On Faculty: 
Gauging Panama's Influence 3 Million Years Ago


Emil Venere
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Homewood News and Information

     Paleontologist Steven Stanley believes he has found an
ironic twist to the human evolution story.

     Stanley, in a paper published this month in the Journal of
Paleontology, cites evidence that the Ice Age and, eventually,
the evolution of the human race were triggered when a chance
movement of plates in the Earth's crust formed the Isthmus of
Panama 3 million years ago.

     There was a time when the Arctic had no tundra; the region
was covered with something resembling evergreen forests. The
seasons blended together, without the extremes of today's annual
weather cycle. On the North American continent, a subtropical
climate reached all the way to present-day Virginia.

     But the Ice Age ushered in a whole new era of more seasonal
weather. This rude global awakening was a product of geological
coincidence--a narrow strip of Earth's crust became a barrier
between the Atlantic and Pacific oceans in a crucial location,
preventing the waters from mixing and eventually leading to an
increase in salinity of water in the Atlantic Ocean, Stanley
says. Heightened salinity, in turn, changed ocean flow patterns,
preventing warm water from reaching the Arctic Ocean, which
started a cascade of events that brought about the Ice Age.

     Meanwhile, thousands of miles away, in tropical Africa, the
chain of climatic and ecological events ultimately resulted in   
the origin of human beings, Stanley theorizes. 

     More arid, and slightly cooler, conditions caused a dramatic
shrinking of the rain forests. Humankind's tree-dwelling
ancestors, suddenly faced with a dwindling forest habitat
produced by climate change, were forced to begin living on the
ground, setting the stage for the evolution of larger-brained,
upright-walking descendants.

     Stanley conceived his theory while writing a book about the
origin of human beings. The book, Children of the Ice Age: How a
Global Catastrophe Allowed Humans to Evolve, will be published in
April 1996.

     His theory is centered on the observation that the present
"conveyor belt" of ocean water flow that keeps the Arctic Ocean
cold could not have existed without the Isthmus of Panama. The
conveyor belt, a huge ribbon of moving water, circulates from the
central Pacific Ocean, through the Indian Ocean, around Africa
and westward across the Atlantic Ocean, where--blocked from the
Pacific by the Isthmus of Panama--it joins the warm ocean current
known as the Gulf Stream on a curved path northward.

     When the conveyor belt reaches the northernmost Atlantic
Ocean, most of the warm water descends and turns back to the
south, traversing the ocean at great depth, passing back around
the tip of Africa to the Pacific Ocean. It then rises in the
Pacific, north of the equator, looping back and retracing its
path to the North Atlantic.

     Previous theories have failed to explain why the Arctic
suddenly grew colder slightly more than 3 million years ago. Some
of those theories suggest that the expansion of glaciers caused
the Ice Age, but, in fact, the glaciers were the result of the
Arctic freeze, not the cause, Stanley argues. He points out that
the Arctic is still frigid today, even though the glaciers have
shrunk considerably.

     Before the Isthmus of Panama and the conveyor existed, warm
water from the Atlantic Ocean mixed freely with the Arctic Ocean.
The conveyor cut off that source of warm water by routing it
deeper and southward, Stanley says.

     "You suddenly rob the Arctic Ocean of its source of heat. It
became what I call the Arctic pond. It just sits there at a high
latitude, with the sun's rays striking at a low angle," says
Stanley, a professor of paleobiology in the Department of Earth
and Planetary Sciences.

     The Ice Age hasn't actually ended. "We are simply in an
interval of glacial recession," Stanley says. The prominence of
glaciers waxes and wanes over long periods of time, with changes
in the Earth's rotational dynamics, such as the angle of the
Earth's tilt on its axis and the shape of its orbit, he says.

     The Arctic pond touched off the Ice Age, sending frigid air
and water southward, and bringing about a dramatic change in
global climate. Increases in snowfall and accumulation of sea ice
in the polar region also caused more sunlight to be reflected,
making it even colder, says Stanley, who studies fossil records
to trace the history of life and ecosystems.


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     Steven Stanley has a knack for upsetting the apple cart of
scientific dogma; his Ice Age theory isn't the first time he has
proposed something controversial. The paleobiologist isn't timid
about wandering outside his chosen field, the evolution of
mollusks.

     His theory explaining why humans evolved from their apelike
ancestors has drawn praise as well as skepticism from the
scientific community. 

     The most popular idea is that evolution took a decisive turn
toward humanity when protohumans climbed out of the trees and
began living on the ground after a global cooling turned much of
Africa's rain forests into savannas. But, while Stanley agrees
with that overall scenario, he has proposed a bold new mechanism
to further explain the evolution of humans. 

     The infants of tree-living apes must be able to fend for
themselves much faster than human infants; they must be able to
cling to their acrobatic mothers and learn how to feed
themselves, so their brains are nearly fully developed at birth.

     Stanley theorizes that when the prehuman apes, known as
Australopithecus afarensis, began living on the ground, infants
could take more time to develop. Like other primates, a human's
brain is about 10 percent of its body weight at birth. But unlike
other primates, humans retain that ratio throughout life; for a
year after birth, the brain continues to develop as fast as the
body does.

     That makes human infants helpless much longer than other
primates. "Our whole development is slowed down," Stanley said.
"It's as if we have a gestation period that goes on for a year
after birth."

     Living on the ground made it possible for that extended
brain development, he said.
--E.V.