11 MAY 2006
If you dig up and date bones, you may notice a peculiar coincidence. Shortly after people reached the western hemisphere, a herd of big mammal species suddenly went extinct. You don't see saber-tooth cats strolling the forests of Mexico these days, where they were once almost as common as taquerias.
At a dig, a fossil mammoth tusk is prepared for
removal. Slap that baby in the carbon dater, and can find out when
it lived. Collect those dates, and you may figure out why it died. Photo: State
of Utah
Up in Alaska, the humans who first arrived from Asia about 13,000 years ago could have posed for postcard photos with the mammoths and horses they found grazing in the area. Nowadays, not even the local taxidermists have specimens of these impressive four-foots.
Why? Did some bizarro new disease suddenly whack large mammals? Did an ecological imbalance sparked by the demise of a "keystone" species (think: mammoth) lead, domino-effect-wise, to widespread animal extinctions?
More provocatively, did humans, acting as "blitzkrieg hunters," exterminate their prey during an sanguineous orgy in their first 1,000 years in the Americas?
The bloodthirsty overhunting hypothesis is complimentary, as it gives lavish credit to the smarts of the two-footed mammal that moved across from Asia. But it's unlikely to have the ancillary virtue of truth, says R. Dale Guthrie, a paleontologist at the Institute of Arctic Biology at the University of Alaska, Fairbanks.
Rather than paleo-folks with spears sparking a butchery worthy of Antietam or Verdun, he thinks a mix of weather and ecological conditions pushed the horse, wooly rhino and mammoth from the stage, during a period when humans, bison and elk did rather nicely, thank you very much.
Hunting for evidence
After dating bones from about 800 sites, Guthrie thinks he has detected a transitory garden of Eden period in the north about 13,000 years ago. Before that period, as the last wave of Pleistocene glaciers was melting, the region surrounding the North Pole was windy, arid, and treeless.
That made it rotten habitat for most grazers, although some, especially mammoth and horse, did cling to life. The sea level was low, because so much water was still frozen in the glaciers, and that exposed a "land bridge" linking Asia to the Americas.
(left) During the last ice age, sea level was 120 meters
or more below its present position, and the Bering land bridge formed. The shallow
continental shelf that
was exposed made a wide connection between northeastern Asia and Alaska, and
many plants and animals (including humans) crossed.
(right) When the ice-age glaciers melted, the ocean rose. This is the modern shoreline of western Alaska and eastern Siberia. Maps from USGS
Scientists agree that humans traveled north in Eurasia during this period, then across the land bridge. Their migration was sparked by climate change, Guthrie says. As the north grew warmer and wetter, edible grasses and edible woody plants flourished. Mammoth, bison and elk took advantage of the situation, by chowing down and multiplying, as you can see from the graph.
In Alaska and the Yukon, the end of the ice age was a
golden age for mammals and the plants they ate: grasses (gramineae) and willows
(salix). Then taiga
(sparse forest) and tundra moved in, bringing "Mr. Yuk" dwarf birch
(betula). Courtesy Dale Guthrie, University of Alaska
But not for long. As mother warned, the problem with good times is that they usually don't last.
Within centuries of human arrival, the vegetation changed again, and inedible tundra plants and dwarf birch trees took over the land. "About the time horses were extinguished and the mammoth was declining, we get an invasion of the whole area with this rather toxic vegetation," Guthrie says. "The decline of these animals corresponds exactly to the peak of dwarf birch."
The ecological change probably whacked the mammoth and horse, Guthrie suggests,
while moose prospered due to its rare talent for browsing tree leaves.
Dwarf birch (Betula nana) grows between tussocks of cottongrass sedge. The shift from ice-age grassy vegetation to sedge-moss, low-shrub tundra coincides with the extinction of many grazing mammals (mammoth, horses, steppe bison, saiga antelope, helmeted musk oxen) around the Bering Sea. Photo: T. Ager, USGS
Let the good times roll!
You can imagine the same change by envisioning the dry, cold Dakotas, Guthrie says, where the short-grass prairie resembles the Pleistocene (ice age) steppes that once carpeted Alaska. Scanty rainfall limits plant productivity. The grazing would improve if the climate turned warmer and wetter, as Alaska's did 13,000 years ago. But after several centuries, you could expect trees to migrate south from Canada and start to starve the grazing animals that can't eat trees.
Something similar happened in Alaska and the Yukon around 13,000 years ago, Guthrie says. The global climate suddenly warmed, local rainfall increased, and willow, grass and other tasty vegetation flourished. Humans trotted across the land bridge from Asia, doubtless drawn by the Bermuda-like forecasts on paleoweather.com, gravy-intensive menus at eatmoremeat.fork, or the rock-bottom real-estate at alaskalandrush.scam.
Whatever.
Then, slightly less than 12,000 years ago, the environment of Alaska and the Yukon pulled another swaperoo. All those years of grazing had created evolutionary pressure in favor of Mr. Yuk plants that would turn even a mammoth's mammoth stomach. In the north, Guthrie notes, a plant has a hard enough time living from one year to the next without becoming a mammoth meal, so plants that make toxins have an evolutionary advantage.
And so evolution through natural selection ushered the mammoth and the hungry horse from the scene. Bison, moose and elk were better adapted to the new tundra, so they prospered, and the clever humans held on as well.
Ultimately, instead of blaming human hunters for the disappearance of horse and mammoth, Guthrie sees both the human appearance and the animal disappearance as consequences of climate change.
The same species of elk, or wapiti, occurs today in Mongolia and North America. Elk crossed the land bridge into Alaska from Asia during the ice ages. Photo: Richard Reading, USGS
Follow the sun
Guthrie raises other questions about the "blitzkrieg" extermination hypothesis. For example, there is no solid evidence that people hunted the mammoth and horses they found in Alaska and the Yukon. But, Guthrie says, there is clear, convincing and continuing evidence that humans hunted bison, elk and moose. And they all survived the transition.
One final thought. The new study found major population increase among bison and elk about 14,000 years ago. "In any other context," Guthrie says, "diet would be part of the explanation" for That increase. The equation is older than the science of ecology. More food translates into bigger consumers. Just ask the star of Super Size Me."
And diet also explains why people moved across the land bridge: There was food on the other side. The blooming of humans and large mammals in Alaska and the Yukon happened in "a much more nutritious time," Guthrie says
-- David Tenenbaum
Bibliography
• New Carbon Dates Link Climatic Change
with Human Colonization and Pleistocene Extinctions, R. Dale Guthrie,
Nature, Vol 441|11 May 2006|doi:10.1038/nature04604.
Related Why Files
• Polar Science
• Extinct Giants
