"Reply the tape a million times ... and I doubt that anything like Homo sapiens would ever evolve again" (Stephen Jay Gould from "Wonderful Life", 1989 p. 289, Harvard University Press.).
With his standard panache, the late Harvard paleontologist Stephen J. Gould argued strenuously that evolution had no inherent directionality. It was a cosmic crapshoot - in no way destined to produce anything complex, self-conscious or human. We are mere accidents; a "tiny twig on an improbable branch of a contingent limb on a fortunate tree" ("Wonderful Life" p. 291). Highly fortunate indeed! Eons ago, a dinosaur-dominated earth held little promise for mammalian ascendancy (let alone primates or humans). Our distant ancestors might have remained little more than scurrying nuisances nipping at the feet of giants if not for a most unlikely calamity - a massive meteor strike which swept away the dinos and forever altered the earth's bio-saga. Who would have guessed?
Evolution's capricious nature seemed to represent a severe stumbling block for the Abrahamaic religious traditions. In their narrative, humans represented the culmination of God's creative work - the very purpose for creation itself. But evolution is an awfully shoddy way of enacting a divine plan. Gould delighted in annoying the faithful by emphasizing this very point:
"Odd arrangements and funny solutions are the proof of evolution - paths that a sensible God would never tread but that a natural process, constrained by history, follows perforce" ("The Panda's Thumb", 1980, pp. 20-1).
Theologians, however, were quick to point out that the chance element in evolution was neither new nor necessarily contrary to the Judeo-Christian view of God. Human history was replete with chance; evolution only extended the theme. Moreover, chance allowed for freedom - a virtue high on God's agenda. However theologically sound these retorts may have been, their force was often lost on the average believer. The accidental nature of human existence provided just another reason to reject evolution altogether in order to preserve God's special concern for humanity.
Gould was a talented science writer, but he overplayed evolution's whimsy. Increasingly, science is showing that the evolutionary process has many built in constraints which limit its possibilities and bias its pathways. Take, for example, the ubiquitous phenomenon of convergence - the tendency for highly diverse species to independently evolve similar adaptive (analogous, not homologous) traits. Most of us are familiar with the saber-toothed tiger, the scourge of our hominin ancestors. Less familiar are a group of South American marsupials called the thylacosmilids who independently evolved similar protruding saber-teeth. Convergence can also be seen in a number of specifically human traits. For example, we share a mode of locomotion, bipedalism, with birds, kangaroos, and some dinos. The lateralized and convoluted structure of our brains can also be found in octopi, this despite the fact that vertebrates and cephalopods diverged from one another over 450,000 million years ago.
In his book "Life's Solution" (2003, Cambridge Press) Cambridge Palaeobiologist Simon Conway Morris documents scores of examples of convergent evolution from insect body designs to the social systems of dolphins and chimpanzees (both fission-fusion). The important lesson is that there are only a limited number of ways that evolution can solve the adaptive problems posed by the earth's ecosystems. Time and again, evolution stumbles upon the same general design features from which to fashion adaptive traits.
Now add to this the Baldwin effect - an idea originally proposed in 1896 wherein organisms are posited to actively shape their own selective forces. For example, suppose some fairly intelligent primates begin fashioning tools, giving them access to new resources and a competitive advantage over non-tool users. Any genetic predisposition facilitating tool use would also be positively selected.
A severe limitation on Baldwin effects has always been the unpredictability of genetic mutation. For any heritable genetic changes to occur (so the thinking has always been) our tool wielding primate would just have to wait around and hope for a lucky "tool use" mutation to pop up. But maybe not. Two recent books, Jablonka and Lamb's "Evolution in Four Dimensions" (2005 MIT press) and Kirschner and Gerhart's "The Plausibility of Life" (2005, Yale University Press) discuss connections between recent work in genetics and Baldwinian processes. What if the primate's tool use actually raised the probability that a tool-relevant genetic change would take place which could then be passed along to offspring?
Recent genetic research (in a field called epigenetics) shows that experiences occurring over one's lifetime can produce heritable genetic changes. For example, mice exposed to two weeks of environmental enrichment (more social interaction, activity, novel objects to explore) show evidence of enhanced memory function (not surprising). More surprising is that their offspring also show evidence of enhanced memory even though they were never exposed to environmental enrichment (Journal of Neuroscience, 29, p. 1496). Thus, the increased environmental stimulation created a genetic change in the parents that was then transmitted to offspring. This change appears to involved altered patterns of gene regulation (how genes are turned on and off during development). Similar effects have been noted in humans (see European Journal of Human Genetics, 14, p. 159).
Convergence, epigenetic inheritance, and Baldwin effects are only a few of the mechanisms serving as directional constraints on evolution's pathways. In his review of the various factors affecting the evolutionary process, anthropologist Melvin Konner concludes:
"There are no intrinsic driving factors in evolution, but there are intrinsic constraints and canalized paths along which either evolution or development may more easily proceed" ("The Evolution of Childhood," Harvard Press, 2010, p. 59, italics in original).
Of course, none of these constraining factors guarantee our arrival on the evolutionary stage. They do, however, raise the odds that in time a complex, rational, self-aware creature capable of entertaining both scientific and religious ideas might emerge.
The more we understand evolution, the less it seems like either the bogeyman creationists fear or the universal God-dissolving acid some atheists crave.