Hypothesis and explanation in human evolution

hypothesis+explainationHumanEvol.pdf

Hypothesis and explanation in human evolution by Donald L. McEachron in J. Social Biol. Struct. (1984)

Abstract: Many attempts to reconstruct the evolutionary history of Homo sapiens have been hampered by a failure to incorporate evolutionary limitations and adhere to proper scientific methodology. Models of human evolution should be strictly derived from general evolutionary hypotheses which have been tested and to some extent verified with living forms. To preserve the testability of such models, researchers should begin by determining the environmental parameters faced by the hominids in the past and then design alternative evolutionary pathways in an attempt to retrodict the characteristics of modern humans. Differences between retrodictions would then provide tests of alternative explanations. Traits of modern humans should not be used, other than determining taxonomic relationships with fossil species, in creating a model, since the model would then become a tautological explanation and not a scientific hypothesis.

 …

 Models of evolutionary processes are limited only by the rules of logic and the researcher’s imagination. Biological evolution itself, however, is subject to numerous constraints, including but not limited to :

(1) the amount, structure, and variability of the available genetic material;
(2) the chronology of the factors mentioned under (1), i.e. what genetic material was available at what times in relation to the selection pressures exerted at those times;
(3) stochastic effects, such as mutation and genetic drift;
(4) the amount and directions of the selection pressures;
(5) the timing of these pressures-when and for how long they operated;
(6) the rate of reproduction.

…

The following steps should be taken when attempting to reconstruct the phylogenetic history of a species with living descendants. First, use fossils to establish the taxonomic status, physical characteristics and limitations in the ancestral species. This provides the range of conceivable models, both by comparison with existant members of related species, and by delimiting the kinds of behavior which would have been possible physically. Next, establish the ecological conditions prevailing at the time and locations in question. Finally, incorporate these parameters in designing several different evolutionary models using the most general (i.e. most widely applicable to modem species) models possible. One should be careful that these pathways be logical and consistent derivations from the general models.

…

Modern species must be used to establish the taxonomic relationships with probable ancestors, but this must be the limit to which they are used to begin construction of an evolutionary model. There are several reasons for this. First, and most importantly, by using the attributes of the modern species to create the evolutionary model, the ability to test the model is lost. The adaptations which should have been predicted by the hypothesis are the very ones used to create it. At that point, the model ceases to be a scientific hypothesis and becomes a tautological explanation. Second, there is a tendency for researchers to develop a single explanation for the evolution of numerous traits which may or may not have evolved simultaneously, thus ignoring the possibility of mosaic evolution. Finally, extrapolating present adaptations onto past species indicates a failure to recognize evolution as a process of change from the ancestral to the modern forms. If the ancestral species had possessed behavioral and morphological characteristics identical to those of the living forms, there would have been no change between then and now (and thus, no evolution).

…

As a further example, consider how these researchers examine the question of the early hominids’ mating system. Morris (1967), Wilson (1978), and Lovejoy (1981) apparently consider modern humans to be monogamous-an assumption, for that matter, which itself is highly debatable (Bermant & Davidson, 1974; Martin & May, 1981). In fact, polygyny is quite common in human societies (Davenport, 1976), and in a survey of one hundred and eighty-five human societies, one hundred and fifty-four were found to be basically polygynous (Ford & Beach, 1951). Even if modern Homo sapiens were monogamous, however, that is not sufficient reason for assuming that the early hominids practiced monogamy; nor does that assumption establish the exact time when monogamy developed, since the pre-hominids may or may not have been monogamous themselves. By deciding that the hominids were monogamous before creating their evolutionary models, Morris, Wilson, and Lovejoy are forced to design their models with the following question in mind: ‘Why were the hominids monogamous, and in what way did human characteristics contribute to this mating system’? Posing the question in this manner severely limits the kind of models that can be offered as answers - all must be based on monogamy. The question should be stated: ‘Given the ecological constraints, habitat, and the modern primates’ range of genetic and social variation, what mating systems could the early hominids have had’? Posed in this way, no possibilities are excluded a priori and various hypotheses can be tested against each other.