Children of the Ice Age: A Review
Steven M. Stanley Explores the Evolution of Homo Sapiens in Africa
Dec 23, 2008
Children of the Ice Age: How a Global Catastrophe Allowed Humans to Evolve (Published by Harmony Books, New York, Copyright 1996 by Steven M. Stanley, ISBN 0-7167-3198-3) takes an approach to the evolution of the genus Homo that is both novel and controversial.
Taking a position held by only a minority of experts in the field of evolutionary experts, the punctuational model, Stanley hypothesizes that two major factors combined to force a rapid evolution of the Australopithecus africanus from a semi-arboreal species into the terrestrially bound Homo rudolfensis, the oldest known member of the Homo genus.
Punctuated Equilibrium
In the early 1970's, two paleontologists published a paper that put forth an idea that is still debated by evolutionary experts in the Twenty-First Century. Stephen Jay Gould and Niles Eldredge built upon earlier work by Ernst Mayr, suggesting that most sexually reproducing species would experience prolonged periods of stagnation, or stasis, and would evolve rapidly into a different species due to environmental pressure. This concept is known as punctuated equilibrium and it stands in stark contrast to the more widely accepted concept of gradualism that suggests evolution is a constant process that works in a slow, smooth progression over long geological timescales.
Richard Dawkins spends an entire chapter on the concept in his book, The Blind Watchmaker. Dawkins points out that unlike some other more radical evolutionary models, punctuated equilibrium is still a form of gradualism, just with varying speed thrown into the mix to explain the seemingly unchanging nature of many species over long timescales.
Timescale is Everything
In evolutionary parlance, the word "rapid" means something very different than it does in conversation. A "rapid" event at the geological timescale is an event that occurs in something like 20,000 to 50,000 years. In the case of a primate with an average generation of 13 to 20 years, a "rapid" event represents something that occurs over 1,000 or more generations. Even a punctuational model posits that speciation occurs over thousands or even tens of thousands of generations.
Rapid Maturity Vs. Prolonged Infancy
Recent reexamination of the fossils of Australopithecus africanus have led to a view that the primate was more of an arboreal species than scientists had first believed. Comparative studies of the anatomy of the brain sizes and growth rates of living primates suggest that Australopithecus africanus would have followed a developmental model similar to living primates like the chimpanzee, as opposed to the Homo sapiens model. In light of a tree climbing existence, it makes sense that unlike Homo infants, Australopithecus infants would mature rapidly so as to be able to cling to their mothers as they sought safety, shelter and sustenance in the trees.
In his view, Stanley feels that the mechanism that allows Homo sapiens to develop the extraordinary large brain that is the definitive feature of the species would only be possible in a species that developed what he calls the "terrestrial imperative" that brought them out of the trees, thus freeing their hands to care for the slowest maturing infants in the mammal kingdom.
It is during this slow maturation that the infants of the Homo sapiens (and likely other Homo species like rudolfensis and erectus) develop the extraordinary brain size while other primates' brain development stalls within a short time of leaving the womb.
The Ice Age Connection
Stanley suggests that the Ice Age that began 2.5 million years ago was responsible for changing weather patterns, thinning the population of trees that had sheltered Australopithecus africanus for millions of years, thus pushing at least part of the population from the safety and stability they had previously enjoyed. His hypothesis is that some part of the displaced population managed to survive and through the mechanism of suddenly inflated pressure in the form of increased vulnerability to predators and decreased availability of resources were forced through the eye of natural selection to come out the other side of a the crisis as Homo rudolfensis.
Questionable Conclusions
The evidence of the fossil record does indeed support Stanley's hypothesis, but with a caveat. The same fossil record also supports a more gradualist view of the transition from Australopithecus to Homo, depending upon how the dating of the fossils is viewed. It is indeed a possibility that the speciation from Australopithecus africanus to Homo rudolfensis occurred rapidly, at least on a geological timescale, but equally possible is the possibility that the rudolfensis evolved much more slowly and the necessary fossil evidence simply hasn't yet been found. As Stanley himself points out, "The absence of recognized intermediate bones does not mean that there never were any, but it does mean that any intermediate forms probably existed during only a brief interval of geologic time and in small numbers within a restricted geographic area."
Stanley writes with the courage of his convictions, but many of his colleagues in the field of paleontology and evolutionary biology do not agree with his ideas. While the book is interesting, challenging and intriguing, the lay reader would be wise to keep that in mind while reading Children of the Ice Age.
 |
