The human species has successfully dealt with twenty or more distinct episodes of global warming, surviving, if not prevailing, but in circumstances that no longer exist.
There is no real difficulty in identifying the most important news of 2015. Global warming is the news that will remain news for the foreseeable future, because our world will continue to warm at a rate that has never been seen before, at least at the moment without a foreseeable end. Writing as a paleoanthropologist, despite the fact that the past is usually a poor model for understanding the present, this time paleoanthropology brings a quite different perspective to the news because global warming has happened before, perhaps twenty or more distinct episodes of warming during the time our genus—Homo—has existed, with important, sometimes quite significant, effects on human evolution. The human species has survived global warming in the past, indeed persisted in the face of global warming, but paleoanthropology is a comparative science and comparing most past episodes of global warming to the global warming in today’s news leads me to question whether (and how) we may survive this one. And this is not solely because of a point the news also recognizes, the rate of temperature change is much faster than humans have ever experienced before.
Prior episodes of significant global warming within the Pleistocene, more-or-less the last 2 million years, have invariably followed cooling periods with glacial advances. During the Pleistocene, the human lineage, within the genus Homo, successfully adapted to changing environments (including climate) and evolved to take advantage of the opportunities afforded by the changes, even as populations of Homo reacted to the constraints environmental changes created. Human populations evolved diverse adaptations to the different climates and ecological circumstances they encountered, but the improvements in communication skills, planning depth, and retention of deep history with tales, poems, song, and other aspects of cultural behavior dispersed throughout humanity.
At any particular time the Pleistocene world population was quite small, 1 to 2 million is estimated during all but the most recent episodes of cooling and warming cycles, half or more living in Africa. Scarce on the ground, with little ecological impact and with vast habitable areas unoccupied by human groups, the human reactions to periods of global warming often were simply population movements. These had important consequences over the course of human evolution because our particular brand of evolutionary change—the unique human evolutionary pattern—began with the initial and ongoing geographic dispersals of human populations. In many mammalian species, significant range expansion such as the human one resulted in geographic isolation for many groups, and the formation of subspecies and ultimately of species. In humans these processes were mitigated by continuing population interconnections created by gene flow, in some cases the result of population movements, and in others because expanding human populations grew to encounter each other. The unique human evolutionary pattern was created as adaptive genes and behaviors, under selection, spread throughout the human range. Genetic changes adaptive for the entire human species were able to disperse throughout it, no matter how different individual populations may have become, because population contacts allowed it and natural selection promoted it. Thus, the unique human pattern of evolution is based on continued population mixtures, and as some populations merged and others became extinct, continued replacements.
But this long-lasting pattern has been disrupted as humans gained increasing control of their food resources and human populations began the accelerated increase in number that is so evident today. This is recent, so recent that a rapidly growing humanity is yet to have encountered significant climate change, until now. Today’s headlines make it clear that the change we are encountering is global warming. It is not at all evident that the adaptive successes of the past, as the human species successfully reacted to many instances of global warming, guarantee a successful human reaction to the changes coming upon us today. The world is quite different. Adaptive strategies that underlay a successful strategy for the human species in the past may no longer be possible; a vastly larger number of humans probably preclude similar success from the same strategies, even without considering the rapidity of the climate changes humanity is encountering.
The fact is that the present is not a simple extension of the past, the conditions are radically different and the strategies that promoted a successful balance of population variation within the human species and a successful adaptation for all populations throughout the Pleistocene may, today, create competition between human populations at a level that could make the lives of the survivors quite unpleasant.
Of course, nothing like this is inevitable, or even necessarily probable, but its possibility looms large enough to be taken seriously. We need to learn from the past without trying to repeat it.