Climate reconstruction and human evolution, 1

I have from time to time written short pieces here about climate fluctuations and their effect on human evolution. This topic was a major theme in the recent Nova miniseries on human origins.

Some scientists think that human intelligence is an adaptation to rapid climate changes, and that brain size increased during the Pleistocene as a direct result of climate fluctuations. I write, “scientists,” because this really is a motley interdisciplinary crew – archaeologists, geologists, psychologists and some paleoanthropologists in the mix.

Some others think that humans mainly suffered through climate by means of a few major events, global catastrophes like the Toba eruption. By decimating human populations, these big catastrophes caused all kinds of accidental changes to our biology. Why just today, you can read a story riffing on a Curtis Marean lecture, making it into this: “How shellfish saved the human race.” Just one of many speculations about how climate brought humans to near-extinction in the last couple hundred thousand years.

These two kinds of thinking about paleoclimate and human evolution are not mutually exclusive, and many people kind of lump them together. Both kinds of arguments depend on models linking fitness, plasticity, and paleoclimate. They also depend on the amplitude of ancient climate change episodes and the relation between this amplitude and the geographic variation in climate known to have been tolerated by ancient humans. Genetic modeling and paleoclimate modeling, working together.

That’s the way that testable science ought to work. But so far, proponents of these ideas have proposed only verbal arguments without any modeling or math at all. Without the numbers, I don’t see any reason to believe that climate specifically affected the evolution of human behavior. Is it bunk? I’d like to know at least if any of it is falsifiable.

I’m going to review some literature in both areas during the next few weeks. I’m reasonably familiar with the literature on selection and plasticity, and it gives me a lot of skepticism about the prospects of explaining human intelligence as a function of climate variability. But I’m less familiar with the paleoclimate literature. Maybe hypotheses about climate forcing in human evolution are untestable – but to arrive at this conclusion both the genetic and paleoclimate aspects need a realistic assessment.

Submillennial-scale variability

How can we start to understand the role of climate variability on Pleistocene human evolution? I’m going to begin with the topic of submillennial-scale climate fluctuations. The global climate fluctuates from year to year, from decade to decade, across centuries, and on much longer scales – up to the 100,000-year duration of Pleistocene glacial episodes.

The longest climate cycles are relatively simple – the paleoanthropological record is just good enough that we might be able to find associations between skeletal evolution, archaeological presence and the major glacials. These very large-scale temporal effects are not my immediate interest, but they will be worth considering very closely. Did the earliest habitation of Europe depend on climatic goodwill? To what extent were humans driven south out of Northern Europe by the last glaciation? What effects did periodic isolation from the Asian mainland have on Pleistocene Javan populations? Those are questions that implicate the largest-scale climate cycles, and I think they may be to a great extent answerable.

They are not, however, the kind of effects argued to have induced a consistent selection pressure on intelligence. Nor do they suggest a global bottleneck due to climate instability.

A sustained selection pressure would require climate instability on a shorter timescale – measured in maybe tens of human generations or less, not thousands. This scale is submillennial at most, and possibly decadal.

The reconstruction of paleoclimate on these shorter timescales has become feasible during the last ten to fifteen years. For the last few millennia, tree ring records provide evidence about climate variation – sometimes at annual intervals, sometimes time-averaged over five or ten year spans. Lake sediments sometimes preserve a record of climate variability, in the thickness or composition of layers laid down over time. Sea floor sediments also can inform about ancient climate, principally because windblown (or waterborne) sediments reflect continental aridity. Ice cores from glaciers and ice sheets also provide a record for reconstruction atmospheric temperatures and precipitation over some regions – and in the exceptional cases of the Greenland and Antarctic ice sheets, climate reconstruction may be possible back to as far as 800,000 years ago.

But these records are not direct indicators of ancient temperature or precipitation; they are proxies. Each of them is localized in a particular place with its own unique regional deviations from the global mean. During the Pleistocene, the places were people lived were pretty far away from most of the available proxy records. There are exceptions, like the Nile delta sapropels and African lake sediments, which I’ll discuss in some detail. But for the most part, our question is whether proxy records can test hypotheses about regional climate variability in the regions occupied by humans.

The same question arises in the context of recent climate change. We have several proxy records for temperature and precipitation during the last 1000 years, and we want to accurately reconstruct global mean temperature means and variability. The question was broached in a 2009 editorial essay by Hughes and Ammann, as follows:

The last millennium paleoclimate reconstruction (PR) challenge Given the suite of uncertainties and limitations connected to proxy and short instrumental data, it is very difficult to derive a reconstruction method that is capable of explicitly dealing with all issues, and thus some compromises must, inevitably, be made. At the same time, many existing methods might be reasonably well suited to study particular aspects of past variability and change (Ammann and Wahl 2007). However, we do not have a solid quantitative understanding of the strengths and weaknesses of each method. When different methods are applied to the same time period and spatial domain, how different are their results and inherent uncertainties, and to what is each method particularly sensitive? Differences could result from limitations in the included proxy climate records ability to record climate across different time scales; the sampling distribution (network); or possibly the method itself or its underlying assumptions. To assess the influence of all these uncertainties, the last millennium paleoclimate reconstruction (PR) challenge is being developed to offer an experimental test bed for systematically gauging the ability of the various methods to recover the true underlying climate. The climate data are drawn from output of fully coupled climate system models that provide geophysically realistic spatiotemporal variations across various climate ?elds. Thus the target of reconstructions will be fully known. ... The key question, ultimately, is to ?nd out how well the suite of reconstruction methods, if applied to a realistic set of pseudo-proxy data, are able to reproduce the underlying climate, not knowing the actual target climate (achieved in a double-blind approach) (Hughes and Ammann 2008:256-257).

In other words, even considering the last 1000 years with its abundance of proxy data, the climate models are not yet capable of replicating regional variability or time-series variance of temperature records with high fidelity. I’ll add (from elsewhere in the editorial) that the general question of the magnitude and duration of external forcing of climate (say, fluctuations in solar output) are also not well integrated.

The last 1000 years (and more broadly, the Holocene) has sometimes been described as relatively stable compared to the higher-amplitude climate fluctuations of the Late Pleistocene. In that light, the prospects seem poor that we could accurately assess the regional effects of Pleistocene climate instability, using many fewer proxies than are available for the last 1000 years.

But for particular times and places, the situation may be better. Also, the general concept of global climate instability may itself allow us to work with genetic models, if we only knew the likely amplitude and periodicity of changes. So I’ll be reading further into the climate variation of the last 1000 years and how it may have been different in the Pleistocene.

Meanwhile, the Hughes and Ammann editorial is interesting in several sections, as the authors discuss possible methods for increasing the fidelity of climate models as applied to paleoclimate data. I was following up on the literature citing one of these methods when I found the essay. On this, I’ll just add that the statistical methods for dealing with time series of temperature proxies are tricky. Some methods may raise the prediction value of a proxy in different regions, while decreasing the appearance of yearly or decadal-scale variation in the global mean. So one must be careful about the choice of statistics when assessing paleoclimate variability.

References:

Hughes MK, Ammann CM. 2009. The future of the past -- an earth system framework for high resolution paleoclimatology: editorial essay. Climatic Change 94:247-259. doi:10.1007/s10584-009-9588-0