After reading this scholarly research on human evolution, I came to believe the authors’ hypothesis: Environmental change caused human change. See if you agree:
Evolution on a Restless Planet: Were Environmental Variability and Environmental Change Major Drivers of Human Evolution?
Peter J. Richerson, Robert L. Bettinger, and Robert Boyd
Two kinds of factors set the tempo and direction of organic and cultural evolution, those external to biotic evolutionary process, such as changes in the earth’s physical and chemical environments, and those internal to it, such as the time required for chance factors to lead lineages across adaptive valleys to a new niche space (Valentine 1985). The relative importance of these two sorts of processes is widely debated. Valentine (1973) argued that marine invertebrate diversity patterns responded to seafloor spreading as this process generated more or less niche space.
He suggested that natural selection is a powerful force and that earth’s biota are in near equilibrium with the niches available on the geological time scale. Walker and Valentine (1984) modeled the evolution of species assuming a logistic speciation rate limited by internal factors and a diversity-independent death rate caused by ongoing environmental change. Fitting this model to the observed evolution of shelled marine invertebrates suggests that the lag between extinctions and the evolution of new species leaves perhaps 30% of ecological niches unfilled. In this model, the biota lag environmental change by perhaps a few million years. However, as Valentine (1985) notes, if adaptive landscapes have whole suites of niches protected by deep maladaptive valleys, the waiting time for some pioneering species to cross the divide may be very long, generating the rare events that set new body plans and generate major adaptive radiations.
Eldredge and Gould (1972) and Gould (2002) championed the idea that internal processes such as genetic and developmental constraints, coupled with the complexity of the adaptive landscape, resulted in a highly historically contingent evolutionary process. On Gould’s account, most of the history of life had to do not with a relatively close tracking of a changing environment but with the halting evolutionary exploration a deeply fissured niche space, mostly by rapid bursts of evolution as a fissure was crossed, followed by long periods of stasis. Note that if the adaptive landscape is deeply fissured for any reason, evolution may take on a progressive character (Stewart 1997).
Imagine that the original simple forms of life began at the foot of a large mountain range of Handbook of Evolution, Vol. 2: The Evolution of Living Systems (Including Hominids) Edited by Franz M. Wuketits and Francisco J. Ayala Copyright © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30838-5 1268vch07.pmd 223 28.02.2005, 08:49 224 7 Evolution on a Restless Planet adaptive topography. Potentially, the whole history of life has been a halting and episodic process of moving first onto to local optima of the near foothills and subsequently filtering across adaptive chasms to higher peaks deeper into the complex topography. Perhaps we have not yet come anywhere near to reaching the highest peaks in the topography on earth, even after perhaps 3.5 billion years of life on our planet.
More complex scenarios are possible. Vermeij (1987) argues that much evolution is driven by the top-down biotic process of predator–prey coevolution, but that the degree of escalation of predator attack strategies and prey defenses is limited by external factors, especially those that control productivity. Vermeij does not commit himself on the issue of how closely the predator–prey escalation process tracks external environmental change. Discussions of the large-scale patterns of evolution typically assume that the overall environmental framework of the earth is static and that changes in features like the size of brains represent a series of progressive changes from simpler to more complex organisms. Billions of years have transpired since the origins of life on earth, and about 540 million years have transpired between the abundant fossil animals of the Cambrian and the evolution of humans.
If the earth’s environment has been essentially constant since either the origin of life or even the beginning of the Cambrian, the growth of organic complexity by natural selection and other evolutionary processes such as species selections would have to have been so limited by internal processes as to be exceedingly slow. On the other hand, since the discovery of seafloor spreading 40 years ago, the role of external factors in macroevolutionary processes has become much clearer (Valentine 1973). Today we have a reasonably clear picture of past continental configurations and past biogeochemistry, especially the chemistry of the oceans and atmosphere (Holland 1984; Scotese 2003). Past environments were very different from those of today.
For example, during the late Paleozoic, high oxygen concentrations in the atmosphere supported gigantic flying forms, including dragonflies a meter long and pterosaurs weighing perhaps 100 kg. (Graham et al. 1995; Dudley 2000). Everyone accepts, we suppose, that external processes are important regulators of the rate and direction of evolution in the very long run, and everyone accepts that evolution is not an instantaneous process. At the most extreme, life on earth could not begin to evolve until the earth formed, and new species do not evolve in one generation. But the gap is very wide between those that argue that most of the history of life, at least since the late Precambrian, is mainly regulated by internal processes and those that think that, for the most part, the earth’s biota are in near equilibrium with existing environmental conditions, aside from a few empty niches resulting from relatively short-term constraints operating on evolutionary processes. (more)