Heterochrony in human evolution a compilation

Repetitio est mater studiorum or as Schwartz (1999 p ix) commented:''... it would be foolhardy to cling unreservedly to a particular set of models and hypotheses without at least occasionally questioning their very bases.'' In Sudden Origins, Schwartz reconsidered and discussed inter alia the different interpretation of the Taung child. While Raymond Dart was influenced by ideas of neoteny and concluded that his fossil occupied a missing link-position, Sir Arthur Keith drew a totally different conclusion and defined the Taung individual as an ape. Ironically, Keith's results were also based on a developmental and even a neotenic chain of ideas (O Figure 3.3).

Furthermore, Schwartz (1999) critically reviewed the suggestions of the Australian anthropologist Andrew Arthur Abbie, who equated neoteny and paedomorphism, and portrayed H. sapiens due to its long legs and a highly arched nose as gerontomorphic (=more differentiated from a fetal standard), although he generally described our species as paedomorphic (=little differences from a fetal standard). Abbie absorbed the heterochronic perspective but failed to sustain the argument that human morphological variability is not fixed to a limit and that any combination of features can be incorporated in the same individual.

Gould (1977, 1991 p 2) called attention to the difference between Haeckel's heterochrony, which describes the pathway of development in an organ relative to

D Figure 3.3

The Taung child (left), compared to the skulls of comparably aged gorilla (middle) and chimpanzee (right). Dart's desire was to show that the apes had begun to grow away from the juvenile state. The Taung specimen has no brow ridges, small jaws, and a very large brain (modified from Schwartz 1999, drawing by T. D. Smith, © J. H. Schwartz)

D Figure 3.3

The Taung child (left), compared to the skulls of comparably aged gorilla (middle) and chimpanzee (right). Dart's desire was to show that the apes had begun to grow away from the juvenile state. The Taung specimen has no brow ridges, small jaws, and a very large brain (modified from Schwartz 1999, drawing by T. D. Smith, © J. H. Schwartz)

the pathways of other organs in the same animal, and current usage, which defines it as "the course of a trait relative to the ontogeny of the same trait in an ancestor (or related form).'' Jena's most famous zoologist was not really interested in elucidating evolutionary mechanisms as a method to explain the patterns of character states. He was rather interested in reconstructing natural history by applying biogenetic laws. Sir Gavin de Beer (1930) opened up new vistas by modifying Haeckel's concept: any change in the timing of a character is compared to the same feature in an ancestor. This established the study of heterochrony within the modern synthesis (Gould 1991).

McNamara (2002 p 1) defined heterochrony essentially as "change to the timing and rate of development'' which produces two major effects: paedomor-phosis (if there is less growth during ontogeny, the descendant adult will resemble the juvenile condition of the ancestor) and peramorphosis (where the descendant undergoes greater development). Three different processes generate paedomor-phosis and peramorphosis, respectively: (1) progenesis (prematurely truncated duration of growth in the descendant), neoteny (a lesser growth rate in the descendant than in the ancestor), and postdisplacement (delayed onset of growth) and (2) hypermorphosis (extension of duration of growth in the descendant), acceleration (an increase of growth rate in the descendant), and predis-placement (earlier onset of growth in the descendant). The whole organism can only be affected by progenesis and hypermorphosis, while the remaining aspects affect certain traits of the organism. McNamara (2002) further stated that peramorphosis and paedomorphosis are the products of varied processes rather than being processes in themselves. Hypermorphosis (extensions of the end of growth) and progenesis (contractions of the end of growth) are considered within a global context, "based on changes to the time of onset of sexual maturity and cessation of somatic growth, with the two frequently coinciding'' (McNamara 2002 p 105).

Different views have emerged on how large a role neoteny plays. While Gould (1977), Anton and Leigh (1998), and Montagu (1989) supported true paedomorphosis (neoteny), Shea (1988, 1989, 1992), McKinney and McNamara (1991), and McNamara (1997) favored some kind of hypermorphosis in time. Bogin (1997) suggested a new developmental stage in between (no heterochrony); Alba (2002) on the other hand, characterized paedomorphosis and peramorpho-sis as interpretative, not descriptive, terms. He emphasized the importance of modifying conventional clock models based on meaningful variables. Alba (2002) called for a double standardization (initial and final developmental stages) of ontogenetic trajectories and suggests a "complete" model including absolute age (at homologous developmental stages), shape, size, and behavior. As he admitted, this is not an easy undertaking.

McKinney and McNamara (1991) have indicated that, during ontogeny, contractions and extensions can occur at transitions between particular life-history stages and that local growth fields can also be modified. The reality that heterochrony may operate at any time during ontogeny (from the point of fertilization until the cessation of growth) has been blurred by the comparison of the cessation of a descendant's growth with the ancestral condition (McNamara 1983; McKinney and McNamara 1991; Maier 1999). Mammals, for example, are characterized by embryonic and postembryonic (infantile, juvenile, adolescent, and adult) growth phases. In his thought experiments, McNamara (2002) (O Figure 3.4) showed how some authors have described paedomorphosis where it was nonexistent. Sequential hypermorphosis (defined by him in 1983 as terminal hypermorphosis), for instance, also effects the offset of growth and therefore implicates greater development within each growth stage based on "scaling effects and probable increase in size, either of the part, or of the whole'' (2002 p 108). Hence, the descendant is compared to the ancestor in a relatively more juvenile state. Paedomorphosis, however, should be defined by adult characteristics.

What kind of consequences does sequential heterochrony have for our understanding of human evolution? The list of "general neoteny—supporters'' is long (I have already mentioned Bolk and Gould, but see also Montagu 1989; Wolpert 1991). Montagu's (1989) favorite ancestor is a form very like the pygmy chimpanzee in order to compensate the incorrect assumption of Bolk that the hominid line has passed through an ape-like stage such as the gorilla-orang type. Provided that human is "essentially neotenous'', hominid evolution would have produced an organism characterized by having a smaller body size, brain, and limbs (Shea 1989; McKinney 1998). McNamara's (2002 p 115) summary described hominid evolution as involving "a mixture of peramorphic and paedomorphic

O Figure 3.4

McNamara's (2002, © Johns Hopkins University Press) representation of the ontogeny of ancestral and descendant species that pass through distinct growth stages. (A-C at times T or T'). (a) A greater sequential hypermorphosis results in failure of the descendant to pass into stage C before cessation of growth at time T3. (b) Sequential hypermorphosis (Seq Hyp) at A and B results in the period spent in stage C being much shorter in the descendant. (c) Terminal hypermorphosis (Term Hyp) and sequential hypermorphosis result in all descendant growth stages being extended. (d) A shows an extreme sequential hypermorphosis and results in stage B being omitted in the descendant

traits.'' Forty years back, Starck and Kummer (1962) had come to a similar conclusion via a totally different method.

Shea (2002, p 95) applied his working hypothesis—size diversification occurs via predominant rate changes (rate hypomorphosis and hypermorphosis) rather than by time changes (time hypomorphosis and hypermorphosis)—to human evolution, and concluded that: "Certainly, at present no emergent data support any genetic or developmental basis for a global or generalized neoteny.'' He accused Godfrey and Sutherland (1996) of revitalizing the idea that hominid evolution has predominantly involved a generalized neotenic transformation. This accusation is problematic in that Godfrey and Sutherland (1996 p 40) freely admitted that "Gould's neoteny hypothesis for human evolution has been criticized on a number of grounds. The thesis of this paper is that both Gould and his critics overstated their cases: Nothing that we have said should be construed as a defense of Gould's hypothesis. Our intention is simply to reopen the dialogue, and to propose a framework for more precise testing of heterochronic hypotheses'' (My emphasis).

In contrast, Hall (2002 p 13) attacked the one-sided concentration on hetero-chrony as the only way to consider development and evolution by claiming that it "seemed that everyone could find evidence for heterochrony or at least justify use of the term to explain phenotypic changes in their favourite organism. Other mechanisms linking development and evolution were ignored or not sought.'' He emphasized the importance of heterotopy, the spatial pendant of heterochrony.

In summary, I give below some important terms and definitions (see also earlier) based on Minugh-Purvis and McNamaras' (2002) summary, to present a brief synopsis:

Acceleration

A heterochronic process that involves a faster rate of development in the descendant and also produces a peramorphic trait in the descendant

Deceleration

A slower rate of developmental events in the descendant. It produces a paedomorphic trait when expressed in the adult phenotype (syn. neoteny)

Hypermorphosis

Isomorphosis Neoteny

Developmental events show a delayed cessation (or offset) in the descendant. Expressed in the adult phenotype, peramorphic traits are produced. Global hypermorphosis can be caused by late sexual maturation (terminal hypermorphosis). But hypermorphosis can also be caused by a delayed cessation in local growth fields Peramorphosis followed by paedomorphosis (or vice versa). The descendant does not show any effective morphological changes. Unusual phenomenon of paedomorphosis

A slower rate of developmental events in the descendant, producing paedomorphic traits when expressed in the adult phenotype (syn. deceleration)

Paedomorphosis Peramorphosis

Progenesis

Sequential heterochrony Sequential hypermorphosis

The descendant adult retains subadult ancestral traits

The ancestral adult shows development of traits beyond the ''usual''

ancestral stage

Developmental events show an early cessation in the descendant. Expressed in the adult phenotype, peramorphosis is produced (syn. time hypomorphosis)

Contraction or prolongation in the descendant relative to the ancestor of life-history stages or ontogenetic growth Life-history stages or ontogenetic growth are prolonged in the descendant relative to the ancestor (syn. proportional growth proportion)

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