Facial growth and remodeling in the australopithecines
- a review
K atarzyna A. K aszycka
AbstractThe purpose o f this paper is to review the previous investigations o f the facial grow th and rem odeling patterns o f early hom inids (the australopithecines), and to present som e o f the problem s that arise from these kinds o f analysis. :
Katarzyna A. Kaszycka 1998; Anthropological Review, vol. 61, Poznan 1998, pp. 7 5 -8 3 , figs 4, ISBN 83-86969-35-0, ISSN 0033-2003
Introduction
Growth o f the face is influenced by a variety o f factors which play different roles in different stages o f development. It appears that during the late prenatal and early postnatal stages, the dominant factor contributing to facial growth is the growth o f the cranial base, including the nasal capsule, and the nasal septum. Later, growth is known to occur by re modeling processes (apposition and re sorption), sutural growth, neural and synchondroseal growth, dental develop ment, and the development o f the maxil lary alveolar process. By age seven, once the anterior cranial base has become stable, the growth o f the midface is in fluenced mainly by the process o f down ward and forward displacement and pe riosteal remodeling [CARLSON 1985],
The basic concept o f the growth and
Institute o f Anthropology . Adam M ickiewicz University
61-701 Poznan, Poland
rem odeling o f the craniofacial skeleton originated more than 200 years ago with the pioneer works o f J. Hunter [H UNTER
1771]. In the 1920s and 1930s Brash revised and developed Hunter’s “remo deling” theory claiming that cranial growth was due to ectocranial apposition and endocranial resorption [BR A SH
1934]. During the 1940s-1960s three hypotheses were put forward to explain the mechanisms influencing midfacial growth. These were: the sutural hy pothesis (see e.g. PRITCHARD ET AL.
[1956]), the nasal septum hypothesis
[SCOTT 1953], and the functional matrix hypothesis [M O SS 1976].
Other m ajor contributions to the study o f facial growth were made in the 1960s and 1970s by Enlow and Bjork. Enlow ’s contribution consisted in making major generalizations about the facial growth o f both humans and rhesus monkeys. In 1966 the author wrote: D etailed studies
o f remodeling patterns in the fa cial skeleton o f extinct members o f the genus Homo as well as other related genera
76 K atarzyna A. K aszycka
and species, both living and fossil, are growth and remodeling: (1) The concept
now needed. It should be determined i f o f “area relocation” (the changing rela t e distinctive resorptive nature o f the tive positions o f all areas within a bone);
fo rw a rd p a r t o f the maxillary and man- (2) The principle o f the “V ” (the enlarg-
dibular arch is a specific characteristic ing V involves deposition on its inner
o f H. sapiens. [E n lo w 1966a, p. 302]. surface and resorption on its outer sur- F ollow ing this directive, T. B rom age face); and (3) The principle o f “cortical performed studies to answer the question drift” (surfaces that face the actual di- o f how bones o f the face grew and rection o f growth are depository, while rem odeled in extinct au stralopithecine surfaces that face away from the direc-
species. tion o f growth are resorptive; the result is
The purpose o f this paper is to review a drift movement o f the bone in the di- tlie previous investigations o f the facial rection o f the depository surface), bone growth processes o f early hominids, One o f the major points made in En- and to present some o f the problems that low’s generalization is that the maxilla arise from these kinds o f analysis. grows downward and forward by a
process o f bone deposition on the
poste-Facial growth of
Homo
rior and superior surfaces. There are two1
and
Macaca
major areas o f maxillary growth: the maxillary tuberosity, and the roof o f the A ccording to Enlow [E N LO W 1968; oral cavity, both o f which receive newENLO W & Ha r r i s 1964; ENLOW & bone deposits. In contrast, the anterior
B A N G 1965; D U TER LO O & ENLOW surface o f the maxillary complex is re-1965], three principles apply to bone sorptive in character. The mandible, like
Fig. 1. T he distribution o f depository (light) and resorptive (dark) surfaces in (A) the rhesus m onkey, and (B) human
77
the maxilla, is continuously repositioned in a forward and downward direction as actual new growth proceeds posteriorly; the mandibular ; condyle represents an important growth site.
ENLOW [1966a, 1970, 1982] com pared and contrasted the remodeling changes that occur during the growth o f the human and rhesus monkey faces. The detailed distributions o f resorptive and depositionary surfaces were determined and mapped in the diagram (Fig. 1). His findings are as follows:
(1) The general plan o f facial growth is similar in both species.
(2) The m ajor differences exist in the m uzzle area. In the m onkey the whole anterior surface o f the m axilla is depositionary, w hile in the human the forw ard part o f the m axillary arch is resorptive. These differences, accord ing to Enlow, provide the basis for the reduced degree o f prognathism which is characteristic o f the human skull.
(3) The anterior part o f the mandible also demonstrates differences. In the rhesus monkey, the entire labial side o f the mandible is depositionary, while in the human mandible the alveolar portion located between the mental foramina is resorptive. This could be one o f the rea sons for chin formation in the human mandible. The opposite (lingual) side o f the human mandible is entirely deposi tory, while in the rhesus monkey it is a combination o f resorption and deposi tion. This combined process produces the “simian s h e lf’.
(4) R em odeling differences in the zygom atic area also exist, although the general pattern, i.e., resorption at the anterior surfaces, rem ains the same for both species.
Facial growth in the
australopithecines
Brom age’s . study [BROM AGE 1985, 1989, 1992] is an “Enlowian” type o f study; his interpretations are based on concepts and principles o f craniofacial growth and rem odeling taken from En- low’s studies o f modern humans [ENLOW
1966b, En l o w & Ba n g 1965, En l o w &
HARRIS 1964], and from a comparative
study o f Homo sapiens and Macaca
mulatto [En l o w 1966a, En l o w 1982, Du t e r l o o & En l o w 1970]. ,
Bromage used bone surface replicas to study bone formation and resorption with the scanning electron microscope. The periosteal surfaces o f immature early hominid specimens were analyzed. The australopithecine specimens were di vided into two groups [BROM AGE 1989]: (1) Australopithecus, in clud in g^, africa-
nus and A. afarensis (10 specimens), and (2 ) Paranthropus, including P. robustus and P. boisei (15 specimens). The age range for the first group o f the early hominids (“gracile” australopithecines) was from 3.3 to 11.3 years, according to Brom age’s estimation based on pongid dental development [BROM AGE 1987, Br o m a g e & De a n 1985], or from 5.0 to
15.6 years based on human-like dental ages at death taken by Bromage from
Ma n n [1975] and Sk in n e r & Sp e r b e r
[1 9 8 2 ], The youngest specimen o f this group is one numbered LH 2 (mandible) with complete deciduous dentition and first perm anent m olars. The oldest specimen, STS 52 (maxilla and mandi ble), has all its permanent dentition with
M 3 ju st erupting. The age range for the second group o f early hominids (“robust” au stralopithecines) is from 1.0 to 11.3
78 K atarzyna A. K aszycka
[BROM AGE 1987], or from 2.0 to 15.4 years using the more conventional method o f Ma n n [1975]. The specim en num
bered SK 438 (m andibular fragm ent) w ith dm 2 w as the youngest specim en, w hile SK 52 (m axilla) w as the oldest specim en - w ith com plete perm anent dentition — considered.
As can be seen in Enlow ’s work, and also according to Bromage, the bones o f the face grow and change shape through a combination o f two processes: deposi tion and resorption. These are said to be the key mechanisms accounting for the
different craniofacial forms in each species. Australopithecus africanus (the Taung child) was shown by BROMAGE
[1985, 1989, 1992] to have an ape-like pattern o f facial growth and remodeling, while the facial growth o f the “robust” australopithecines (in Brom age’s papers
Paranthropus) resembles that o f humans.
The different degrees o f prognathism between these early hominids can be explained on the basis o f different re modeling patterns. M aps o f distribution o f bone deposits and locations o f bone resorption are presented in Fig. 2.
B
Fig. 2. Frontal and lateral view s o f (A) A. africanus, and (B) A. robustus facial rem odeling. Light represents areas o f deposition, and dark represents areas o f resorption. A fter BROMAGE [1989]
79
As can be seen from Figure 2, Austra
lopithecus africanus’ facial growth is. char
acterized by bone deposition on the whole of the anteriorly-facing aspect o f the face. The “robust” australopithecines differ from the former by having resorptive areas on the anterior wall o f the temporal fossa, the “nasoalveolar clivus”, and the anterolateral comers o f the mandibular corpus. The two latter features are said to correspond to remodeling patterns that are characteristic o f modem humans [BROMAGE 1989],
Downward facial growth in the “robust” australopithecines is opposed to downward and forward facial growth in A. africanus.
Remodeling fields have four main at tributes, according to BROMAGE [1985]: size, shape, placement, and rate o f activity. Bromage suggests that the first three are the same for the Taung baby and the chimpan zee. Differences in the last one, i.e., the rate of activity o f remodeling fields, show that the Taung baby is not an ape.
Discussion
As was mentioned before, Enlow ’s contribution to the study o f facial growth and remodeling was making generaliza tions about these processes. His studies were based on looking at the surface o f the bone to establish whether it was re sorptive or depository. According to Enlow [e.g. ENLOW & B A N G 1965], not much growth occurred in the sutures, and therefore practically all the growth was the result o f remodeling (deposition and resorption). A fter SCOTT [1953] put for ward the hypothesis that the maxilla grew in length by apposition o f its ante rior^, surface, Enlow [e.g. ENLOW 1966b] was"the first to report that the surface o f the human maxilla was not depository but resorptive.
Another series o f major contributions to our understanding o f facial growth and remodeling was made by BJORK and
SKIELLER [1972, 1976], Their longitudinal studies o f a group o f children, using metal lic implants as reference points, focused on individuals, and not on averages. Their findings were that while remodeling played a role in craniofacial growth, sutures were important too. The midface increases in height (vertically), depth (anteriorly) and width. An increase in maxillaiy height takes place by sutural growth and by ap position. Growth in width is not only a result o f remodeling processes but occurs by growth o f the median suture as well. In addition, growth in length o f the maxilla occurs suturally (toward the palatine bone) as well as appositionally in the maxillary tuberosities.
B jo r k ’s n ex t fin d in g [BJORK & SKIELLER 1976] w as that the anterior sur face o f the m axilla w as neither depository nor resorptive, but sagittally stable (from the age o f about 4 to 21). A lso, the anterior surface (contour) o f zygom atic process appears to be stable, in contrast to ENLOW and BANG’S [1965] classical v iew that the anterior surface o f that process is resorptive and the posterior surface is depository. E nlow also postulated that deposition took place on the posterior maxillary surface, Bjork argues, however, that this is not the case because sutures are able to grow.
There are other problems with the point o f view o f Enlow and Bromage. Firstly, these authors establish which portion o f a bone is depository and which is resorptive at the point o f death o f an individual. This does not allow a knowledge o f what hap pened during the lifetime o f the individual. Everything, which can be observed, would have happened only recently. The next problem arises from looking at dead bones
80 Katarzyna A. K aszycka to determine how much, for how long and
how long ago growth occurred. These ¡questions simply cannot be answered from observations o f the bones. Yet another problem is that there is age variation in growth patterns. In order to account for this, growth series were studied, but then one general • pattern o f facial remodeling was presented as if there was no variation, and as if areas o f deposition and resorption were immutable for the entire life o f an individual. Finally, there is the problem o f individual variation. Bjork’s studies
[BJORK & SKIELLER 1972, 1976] clearly reveal that not all individuals have the same growth and remodeling patterns.
Bromage has said that the face becomes more prominent during growth if deposi tion o f bone on the front surfaces is com bined with resorption o f the back. There fore, according to him [ Br o m a g e 1992],
the Taung face, for example, is typical o f a monkey (or ape) because it does not have resorption areas as seen in modem humans. However, as BJORK and SKIELLER [1976] noticed, maxillary projection in the face (alveolar prognathism) depends not only on its growth in length, but may also be a re sult o f displacement and rotation o f the maxilla. In addition, there is little evidence that surface deposition could account for downward and forward midfacial growth in humans during the first seven years o f life
[CARLSON : 1985]. It has, however, been noticed that in prognathic apes after a cer tain age, all maxillary growth in length is by surface deposition. In the phylogenetic development o f primates, prognathism decreases as a result o f shortening of. the jaw s [BJORK 1951].
Among the australopithecines there are d ifferences in the degree o f facial pro gnathism . T he A. africanus face is pro gnathic, w hile the face o f the “robust”
specimens is considered to be more ortho- gnathic,'and therefore more like the Homo face. However, as RAK [1983] has pointed out, similar degrees o f prognathism in
Homo and A. boisei reflect two different
configurations (Fig. 3). In modem Homo, it
Fig. 3. A comparison between (A) a prognathic chimpanzee, and two orthognathic primates - (B) “robust” australopithe-
81
is not a result o f the retraction o f the palate as seen in the “robust” australopithecines, but a result o f reduction in the length o f the palate. In fact, when the length o f the palate is expressed as a percentage o f independent measurements, the index value in A. boisei does not differ from that o f a chimpanzee. In contrast, A. ajriccmus has strong facial prognathism - the index o f prognathism can be as high as 137% [WOLPOFF 1975],
The upper jaw and therefore the palate, project far anteriorly. The medial part o f the midface is located more anteriorly than its lateral (peripheral) portion. Although all the australopithecine faces are prognathic, this feature in the “robust” forms is not as pronounced. The most orthognathic face is that o f A. boisei, where the index o f pro gnathism can be as low as 113%.
Fig. 4. Lateral view o f the face o f (A) A. africanus, and (B) A. boisei. A fter R a k [1983]
, Generally, the medial part o f the face and the dental arcade are located more posteriorly in the “robust” specimens, while the lateral part o f the face is lo cated more anteriorly. W ith such a posi tion o f the peripheral face, i.e., the zy gomatic bones (see Fig. 4), the medial part becomes concave and the nose, in lateral view, is hidden behind it. This configuration is also a reason why the face o f the “robust” australopithecines seems to be more orthognathic.
* * *
To sum up, one can say that although Bromage is probably right in what he observes, there are certain problems, as presented in this paper, with that kind o f analysis. W hile bones reflect remodeling processes, the whole o f the growth his tory o f bone is not reflected in the tissue itself. Detailed sequences o f growth changes, therefore, cannot all be recon structed.
One additional point can be made here. Even if certain early hominids ex hibit remodeling patterns more consistent with macaques, and even if Enlow and Bromage are correct in claiming that this leads to, or is consistent with, greater maxillary prognathism, this means noth ing as far as human lineage and adapta tion/evolution are concerned. It may simply mean that in order to have pro gnathism one must have more bone deposition at the labial surface o f the maxillary complex. We already know that monkeys and apes tend to be more prognathic than modern humans, so by definition they must exhibit greater bone deposition along the surface o f the maxil lary complex, if we assume Enlow to be correct. One test o f this, which would still not be definitive with respect to
82 Katarzyna A. K aszycka questions o f lineage, would be to look at
rem odeling patterns in prognathous vs. ortoganthous primates. Also, it would be interesting to examine the remodeling patterns in different humans with age: prognathous vs. orthognathous.
References
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83
Streszczenie
N a w zrastanie szkieletu tw arzy w pływ a wiele czynników . W fazie okolourodzeniow ej głów nym czynnikiem je s t pow iększanie się podstaw y czaszki, w tym ścian i przegrody ja m y nosow ej. N ieco później, rozrost tw arzy odbyw a się dzięki procesom rem odelow ania (apozycji i resorpcji), w zrastania śródszw ow ego i w obrębie chrząstkozrostów , rozw o jow i zębów i w yrostka zębodołow ego szczęki. Od 7 roku życia n a rozrost tw arzy zasadniczo oddziałuje ju ż tylko proces dyslokacji ku dołowi i ku przodow i oraz rem odelow anie odokostnow e. Podstaw ow a koncepcja w zrostu i rem odelow ania szkieletu kraniofacjalnego liczy ponad 2 0 0 lat, a rozwój badań nad tą problem atyką nastąpił w naszym stuleciu dzięki pracom m .in. Brasha, P ritcharda, Scotta, M ossa, E nlow a i BjOrka.
C elem przedstaw ianej pracy je s t ocena dotychczasow ych badań nad zagadnieniem w zrostu i rem odelow ania szkieletu tw arzy u naszych przodków - australopiteków i zaprezentow anie niektórych problem ów , które pow stają przy tego rodzaju analizach.
W kład Enlow a do badań nad w zrastaniem szkieletu tw arzy (lata 60. i 70.) polegał na stw orzeniu uogólnień doty czących zarów no człow ieka ja k i m ałp. Jak w ynika z je g o prac, kości tw arzy rosną i zm ieniają kształt poprzez kom bi nację dwu procesów : apozycji i resorpcji. Jest to m echanizm w arunkujący m iędzygatunkow e zróżnicow anie kształtu twarzy. En lo w [1966a, 1970, 1982] porów nał i zestaw ił przem iany w ynikające z rem odelow ania, które pojaw iają się podczas wzrostu tw arzy człow ieka i m ałpy rezusa. O znaczył obszary resorpcyjne i apozycyjne (przedstaw ione na rys. 1). W nioski Enlow a są następujące. (1) O gólny plan w zrostu tw arzy u obu gatunków je s t podobny. (2) Podstaw o w e różnice dotyczą okolicy pyska (szczęk). U m ałp cała przednia pow ierzchnia szczęki je s t m iejscem apozycji, pod czas gdy u człow ieka w przedniej części łuku zębodołow ego szczęki zachodzi resorpcja. (3) U rezusa cała policzkow a strona żuchw y je s t m iejscem apozycji, podczas gdy w przedniej części ludzkiej żuchw y, pom iędzy otw oram i bródko- w ymi, kość ulega resorpcji. Przeciw na - językow a - strona żuchw y u człow ieka całkow icie podlega apozycji, u m ałp natom iast je s t częściow o resorpcyjna, a częściow o apozycyjna, co pow oduje utw orzenie tzw. „m ałpiej półki” .
W latach 80. T. Brom age, w zorując się na pracach Enlow a, przeprow adził bad an ia nad w zrostem i rem odelow a- niem kości tw arzy u w czesnych hom inidów . Brom age użył odcisków pow ierzchni kości tw arzoczaszek do badań form acji i resorpcji kości, korzystając z elektronow ego m ikroskopu skaningow ego. W ydzielił dw ie g rupy wczesnych hom inidów - „m asyw ne” i „gracylne” australopiteki. U A. africanusBr o m a g e [1985, 1989, 1992] stw ierdził m ałpi typ rozrostu i rem odelow ania kości twarzy, podczas gdy u „m asyw nych” australopiteków - ludzki. Z różnych w zorów rem odelow ania w ynika zróżnicow any stopień prognatyzm u u w spom nianych grup hom inidów . M apy rozkładu apo zycji i resorpcji na kościach tw arzy u australopiteków prezentuje rys. 2.
Prace i uogólnienia Enlow a nie w yjaśniły je d n a k w szystkiego. BjOrk i Sk ie lle r [1972, 1976], w w yniku badań longitudinalnych grupy dzieci, zwrócili uw agę na dodatkow y aspekt wzrostu i rem odelow ania kości tw arzy - znacze nie wzrostu śródszw ow ego. Stw ierdzili także, że przednia pow ierzchnia szczęki nie je s t ani apozycyjna, ani resorpcyj na, ale strzałkow o stabilna (w w ieku od 4 do 21 lat). S ą też inne problem y w ynikające z przyjęcia punktu w idzenia Enlowa i B rom age’a. Po pierw sze, autorzy ci posługiw ali się kośćm i zm arłych osobników , z których m ożna odczytać procesy zachodzące krótko przed śm iercią. N a podstaw ie kości nie m ożna ustalić w ja k ie j fazie życia i ja k długo trw ały procesy rem odelacyjne. N astępnym problem em je s t fakt, że istnieje ontogenetyczna zm ienność w zorca w zra stania tw arzy - nie m ożna w zw iązku z tym przedstaw iać je d n eg o ogólnego w zorca rem odelow ania kości tw arzy oraz obszarów apozycji i resorpcji. Jest też problem indyw idualnej zm ienności - prace BjOrk a i Sk ie lle r a [1972, 1976] dobitnie dow odzą, że nie w szyscy osobnicy m ają ten sam w zór wzrostu i rem odelow ania.
A ustralopiteki m ają zróżnicow any stopień prognatyzm u tw arzow ego. Tw arz A. africanus je s t prognatyczna, pod czas gdy tw arze „m asyw nych” australopiteków są bardziej ortognatyczne i w zw iązku z tym bardziej przypom inają twarze rodzaju Homo. Tym czasem , n a co w skazuje R a k [1983], podobny stopień prognatyzm u Homo i A. boisei
w ynika z dwu różnych konfiguracji (rys. 3) - u w spółczesnego człow ieka je s t w ynikiem redukcji długości podniebie nia, a u „m asyw nych” australopiteków , rezultatem cofnięcia podniebienia. R ów nież bardziej przednie położenie bocznych części tw arzy (kości jarzm ow ych) u A. boisei potęguje w rażenie ortognatyczności (rys. 4).
C hociaż Brom age m a praw dopodobnie rację w tym, co pisze o rem odelow aniu, pozostają pew ne problem y. Kość odzw ierciedla procesy rem odelow ania, je d n a k nie cała historia w zrostu kości je s t utrw alona w tkance. W zw iązku z tym , nie w szystkie sekw encje zm ian w zrastania m ożna szczegółow o zrekonstruow ać. Poza tym , naw et jeśli niektóre wczesne hom inidy m ają w zorce rem odelow ania zgodne z m ałpam i (co zdaniem Enlow a prow adzi do w iększego prognatyzm u), nie m a to znaczenia dla rekonstrukcji zm ian ew olucyjnych w ludzkiej linii.
