About 23,000 years ago, in a period of intense
cold that preceded the end of the last ice age,
glaciers from west and east merged to cut off
Alaska from North America. With so much of
the world’s water locked up in ice, sea levels
were much lower and a now-lost continent
, Beringia, stretched across what is now the
Bering Strait to join Siberia to Alaska. But
people who had trekked across Beringia to
Alaska could go no further because of the
ring of glaciers that blocked their way south.
to retreat and an ice-free corridor, roughly
900 miles long, opened between Alaska and
the Americas. In the middle of the corridor
lay a body of water, 6,000 square miles in
area, fed by the melting glaciers and known
as Glacial Lake Peace. Not until the lake had
drained away, and plants and animals had
recolonized the corridor, would early peoples
have been able to support themselves as they
traversed the corridor between the glaciers.
DNA, the two teams of scientists have each
developed ingenious ways to calculate the
date at which the corridor first became fit
for human travel. A group led by Peter D.
Heintzman and Beth Shapiro of the
University of California, Santa Cruz,
regards bison as the ideal proxy for
assessing human travel through the
corridor, given that bison were a
major prey of early hunters.
ago, the bison populations in Alaska and
North America were separated and started
to evolve minor variations in their
mitochondrial DNA, a genetic element
that survives well in ancient bones. Dr.
Shapiro’s team collected ancient bison
bones from up and down the corridor,
analyzed their mitochondrial DNA and
looked for Alaskan bison that had
traveled south through the corridor
and American bison that had traveled
The corridor was “fully open” for bison
traffic about 13,000 years ago, Dr.
Shapiro and colleagues reported on
June 6 in the Proceedings of the
National Academy of Science, and
human populations could have
traversed it at the same early date.
“Our chronology supports a habitable
and traversable corridor by at least
13,000” years ago, “just before the
first appearance of Clovis technology
in interior North America,” they write.
to belong to the first people to reach
the Americas. But archaeologists have
now detected human presence in the
Americas as early as 14,700 years ago.
Since the corridor was closed at that
time, presumably those first
immigrants took a coastal route and
arrived by boat. But the Clovis people
could have arrived later through the
corridor. Also, Dr. Shapiro’s team
notes, people already in North
America could have used the
corridor to travel north.
with Dr. Shapiro on the general
chronology of the corridor but puts
ts earliest possible opening some
500 years later, enough to tilt the
scales against any significant use
of it by the Clovis people. A team led
by Mikkel W. Pedersen and Eske
Willerslev of the University of
Copenhagen has examined ancient
DNA and pollen from sediments
of lakes thought to be the remnants
of Glacial Lake Peace.
have now been decoded that the snippets
of ancient DNA can be identified by looking
for matches in DNA databanks.
The researchers infer that as the lake
shrank, grasses and sedges started growing
around it, followed by sagebrush,
buttercups, birch and willow. About
12,500 years ago, DNA from bison,
voles and jack rabbits appears in the
lake sediments, Dr. Willerslev’s team
reports in Wednesday’s issue of Nature
date at which the corridor would have been
able to supply bison for human travelers.
The corridor therefore “opened too late to
have served as an entry route for the
ancestors of Clovis,” who were present
in North America by 13,400 years ago,
the Willerslev team states. It prefers a
date 400 years earlier for the Clovis
culture than that of the Shapiro team.
the general date for the opening of the
corridor, have each found reason to
suppose the other is wrong on the
issue of its use by the Clovis people.
and American bison lineages analyzed
by the Shapiro team could have become
distinct before, not during, the merger of
the glaciers 23,000 years ago. “If bison
could move north and south through the
interior ice-free corridor, why should
they not also have been able to do so
before the ice caps completely blocked
the way?” he said. If so, the presence of
northern bison in the south or vice
versa cannot be used to date the
opening of the corridor.
W. Ives of the University of Alberta,
said its dating of the split in bison
lineages was more plausible. He also
questioned whether the present-day
lakes sampled by Dr. Willerslev were
true remnants of Lake Peace. They
could have formed many hundreds
of years after Lake Peace disappeared
, in which case they would omit the
earliest sediment layers and evidence
of an earlier opening of the corridor, Dr
. Ives said.
Americans concluded that their ancestors
had arrived in the Americas as part of a
single migration but that this group had
split in two by around 13,000 years ago.
The Shapiro and Willerslev teams agree
that this migration must have arrived by
some route other than the corridor,
presumably along the coast. It remains
to be seen what role, if any, the corridor
played in the population split that occurred
around the time of its opening.