Rise in atmospheric oxygen and patterns of mammalian evolution
In Science this week (9/30/05), there was an article by Paul Falkowski and colleagues, including Michael Novacek of the American Museum, which documented the rise in atmospheric oxygen over the past 205 million years and suggested that this rise may have allowed the evolution of large placental mammals.
The introduction is more informative than the abstract:
It has long been recognized that atmospheric oxygen levels play a key role in the evolution of metazoans (1), yet our understanding of precisely how oxygen concentrations influence specific animal evolutionary traits is limited. Although many metazoans are capable of acclimating to hypoxic conditions by lowering metabolic rates and/or operating the tricarboxylic acid cycle partially in reverse (2), these physiological modifications cannot be sustained indefinitely. Controls of atmospheric oxygen by the carbon and sulfur cycles (3, 4) have led to models based on analyses of the isotopic composition of carbonates and sulfur (3, 5) or on the relative abundance of different rock types (6), which suggest that atmospheric oxygen concentrations varied throughout the Phanerozoic, with a maximum 300 million years ago (Ma), a minimum 200 Ma, and an overall rise from 200 Ma to the present (5, 6). However, the range and underlying causes of these variations in oxygen are not well understood. Here, we provide an isotopic record for organic carbon, which we analyzed in conjunction with isotopic records for carbonates and sulfates for the past 205 million years (My). Our analysis suggests that ambient oxygen levels approximately doubled from 10% by volume (76 Torr) to 21% (160 Torr) over this period. Concurrent examination of the fossil record suggests that this change in oxygen tension was potentially a key factor leading to the evolution of large placental mammals in the Cenozoic (Falkowski et al. 2005:2202).
The main idea is that an increase in oxygen availability was essential to the evolution of large mammals, because the placental environment is hypoxic compared to the ambient air, placing limits on the metabolism and growth of developing fetuses, and because the density of capillaries scales with negative allometry with body size, so that larger animals benefit from higher atmospheric oxygen levels.
The authors don't attempt to correlate the dates with the evolutionary history of the globin genes. This seems to me to be an important comparison, since if it is true that the placental environment is more similar to the Jurassic atmospheric conditions, then the fetal hemoglobin molecule might be expected to be a closer functional analogue to the common ancestor of adult mammalian hemoglobin than are the current adult forms. The globin genes have undergone multiple duplications over vertebrate history, some of which have resulted in functionally different genes (the subunits of fetal hemoglobin included). They ought to fall into this story somehow.
Most of the increase in atmospheric oxygen is pre-primate, so it isn't exactly relevant to paleoanthropology. But there was an apparently rapid increase across the Eocene from around 18 percent to around 23 percent. This is possibly very relevant to primate evolution, because this was a time of radiation of today's primate lineages, including the living branches of prosimians (lemurs, lorises, and tarsiers) and the first anthropoids.
The paper relates the increase in oxygen to increases in body size, noting that the average mammalian body mass grew across this time period. But for primates, there was no great increase in body mass (although some lineages did ultimately include larger representatives), because arboreal adaptation ultimately imposes limits on how big primates are.
What may have changed in primates is the relative size of the brain. Primates today have larger brains for their size than most other mammalian lineages. This increase in relative brain size was not evident in the Paleocene relatives and ancestors of primates, such as the plesiadapids. Perhaps the Eocene increase in oxygen availability allowed the overall increase in metabolic rate that a larger brain would require in primates.
Of course, that leaves unanswered whether the subsequent decrease in atmospheric oxygen to today's 21 percent had any effect on our evolution. This decrease dominated the Miocene, which was not exactly a time of brain size or body size reduction. So maybe the whole thing is a red herring.
References:
Falkowski PG et al. 2005. The Rise of Oxygen over the Past 205 Million Years and the Evolution of Large Placental Mammals. Science 309:2202-2204. Full text (subscription)
Pleistocene Park, USA
Should we return proboscids, lions and other megafauna to North America's Great Plains? Nature is running this commentary (subscription required) by Josh Donlan and colleagues that argues just that.
Our vision begins immediately, spans the coming century, and is justified on ecological, evolutionary, economic, aesthetic and ethical grounds. The idea is to actively promote the restoration of large wild vertebrates into North America in preference to the 'pests and weeds' (rats and dandelions) that will otherwise come to dominate the landscape. This 'Pleistocene re-wilding' would be achieved through a series of carefully managed ecosystem manipulations using closely related species as proxies for extinct large vertebrates, and would change the underlying premise of conservation biology from managing extinction to actively restoring natural processes.
I can attest to the dandelions, although we have no rats here at the house.
The idea is to start small with species that are already here, such as the Bolson tortoise and wild horses and asses. Then more exotics, like Bactrian camels, Przewalski's horses, and ultimately all the African and Asian animals with extinct American counterparts, including elephants, lions, and cheetahs. They propose starting on large private ranches, and then expanding outward from there:
Large tracts of private land probably hold the best immediate potential for such studies, with the fossil record and research providing guideposts and safeguards. For example, 77,000 large mammals (most of them Asian and African ungulates, but also cheetahs, camels and kangaroos) roam free on Texas ranches, although their significance for conservation remains largely unevaluated.
The third stage in our vision for Pleistocene re-wilding would entail one or more 'ecological history parks', covering vast areas of economically depressed parts of the Great Plains. As is the case today in Africa, perimeter fencing would limit the movements of otherwise freeliving ungulates, elephants and large carnivores, while surrounding towns would benefit economically from management and tourismrelated jobs. A system of similar reserves across several continents offers the best hope for longterm survival of large mammals.
It's a similar idea to the mammoth tundra restoration project I referred to last month. Of course, if they had already gotten Ted Turner on board, I'm sure there would be no need for publicity. As they say, there are already plenty of semi-wild non-native animals on ranches in this country.
I have my doubts about whether the economics will work out in their favor anytime soon, at least across most of the Great Plains. It is certainly true that the population in that area is both aging and decreasing. Hey, I'm a prime example. But the main economic change has been the use of more land by fewer farmers, equipped with better technology, better farming practices, and in many cases greater diversification. There may be some areas of the plains where land values are low enough to make a go of megafauna tourism, but those areas tend to have low carrying capacity for stock.
And it seems pretty implausible to me that ranchers are going to want elephant herds around because they keep the woody plants down (the commentary alludes to this idea). A couple of ranch hands in Montana are not going to sit idly on their horses while a couple of lions attack an elephant on their land.
Then again, it would be entertaining to see the look on a pronghorn's face the first time a cheetah gave it a good chase. As in, "12,000 years of easy living, and now this?!?"
References:
Donlan J et al. 2005. Re-wilding North America. Nature 436:913-914. Full text (subscription required)
Sahara at least 7 million years old
A concise 4-paragraph article by Mathieu Schuster and colleagues reports on dune deposits that show the Sahara formed during the Late Miocene.
After the mid-Holocene humid period (6000 years ago), arid conditions developed throughout North Africa, culminating in the formation of the Sahara, which is the largest warm-climate desert on Earth (9,000,000 km2). However, earlier desert recurrences in the region are also documented. Direct evidence for eolian deposition is given by thermoluminescence dating for the Late Pleistocene; e.g., in Mauritania [25 to 15 thousand years ago (ka)] (1) or in Tunisia (86 ka) (2). The latter is currently considered as the oldest terrestrial record for desert conditions in the Sahara (2), even if firm evidence exists for a pre-Quaternary Great Western Sand Sea in Algeria (3). Some earlier arid episodes (Miocene-Pliocene) were also suggested by marine records off West Africa (4); but until now, no contemporary in situ eolian deposits were known in the Sahara region. In the northern Chad Basin, we recently identified and dated widespread outcrops of eolian dune deposits that are distributed over an area more than 2000 km2. Our results testify that the onset of recurrent desert conditions in the Sahara started at least 7 million years ago (5-7) (Schuster et al. 2006:821).
The desert comes and goes, expanding and contracting -- and those vacillations are recorded by this earliest evidence, also:
In the Toros Menalla region, these eolian sandstones are conformably overlain by a horizon bearing abundant vertebrates fossils, including Sahelanthropus tchadensis, the earliest known Hominid [sic] (5, 7). In this horizon, named the Anthracotheriid Unit, biostratigraphic correlation of the mammalian fauna indicates an age of 7 Ma (5â7).
Now, this isn't news (which I'm sure Science didn't bother to check) since Vignaud and colleagues (2002) published the same evidence, complete with the wind direction chart:
The lower part of the section (at least 4 m thick) is composed of fine to very fine white sands, poorly cemented, and is mainly constituted by numerous quartz grains, without matrix. The grains are well sorted, well rounded, matt and frosted, and are strong evidence for aeolian modelling. The foreset laminations (avalanche laminations in front of the aeolian dune) represent a typically aeolian deposit. These sands show cross-beddings that progressively decrease in size from the bottom (1 - 2 m) to the top (20 cm). This facies exhibits typical alternations of grain-fall and grain-flow laminations, characteristic of aeolian dune deposits. Our interpretation is confirmed by frequent wind ripples at the foot of the fossil dunes, whose crests are perpendicular to the direction of dune progradation. These fossil dunes are, to our knowledge, the oldest evidence for desert conditions in the southern Sahara area (Vignaud et al. 2002:152).
I guess this is the science journal equivalent of getting "punk'd" -- "Ha ha! You published what we printed four years ago!"
I opened up the Vignaud paper to double-check the paleoenvironment in the fossil-bearing layer. From the faunal list, they conclude this:
The oldest known East African hominids (Ororrin [sic], Ardipithecus) are contemporary with faunas associated with wooded environments. Younger australopithecines lived in a wider range of habitats. In contrast, the TM 266 vertebrate fauna contemporary of the Toros-Menalla hominid suggests a mosaic of environments from gallery forest at the edge of a lake area to a dominance of large savannah and grassland. Determining the precise habitat of the TM 266 hominid locality among the mosaic of environments available to it constitutes a research challenge to be met by further laboratory and field studies currently in progress (Vignaud et al. 2002:155).
They (Vignaud et al. 2002) interpreted the succession of dune and lacustrine deposits to mean that the hominids lived in a mosaic environment near sandy desert, but locally including marshy/swampy, lake, and gallery forest. An alternative interpretation might be that the desert really receded (or disappeared) during the later time period when the hominids were there. In either case, the paleoenvironment is interesting, because it means that the Sahelanthropus-like primates colonized (and possibly repeatedly recolonized) areas that were periodically dune desert (and therefore probably not habitable by large primates). This may not mean much in terms of locomotion -- the hominid-bearing unit is clearly water-rich, and we can't refute the idea that the surroundings were as woodland-like as those preserved in the Late Miocene Middle Awash localities.
But I think it is a good hypothesis that all of these apes (or hominids) were very cosmopolitan compared to extant chimpanzees and gorillas. The question is whether their actual dispersal abilities were different from chimpanzees. Prehistorically, genetics would seem to indicate that chimpanzees had long-distance dispersal; the only fossil evidence of chimpanzees has been found in a region that historically did not support chimpanzees; and they today successfully utilize relatively open savanna at the eastern end of their range.
So it is by no means obvious that the cosmopolitan nature of these Late Miocene lineages would have required a specialized terrestrial adaptation -- at least not beyond the specialization of knuckle-walking. So why become bipeds?
References:
Schuster M et al. 2006. The age of the Sahara Desert. Science 311:821. Full text (subscription)
Vignaud P et al. 2002. Geology and paleontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Full text (subscription)
Unappreciated supervolcanoes
I just love that line from this GSA press release (via Science Blog):
North America isn't the only continent that's experienced super-colossal volcanic eruptions in the recent geologic past. The massive explosion of the almost unknown Vilama Caldera in Argentina appears to have matched Yellowstone's last continent-blanketing blast. It may, in fact, be just one of several unappreciated supervolcanoes hidden in a veritable mega-volcano nursery called the Eduardo Avaroa Caldera Complex, located in the inhospitable Puna-Altiplano region near the tri-section of Argentina, Bolivia, and Chile.
"Vilama Caldera formed during a single event that emitted approximately 2000 cubic kilometers (almost 500 cubic miles) of pyroclastic material," said geologist Miguel M. Soler of the National University of Jujuy in San Salvador de Jujuy, Argentina. The volume of ash and pyroclastic material, called ignimbrites, produced by the 8.4 million-year-old eruption, and the size of the associated caldera, put it among the world's largest known eruptions, he says.
"In contrast, for example, Yellowstone produced its important volumes of ignimbrites and lavas in three cataclysmic events. Eruptions at 2.0, 1.3, and 0.6 million years ago ejected huge volumes of rhyolite magma, and each formed a caldera and extensive layers of thick pyroclastic flow deposits," said Soler.
Of course, I suppose it's rather uninspiring to be working on ancient non-super-volcanoes, the plain vanilla volcanoes of the past. I have always thought the Yellowstone events are fascinating, partly because there are pumice mines near my hometown, some 800 miles away (!). They must have been a whole lot worse than Toba, and every one of them could have affected hominids.
The mammoth tundra
Geophysicist Sergey Zimov had an essay in the May 6 edition of Science concerning the ecological pressures that may have changed the Arctic ecosystem at the end of the Pleistocene. I missed it at the time, but courtesy of a reader I include some quotes here. About Zimov:
In 1989, Zimov initiated a long-term project known as "Pleistocene Park," which he now is pursuing with a number of partners. The goal of the project is to reconstitute the long-gone ecosystem of the Pleistocene epoch that supported vast populations of large animals including mammoths, horses, reindeer, bison, wolves, and other large predators. If the effort succeeds in the park, Zimov and his co-workers would like to see the ecosystem restored over much larger areas in an effort to stave off what otherwise could be a massive release of carbon that now is sequestered in the permafrost but that could be released into the atmosphere as global temperatures rise. His hunting of mammoth remains in the tundra and his bold vision of controlling and restoring ecosystems have earned him coverage in books, documentaries, and other media.
The interesting thing to me is what Zimov has to say about the forces internal to the ecosystem that may have led to its decline:
The physiological traits associated with Holocene vegetation partially explain the vegetation changes that coincided with loss of the Pleistocene megafauna. Plant transpiration accounts for most of the water loss from landscapes, and high transpiration rates are associated with more productive plants. Rates of water loss must therefore have been high in the north when productive Pleistocene meadow and steppe vegetation prevailed. As a result, vast amounts of water were sucked up from the ground, resulting in dry conditions, while the plants themselves sequestered nutrients to drive their own productivity.
Holocene vegetation, in contrast, is dominated by unproductive moss and shrubs. This type of vegetation does not transpire enough moisture to dry out the soil. Moss does not even have roots. This leads to wet conditions conducive to the growth of mosses, which account for a substantial proportion of the northern Siberian biomass. Water-saturated soils inhibit decomposition of biomass and therefore the availability of nutrients to support plant growth. What's more, mosses insulate the ground efficiently--a 20-cm layer of moss prevents the underlying frozen soil from thawing. This also has the effect of sequestering nutrients and preventing their cycling through the ecosystem. All of these factors indicate that moss communities, once they are in place, create and sustain their own environment and do not depend so much on particular climate conditions.
They are quite vulnerable to physical disturbance, however, and this is where their ecological connection to herbivores comes in.
When mosses are destroyed on loess soils, the site becomes overgrown with grasses within 1 to 2 years. The grasses then dry out the soil through their high transpiration rates, creating a steppe-like ecosystem. But when herbivore populations are low, grass productivity begins to decrease within a few years, because grass litter accumulates on the soil surface, shading and insulating the soil. In turn, soil fertility declines. As a result, shrubs and mosses, which have lower nutrient requirements than grasses, ultimately become dominant (Zimov 2005:797).
Thus, Zimov believes that the herbivores themselves maintained the grasslands, and when human predation became a severe enough pressure on the herbivores, the ecosystem lost its major regulator.
I can imagine that substantial hunting pressure may have induced an ecological collapse. A reduction in the population density of large herbivores would have allowed them to become less mobile, possibly removing their influence from large areas of the tundra-steppe. The invasiveness of the moss-lichen tundra ecosystem then replaced the grasses in such areas. These high-moisture plants might have colonized the areas originally covered by glaciers more rapidly anyway, so that climate change, human predation, and plant ecology all interacted in the terminal Pleistocene.
On the other hand, I wonder where fire comes in, since it is a major reason for the maintenance of many grassland ecosystems today. Perhaps grassland staves off forest because of fires, but cannot stave off moss?
In any event, Zimov is persuasive when he writes about the non-simultaneity of climate changes and extinctions:
Twenty years ago, scientists explained the disappearance of numerous animals in the northern grasslands very simply--the arid steppe climate changed into a humid one, and when the steppe vanished so did the steppe's animals. In short, the moist Holocene climate was a catastrophe for them. In the last few years, however, a growing accumulation of radiocarbon dates of animal remains has been suggesting a different story. It appears now that mammoths survived the Pleistocene-Holocene shift. For the first 7000 years of the Holocene, they persisted on Wrangell Island in the Arctic Ocean. Bison, horses, and musk oxen also lived in the north of Siberia in the Holocene. Horses and musk oxen lived there even up to historical times (Zimov 2005:797).
And:
But overall, if climate were the only controlling factor, the total pasture productivity and the number of herbivores should have increased in the Holocene. Support for this view comes from the climate history that is chronicled in the Greenland ice sheet. It shows a sharp warming and dramatic increase of precipitation ~14,700 years ago, leading to conditions that resemble the present climate. Even so, in the north of Siberia, mammoth populations soared at this time (Zimov 2005:798).
The question for me is not whether people could have reduced the number of mammoths enough for this to happen. The mammoth tusks that show repeated pregnancies in females, presumably caused by humans killing the offspring, are enough to persuade me of that. The question is whether the mammoths themselves were really enough to maintain the whole ecosystem. Looking at the activity of elephants in Africa, I can see the possibility of it. But it is surely a difficult problem to wrap the mind around.
The essay goes into some detail about the Pleistocene Park project and its attempt to reconstruct the ecosystem. The main struggle is in reducing hunting pressure, which is still depressing wild herbivore populations in Siberia.
If they get it going, I wonder if it would be useful to bring in elephants. It probably wouldn't take too long to get furry ones, with some selection for cold.
Meanwhile, don't even mention bringing back the Neandertals. A little selection and they might evolve into more of us!
References:
Zimov SA. 2005. Pleistocene Park: return of the mammoth's ecosystem. Science 308:796-798. Summary
Nothing says "Christmas" like a dodo death assemblage
I'm sure you've seen the story:
A team of Dutch and Mauritian scientists discovered the bones in a swampy area near a sugar plantation on the south-east of the island.
The bones were said to have been recovered from a single layer of earth, with the prospect of further excavations to come.
Sections of beaks and the remains of dodo chicks were thought to be among the find.
What more can I say? Oh, yeah: now begins the dodo genome project.
A hard bolide to swallow?
For those of you who may be wondering what is wrong with paleoanthropology that we can't just resolve the hobbit problem, I can only say one thing: We are not alone.
For instance, there is the idea that a "mammoth-killing" impact caused the Younger Dryas, suggested in a paper last year by R. B. Firestone and colleagues.
If you like the idea, this seems like a bad sign:
Archaeologist Vance Haynes, professor emeritus at the University of Arizona, Tucson, is finding likely looking magnetic spherules in the darnedest places. He has spent 30 years studying Clovis sites, many of which the Firestone group sampled. As a check on his own ongoing independent analysis of YD samples, he collected a modern sample. "I got 300 grams of dust off the roof [of my house], and it's full of magnetic microspherules," he says. Whether they are the melted, iridium-rich micrometeorites that continually drift down from the upper atmosphere or the product of high-temperature industrial processes such as coal burning, he doesn't yet know. Either way, they could be trouble. The cosmic dandruff of microspherules could have salted sediments forming 12,900 years ago with iridium, while the humanmade variety might have settled on modern outcrops before sampling.
That's from a long news article in Science by Richard Kerr, titled "Experts find no evidence for a mammoth-killer impact."
Like almost every other temperature fluctuation of the last 80,000 years, the Younger Dryas has been attributed with quasi-magical power: in this case, the power to kill mammoths and extinguish cultures. And hey, maybe it really did...but I have a lot of skepticism when associations are made on the basis of dating uncertainty. It would help if these climate changes would affect every species and culture, rather than showing catastrophic effects on particular ones without showing any signs of affecting others.
Of course, without the biological and cultural fallout, the story of a particular climate cycle isn't very interesting. Nobody would care. With the Younger Dryas, we have human archaeological evidence from all over the world 12,900 years ago. Surely something will match! Of course, the megafauna didn't become extinct precisely then, exactly. But no matter -- it's close enough. Surely these events would have been devastating for animals and plants, right? Most of them were lucky enough to escape the devastation with their lives, but a few unlucky victims because extinct, thousands of years afterward.
A meteor helps to spice it up, and in this case we have the whole package:
The catastrophe had taken place a geologic instant ago--closely coinciding with the disappearance of North America's mammoths and the continent's earliest human culture (Science, 1 June 2007, p. 1264). Then came the 26-author paper last October in the Proceedings of the National Academy of Sciences (PNAS), not to mention the hourlong National Geographic Channel documentary running on cable since last October, with more coverage on the way from the History Channel and PBS's prestigious program NOVA.
Uh, yep. Sounds familiar so far. Now is the time to bring out the spurned skeptics:
"The whole thing is contrived," says geochemist and impact specialist Christian Koeberl of the University of Vienna, Austria. "Their data don't agree with anything we know about impacts. It just doesn't make any sense. Occam's razor has been put safely in a drawer somewhere."
Kerr lists an impressive array of scientists who think the evidence for an impact doesn't add up. I don't have any particular opinion -- my main skepticism concerns the proposed link between climate change and Clovis, which is a separate issue from whether an impact occurred.
But the story is a good one to watch, and has many parallels to the Flores hobbit story, considering different standards of evidence, the interactions of specialists from different scientific specialties, and the use (or abuse) of the press.
P. S. A number of abstracts from the 2007 AGU meetings pick up the question.
References:
Firestone RB and lots of others. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Nat Acad Sci USA, 104:16016-16021. doi:10.1073/pnas.0706977104
Kerr RA. 2008. Experts find no evidence for a mammoth-killer impact. Science 319:1331-1332. doi:10.1126/science.319.5868.1331
Metagenomics under 2 kilometers of ice
Andrew Curry profiles ancient DNA researcher Eske Willerslev, of the University of Copenhagen. Willerslev is best known for the characterization of chloroplast and mitochondrial DNA from old tundra cores, and this week's paper interpreting local biotic diversity from Greenland ice cores.
Willerslev began to wonder about the ignored ice core bottoms in the building his lab shared with Steffensen's climate research group. "I did the permafrost stuff, and then suddenly it hit me: Silty ice is icy permafrost, right?" Judiciously cutting and melting the core bottoms, Willerslev and his colleagues analyzed the resulting water for signs of DNA. What Willerslev found, and reports on page 111, broke his own record for the oldest DNA ever recovered, and promises to rewrite the history of Greenland's climate. His team identified and dated genetic sequences from coniferous trees, butterflies, beetles, and a variety of other boreal forest plants--traces of ancient forests that Willerslev says covered southern Greenland perhaps as far back as 800,000 years ago.
The paper itself is interesting and brief -- Willerslev and colleagues found DNA from boreal forest tree and insect species at the bottom of a 2-km ice core from south-central Greenland. The trick was dating the thing, and using 4 different methods they conclude the ice is between 500,000 and 1 million years old.
References:
Willerslev E and 29 others. 2007. Ancient Biomolecules from Deep Ice Cores Reveal a Forested Southern Greenland. Science 317:111-114. doi:10.1126/science.1141758
Tropical African climate after LGM
Johan Weijers and colleagues (2007) found that rainfall likely increased in tropical Africa at the end of the last glaciation and during the Holocene, with a brief interruption that approximately corresponds to the Younger Dryas.
We analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the Congo River outflow, and the results enabled us to reconstruct large-scale continental temperature changes in tropical Africa that span the past 25,000 years. Tropical African temperatures gradually increased from 21° to 25°C over the last deglaciation, which is a larger warming than estimated for the tropical Atlantic Ocean. A direct comparison with sea-surface temperature estimates from the same core revealed that the land-sea temperature difference was, through the thermal pressure gradient, an important control on central African precipitation patterns.
Their reconstruction indicates a greater temperature rise over land than over the Atlantic with Pleistocene and Holocene global warming, which is expected to draw moisture onto the land. Tropical rainforest area was reduced during the glaciations, which seems to have been known from climate modeling:
This difference between continental and oceanic deglacial warming in the tropics is in agreement with climate model studies, which suggest that the average continental deglacial warming in the tropics was about 1.5 times stronger than the deglacial warming of the tropical oceans (23, 24). This amplified continental warming may have been because the continents cool more during glacial times. At high and mid-latitudes, the presence of a changed and often reduced vegetation cover during glacials results, through increased albedo, in enhanced cooling of the land surface. However, in tropical areas, this effect is counteracted by a negative feedback from reduced evaporation, which results from decreased tropical rainforest area [again, during glacials]. The reduced evaporation leads to a decreased loss of latent heat and thus relatively warmer surface temperatures. As a result, vegetation changes in tropical Africa are thought to result eventually in a negligible temperature effect (25). A more likely explanation for the enhanced glacial cooling of the continental tropics may thus be an increased pole-to-equator temperature gradient, resulting in a strengthened and enlarged Hadley Cell circulation (26) and, consequently, an increase in relatively cool air that flows from higher toward lower latitudes.
The paper also includes a good review of other data on African climate during and after the last glacial maximum. They claim that tropical Africa reflects global climate changes because it receives airflow from both Northern and Southern hemispheres, and the data connect to similar patterns elsewhere.
References:
Weijers JWH, Schefuss E, Schouten S, Damsté JSS. 2007. Coupled thermal and hydrological evolution of tropical Africa over the last deglaciation. Science 315:1701-1704. doi:10.1126/science.1138131
Leading me to climate frustration
Science seems to have had a stealth theme going last week on climate change, and it included this perspective by Anna Behrensmeyer on climate change in human evolution.
The central insight is the great difficulty of finding causes among temporally correlated events:
Another challenge is deciding what constitutes a strong case for a causal link between a climate change and an evolutionary event. We can't step into a laboratory to test the impact of climate change on the human genome, but we do have the results of natural experiments--the proxy evidence for environmental changes in continental rock sequences, as well as many fossils of hominins and other organisms that were evolving on different continents during that same time period. There is a rich body of data to draw upon, but hypotheses are often structured around an assumption that "synchronous" events in the geological and paleontological record constitute evidence for cause and effect. These hypotheses, while seductive in their simple explanation of how our species came to be, do not do justice to the complexity of the climate-evolution problem (see the figure) or to the full range of evidence and scientific methodologies that now can be brought to bear on this problem.
I'd say that sums up some frustrating problems very well. There is no climate-altering event during the past seven million years small enough that some paleophile hasn't offered it up as the key factor in human evolution. But how can you prove anything? How can you even test the hypothesis of causation for most of these?
I had to read this sentence a few times, but I think it circumscribes an essential problem:
The related notion that fluctuating lake levels provided environmental stress that drove speciation does not provide a mechanism for how this could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of a new genus and species.
This is the problem with almost any climate-driven hypothesis. How did the change in climate cause anything to happen? Especially considering the huge bias in the sites we have to sample. Sure the Rift Valley paleoenvironment was changing during the Late Pliocene, but how central was that area to the hominid range as a whole? It's like diagnosing the causes of the American Revolution only knowing what happened in Charleston.
What we sorely lack is mechanisms that would link climate change to fitness in hominid populations. So far, there are generally two serious options: "Trees Too Far To Walk Between", and "Volcanic Winter".
The story is especially bad for the origins of Homo in the Late Pliocene. The received wisdom is that the global climate got cooler, and East Africa generally got drier. We know the robust australopithecines appeared, as did stone tool manufacture and presumably Homo. But how are these linked? It would of course help if we knew what robust australopithecines ate! If we link stone tools to meat eating (reasonably), at least we have a mechanism for dietary change in Homo. But what does a drier climate have to do with that? More antelopes?
In that paragraph, Behrensmeyer is discussing the range of climate-change hypotheses for the origin of Homo, it continues:
Other proposals instead have linked human evolution with increasing aridity and climate variability. Finally, other paleoclimatic evidence indicates drier rather than wetter climatic conditions between 2.7 and 2.5 Ma [see the figure, land record (center)], bringing into question the extent of a prolonged high lake phase throughout East Africa. Although the multibasin approach to establishing regional paleoclimate trends is commendable, the proposed causal link between a wet climate phase and the origin of Homo is not yet supported by sufficient evidence to establish its credibility.
Aaarrggh! Are the lake levels a red herring? Are the global climate figures a red herring?
The stinky fish are our only trail. What we've got is the sites we've got and the climate records as they are. Is there a good reason to think they say anything interesting about the human lineage? That depends how much of the lineage we've sampled with those sites. Yuck!
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks