Where Did The Mammoths Really Go?
Dung Spores Offer New Insight Into An Old Debate
Fifteen thousand years ago, North America was home to roughly 35 different types of large mammals which no longer exist today, such as saber tooth cats, giant ground sloths, and mammoths. Many species weighed over two thousand pounds; a full-grown adult male mammoth could weigh up to 20,000 pounds, the weight of a city bus. Why did so many large animals die off by the end of the last ice age? It's rare to find a mystery as enigmatic or hotly debated as the cause for the disappearance of North America's largest known mammals. Scientists have worked to elucidate the cause of this extinction for over fifty years. Investigation of fungus that lived in megafaunal dung has led to a new timeline for the extinction, and the results were published in the November 2009 issue of Science.
Several major events occurred in North America during megafaunal population decline which have fueled speculation about the causes of extinction. Climate changes, over hunting by humans, and a purported cosmic impact are all posited as causing megafaunal extinction. While there is some support to link these events to extinction, there is not overwhelming evidence for any one hypothesis. Furthermore, parsing correlation from causation for events ten thousand years in the past is difficult, to say the least.
Traditionally, tracking the decline of these giant animal populations has relied on radiocarbon dating of fossils. Based on such evidence, most extinctions are dated somewhere between 13.3 to 12.9 thousand years ago. Dung fungal spores, a different proxy to measure megafaunal populations, were analyzed by Jacquelyn Gill, John Williams, and colleagues at the University of Wisconsin-Madison. The fungus that lived in the dung of plant-eating megafauna thousands of years ago left hardy spores that may provide an indirect measurement of the number of animals present at that time. While not a novel technique, Gill's innovation was tying the spore data to other ecological indicators in the same sediment layers including plant matter, pollen, and charcoal.
Gill's data indicate that megafauna populations began to decline earlier than previously expected: beginning at 14.8 thousand years ago. "By approximately thirteen point five thousand years ago you see a megafaunal collapse that doesn't recover. We thought, wow, that's quite a bit earlier than the fossils indicate." Gill describes the population decline between 14.8 and 13.5 thousand years ago as the functional extinction. The final extinction didn't occur until about 11.5 thousand years ago, suggesting that it took longer than previously thought for the megafaunal populations to dwindle. "Because of the timing, we can rule out geologically instantaneous events" such as cosmic impact or quick human overkill. Human hunting over a brief time period--first referred to as blitzkrieg by Paul Martin in 1967--is usually attributed to the Clovis people who arrived on the scene about 13.3 thousand years ago.
As expected in a field that is so hotly contested, Gill's paper has had varied reception in the scientific community. While many say her results make sense, there are still questions left unanswered. According to Gary Haynes, archeologist at the University of Nevada-Reno, the Gill article "holds water" and population decline could have begun a thousand years before the final extinction. Russell Graham, professor of geosciences at Pennsylvania State University, agrees that the paper has merit, but points to mastadon and mammal fossils found in the same area that date later than the spores indicate.
Because analysis of fungal spores buried in such deep, ancient, sedimentation layers is still a relatively new technique, the various factors controlling the deposition and preservation of the dung-fungus spores are poorly known, points out Eric Grimm, curator and chair of botany at the Illinois State Museum. Grimm questions whether the decline in spore levels at one site can be necessarily attributed to the decline in megafauna, or whether other factors affecting the transport and deposition of the spores could muddy interpretation of the spore analysis.
The relative timeline within Gill's study comparing the spore, pollen, and charcoal samples makes sense, but the absolute chronology is difficult to establish, says Paul Koch professor of earth and planetary sciences at the University of California-Santa Cruz. Unfortunately, in this case "there's no way to compare apples to apples," says Gill; the spore data cannot be easily compared to fossil data.
Everyone, Gill included, agrees that the study is limited in scope since samples were only taken from three lakes. It is unknown whether the results can be generalized across North America. "It's still early in the game," asserts Koch.
Although Gill's paper refutes the Clovis blitzkrieg hypothesis, it leaves a window open for a gradual human over-hunting hypothesis, perhaps by pre-Clovis people. Haynes points out that as soon as modern humans arrive in locations all over the world, there is very quick collapse of megafauna populations. "Almost everything that's big gets wiped out immediately." While the medium-sized fauna can survive for longer, "we are bad news for big animals." Haynes admits the possibility that there may have been pre-Clovis hunters who played a role in megafauna decline, but he says the evidence for their existence is sparse, describing them as "archeologically invisible people." Although climate may play a role, Haynes thinks that humans are the key factor.
A small cadre of scientists have recently proposed a cosmic impact 12.9 thousand years ago as the cause for megafaunal extinction. The impact hypothesis is inconsistent with Gill's research establishing an earlier date for population collapse beginning at 14.8 thousand years ago that declined over a longer period of time. Cosmic impact proponent James Kennett, professor of earth science at the University of California Santa Barbara, says "I have not seen anything yet, including the Gill article that would compel me to change my mind." The flashiest of evidence supporting the impact hypothesis is the discovery of nano-diamonds found in the sediments and glacier layers deposited near the time of the megafauna extinctions. The carbon of these minute diamonds is crystallized in an unusual hexagonal geometry, invoking an extraterrestrial origin such as an asteroid impact.
Most scientists studying the mystery of megafauna decline do not accept the newer cosmic impact hypothesis. Critics point to the absence of an appropriately aged impact crater. In addition, Donald Grayson, archeologist at the University of Washington, points out that not all of the big mammals died off at the same time. The giant sloth populations may have declined as far back as 28 thousand years ago "so the idea that this impact could have caused the extinctions of all these animals just does not make sense," Grayson said in an interview with NOVA.
Climate change is another potential culprit for megafaunal demise. At the end of the last ice age, in the Pleistocene, there was a series of warming and cooling cycles. The Bølling-Alerød warming period occurs at about the same time the megafaunal population began to decline according to Gill's work. The Younger Dryas, a colder era, lasted from 12.8 to 11.5 thousand years ago, close to the date of final megafaunal extinction. One mechanism by which climate change could alter the survival of large mammals is by changing the type or availability of vegetation, and hence the food source of the megafauna. Pollen and charcoal data collected by Gill and colleagues demonstrate that the drastic vegetation changes from parkland to a more woody ecology occur after the megafaunal population decline. Because of the sequence of events determined by their analysis, they reason that the vegetation changes are due to the loss of herbivory when the large animals died out, not the the other way around.
However, Gill's work does not rule out other consequences of climate change as causes for megafaunal extinction. Climate warming or cooling could reduce the geographic range available to animals, leaving them more vulnerable to chance events such as fire, hunting, or storms according to Graham. "Climate is complicated" and could have many affects including vegetation, habitat range, precipitation, and seasonal changes--all of which could influence the survival of megafauna, says David Meltzer, professor of prehistory at Southern Methodist University.
Hybrid models that combine climate change and human hunting also have support in the scientific community. Megafauna may have been stressed by the various consequences of climate change, but human hunting, even mild hunting, may have pushed the massive mammals over the edge, says Haynes. "There would be no extinction without people, but of course environment matters," adds Koch.
Gill's work was not originally intended to crack the extinction case, but rather to investigate the ecological impacts due to disappearance of large plant-eating animals. However, Gill found herself front and center of the debate after publishing her results in Science last November. "No young scientist wants to develop a reputation for such controversy," says Gill, currently in the middle of her graduate program. Gill describes herself as "deliberately agnostic" about the cause of the megafauna extinction and is focusing instead on the ecological consequences of animal population decline. Like the scientists who have walked this path before her, she is careful not to jump to conclusions.
In a debate such as this, about events so long ago, it is difficult to make conclusions with full certainty. "The process of science is still ongoing," as Meltzer says. Scientists continue to tease apart the clues as they are uncovered, whether they appear in the form of dung or diamonds, to investigate the mystery of North America's biggest beasts.
Kate Stoll is a graduate student in biochemistry at the University of Washington.
Top: Large mammals, some weighing over a ton, grazed on the once grassy parklands tens of thousands of years ago. Image: Barry Carlsen, the University of Wisconsin Board of Regents
Middle: Jacquelyn Gill, graduate student at the University of Wisconsin-Madison, examines lake sediment cores to investigate megafauna extinction of the last ice age. Photo: Jeremy Parker
Bottom: Dung fungal spores can provide a proxy for large mammal populations. Image: Guy Robinson