In summary, the history of evolutionary neuroscience features some serious missteps, such as the idea that brains evolved in a phylogenetic series and Ariens Kappers' law of neurobiotaxis, but it also reveals considerable progress. The scala naturae has ceased to guide the research of evolutionary neuroscientists and the idea of neurobiotaxis has quietly disappeared. The once stagnant field of brain allometry is showing signs of revival, largely because of new statistical techniques and a new emphasis on absolute brain size. The debate about concerted versus mosaic evolution persists, but directions for rapprochement are emerging. In general, the field has flirted with a broad variety of theoretical ideas and found some of them wanting and others promising. In terms of theory, the field is still quite young, but it is poised to mature now.
Predicting directions of growth for any science is problematic, but I believe that most future developments in evolutionary neuroscience will parallel developments in other, non-neural domains of evolutionary biology. After all, the history of evolutionary neuroscience is full of ideas that originated in non-neural areas of biology. For example, the methodology of phylogenetic reconstruction or cladistics (which I did not discuss in this article but have treated elsewhere; see Striedter, 2005) was originally developed by an entomologist (Hennig, 1950; see also Northcutt, 2001). Similarly, evolutionary developmental biology was burgeoning before it turned to brains (Hall, 1999). Therefore, I think it likely that the future of evolutionary neuroscience has already begun in some non-neural field. Maybe molecular genetics, with its new emphasis on evolutionary change (Dorus et al., 2004), will soon take center stage. Maybe the excitement about linking physiological allometries to metabolic parameters (West and Brown, 2005) will infect some mathematically inclined evolutionary neuroscientists. Or perhaps the next big thing in evolutionary neuroscience will be microevolutionary studies that integrate across the behavioral, physiological, and molecular levels (Lim et al., 2004). Maybe the future lies with computational studies that model in silico how changes in neuronal circuitry impact behavior (e.g., Treves, 2003). It is hoped that all of these new directions - and more - will bloom. If so, the field is headed for exciting times.
On the other hand, evolutionary neuroscientists are still struggling to make their findings relevant to other neuroscientists, other biologists, and other taxpayers (see Relevance of Understanding Brain Evolution). It may be interesting to contemplate the evolution of our brains, or even the brains of other animals, but can that knowledge be applied? Does understanding how or why a brain evolved help to decipher how that same brain works or, if it does not work, how it can be repaired? Are advances in evolutionary neuroscience likely to advance some general aspects of evolutionary theory? All of these questions remain underexplored (see Bullock, 1990).
Near the end of the nineteenth century, Jackson (1958) attempted to apply evolutionary ideas to clinical neurology, but his efforts failed. It has been pointed out that some species are far more capable than others at regenerating damaged brain regions (e.g., Kirsche and Kirsche, 1964) and that nonhuman apes tend not to suffer from neurodegen-erative diseases such as Alzheimer's (Erwin, 2001). Such species differences in brain vulnerability and healing capacity might well help us elucidate some disease etiologies or lead to novel therapies. Unfortunately, this research strategy has not yet succeeded. Thus far, evolutionary neuroscience's most important contribution has been the discovery that human brains differ substantially from other brains, particularly nonprimate brains, which means that cross-species extrapolations must be conducted cautiously (Preuss, 1995). This is an important message, but it can be construed as negative in tone. Hopefully, the future holds more positive discoveries.
Work on justifying evolutionary science is especially important in the United States, where anti-evolutionary sentiment is on the rise. Many conservative Christians believe that evolution is a dangerous, insidious idea because it makes life meaningless (Dennett, 1995). Add to this fear the notion that our thoughts and feelings are mere products of our brains (e.g., Dennett, 1991) and evolutionary neuroscience seems like a serious threat to God's supremacy. Although this line of argument is well entrenched, Darwin and most of his immediate followers were hardly atheists (Young, 1985). Instead, they either distinguished clearly between God's words and God's works, as Francis Bacon put it, or argued that God's creative act was limited to setting up the laws that control history. Either way, God was seen as quite compatible with evolutionary theory. Moreover, Darwin's view of life need not produce a meaningless void. Instead, it helps to clarify our relationships with other humans, other species, and our environment. Those relationships, in turn, give meaning to our lives, just as linguistic relationships give meaning to our words. Thus, Darwin knew - and we would do well to recall - that evolutionary biology can be useful even if it yields no direct medical or technological applications. Even Huxley (1863), who was a very pragmatic Darwinian and coined the word 'agnostic', knew that the uniquely human quest to comprehend our place in nature is not driven by mere curiosity or technological imperatives, but by a profound need to understand ourselves, our purpose, our existence. Within that larger and enduring enterprise, evolutionary neuroscience will continue to play a crucial role.
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