快猫短视频

WHEN Colombus 鈥渄iscovered鈥� the Americas half a millennium ago, it was a
profound shock for European intellectuals. As well as redrawing their maps of
the world, they were forced to reassess their place in it. And native Americans
were equally stunned by the unexpected arrival of people from unknown shores.
Both groups struggled to adapt their world view to incorporate the other鈥檚
existence, often through wild speculation and mythology.

This century, anthropologists wondering when and how the first humans set
foot on the vast continent, came up with a more prosaic explanation. Diverse as
Native Americans are, they all seemed to originate from eastern Asia. Their
pioneering ancestors supposedly arrived in the New World little more than 11 300
years ago on foot, in three waves of migration over the Bering land bridge that
used to link Siberia with Alaska.

Evidence for this view started accumulating in the 1920s and went
unchallenged for decades. But anthropologists have now been forced to reassess
their ideas. 鈥淲e are in the middle of a paradigm in the process of shifting,鈥�
says Richard Jantz, an anthropologist at the University of Tennessee at
Knoxville. Driving this shift is a confection of evidence, including powerful
new genetic data and dogged analysis of a once highly controversial palaeoindian
settlement in South America. And there are tantalising hints in the anatomy of a
9300-year-old skeleton that was found in Oregon two years ago. 鈥淲e are looking
at a much more complex scenario emerging,鈥� says Jantz. The picture is of the
first settlers reaching the Americas at least 25 000 years ago. It seems likely
that people arrived by boat as well as on foot, and, most intriguing of all,
east Asians may have had European companions on the virgin continent.

Early this century, anthropological dogma had it that the New World had been
inhabited for only a short time, perhaps a couple of millennia, and by
technologically naive foragers. But a discovery in 1927 at Folsom, New Mexico,
exploded that view. A stone projectile point was found embedded between the ribs
of a bison that had become extinct at the end of the last ice age, 10 000 years
ago. Five years later, archaeologists working in a gravel pit near the town of
Clovis, again in New Mexico, unearthed more primitive stone points, this time
alongside the bones of mammoths. The image of the first Americans as simple
foragers quickly gave way to one of a people who were big game hunters par
excellence, sweeping down from Alaska to the far reaches of South America,
leaving a trail of carnage as they went. When radiocarbon dating showed that
these colonisers arrived more than 11 000 years ago, when the Earth was still in
the frigid grip of an ice age, the image of a heroic, pioneering people was
further enhanced.

A way through the ice

As it has turned out, big game hunting was only one of the ways that
palaeoindians got by. Some were simple foragers. However, they did indeed have
to struggle with the challenge of making their way from the Old World to the New
while much of North America was buried under ice sheets almost two kilometres
thick. For periods from 75 000 to 10 000 years ago, during the last throes of
the Pleistocene ice age, the oceans were at least 50 metres lower than they are
today, low enough to expose the Bering land bridge. Nevertheless, this wasn鈥檛 a
continuous green light for entry into the Americas, because the fluctuating size
of the eastern and western ice sheets of North America left only two windows of
opportunity for safe passage through an ice-free corridor between them: before
20 000 and after 12 000 years ago. The age of the many Clovis sites鈥攁ll
clustering around 11 000 years鈥攅ncouraged the widespread conviction that
the first Americans had entered through the second window. And the paradigm for
the event was 鈥淐lovis first鈥�.

However, this paradigm did not discourage a blizzard of claims for
archaeological sites predating Clovis, some by as much as 200 000 years. But, as
David Meltzer, of Southern Methodist University, Dallas, pointed out recently,
鈥淥f the scores of pre-Clovis archaeological finds made in the last sixty years,
none so far has withstood the harsh glare of critical scrutiny.鈥� The claims have
been rejected on various grounds including erroneously old dates and naturally
shaped stones mistaken as human handiwork. 鈥淪o many pre-Clovis claims have
failed that the archaeological community grew sceptical of any and all
pre-Clovis claims,鈥� says Meltzer. Until Monte Verde, that is.

Between 1977 and 1985, Thomas Dillehay of the University of Kentucky led an
excavation of a palaeoindian site at Monte Verde in southern Chile. His team
found that about thirty people had lived on the sandy bank of a small creek,
where they erected hide shelters and lived mainly by foraging. They ate roots,
stems, fruits and nuts, many of which are still used by the local Mapuche people
today, as food, drink and medicine. Animal bones large and small, including
those of mastodons, testify to the Monte Verde group鈥檚 taste for meat. Stone
points and grinding stones, digging sticks and bola stones (still used by
technologically primitive people today to disable small animals) yield a hint of
the settlers鈥� technology. Most poignant of all is the print of a child鈥檚 foot
near a hearth.

The Monte Verde site is well preserved because soon after its people left,
the site was covered by water and fibrous peat which kept out oxygen and so
slowed decay. The richness of the site makes Monte Verde important enough, but
more astounding is the age of more than 30 samples of charcoal, wood and ivory.
Radiocarbon dating sets the time at 12 500 years, more than a thousand years
earlier than Clovis. If true, this would undermine the dominant Clovis paradigm.
No wonder Dillehay had such a hard time persuading his fellow archaeologists to
take him seriously, even after a decade of meticulously recording and analysing
the material recovered from Monte Verde. Finally, he effectively said: 鈥淐ome and
see for yourself.鈥�

In January last year, a team of nine archaeologists鈥攕taunch sceptics
among them鈥攄id just that, spending a week poring over the site. Convinced
by what they saw, they admitted collectively that their scepticism had been
misplaced. Dennis Stanford, an archaeologist at the Smithsonian Institution and
a member of the team, said, 鈥淚t totally changes how we think of the prehistory
of America. Our models clearly are not right.鈥� Vance Haynes of the University of
Arizona, who has made it his business to keep pre-Clovis claims honest,
described Monte Verde as a 鈥減aradigm buster鈥�. The site had clearly met the three
criteria that archaeologists demanded for accepting a pre-Clovis claim: the
tools are undeniably made by humans; the setting is undisturbed, as the
occupants had left it; and the dating is beyond criticism. Vindicated, Dillehay
said, 鈥淚 knew if they would only come to the site and look at the setting and
see the artefacts they would agree that Monte Verde was pre-Clovis.鈥�

Off the radar screen

So, the Clovis people were not the first Americans. Although some researchers
stick doggedly to the old theory, the widespread acceptance of Monte Verde has
led to a scramble for new scenarios. Ryk Ward, a geneticist at the University of
Oxford, warns that this enthusiasm is leading to less than exacting standards of
assessment of some of the data and to premature conclusions. 鈥淲e need to be
rolling up our sleeves and collecting new data, and be more rigorous about our
analysis of existing data,鈥� he says.

Theories about the peopling of the Americas will, no doubt, get much more
complex before consensus is reached. One problem is the lack of archaeological
evidence. If the dates for the ice-free corridor are correct, then people must
have entered the Americas at least 25 000 years ago. So where are all the signs
of occupation before 11 300 years ago? Aside from a rock shelter site near
Pittsburgh, known as Meadowcroft and claimed鈥攂ut not universally
accepted鈥攖o be at least 14 500 years old, there is precious little else on
the archaeological radar screen that is older than Clovis. 鈥淚t鈥檚 a question of
archaeological visibility,鈥� suggests Meltzer. 鈥淧eople entered a virgin land,
rich in resources and space, and so they probably didn鈥檛 stay in any one place
very long, which is what you need to create archaeological visible sites.鈥� It
could easily have taken 10 000 years or more before populations reached a level
that would start to be detected on the radar screen, he speculates.

With the Clovis bulwark fallen, what of the mode of entry to this new land? A
little more than a decade ago, archaeologists were all abuzz over the so-called
Greenberg hypothesis. It was named after the Stanford University linguist Joseph
Greenberg, who argued that people had entered the Americas in three waves
beginning around 12 000 years ago, each giving rise to a distinct linguistic
group. Two of the linguistic groups, the Na-Dene in the northwest coastal region
(with a pocket in the southwest, the Apache and Navajo), and the Eskimo-Aleut in
the northeast coastal region, are accepted by most scholars. But Greenberg鈥檚
proposed third group, the Amerind in the rest of the continent, raised American
linguists鈥� collective eyebrows. The languages in this putative group are too
diverse to be part of a single family, they insisted. Greenberg stuck to his
guns, as he has many times in his distinguished but often controversial
career.

Then, in 1992, Douglas Wallace and his colleagues at Emory University in
Atlanta announced the results of their analysis of mitochondrial DNA from living
Native Americans. Mitochondria are the cells鈥� energy factories and contain many
copies of a circular piece of DNA. Using enzymes to cut the DNA at certain
specific sequence sites, the researchers found four distinct types, or lineages,
of mitochondrial DNA, which they called A, B, C and D. These lineages are also
found in Asian populations, but not in Europeans or Africans, thus supporting
the theory of native Americans having an ancestral link to Asia.

Moreover, Wallace and his colleagues found that all four lineages exist in
populations of Amerindian people, but in the Na-Dene only A is present, while in
the Eskimo-Aleut people only A and D appear. This distribution of mitochondrial
lineages is consistent with Greenberg鈥檚 proposal of three waves of migration. By
counting the differences in sequences between Asian and American lineages, and
using a generally accepted rate at which mutations accumulate, the Emory team
came up with times of entry for the three groups: 25 000 years for the Amerind
and about 12 000 years for the Na-Dene and the Eskimo-Aleut.

Linguistic clues

The Emory dates fit nicely with the estimated times of the two ice-free
corridors, but for the Amerind, Wallace鈥檚 date was roughly twice as early as
Greenberg proposed. Just recently, another linguist, Johanna Nichols at the
University of California, Berkeley, calculated that the time of first entry must
be as ancient as Wallace says, based on the estimated amount of time required to
produce the diversity of languages present in the Americas, a technique known as
glottochronology. Greenberg disagrees, and sticks with his original calculation.
However, with the acceptance of Monte Verde鈥檚 age there is now no theoretical
block to the earlier dates.

This relatively simple picture has grown more complicated over the past few
years, as more data on mitochondrial DNA have flowed in from genetics
laboratories in the US and Europe. More mitochondrial lineages have turned up,
and all four major types鈥擜, B, C and D鈥攈ave been found in people
from all three linguistic groups, although lineages B, C and D are rare in the
Na-Dene as are B and C in the Eskimo-Aleut. Andrew Merriwether of the University
of Michigan interprets these new data as indicating a single migration, rather
than three.

According to Merriwether, a single wave of genetically diverse people entered
Alaska about 25 000 years ago, probably as 鈥渁 trickle of people over a period of
a thousand years or so鈥�. Some of these people headed south, forming what
Greenberg calls the Amerind family, while others remained in isolated patches in
the north. This latter group must have been living in a gruelling environment
surrounded by glaciers, Merriwether suggests. As a result, their numbers shrank.
However, isolated populations persisted for thousands of years, finally bouncing
back and migrating to (mainly) northern regions. Their reduced genetic diversity
would have produced the skewed distribution of their mitochondrial lineages.

Wallace disagrees. He suggests that the existence of low frequencies of
lineages B, C and D in the Na-Dene, and B and C in the Eskimo-Aleut, are the
result of the later interbreeding of these people with Amerinds. One line of
evidence he invokes to support this view is that lineage A in the Na-Dene is
only 9500 years old while in the Amerind population it is close to 30 000 years.
鈥淚f the Na-Dene and the Amerind people descended from the same population, the A
lineages would be the same age,鈥� he says.

The latest mitochondrial DNA data from the Emory team, to be published later
this year, has produced the biggest surprise: a putative link between Native
Americans and Europeans. A few years ago Wallace and his colleagues found a new
mitochondrial lineage in Amerindians in the Central Great Lakes region in North
America, which they called X. The same lineage is present, at low frequency, in
European populations, but is absent in Asian populations. 鈥淚nitially we thought
X was present in Native Americans as a result of interbreeding between them and
Europeans, post-Columbus,鈥� says Wallace. 鈥淏ut that turned out not to be the
肠补蝉别.鈥�

If such interbreeding had occurred then other European lineages should be
present, not just X. But data from Wallace鈥檚 own lab and from that of Antoni
Torroni at the University of Rome showed that there were no other European
lineages, which implies that X has been present in Native Americans from the
beginning. That implication was strengthened last year when Anne Stone of the
University of Arizona found lineage X in the tooth enamel of people in a
pre-Columbian burial in Illinois. The lineage therefore cannot have been
introduced by interbreeding after Europeans arrived in the Americas and so must
have come over during an earlier migration.

Disputed bones

鈥淚t looks as if a European population moved up through Asia and was part of
the wave of east Asian people who moved across the Bering land bridge,鈥� says
Wallace. This population movement must have been occurred than 30 000 years ago
if the early date for entry into the Americas is correct. And these people
evidently did not interbreed with populations in Asia, or at least not to any
great extent. There are alternative explanations for the absence of the X
lineage in Asia, such as the possibility that it was once there but has been
lost through genetic drift. Wallace believes his interpretation is the most
parsimonious, however.

A European link with Native Americans has been made before, in the 1960s,
based on purported similarities between certain stone tool technologies in
France and the Clovis fluted points. Modern archaeologists are not persuaded by
this argument. The new mitochondrial data, however, are much more persuasive.
And they may cast light on the interpretation of a skeleton found in Oregon a
little more than two years ago. Known as Kennewick Man, the skeleton was
initially thought to be very modern. James Chatters, an independent
anthropologist, suggested the skeleton belonged to a European settler from the
19th century, based on what he described as Caucasoid features. These include a
narrow skull, a light-boned face and a receding forehead.

But when the bones of Kennewick Man were dated to 9300 years ago, it was
obvious that he wasn鈥檛 European but a Native American. As soon as the skeleton鈥檚
age was known, it became embroiled in a political mire that has effectively
removed it from the gaze of any immediate detailed study. The Umatilla tribe in
Oregon claimed Kennewick Man to be one of their ancestors and, under the 1990
Native American Graves Protection and Repatriation Act, sought to rebury him.
Anthropologists have been pressing the courts to let them have an opportunity to
study the skeleton before it goes back into the ground. Last month a judge
ordered that the skeleton be moved to the Burke Museum at the University of
Washington, thus brightening the hopes that one day it will be available for
study.

Could Kennewick Man鈥檚 purported Caucasoid features be an anatomical
manifestation of the genetic message encoded in the mitochondrial DNA of some
Native Americans? 鈥淭hat鈥檚 an interesting speculation,鈥� says Wallace, 鈥渂ut unless
we can get a small piece of tooth enamel for genetic analysis, we can never be
sure.鈥� Jantz, who has seen a cast of the skeleton, agrees that it could signal
the presence of European blood in the earliest Americans, but is cautious, as
are most anthropologists, principally because the skeleton has had so little
scientific scrutiny.

One thing Jantz is certain of, however, is that Kennewick Man underscores the
high degree of anatomical variation that existed among the earliest Americans.
That variation tells you that the settlers of the New World came from many
diverse populations in the Old World, he says. It also could include people who
arrived by boat rather than on foot. Evidence suggesting a maritime route for at
least some of the first Americans was published last month in Science,
in the form of archaeological findings from a coastal settlement in southern
Peru where the people were evidently skilled fisherfolk as much as 13 000 years
ago. Their presence and their dedication to a maritime way of life suggests that
these people might have been seafarers for a long time, rather than originally
being landlubbers who later took to the sea in search of food.

But whatever this group鈥檚 history, their existence is yet another piece of
evidence all supporting the same theory: that the New World is a lot
older鈥攁nd its origins more diverse鈥攖han we鈥檝e been led to believe.
Although this diversity decreased over time as a result of interbreeding and
extinction of local populations, the pioneering Americans came from many
homelands and not all at one time. 鈥淎ll the evidence suggests that the peopling
of the Americas was much more complicated than just a few people walking across
the Bering Strait,鈥� says Jantz.