Punctuated equilibrium, as proposed by Gould and Eldredge in 1972, is a model of species evolution to counter the explanation of evolutionary change through phyletic gradualism, or the slow accumulation of novel adaptations over evolutionary time, culminating in the appearance of new species (speciation). Gould and Eldredge argue that evidence of gradualism would require fossil evidence of intermediary stages in a species’ evolution, with different specimen of an evolving species exhibiting ancestral traits and an increasing number of derived traits leading towards the present form. Paleontologists, however, had long noted that the fossil record was marked by long periods of stasis (on a geological scale) with the sudden appearance of new species, though these gaps were interpreted as simply missing information (Gould and Eldredge 1993). While fossil preservation is notoriously problematic (Wood et al. 1992), Gould and Eldredge (1972, 1993) proposed that the gaps in the record arise not from artifacts of preservation, but from an alternate mechanism of evolution by which small populations become isolated from parent stocks and rapidly accumulate novel adaptations leading to speciation. Thus, parent stocks remain in the geological record parallel with newly adapted species. Successfully adapted species would thereafter remain in the fossil record as well. Thus, most species in the fossil record appear in a state of equilibrium across geological time (i.e. not exhibiting any significant morphological changes), after a seemingly spontaneous appearance. The theory rests on two central tenets: (1) that established species remain static, and (2) that macroevolutionary processes, rather than resulting from continuous processes across time within individual lineages, occur at discreet, high-level intervals as stable species achieve different evolutionary success (Gould and Eldredge, 1993).
Fossil evidence for punctuated gradualism in human evolution is given by the apparent bursts in the fossil record of novel and diverse African antelopes and primates about 5 million and 2.5 million years ago. These speciation events correlate with dramatic climactic and environmental changes estimated for those times, which could have led to widespread geographic alterations and population isolations, supporting conditions for large scale punctuated events (Klein 1989). At the same time, certain aspects of hominin evolution, such as linear increases cranial capacity and decreases in dentition (from Australopithecus to anatomically modern humans, with the exception of enlarged dentition in Paranthrapus sp.) support phyletic gradualism as a mechanism for evolutionary change. Here though, Gould and Eldredge (1993) argue that, brain growth in Homo actually occurred early in its lineage (punctuation), and that the intervening time (200, 000 years) has been too short to compare with static traits on larger geological scales. They argue that, if Homo sapiens had emerged 2 million years earlier, we would see a clear pattern of punctuation followed by long stasis consistent with the theory. The same argument can be turned around, however, and we might ask if 2 million years from now, human brains might be incrementally bigger. In essence, fossil evidence for either gradualism or punctuated equilibrium are essentially interpretations of scant evidence, with punctuated equilibrium derived solely from the available evidence, and gradualism derived from the veritable absence of evidence. While punctuated equilibrium may be the more pragmatic in terms of examining fossil evidence, it is plainly short-sighted to ignore vast swaths of missing evidence given the realities of fossil preservation (Wood et al. 1992). Just as Gould and Eldredge criticize assumptions made about the fossil record to support gradualism the criticisms could be levied against interpreting the record to support punctuated equilibrium. Writes Klein (1989), “it is the nature of the record, not logic or experiment, that most strongly supports punctuated equilibrium.”