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Evolutionary change in the primate wrist and inferior radio-ulnar joints.

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Evolutionary Change in the Primate Wrist and
Inferior Radio-ulnar Joints
0. J. LEWIS
D e p a r t m e n t of A n a t o m y , St. Bartholomew's Hospital Medical College,
Charterhouse Square, L o n d o n E.C.1.
ABSTRACT
The joints at the junction of the forearm and hand are described in
a number of Primates, including Man. It is shown that the original lower articular
extremity of the ulna recedes from its primitive articulation with the triquetral and
pisiform to become the ulnar styloid process, while a neomorphic ulnar head is
elaborated entering into a new synovial inferior radio-ulnar joint. The lower capsule
of this joint becomes the triangular articular disk. In the anthropoid apes a meniscus
(with a laterally-directed concave free border bounding the ulnar styloid) develops in
the interval between the receding ulna and the carpus. In gibbons this meniscus presents a lunula (0s Daubentonii) which is also present in the human fetus as a transient cartilaginous nodule often erroneously homologized with the 0s intermedium. The
upper articular surface of the human wrist joint is formed by the radius, the triangular articular disk and the homologue of the ape meniscus. Between the two latter
components is a n aperture, similar to that of apes, leading into a pea-sized diverticulum of the wrist joint - the pre-styloid recess. The mechanism of the evolution of
new synovial joints is discussed.
Despite numerous observations recorded
during the past two centuries concerning
the comparative morphology of the joints
between the primate forearm and hand, no
clear or coherent understanding of these
structures has as yet emerged. Even more
surprisingly the modern textbook accounts
of the human wrist joint are often difficult
to reconcile with the findings of direct observation in the dissecting room. In Man
and in other Primates a number of such
findings call for intelligible and convincing
explanation.
Primate evolution from the Prosimii to
the Hominoidea has been characterized by
an increased mobility of the hand, effected
by two major skeletal modifications. Firstly
the ulna has become withdrawn from its
primitive articulation with the pisiform
and triquetral bones (Flower, 1885; G r a d ,
'55), its original carpal articular extremity
being finally represented by the human
styloid process; this change has increased
the range of hand adduction (Wood Jones,
'49). Secondly, and correlated therewith,
pronation and supination have been enhanced by the elaboration of a new ulnar
capitular articulation within a synovial inferior radio-ulnar joint. This neomorphic
ulnar head is excluded from the wrist joint
cavity by a triangular articular disk arisANAT. REC., 151: 275-286.
ing, apparently, as a modification of the
lower part of the capsule of the inferior
radio-ulnar joint (Wood Jones, '49).
Two questions therefore pose themselves, viz. (1) by what agency has the
ulnar styloid been excluded from the wrist
joint ( a process recapitulated - save very
exceptionally - in human ontogeny) and
( 2 ) has there been evolution of an entirely new, diarthrodial, inferior radioulnar joint?
The gibbon carpus contains an additional bony element, situate between the
ulnar styloid and the triquetral and pisiform bones, apparently first described by
Daubenton (1766) and hence often known
as the 0 s Daubentonii; it was subsequently
observed by Leboucq (1884) and by Kohlbrugge (1890) (fig. 1A). It is also established that the human embryo, during the
second to fourth month period, exhibits a
transient nodule of hyaline cartilage at a
comparable carpal site, i.e., in the lose connective tissue immediately distal to the
developing triangular articular disk (Leboucq, 1884; Corner, 1898). Even in the
adult a radiographic opacity is sometimes
observable in this same situation (Wood
Jones, '49; Oliver, '62), while Pfitzner
(1895) described an actual bone (trique275
276
0. J. LEWIS
B
Fig. 1 A, the carpus of Hylobates leuciscus (from Kohlbrugge, 1890). B, a tracing of a
normal arthrogram of the human wrist joint, showing the pre-styloid recess (from Kessler
and Silberman, '61). Lu, lunate; OsD, 0 s Daubentonii; Pi, pisiform; Ra, radius; Sc, scaphoid;
Tr, Triquetral; U1, ulna.
trum secundarium, triangulare) here in an
osteological preparation.
The gibbon carpal element and the identically located cartilaginous nodule of the
human fetus have been interpreted as serially homologous with the pedal 0s trigonum (Leboucq, 1886); and just as the
0s trigonum has been said to represent the
0s intermedium tarsi so this carpal element has been interpreted as an 0s intermedium antibrachii (Poirier and Charpy,
'11; Testut, '04).' Since it is obvious that
an 0s Daubentonii can only appear subsequent to the retreat of the ulna from its direct articulation with the carpus, it is
tempting to suppose that such appearance
is in some fashion directly associated with
this retreat.
Now Lewis ('64) has shown that the
comparable free "0s intermedium tarsi" of
the marsupial (held to correspond to the
anomalous human 0s trigonum) is nothing
more than a lunula, i.e. an ossification
within the thick part of an intra-articular
meniscus. The likelihood suggests itself
therefore that the correspondingly placed
ossicle in the gibbon carpus is nothing
more than a similar functional adaptation.
During the last century anatomists
(Henle, 1856; Morris, 1879) described the
proximal articular surface (the receptive
cavity) of the condyloid human wrist joint
as a smoothly concave ellipse whose transverse extent was limited by attachments to
the margins of the upper articular surfaces
of the scaphoid and triquetral bones. But
clearly the medialmost part of this surface
(that part related to the triquetral facet)
cannot be the triangular articular disk itself. Henle (1856) described this disk as
undergoing an apical cleavage into upper
and lower laminae, the former attaching to
the root of the ulnar styloid, the lower continuing as part of the curved articular surface. Henle's account does not accord with
the modern textbook descriptions which regard the entire apex of the triangular articular disk as attached to a pit at the base
of the ulnar styloid and whose accompanying illustrations are not always reconcilable with their text. (Pictures frequently
show increasing thickness of the disk on
being traced medially. )
French anatomists (Testut, '04; Poirier
and Charpy, '11) have described a promi1 Actually two additional ossicles have been described just proximal to the first row of carpalia in
the aibbon. viz.. the aDDarentlv constant 0 s Daubentonii- and ' another (rkiatively uncommon) ossicle,
more laterally adjacent to the radius, the 0 s Camperi
(Kohlbrugge, 1890). Thilenius (1895, 1896) interpreted the 0 s Camperi and the human embryonic
carpal nodule as homologues and representative of
the 0 s intermedium, but this distinction between the
two supernumerary gibbon ossicles has not been retained in later derivative accounts.
277
EVOLUTION O F WRIST JOINT
nent cul-de-sac (the pre-styloid recess)
establishing a relationship between the
wrist joint cavity proper and the ulnar
styloid process. This pea-sized diverticulum of the wrist joint cavity has recently
been demonstrated constantly in arthrograms (Kessler and Silberman, '61) (fig.
1B). Yet it escapes notice in British anatomical treatises, and its precise topography is ignored by the French anatomists,
although Poirier and Charpy state that occasionally the wrist joint's internal lateral
ligament is of a tubular nature, occupied
by the pre-styloid recess and attached to
the base of the ulnar styloid, whose tip
plays freely within its synovially lined
cavity.
It seems not unlikely that the pre-styloid
recess is related in some way to the phylogenetic and ontogenetic retreat of the
ulnar styloid from the carpus. It is shown
below that a clear concept of wrist joint
phylogeny clarifies the present unsatisfactory morphological picture and necessitates
some reappraisal of traditional interpretations of human wrist joint morphology.
crown-rump length and of serial coronal
sections of the hand and forearm of a
fetus of 13 cm crown-rump length.
OBSERVATIONS
Trichosurus vulpecula
Arrangements herein typify the basic
mammalian carpus pattern. The large
and elongate pisifonn articulates with the
triquetral : a concave articular surface,
largely triquetral but completed anteriorly
by the pisiform, receives the convex articular lower extremity of the ulna, no meniscus intervening. Immediately proximal to
its spherical lower end the ulna constricts
as a neck, proximal whereto it expands
into an evenly fusiform enlargement traversed about mid-way by the distal epiphyseal line. This ulnar enlargement is intimately united to the radius by ligamentous
tissue representing a modification of the
distal portion of the interosseous membrane. No synovial cavity is present: the
inferior radio-ulnar joint is a syndesmosis.
Lemur fulvus (fig. 2A)
In many respects the arrangements are
The wrist and inferior radio-ulnar joints similar to those in Trichosurus but the
were first studied in the common brush- articular surfaces of ulna on the one hand,
tailed possum (Trichosurus vulpecula) and triquetral and pisiform on the other,
which demonstrated the generalized mam- show a lesser degree of curvature and the
malian arrangement. Against this estab- pisiform makes a greater relative contribulished basic pattern the articulations were tion to the lower joint surface. The ulna
dissected in examples of the following pri- again presents a fusiform swelling, travmate species : brown lemur (Lemur ful- ersed by the lower epiphyseal line, and
v u s ) , weeper capuchin monkey (Cebus united by a ligament to the radius. In this
nigrivittatus), olive colobus monkey (Proco- case, however, a small synovial cavity ocZobus v e r u s ) , white-thighed colobus mon- cupies the interior of the ligamentous
key ( Colobus polykomos vellerosus), white- bond, and each bone presents a tiny cartinosed monkey (Cercopithecus nictitans), lage-clothed articular facet on its epiphysilvery gibbon (Hylobates lar leuciscus), sis. Two prominent ligaments, converging
lar gibbon (Hylobates lar) and chimpanzee on the lunate from the neck of the ulna
and the front of the radial styloid process,
( P a n satyrus).
The corresponding joints were carefully are seen on the inner aspect of the anteexamined in 19 adult human arms; in five rior capsule of the wrist joint; these are the
of these specimens the radiocarpal joint palmar radiocarpal and ulnocarpal ligacavity was injected with neoprene latex ments.
prior to dissection. The joints were also
Cebus nigrivittatus
examined, with the aid of the dissecting
Again a large pisiform forms, with the
microscope, in human fetuses of 15 cm
and 16 cm crown-rump length; the find- triquetral, a concave facet receiving the
ings here were confirmed by the examina- convex lower extremity of the ulna, withtion of serial transverse sections of the out the intervention of any meniscus.
forearm and hand of a fetus of 24 cm Again the ulna has a constricted neck just
MATERIAL AND METHODS
2 78
0. J. LEWIS
A
B
Fig. 2 A, the right wrist of Lemur fulvus. B, the right wrist of Cercopithecus nictitans.
In each case the hand has been flexed on the forearm after opening the wrist joint from
behind. In each case the palmar ulnocarpal and radiocarpal ligaments are shown converging on the lunate and in B is shown the neomorphic ulnar head and the emergent triangular
articular disk. In each case the lower ulnar epiphyseal line is represented by the series of
short vertical lines. The lettering is as in figure 1.
above its distal extremity (the future styloid process) but, instead of a fusiform
swelling related to the radius, manifests an
asymmetrical enlargement - a laterally
placed prominence which clearly heralds
the appearance of the “head” of human
anatomy. (To avoid confusion, the terms
of human anatomy - styloid process and
head, respectively - will be used in the
following descriptions for the original carpal articular extremity and for this newlyevolved enlargement.) A tough ligament
unites this ulnar head to the radius without the intervention of any synovial cavity.
The lower aspect of this ligament is
smooth, attaches medially to the neck of
the ulna, and forms part of the upper articular surface of the wrist joint, just as
does the triangular articular disk of the
human inferior radio-ulnar joint. A separate ligamentous band (the palmar ulnocarpal ligament) joining the neck of the
ulna to the lunate, with attachment to the
pisifonn between, is related to the anterior
margin of this homologue of the human
articular disk. There is also a prominent
palmar radiocarpal ligament joining the
front of the radial styloid to the lunate.
Procolobus verus
In all essentials the arrangements reflect those described for Cebus.
Colobus polykomos vellerosus
The articulations under consideration
closely resemble those of the preceding
species with one further modification,
namely the appearance of a small synovial
cavity (essentially a bursa) within the
interosseous ligament between the ulnar
head and the radius. The cavity of this
bursa is related particularly to the ulnar
head, and the radius here bears no true
articular facet, hence the joint could not
justifiably be termed a true diarthrosis.
Cercopithecus nictitans (fig. 2B)
The joints resemble those of Colobus
polykomos vellerosus save that the elaboration of a synovial inferior radio-ulnar joint
has progressed further. There is a considerable radio-ulnar synovial cavity and
the ulnar head now has a convex, carti-
-.-l,..l
JLVULU l l U l Y
279
ur WRIST JOINT
lage-clothed, articular surface; the articular facet on the radius is more rudimentary.
Hylobates Zar leuciscus (fig. 3)
There is a fully elaborated synovial inferior radio-ulnar joint, separated from the
wrist joint by a triangular articular disk.
The original ulnar articular extremity, now
transformed into a recognizable styloid
process, has retreated somewhat from the
carpus. Correlated with this is the appearance in this interval of a very large, obtrusive intra-articular meniscus, which thus
intervenes between the cartilage-clothed
ulnar styloid process and the triquetral,
where the latter bears the pisiform articulation. The meniscus has a concave free
border facing laterally and a convexity attached to the wrist joint capsule. Its two
horns have important attachments: the
posterior is inserted on the radius behind
the inferior radio-ulnar joint; the anterior
is affixed partly to the radius, but mainly
to the lunate which it reaches by merging
with the palmar ulnocarpal ligament observed in monkeys spanning the interval
between ulnar neck and lunate at the anterior margin of the triangular articular
disk. Within its very thick medial portion
the meniscus is ossified, i.e., it contains a
lunula. The size and position of this ossicle proclaim it as the 0s Daubentonii. Nothing corresponding to the 0s Camperi was
observed. A strong palmar radiocarpal
ligament is present.
Hylobates lar
(This is a young specimen and at the
very start of the Juvenile I1 period, according to the criteria of Schultz, '44: the
permanent first molars and central incisors
are all erupted and the permanent lower
lateral incisors are just erupting; the coracoid process is not yet united to the remainder of the scapula and ilium and
pubis are ununited.)
TrD
.
RcL
Fig. 3 The right wrist of HyZobates lar Zeuciscus with the posterior joint capsules removed, and the hand flexed on the forearm. Lun, lunula; Men, intra-articular meniscus;
RcL, palmar radiocarpal ligament; TrD, triangular articular disk; other lettering as in
figure 1.
280
0. J. LEWIS
The joints of this specimen resemble
those of the previous species, but the unossified intra-meniscal lunula consists of
a well-dehed, circumscribed, nodule of
hyaline cartilage.
Pan satyrus (fig. 4 )
The arrangements here present a clear
transition between those in the gibbon and
man. A meniscus is present having similar
attachments to that of the gibbon, but containing no lunula. The wrist joint characteristics of this species, and their correlation with the human condition, can be
best demonstrated by opening the wrist
joint from behind so that the meniscus
(whose posterior horn, in any case, attaches to the radius) is folded forward with
the forearm bones, and thus appears as
part of the receiving cavity for the carpal
condyle. A deficiency is then apparent in
this upper articular surface, located near
the apex of the triangular articular disk:
it is in fact the gap within the concavity of
the meniscus, and here is exposed the
primitive ulnar articular extremity (the
styloid process). This gap leads into what
is virtually an upper compartment of the
wrist joint wherein lies the still articular
cartilage-covered ulnar styloid process.
This upper synovial cavity has its greatest
proximal extent on the anterior aspect of
the styloid process, and its lining membrane presents prominent synovial fringes,
some of which protrude into the lower
compartment through the connecting aperture. The constitution of the human upper
articular surface is thus clearly foreshadowed, the upper synovial compartment being the precursor of the human pre-styloid
recess.
Homo (fig. 5 )
The adult joints. Understanding of the
chimpanzee condition elucidates the human pattern. The ulna has withdrawn
further from its primitive carpal articulation and no longer presents itself, as in
Pan, through the orifice of a meniscus.
But even in Homo the continuous curve
of the upper articular surface presents an
opening - the aperture leading into the
pre-styloid recess. This opening is found
close to the apex of the triangular articular disk and is easily overlooked since its
margin is crenated and not sharp and
TrD
I
I
- -RcL
Fig. 4 The right wrist of Pan satyrus with the posterior joint capsules removed and the
hand flexed on the forearm. Men, intra-articular meniscus; TrD, triangular articular disk;
UlSt, ulnar styloid; other lettering as in figure 1.
281
EVOLUTION OF WRIST JOINT
UI S t
I
I
I
I
TrD
I
I
- - - -RcL
Fig. 5 The right human wrist joint opened from behind; the dorsal radiocarpal and the
palmar ulnocarpal ligaments have been cut and the hand flexed on the forearm. The broken
line indicates the position of the pre-styloid recess whose opening into the wrist joint, with
its crenated margin, is shown in solid black. RcL, palmar radiocarpal ligament; TrD, triangular articular disk; UlSt, ulnar styloid; other lettering as in figure 1.
prominent as in Pan. It readily admits a
blunt probe into the pre-styloid recess, the
homologue of the upper compartment of
the chimpanzee wrist joint cavity, and
similarly situated in relationship to the
ulnar styloid; the major prolongation of
this diverticulum is anterior to the ulnar
styloid. The upper articular surface of the
human wrist joint is thus a composite
structure formed by the amalgamation of
three structures - articular surface of the
radius, triangular articular disk and the
anthropoid ape meniscus. The last component has become that portion of the surface which lies in contact with the triquetral articular facet. Variations in size of
the human ulnar styloid are associated
with corresponding variations in the prestyloid recess, which may project more or
less horizontally from the wrist joint cavity
or may be more vertically aligned. The
ulnar styloid, where related to the recess,
may present no special modification, but
as a cartilage-covered projection may
sometimes actually invaginate the synovial
cavity. The recess was present in all the
hands examined and was clearly demonstrated in all specimens injected with neoprene latex: its extent varied from 6 mm
to 10 mm.
Not to be confused with the opening into
the pre-styloid recess is a common deficiency (apparently pathological) in the triangular articular disk and an additional
frequent communication with pisotriquetral articular cavity; the latter is clearly a
retention of the normal communication
found here in anthropoid apes.
In Man, as in the other Primates described, a strong ligament passes from the
282
0 . J. LEWIS
radius adjacent to its styloid process, to attach to the front of the carpus and particularly to the lunate. This ligament is clearly
apparent only when viewed from the joint
interior and few of the authoritative descriptions give it adequate emphasis.
The human fetus. In both specimens
( 15 cm and 16 cm crown-rump length) examined by dissection the arrangements
provided a connecting link between the
features observed in Pan and those in
Homo and confirmed the morphological interpretation given above : thus the ulnar
styloid approached the triquetral quite
closely and the intervening homologue of
the ape meniscus was more clearly apparent as a separate entity, with a more
clearly defined margin than obtains in the
adult condition. This thin free margin was
closely applied to the under surface of the
triangular articular disk, and formed a
valve-like entrance, which freely admitted
a probe, to the pre-styloid recess and the
adjacent styloid process.
Examination of transverse serial sections of the hand and forearm of a fetus
of 24 cm crown-rump length confirmed
these findings. The coronal serial sections
from the 13 cm fetus also demonstrated
the pre-styloid recess with well-defined margin of entrance and the flap-like homologue of the ape meniscus, with attachment
in the most anterior sections to the lunate.
These sections revealed the great thickness
of the triangular wedge of tissue intervening between the ulna and triquetral and
containing the ulnar styloid and the prestyloid recess. The synovial membrane
walling the recess was extremely vascular.
DISCUSSION
Evolution of the wrist joint. An important feature of primate evolution, viz. increasing mobility of hand on forearm,
centers around the retreat of the ulna from
its primitive carpal articulation. Pari
passu with this, a thick meniscus develops
in the interval so created, which is not
surprising in the light of Whillis's ('40)
work on joint development. At all sites of
such development a mesenchymal joint
disk forms, and where not subjected to
pressure may remain as an intra-articular
structure; at an early stage therefore a
meniscus is present at the periphery of
most joints. Retention of a meniscus at
the wrist of adult anthropoid apes thus follows as a logical consequence upon the
withdrawal of the ulna. The general failure to recognize this meniscus, or to distinguish it from the morphologically different
triangular articular disk, has occasioned a
great deal of confusion. In gibbons the
thickest part of this meniscus exhibits an
ossification, i.e., a lunula. (Grassk's ('55)
statement that the supernumerary gibbon
carpal ossicle is a sesamoid in the tendon
of M. extensor carpi ulnaris is without
justification). The work of Pearson and
Davin ( Y l ) , Pedersen ('49) and Barnett
('54) has revealed that such lunulae,
found in many intra-articular menisci, are
functional modifications apparently preventing compression of the thickest portions.
There can be little doubt that the transient cartilaginous nodule of the human fetal wrist represents an ontogenetic recapitulation of such a lunula. Its appearance
in the connective tissue distal to the triangular articular disk and its early attachment to the lunate (Leboucq, 1884;
Henckel, '31) support this view. The recognition of these primate skeletal elements
as intra-meniscal structural modifications
seems effectively to dispose of the commonly held view that they represent an 0 s
intermedium. (Discussion as to whether
the 0 s Daubentonii or the 0s Camperi'
represent the intermedium is likewise redundant: more than one lunula may be
found in a meniscus and there is little
doubt that the rare 0s Camperi is such a
supernumerary lunula.) The view of
Leboucq (1884) and of Corner (1898) that
these structures represent the carpal element of a lost post-minimal digit, of which
the pisiform is the sole remaining part, is
no longer tenable. Both Henckel ('31) and
Olivier ('62) have earlier rejected interpretation of the elements in question as
part of the primordial skeleton and have
considered them to be "secondary structures." This vague alternative is given
substance by the present demonstration
that they are actually intra-meniscal lunulae .
The gibbon wrist meniscus, with its contained lunula, corresponds in position to a
~~
2
~
See footnote 1.
EVOLUTION OF WRIST JOINT
similar meniscus, also containing an ossicle, found in the marsupial foot (Lewis,
’64). Leboucq (1886) had some justification for his comparison of the pedal 0 s
trigonum with the additional skeletal element of the hominoid wrist. Unfortunately
he was unaware that both represent functional adaptations to similar changes in
upper and lower limb, readily appreciable
on the basis of the general principles of
joint development : thus the initiating factor in the appearance of the forelimb meniscus is retreat of the ulna from the triquetral (ulnare) ; the analogous change in
the foot is withdrawal of the fibula from
articulation with the calcaneus (fibulare).
Since mere regression of the ulna automatically initiates the changes leading to
the appearance of a meniscus and lunula,
it would not be surprising if these structures had appeared more than once in
mammalian forms. Indeed, Daubenton
(1766) noted the ossicle bearing his name
to be present in the magot (Macacus
inuus ) .
T h e h u m a n wrist joint. The human
joint bears the clear imprint of its evolutionary history, in the light of which a
number of its features gain emphasis and
added significance. The upper articular
surface is formed from three components
- the radius, the triangular articular disk
and the homologue of the ape meniscus.
The latter two components are fairly well
blended, thus obscuring somewhat the
identity of the most medial part of the
receptive cavity (that part in contact with
the triquetral) as the homologue of the ape
meniscus. The opening bounded by the
meniscus in Pan remains in Man as the entrance to the pre-styloid recess. This synovial diverticulum enters into relationship
with the ulnar styloid process and perhaps
invests its lower part, in which event the
source is clear of the description by Poirier
and Charpy (’11) of the occasional synovial-lined tubular form which may be assumed by the internal lateral ligament of
the wrist joint. It was noted above that the
wall of the pre-styloid recess is very vascular: from this feature, presumably, is derived Henle’s (1856) description of an aggregation of blood vessels separating the
apex of the triangular articular disk into
283
two ligamentous laminae - the “ligamentum subcruentum.”
The human palmar ulnocarpal ligament
and the palmar radiocarpal ligament are
both significant structures throughout primate evolution.
Some of these features would seem to
merit clinical consideration. Thus, the relationship of the pre-styloid recess may be
of significance in those cases of Colles’
fracture in which the ulnar styloid is
avulsed. There is also little doubt that this
blind cul-de-sac is a common site for pathological change. The two palmar ligaments converging on the lunate are worthy
of note when considering the mechanism
of dislocation of the lunate bone.
Evolution of the inferior radio-ulnar
joint. The Primates exhibit the transition
in this situation from a mere bursa to a
typical diarthrosis. Elaboration of this diarthrosis can only occur as a sequel to
prior changes in the wrist joint, for full
pronation and supination are possible only
after the carpus is freed from its direct
ulnar articulation. The lower capsule of
the new diarthrosis, broadened by development of the neomorphic ulnar head, becomes the triangular articular disk.
The appearance of new diarthroses has
considerable interest. Other examples are
known: Bock (’59) has described a secondary jaw articulation found in certain
birds, and Lewis (’59) has described a
coraco-clavicular diarthrosis (probably present at birth) occurring as a variant in
Man, It is of course well known that pseudoarthroses, with all the attributes of normal diarthroses, may arise at fracture
sites; clearly, the capacity to form a new
synovial joint is a normal bodily response
which may be realized during an individual’s lifetime in response to use. But the
joint under consideration is inherited. It
fits into the category of adaptations
known as pseudoexogenous, i.e., those
which could arise as an adaptive response
during an individual’s life time but which,
in fact, are of developmental origin. This
type, at first sight, appears to require a
Lamarckian interpretation, but, in fact,
such contingencies are adequately covered
in the modern synthetic theory of evolution; it is clear that developmental preemption of such a character is of selective
284
0. J. LEWIS
advantage (Medawar, ’57). The underlying genetical and natural selection basis
of such adaptations, which has been called
“genetic assimilation,” has been experimentally supported by the work of Waddington (’57, ’60).
There is a great contrast between the
joints at the junction of the forearm and
hand in the more primitive of the Primates and those in Man. This might
suggest a change of macroevolutionary proportions, demanding a mutationist explanation. But, as shown here, the changes
have been produced by gradual phylogenetic modification of existing structures,
following the usual pattern in the elaboration of so-called evolutionary novelties
(Mayr, ’60).
Observations presented above demonstrate that some progress, at least, towards
a diarthrodial inferior radio-ulnar joint has
occurred more than once among the Primates, yielding yet another example of the
frequency of parallel evolution. Similarly,
the additional basi-temporal articulation of
some birds has evolved independently several times (Bock, ’59) as has the mammalian jaw articulation itself (Simpson,
’59). The mechanism of genetic assimilation furnishes excellent opportunities for
parallel evolution whereas a mutationist or
saltationist explanation carries with it an
inherent improbability for frequent repetition. The temporary, and functionally useless, recurrence in Man of the ape lunula
provides another example of genetic assimilation of a functional modification.
ACKNOWLEDGMENTS
I should like to thank Professor A. J. E.
Cave for providing the material used in
this study and for valuable advice in the
preparation of the manuscript.
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