john hawks weblog

paleoanthropology, genetics and evolution

climate change

  • Rewilding Siberia

    Tue, 2010-12-07 07:30 -- John Hawks

    The Associated Press ran an article last week about Sergey Zimov and his attempts to "rewild" a small corner of Siberia:

    Of his first herd [of Yakutian horses], Zimov said 15 were killed by wolves and bears, 12 died from eating wild hemlock that grows in the park, and two slipped through the perimeter and made their way back some 1,000 kilometers (600 miles) to their original pastures.

    It's tough to manage these animals without acting as if they were domesticated. He's talking about bringing in bison from North America, of course most of these are extensively managed. It would probably be more realistic to pursue an economic model where the "megafauna" paid for its own management. Maybe that would get in the way of tigers and bears, though.

    Tags: 
    climate
    megafauna
    climate change
    Pleistocene

  • The Neandertals of Mount Doom

    Mon, 2010-10-11 16:07 -- John Hawks

    Well, I already snarked on the science headlines that have been claiming volcanoes "wiped out" the Neandertals. Some variation of this story, swapping in a different Neanderkiller, has been circulating since around 1890. But is there any truth to the headlines?

    (see UPDATE below)

    The source of the story is a paper in the October issue of Current Anthropology, by Golovanova and colleagues [1]. The paper reviews the chronology of Mezmaiskaya Cave, a site occupied by Neandertals and successive Upper Paleolithic peoples, in the Russian Caucasus. This site produced the skeleton of an infant, from which DNA evidence has been recovered. As Golovanova and colleagues describe, the deposit additionally contains volcanic ash from two eruptions that happened around 40,000 years ago.

    The latter of the two eruptions appears to coincide with a long abandonment of the site:

    Hominin occupation of Mezmaiskaya Cave changed dramatically after the later volcanic eruption represented in layer 1D. This eruption was probably more powerful than that in layer 2B-1. Layer 1D has a thickness up to 0.7 m and in some areas is composed of a relatively clean sediment lacking any inclusions (fig. A10). Limestone fragments, bones, and lithic artifacts are absent, and even pollen grains are rare. Pollen data show that extreme deterioration to a very cold and dry climate occurred in this time period....A chemical analysis of layer 1D indicates that the volcanic ash apparently derives from an eruption in the Kazbek volcanic province that occurred around 40,000 years ago. Because no Neanderthal specimens or MP lithic industries postdate layer 1D at Mezmaiskaya, this eruption seems to have significantly disrupted the ecological niche of local Neanderthals, possibly resulting in their rapid disappearance in this region.

    The Kasbek volcanic province is in the Caucasus, so we're talking about a large eruption relatively local to the site. This is the sort of event you might well expect to have a strong impact on a dispersed hunter-gatherer population. The Middle Paleolithic people (presumably Neandertals) might have locally declined in numbers, or they might have moved on. The region need not have been abandoned entirely; a new population might have entered the area without using the same site. In this case, when new people began to use the site much later, the newbies were using an Upper Paleolithic industry.

    A relatively local effect of volcanism in the Caucasus is one thing, but the extinction of Neandertals across western Eurasia is quite a bit more. How does the paper go from local event to a regional extinction?

    The local eruption was the second event to leave ash in the Mezmaiskaya sequence. The first was a different eruption from Mt. Elbrus, which had a smaller impact than the second, as discussed below.

    At issue in the paper is the possible coincidence of the second eruption and consequent abandonment of the site with a much larger volcanic event in Italy:

    The CI [Campanian Ignimbrite] eruption from the Phlegrean Fields, southern Italy—the largest eruption documented in the Mediterranean region during the past 200,000 years (Wohletz, Civetta, and Orsi 1999)—drastically impacted European ecosystems. The most recent numerical (40Ar/39Ar) age determinations for CI eruption vary from to BP and cluster around 40,000 BP (Fedele, Giaccio, and Hajdas 2008:839).

    This eruption produced the CI in Italy and Y5 tephra in Central and Eastern Europe and Eastern Mediterranean (Fedele et al. 2003; Fedele, Giaccio, and Hajdas 2008). High-altitude clouds of volcanic ash from this eruption had a significant effect on global climate. The resulting ash fall covered km2 of land and sea (fig. 1), and the Y5 tephra layer accumulated in the Eastern Mediterranean as far as Cyprus—more than 1,500 km from its source (Mussi 2001:191). The Y5 tephra is also identified in the EUP sequence at Kostenki in the Middle Don River in Russia (Holliday et al. 2007). In Eastern Europe, the ash layer varies from 1–2 cm in the eastern limit (between Penza and Rostov) to 5–8 cm in the west and southwest (southern Ukraine and Moldova) and averages about 3–4 cm (Laverov et al. 2005:51). Obviously, the area affected by this ash fall was much larger than the documented Y5 tephra sites.

    Golovanova and colleagues propose the hypothesis that the climate effects of this CI event caused the demise of the Neandertals:

    Our new data provide support for the hypothesis that the MUP transition in western Eurasia coincides with one of the most globally significant volcanogenic catastrophic events in the recent history of the earth. The large and coeval volcanic eruptions (from an unusually large CI eruption in the Apennines to a smaller eruption in the Caucasus) had a sudden and devastating effect on the ecology and forced the fast and extreme climate deterioration (so-called volcanic winter, perhaps comparable to the effect of nuclear winter) of the Northern Hemisphere in the beginning of Heinrich Event 4. We guess that this catastrophe likely may have both drastically destroyed the ecological niches of Neanderthals, possibly resulting in the mass death of hominins and prey animals and the severe alteration of foraging zones, and caused Neanderthal depopulation from Central Europe to the Caucasus.

    That's a very clearly stated hypothesis. A volcanic eruption initiated climate effects that the regional population of Neandertals could not survive.

    However, Golovanova and colleagues include in their paper several critical facts that run against this hypothesis:

    1. The Mezmaiskaya sequence itself shows Middle Paleolithic people returning and proliferating after a large relatively local eruption. The Elbrus eruption apparently left ash in layer 2B-1, with a low density of bones and a very low frequency of bison compared to caprids. The excavators interpret the layer as a very low-intensity use of the cave. The pollen evidence suggests a "cold, dry climate". In other words, the paleoclimate and faunal evidence are both consistent with the hypothesis that the eruption had effects on Neandertal populations in the Caucasus. But then the Neandertals apparently returned in force:

    The intensity of site use increased, however, during the accumulation of the upper MP layers 2A and 2 when the climate become cool and wet. Although the lithic industry changed slightly after the environmental crisis of layer 2B-1, it still remained typically MP Eastern Micoquian. Skeletal and mtDNA evidence indicates that Neanderthals produced both the earlier and the later MP industries at Mezmaiskaya (Briggs et al. 2009; Golovanova et al. 1999; Green et al. 2010; Ponce de Leon et al. 2008). Thus, the late MP environmental crisis at the cave had repercussions for local Neanderthals but did not cause a break in the continuity of occupation or technology.

    That makes it seem pretty unequivocal. Neandertals survived and effectively adapted to at least one volcanic event in this area. That eruption did not kill them off, and it did not leave the area devoid of Neandertals in a way that facilitated a "modern human invasion."

    It was only after the second volcanic event that Middle Paleolithic people declined at the site.

    At issue is whether this second event was coincident with the CI eruption. The ash in the Mezmaiskaya sequence is not from the Y5 tephra, it is attributable to a much nearer source. I do not fully understand why the authors attribute this second event to the same time as the CI event; the time between layers 2 (terminal Mousterian) and 1C (early Upper Paleolithic) appears to have occupied a few hundred years, between 32,000 and 34,000 radiocarbon years BP. Calibration will move those dates older by a few thousand years (I discussed radiocarbon calibration a few years ago). But I think the CI eruption, around 40,000 years ago, doesn't fit well with this later event. It might fit with the earlier eruption, in my view, as Elbrus lava flows include 40,000 BP.

    In any event, I think the associations of either local volcanic event with the larger CI event is at best uncertain. The record at the site makes it pretty clear that Neandertals were effectively adapting to the changing local climates and faunal abundance that coincided with the first eruption.

    2. The initial Upper Paleolithic of Kostenki had appeared before the Y5 tephra was deposited. I wrote about the identification of this Y5 tephra at Kostenki a couple of years ago ("An earlier initial Upper Paleolithic at Kostenki"). As my post indicated, the identification of the ash layer with the Campanian Ignimbrite event suggested an earlier date for the initial Upper Paleolithic on the Russian Plain.

    From the standpoint of the Neandertal volcanic winter hypothesis, this sequence of events is a problem, which Golovanova and colleagues discuss:

    In any case, with or without the Kostenki addition, the few CI-bearing sites show that this eruption could have also extinguished the first wave (Proto-Aurignacian) of EMH expansion into Europe (Fedele, Giaccio, and Hajdas 2008): “At all key sites, where sedimentary resolution is good, the CI tephra directly seals archaeological layers that contain assemblages of the MUP mosaic, often variants of ‘Aurignacian’-like or so-called Early Upper Paleolithic Industries. … The layers above the CI tephra, where they are not culturally sterile, contain later and often much later properly defined Upper Paleolithic industries” (841). Thus, the CI-bearing sites demonstrate clear evidence of the break in habitation and culture change—a whole gamut of archaeological attributes for population replacement.

    The volcano is supposed to explain the MUP transition, but occurs earlier than the MUP transition in some areas, but later in others. Golovanova and colleagues propose an ad hoc hypothesis to account for this mismatch: some early Upper Paleolithic modern humans were also wiped out.

    Many researchers might find this idea tempting. It might, for example, explain why the (few) skeletal remains of the earliest Aurignacian people have such a high proportion of Neandertal features. We could propose that the initial Upper Paleolithic represents a degree of population mixture that later populations do not; the discontinuity between them could have been caused by climate extremes.

    But we don't need climate or volcanism. Later Upper Paleolithic people retained similarities to Neandertals, which reduced in frequency over time. This is most readily explained by continued gene flow into a sparse European population from West Asia. A volcano-induced climate catastrophe is superfluous: It doesn't add to the explanation of a sustained genetic transformation of Europe that continued through the later Upper Paleolithic and Neolithic.

    3. "Catastrophes" are not rare. The record of climate change during the last glaciation shows frequent strong oscillations. Some of these occurred at the same time as known eruptions, and so might be associated with them, but most climate oscillations have no obvious cause. Up to 40,000 years ago or so, the Neandertals survived them all. They survived the Toba event, largest eruption by volume in the Pleistocene, with no evidence of ill effects.

    The "intensity of occupation" of archaeological sites naturally fluctuated for many reasons. In Paleolithic contexts, sites were almost never inhabited continuously. We usually don't know why a local population returned more often to a site, or why the later population may have used the site less often, but those changes in pattern will make a big difference to the occupation intensity. It's not enough to show that a fluctuation in occupation intensity was coincident with an eruption or climate event -- such coincidences are inevitable even when "occupation intensity" changed randomly.

    What role volcanoes?

    Bad things happened in the past. Many of those bad things -- megadroughts, volcanoes, asteroid impacts, flesh-eating bacteria -- probably killed a lot of people.

    But our ability to find the effects of these death-dealing events is a lot more limited than you might assume. Less than a thousand years after the Black Death, how many signs of it are still evident today? To the exceedingly clever, who know where to look, there are a few. If we discount historical records, which do not exist for the Pleistocene, and limit ourselves to very small samples of bone and stone remains, it becomes very difficult to demonstrate this widespread epidemic, which reduced the population of some parts of Europe by up to half.

    Most Paleolithic sites document exceedingly low-intensity use of an area by ancient people, and have gaps of thousands of years. The hope of finding a single event with a short duration is near zero, unless it affected many sites in the same way.

    The extinction of a widespread group of hominids would be one kind of event we might test. In the current example, I think the data point to a clear conclusion: Not all Neandertals were killed, starved, or slowly declined due to the effects of any single volcanic eruption. Too many of them clearly survived the time of the large eruptions, and the available archaeological indicators suggest that their populations tended to recover after climate extremes had been reached. They were very resilient to climate change, more than many other mammals.

    It's not possible to rule out that one or more eruptions may not have had important effects, even ones that may have devastated some local populations. This is possibly the case at Mezmaiskaya. Nor is it possible to exclude the hypothesis that climate changes of greater and greater amplitude may have stressed their populations, contributing to the Neandertal demise.

    That's one of the returning frustrations of the archaeological record. An event might have been a major tragedy in human terms, but essentially invisible to us today. Meanwhile, the large-scale dynamics of human populations, including speciation and extinction, do not appear to fit the record of catastrophic eruptions. I don't see that as the end of the story, but a more interesting prologue to our understanding of ancient human dynamics.

    UPDATE (2010-10-16): I received a note from Golovanova and Doronichev, kindly pointing out a serious error in my post. I had misread their paper -- I described it as supporting a coincidence of CI with the first ash evidence at Mezmaiskaya, but the paper clearly argues that the CI event was "coeval" with the second ash, in layer 1D of the site.

    I have extensively updated the part of the post that refers to the CI eruption.

    I'm skeptical that the CI and Kazbek ashfalls could have happened near the same time, because the latter seems by radiocarbon evidence to be 2000 or more years later than 40,000 years ago. But the ESR dates are arguably consistent with the idea that the two eruptions coincided. I wouldn't push a chronology argument very far, not without a list of calibrated radiocarbon and ESR/TL dates from the relevant eruptions. But the multiplicity of events helps to reiterate the basic point that geological events happened, and fluctuations of site intensity happened, and it will take a coincidence across many sites to correlate the two.

    References

    1. Golovanova LV, Doronichev VB, Cleghorn NE, Koulkova MA, Sapelko TV, and Shackley SM. 2010. Significance of Ecological Factors in the Middle to Upper Paleolithic Transition. Current Anthropology [Internet] 51:655–691. Available from: http://dx.doi.org/10.1086/656185
    Tags: 
    Middle Paleolithic
    paleoclimate
    Europe
    Kostenki
    Mezmaiskaya
    Neandertals
    climate change
    Upper Paleolithic
    extinction
    Late Pleistocene
    Synopsis: 
    Eruptions in the Caucasus are claimed to explain Neandertal disappearance in that area. I demur.

  • Neandertal headlines

    Wed, 2010-10-06 17:29 -- John Hawks

    For two weeks I've been reading news feeds about how volcanoes killed the Neandertals. I mean, seriously:

    USA Today: "Volcanoes wiped out the Neanderthals?"

    National Geographic: "Volcanoes Killed Off Neanderthals, Study Suggests"

    NY Times: "Neanderthals’ Big Loss in Battle of the Elements"

    UPI: "Volcanic Winter Blamed for Neanderthal Extinction"

    The other stories of the last couple of weeks are some variant of "Neanderthals Were Not So Stupid After All". They make quite a juxtaposition.

    I think they could save some print by combining the stories: "No Longer Rock-Brained, Neandertals Kiss Ash Goodbye". Or "Neandertals Outwit Ice, Die By Fire."

    Oooh, oooh! "Misunderstood Neandertals Pursue Morose Creator To North Pole".

    Tags: 
    science writing
    Neandertals
    climate change
    humor
    media

  • Megafaunal meth

    Mon, 2010-05-24 22:30 -- John Hawks

    I've seen this story around the net today, so I thought I would link to the short research paper by Felisa Smith and colleagues: "Methane emissions from extinct megafauna." The idea is that the extinction of the mammoths, mastodons, horses, camels, steppe bison and other big herbivores may have helped trigger the Younger Dryas:

    Our calculations suggest that the late Pleistocene extinction resulted in a decreased annual methane flux of ~9.6 teragrams (Tg) (range 2.3 to 25.5 Tg; Supplementary Tables T1 and T2). Interestingly, ice-core records of methane concentration reveal an abrupt drop of >180 ppbv at the onset of the Younger Dryas cold event, about 12,800 years ago (Fig. 1). The drop seems to be synchronous with the extinction of New World megafauna.

    The reduction in atmospheric methane is documented by ice cores, and the paper shows that the loss of megafauna ight be enough to explain all of the reduction, or at least a large fraction of it. Methane is a more powerful greenhouse gas than carbon dioxide or water vapor. How much difference would it have made? That's the unanswered question.

    In light of my "Anthropocene" comments, I have to mention the way they end their letter:

    Thus, we propose that the onset of the 'Anthropocene' should be recalibrated to 13,400 years before present, coincident with the first large-scale migrations of humans into the Americas.

    Holy unnecessary interval, Batman! If we're going to make a geological epoch begin at the onset of the Younger Dryas, shouldn't it obviously be the Holocene?

    UPDATE (2010-05-25): From a reader:

    Given this new hypothesized cause for the onset of the Younger Dryas ends up being validated, a more precise/appropriate name should then be "Flatulocene".

    Yes, well, everyone, just tell them you caught wind if it here.

    References:

    Smith FA, Elliott SM, Lyons SK. 2010. Methane emissions from extinct megafauna. Nature Geosci (in press) doi:10.1038/ngeo877

    Tags: 
    climate change
    paleoclimate
    extinction

  • Anthropocene redux

    Thu, 2010-05-20 21:46 -- John Hawks

    If you're a regular reader, you may remember my comments on some geologists' attempt to define an "Anthropocene" epoch to recognize the world-changing scope of human activities -- sort of like a global anthill ("'Anthropocene'? WhaAAAH?!").

    If so, you were two years ahead of the trend. The geologists have continued to organize, and the tipping point may be near, as Elizabeth Kolbert reports ("The Anthropocene Debate: Marking Humanity’s Impact"). I've seen a lot of links to this article, and it does capture the arguments of the idea's proponents.

    I continue to think that "Holocene" marks our impact pretty well, and since we can't predict how massive human impacts will be in the next few hundred years, it hardly makes sense to mark the last couple hundred as a new epoch. But politics are driving the issue:

    In general, Williams said, the reaction that the working group had received to its efforts so far has been positive. “Most of the geologists and stratigraphers that we’ve spoken with think it’s a very good idea in that they agree that the degree of change is very significant.”

    I'm skeptical that there is any scientific value to the concept. I do see the opportunity to reflect on the question of what makes an epoch boundary worth noting. But I don't think we should presuppose the answer, and I favor conservativsm. Still, maybe they can get the radiocarbon people to change "B. P." to "B. A." That would be fun.

    I also question whether "Anthropocene" has the political value that its proponents perceive.

    Tags: 
    climate change
    metascience
    climate
    geology
    politics

  • The bugs will out

    Tue, 2010-02-09 17:54 -- John Hawks

    Following up on the editorial by Ralph Cicerone, calling for more effective data sharing, an editorial in the Guardian by computer scientist Darrel Ince reinforces the point about openness for software used in scientific models:

    So, if you are publishing research articles that use computer programs, if you want to claim that you are engaging in science, the programs are in your possession and you will not release them then I would not regard you as a scientist; I would also regard any papers based on the software as null and void.

    When I have published work based on computer models, all algorithms have been included in an appendix or cited directly in previous publications. Of course, population genetics algorithms are relatively simple compared to some kinds of models. But as Ince points out, it is expected in complex fields like econometrics and mathematics that such algorithms will be deposited with the journal or in an open access source.

    The most important fact is that large software projects inevitably contain errors. If scientists do not use standard methods for testing and validating software, then their code will invariably be worse than computer industry norms:

    There is enough evidence for us to regard a lot of scientific software with worry. For example Professor Les Hatton, an international expert in software testing resident in the Universities of Kent and Kingston, carried out an extensive analysis of several million lines of scientific code. He showed that the software had an unacceptably high level of detectable inconsistencies.

    For example, interface inconsistencies between software modules which pass data from one part of a program to another occurred at the rate of one in every seven interfaces on average in the programming language Fortran, and one in every 37 interfaces in the language C. This is hugely worrying when you realise that just one error — just one — will usually invalidate a computer program.

    If someone demonstrated ESP but the experiment wouldn't work without one particular experimenter standing in the room, we would rightly judge that it is not science. Likewise, if no one else can use your computer program, its results aren't science. Simple as that.

    Tags: 
    climate change
    open access

  • NAS president calls for data sharing

    Sat, 2010-02-06 22:58 -- John Hawks

    Science has a one-page editorial by National Academy of Science President Ralph Cicerone. He alludes to the climate change scandals of the last few months, and points to a significant loss of public confidence in science as a result:

    In the wake of the [University of East Anglia] controversy, I have been contacted by many U.S. and world leaders in science, business, and government. Their assessments and those from various editorials, added to results from scattered public opinion polls, suggest that public opinion has moved toward the view that scientists often try to suppress alternative hypotheses and ideas and that scientists will withhold data and try to manipulate some aspects of peer review to prevent dissent. This view reflects the fragile nature of trust between science and society, demonstrating that the perceived misbehavior of even a few scientists can diminish the credibility of science as a whole.

    Cicerone argues that scientists need to shape up. The only way to maintain confidence in the scientific enterprise is to establish "clarity and transparency":

    Clarity and transparency must be reinforced to build and maintain trust—internal and external—in science. Scientists are taught to describe experiments, data, and calculations fully so that other scientists can replicate the research. Last year, the Committee on Science, Engineering, and Public Policy (COSEPUP) of the National Academy of Sciences (NAS), National Academy of Engineering, and Institute of Medicine put forth a framework for dealing with research data,* emphasizing that "Research data, methods and other information integral to publicly reported results should be publicly accessible." Some journals have established policies that require the sharing of materials and data. However, post-publication complaints regarding data sharing persist. Despite many efforts, the scientific community has failed to uniformly integrate these standards into their practices.

    Access to data may not be enough. In the case of climate research, open access to models and software is equally important -- otherwise, results are not replicable. This means greater support must be given from grant agencies for public accessibility and publication of research methods, including software archives.

    It also means that data sharing policies must have some teeth in them. At a minimum, funding renewal should be contingent on meeting the guidelines for data sharing proposed in grant applications. In 2010, there is no reason in the world why these cannot be downloaded freely from third parties, so that the scientists do not feel "harassed" by requests for information.

    References:

    Cicerone RJ. 2010. Ensuring integrity in science. Science 327:624. doi:10.1126/science.1187612

    Tags: 
    data access
    climate change
    open access

  • Defenestrating deforestation

    Sun, 2010-01-10 07:30 -- John Hawks

    Lots of people have written about the collapse of the ancient Maya, often as some kind of "lesson" about how present-day society needs to change for its own survival. A recent theme, pushed by Jared Diamond in particular, but also others, has been that the Maya failed to manage natural resources sustainably. Their political structure couldn't deal with the growth of their population, and short-term decision-making led to ecological collapse.

    Well, it's easy enough to propose such a sweeping hypothesis, but devilishly hard to test it. And so it's easy to forget that it is just a hypothesis.

    In the early bin at PNAS, McNeil and colleagues report on a test of the hypothesis for one locality, Copan, Honduras:

    Archaeologists have proposed diverse hypotheses to explain the collapse of the southern Maya lowland cities between the 8th and 10th centuries A.D. Although it generally is believed that no single factor was responsible, a commonly accepted cause is environmental degradation as a product of large-scale deforestation. To date, the most compelling scientific evidence used to support this hypothesis comes from the archaeological site of Copan, Honduras, where the analysis of a sediment core suggested a dramatic increase in forest clearance in the Late Classic period (A.D. 600–900). By contrast, in the work presented here, the authors’ analysis of a longer sediment core demonstrates that forest cover increased from A.D. 400 to A.D. 900, with arboreal pollen accounting for 59.8–71.0% of the pollen assemblage by approximately A.D. 780–980. The highest levels of deforestation are found about 900 B.C. when, at its peak, herb pollen made up 89.8% of the assemblage. A second, although less pronounced, period of elevated deforestation peaked at approximately A.D. 400 when herb pollen reached 65.3% of the assemblage. The first deforestation event likely coincided with the widespread adoption of agriculture, a pattern found elsewhere in Mesoamerica. The second period of forest clearance probably was associated with the incursion of Maya speakers into the Copan Valley and their subsequent construction of the earliest levels of the Copan Acropolis. These results refute the former hypothesis that the ancient Maya responded to their increasingly large urban population by exhausting, rather than conserving, natural resources.

    I admire this kind of close empirical work -- identifying pollen in sediment cores may not be glamorous, but it's maybe the best way we have to document human impact on these ecologies.

    References:

    McNeil CL, Burney DA, Burney LP. 2009. Evidence disputing deforestation as the cause for the collapse of the ancient Maya polity of Copan, Honduras. Proc Nat Acad Sci USA (online early). doi:10.1073/pnas.0904760107

    Tags: 
    climate
    complex societies
    climate change
    Maya

  • Climate reconstruction and human evolution, 2: Tracking climate change

    Wed, 2010-01-06 22:50 -- John Hawks

    I am examining the pathways that climate might have influenced human evolution, and as I wrote earlier, I'm focusing first on the issue of relatively short-term climate fluctuations. How could submillennial-scale climate variability plausibly impose selection on human populations? I can think of two mechanisms:

    1. Climatic zones might shift rapidly across geographic space, meaning that humans and their prey species must constantly move to track suitable habitat. Movement and periodic habitat contraction tends to increase competition; it also favors traits that expand habitat tolerance.

    2. Global climate fluctuations might decrease the spatial autocorrelation of climate and geography. In essence, local environments become less stable, so that species fine-tuned to local habitats have lower fitness. Greater habitat tolerance is favored.

    In both cases I wrote "might", because even though global climate changes had a high amplitude during the Pleistocene, only the local consequences of those changes would have mattered to human populations. It is a matter of conjecture that those local consequences were (a) of greater amplitude or rapidity than, say, Pliocene climate changes, and (b) large compared to the ordinary year-to-year fluctuation of local environments.

    Submillennial Pleistocene climate changes could well have had large effects on the climate of Africa (or Asia, or Europe), and yet those effects might still have been minor compared to the ordinary variation. That is the effect claimed for the last 100 years, where the global temperature trend is large, but the ordinary year-to-year variability is much larger.

    How fast does the gradient move?

    Conservation biologists are very worried about climate change over the next few hundred years. Human land use has shrunk the geographic ranges of most land species. Many are now limited to small fragments of habitat, sometimes protected by governments in national parks and reserves, sometimes not. Any population that consists of a very small number of individuals is in danger of extinction, both due to casastrophes like disease and fire, or due to the intrinsic effects of strong genetic drift -- a so-called "mutational meltdown". Some abundant species are at risk of extinction if a few key habitat fragments should be lost -- the Mexican wintering grounds of monarch butterflies, for instance. If the local climates of these habitat fragments should change, the local biota may shift in ways that compromise the survival of the endangered populations.

    So, there is a lot of interest in modeling how global temperature changes may influence local environments. How will rainfall and temperature patterns in small habitat fragments shift as the global system changes?

    The recent paper by Loarie and colleagues (2009) is a high-profile example. These researchers attempt to identify a "velocity" of climate change as applied to different terrestrial ecosystems. The idea of a velocity rests on the assumption that local environments exist along a temperature and rainfall gradient, such that getting a little warmer or cooler will shift the boundary between adjacent habitats. If so, then one might determine how fast a biotic boundary will move with a given rate of temperature increase. Plug the global world temperature model into this ecogeographic model, and you might work out the velocity of change expected in the near future.

    The result is a range of velocities for different biomes:

    Using temperature change calculated from 2000–2100 under the intermediate A1B emissions scenario, the geometric mean velocity was 0.42 km yr-1 (0.11–1.46). (Throughout, we summarize uncertainty in the mean by listing upper and lower, ± 1 s.d., estimates in parenthesis.) See Supplementary Fig. 17 for other emissions scenarios. We summarize velocity for biomes of the globe and rank them by increasing mean velocity (Fig. 3). Doing so shows that mountainous biomes require the slowest velocities to keep pace with climate change. In contrast, flatter biomes such as flooded grasslands, mangroves and deserts require much greater velocities.

    Using these projections, Loarie and colleagues focused on a very specific question. Consider small protected areas where species now find refuge from human habitat disturbance. How long does it take one of these habitat isoclines to traverse the length of such a refuge?

    To explore the interaction between protected area sizes and velocities required to keep pace with climate change, we calculated residence times, defined as the diameter of each protected area divided by velocity (km/km yr-1 = yr). Assuming that protected areas are circular and disconnected, this index can be interpreted as the time for current climate to cross a protected area. Such residence times exceed 100 years for only 8.02% (2.67–16.49) of protected areas.

    This is a kind of modeling that I approach with heightened skepticism. Remember the "Bigfoot biogeography" paper from this summer? The ecogeographic models will accept any data and spit out an answer. The input data in this case come from a climate model, with its own intrinsic error. The ecogeographic data have additional observation/classification error and variance. Those errors add up.

    One thing working in favor of their conclusion is that the northward (and upward) extension of the range of tree species did progress around 1 km/year in many parts of the Northern Hemisphere. They point this out in their discussion, and it brings to mind a couple of very interesting historical examples in evolutionary biology, that I'll have to review another time.

    How would this mode of change affect Pleistocene humans?

    Right now, though, I just want to tackle a more limited question: Would this "velocity" of habitat change really have been any challenge for Pleistocene humans?

    Historic hunter-gatherers were generally very mobile people. Most of them maintained large home ranges and lived at relatively low population densities, under 4 people per square kilometer. In northern habitats, their density was substantially lower, down to as low as 1 person per 100 square kilometers. There are lots of reasons to doubt that Pleistocene hunter-gatherers could have survived in the high-latitude grassland and tundra, places where people lived recently only with sophisticated logistical strategies, at very low densities. Loarie and colleagues mention high-latitude, low-topography regions as those predicted to have high velocities of biotic change. The temperate ecologies, on the other hand, have very broad distributions of predicted velocities, with a mean around the global average, 0.4 km/year.

    That adds up to around 10 km per human generation, which is, give or take, the diameter of a large home range for a hunter-gatherer band. If Pleistocene climates shifted with velocities like those predicted for the near future, a value of 10 km/generation would imply that home ranges at the edge of a habitat regime would be at risk of disappearing (or at least, substantially changing in resources) on a human timescale.

    Moving to track climate would be very easily within human capabilities. These kinds of velocities are orders of magnitude smaller than those maintained by people during long-distance migrations. If we imagine a kind of Brownian motion of individuals within the matrix of hunter-gatherer groups, any one individual probably had a larger residence shift during her lifetime than would be necessary to keep up with the secular climate trend. If we assume that people couldn't move into their neighbors' home ranges, then the worst effect of this kind of secular climate change would be forcing them to adjust to the resources that exist 10 km away. Not so terrible-sounding.

    But maybe the direct effects of climate change were not the important factor. Resource stress in a linear swath of hunter-gatherer groups might have increased social frictions, intensifying the competition for more stable parts of the preferred habitat. If, as I think likely, human groups were embedded in a source-sink metapopulation, climate change would likely have increased the fraction of long-term sink habitat. A nice-looking home range at the edge of a favored habitat would be at high risk of shifting to a different habitat type on the timescale of a few hundred years. Eventually, climate oscillation would return this edge of the range, but possibly at the cost of area on the opposite edge.

    Such changes would not be fatal to human populations -- after all, people moved much farther. But speaking on the scale of generations, people may have tracked habitat mainly by reproducing more in favorable regions, deciding the issue by reproduction instead of migration. If so, climate change would have reduced effective population size to some extent. How much? Depends on the granularity and sizes of long-term favorable ranges, and the intensity of territory defense by Pleistocene groups.

    UPDATE (2010-01-07): For threatened species today, the question of year-to-year variation may be less critical. We assume they are already adapted to year-to-year climate fluctuation, otherwise they'd be dead already. The species that make up their habitat need to have survived that year-to-year fluctuation also. The secular trend increases the number of bad years and makes them on average slightly worse, which may stress some resident species beyond the point they can survive. All this is much worse if genetic variation is already low -- then every stochastic fluctuation in population size becomes a chance of extinction.

    For Pleistocene humans, adaptation to the year-to-year variation can't be assumed constant. The extinction of particular groups over a 100-year span is only relevant to our evolution if there was differential survival determined in part by some heritable traits. One way to adapt to year-to-year climate fluctuation is to broaden the resource base -- which is also a good strategy to deal with long-term secular trends in climate. But the very ability to broaden resource dependence will tend to reduce demographic stress during bad years.

    In other words, I don't see how these kinds of climate fluctuations are going to lead to runaway or autocatalytic selection. The system seems to have a built-in buffer, at least on the timescale of a few generations.

    References:

    Loarie SR, Duffy PB, Hamilton H, Asner GP, Field CB, Ackerly DD. 2009. The velocity of climate change. Nature 462:1052-1055. doi:10.1038/nature08649

    Tags: 
    paleoclimate
    source-sink dynamics
    climate
    metapopulations
    population structure
    climate change

  • Some future evolution scenarios

    Fri, 2009-12-04 14:12 -- John Hawks

    Because of my work on recent human evolution, people ask me a lot -- I mean, an awful lot -- what our evolution will be like in the future.

    This is not a silly question. Evolution is a process, and like many processes we can examine its course in the past and make some observations about whether it was jerky or smooth, fast or slow. Population and quantitative genetics let us predict what will happen in populations under given patterns of selection and drift. So it seems like we ought to be able to make some intelligent predictions about where things are going.

    Yet as a system, evolution is a lot like the stock market. We can make a few sensible predictions, both over the short and long terms. If there's a crisis in the Middle East, then stocks in ethanol producers will rise. Three decades from now, the S&P 500 will be higher than it is now.

    But when we reduce down to particular observations, the stochastic factors become more and more important. Technological innovations drive entirely new business models and industries, yet are hard to predict. Wars have uncertain outcomes, as do elections. Even index investors in for the long term may lose money over a decade or two -- just ask anyone who started investing in 1999. The results are, we say, more volatile.

    So I often cringe when somebody asks me where we're going. There are many possible futures. Some present trends might allow one to extrapolate a bit into the future, but it's hard to see how they will interact with each other.

    That would be bad enough, but there's another problem. Anybody who writes about this problem always exaggerates the wacky possibilities. Like humans diverging into Morlocks and Eloi. Or how women are going to get more beautiful. Or how we're all going to converge into a mass of uniform brown.

    It almost makes me want to turn into Steve Jones. OK, well, that's not going to happen. But it's frsutrating.

    A couple of months ago, Carl Zimmer told me he had been commissioned to write an essay about where human evolution is going in the future, as sort of a conclusion of the year of Darwin. My immediate reaction was, "Finally, somebody who has a chance of describing this incredibly complex problem and getting it right!"

    My second reaction was, "Wow, I'm glad I'm not him."

    His essay appears today in Science: "On the origin of tomorrow" (a reader points out that Zimmer has a free copy in his archive). I think he's done a good job of it. He avoids the sensational, and talks in a sensible way about the relation of recent selection to future change (maybe very little, for many of the recently selected alleles).

    On the other hand, civilization has also blunted some of natural selection's power over humans, particularly in the 150 years since Darwin published On the Origin of Species. Back then, for example, some children had the misfortune to be born with defective copies of a gene for an enzyme that breaks down amino acids in the food they ate. This disorder, known as phenylketonuria, generally led to severe brain damage. Few people with severe phenylketonuria were able to pass on their genes. But today, now that scientists know what causes the disease, people with phenylketonuria can enjoy fairly normal lives simply by being careful about the foods that they eat, and they pass their genes on to their children. Other medical advances, from eyeglasses to antibiotics, may also allow some potentially detrimental genes to become more common than in the past.

    Zimmer was handed a great gift in the recent report on the Framingham Heart Study, which shows rather strong changes in the composition of the population over the last few decades:

    The scientists discovered that a handful of traits are indeed being favored by natural selection. Women with a genetic tendency for low cholesterol, for example, had more children on average than women with high cholesterol. A greater body weight was also linked with greater reproductive success, as was shorter height, lower blood pressure, an older age at menopause, and having one's first child at an earlier age.

    These changes aren't mortality-driven; they're fertility driven. Which is pretty interesting, since many of them -- blood pressure, cholesterol -- we wouldn't classically link with fertility outcomes. But fertility selection is really the only strong factor that can operate on Westernized populations today.

    He also took the opportunity to broaden the question beyond human evolutionary changes to human-induced changes in the evolution of other species. This move has two great advantages for his essay: it puts many good empirical cases into reach, and it allows him to posit strong directional selection -- making evolution plausible in the short term. These examples include both intentional (genetic engineering synthetic microbes, fisheries biology) and unintentional (alien species, effects of ocean acidification).

    I think it's a nice pairing -- the uncertainty of future human changes helps to underline the uncertainty of predicting what will happen in a human-altered nature.

    References:

    Zimmer C. 2009. On the origin of tomorrow. Science 1334-1336. doi:10.1126/science.326.5958.1334

    Tags: 
    climate change
    future

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