologs of separate bones found in the adults of other vertebrates.)  The occipital bone, for example, forms from a number of ossification centers which correspond to separate individual boens in more primitive vertebrates.  Although this set of occipital ossification centers usually fuses to form one solid bone in humans, there are common variations of this pattern.  Ossification centers which do not fuse with the rest of the occipital bone during development can be called sutural, Wormian, and Incan bones.  Although they may be present in normal individuals, they are often associated with other abnormalities, such as osteogenesis imperfecta (Cremin, 1982).   Occasionally, the pars interparietalis doesn’t fuse to squama and forms an Incan bone. (Redfield, )  In rim of foramen magnum, ossicle of Kerkring may form which may, or may not, fuse to squama. (Redfield, )

wormian wormian

In the following sacra, the fusion of the fetal vertebrae has produced variations in which portions of the posterior region is encased in bone.

The sacrum can involve differing numbers of vertebrae (and thus have differing numbers of sacral foramina).
Vertebrae centra develop from 2 lateral chondrification centers.  A hemivertebra results if only one side forms; a butterfly vertebra if the two fail to fuse (Sonel, 2001).  In the following photos, note that the neural arches of one of the vertebrae have not fully fused and that the same vertebra possesses an extra pair of transverse processes. 
axis axis

     In adults, cervical ribs usually do not occur (other than the portions of the cervical ribs which fused to the cervical vertebra during fetal development to form the transverse processes of cervical vertebrae).  The rib associated with vertebra C7 usually fuses with transverse process but it can remain as a separate rib.  The observed incidence of this cervical rib in the general population has varied from .3 to 1%  (McNally, 1990).

     The following photos depict a skull which possesses an anomalous process and foramen in the sella turcica region of the sphenoid bone. 

sella turcica sella turcica
The sutures of the human skull can vary.  Some humans possess sutures between two fetal elements which usually fuse seamlessly in most people during their development.  Other people have one solid bone while most possess two distinct bones separated by a suture.
In the following images, the first vertebra has fused to the occipital bone.
fused first vertebra fused first vertebra

     Abnormalities of the nasal region include the formation of a double nose and an accessory nostril (Williams, 1998).

     The number of human toe bones can vary.  Instead of the normal 3 phalanges on toes 2 through 5, only 2 phalanges may exist.  Biphalangeal toes are present in 41% of the population on the 5th toe, 2.5% on the 4th toe, .2% on the 3rd toe, and .1% on the 2nd toe.  This reduction of phalanges in humans is relatively common and may be part of the ongoing adaptation to bipedalism (LeMinor, 1995).

     One in about one thousand human births suffers a limb anomaly.  Limb abornmalities include extra digits, extra carpals, fused carpals, extra phalanges, the loss of metacarpals, proximal and middle phalanges, the fusion of forearm bones, the absence of a patella, and the development of paddle-like structures attached to girdles.

limb abnormalities