The
Pleistocene is the time when humans evolved
in the old world. The first hominids (i.e. creatures more closely related to
humans than to apes) lived in Africa about 7
million years ago. Scientists have now identified about 14 species, of which
the best known is Australopithecus. They were 1-1.5 meters tall, walked
upright, had a protruding jaw, prominent eyebrow
ridges and a small braincase.
2.5 million years
ago, Homo erectus appeared in Africa, with a
brain as big as the smallest modern human brain. H. erectus differed from
modern humans by the prominent brow ridges and receding chin. They made
sophisticated stone hand-axes with sharp edges, possibly made spear points, and
probably used fire. They spread over Africa
and Asia and survived until about 400,000
years ago.
The first fossils
that are classified in the modern species Homo sapiens date from about 200,000
years ago (Nat. Geog. , Jan. 1996) and are called neanderthals
(a subspecies of Homo sapiens). The neanderthals
still looked primitive, with prominent brow ridges, low foreheads, and receding
chins, but their brains were, on average, slightly larger than ours. They
hunted woolly rhino and cave-bear and disappeared about 30,000 years ago.
About 30,000 years
ago, fully modern humans called Cro-Magnon
evolved from the neanderthal-like forms of the Near
East and spread into Asia and Europe, rapidly
replacing the more primitive neanderthals.
They had domed heads, smooth brows, and prominent chins. They made precision
tools, including definite spearheads, and they produced spectacular works of
wildlife art on the walls of caves, which provide some glimpses of how the big
game was hunted - with spears and rocks and probably also traps and fire. One
painting shows an eviscerated bison about to gore a human.
Last year scientists
reported
discovering remains of an entirely new species of dwarf human, Homo floresiensis. These people were about one meter tall
and lived about 18,000 years ago on the island of Flores,
east of Java. Not only were they hobbitt-sized, their
brains were the smallest of any known hominid. Remains of dwarf elephants were
found in the same deposits as the humans.
During
the last glacial period (the Wurmglaciation
of 120-20,000 years ago), so much sea water was frozen into the ice caps that
the sea level fell about 300 feet, and Asia and North America were connected by
a strip of land called the "Bering Land Bridge".
However, during this time a huge glacier (the Laurentide
Glacier) stretched all the way from the Arctic
to the Pacific Ocean, blocking animal and
human movement between the continents. At about 14,000 years ago a warming
trend opened up an ice-free corridor between the main Laurentide
glacier and the smaller Cordilleran ice sheet along the coast. This made it
possible for early humans to migrate from Asia
into North America. Then during the present interglacial,
melting ice caused the sea level to rise, cutting off the land connection
between Asia and North
America.
North and South America were free of humans until they arrived from
Asia about 11,500 years ago, soon after the
ice-free corridor opened up. These Clovis people are named after
the town in New Mexico
where their distinctive spear points were found in 1932. Clovis
points have been found in association with mammoth bones at several locations
in North America, suggesting that the Clovis people hunted these animals. At some sites (e.g. Murray Springs)
bones of other large mammals have been found, including Horse, Camels, Bison,
Lion, and Dire wolf.
It has long been assumed that the Clovis
people were the ancestors of American Indians, but this assumption is now being
questioned. There are no human skeletal remains from the earliest arrivals, and
only about ten sets of remains more than 9,000 years old. Surprisingly,
physical anthropologists have concluded that the skulls of two of the oldest
skeletons (Spirit
Cave Man from Nevada, 9,400 years old and Kennewick Man
found in Washington State in 1996, 9,300 years old) are quite different from
those of modern American Indians. Their cranial vaults are long and narrow
rather than round, their faces are slender rather than broad, and they do not
have prominent cheekbones. These skeletons resemble those of Polynesians,
Europeans, and the Ainu
of Japan, more than they resemble American Indians, leading to the idea
that American Indians may be derived from one or more distinct groups of
settlers arriving some time after the Clovis
culture. The third specimen (Wizards Beach Man from Nevada, 9,200 years old), is more similar to
contemporary American Indians.
The base sequence of
mitochondrial DNA
(mDNA) from living American Indian populations is
similar to that of samples from Asia and Siberia, confirming the earlier conclusions from
archeology. But some mutations in American Indian mDNA
are found elsewhere only among the aboriginal populations in Southeast Asia and in the
islands of Melanesia and Polynesia, providing a suggestion that the
ancestors of American Indians may have arrived by sea. Unfortunately,
three laboratories were unable to obtain any DNA evidence from Kennewick man.
The legal
requirement to return human remains to tribes that can show an affiliation,
so that they can be reburied, has led to enormous controversy over how much
scientific analysis of these samples will be possible. In the case of Kennewick man, after the
failure to obtain DNA evidence the Department of the Interior in 2000 decided
to abandon scientific research and give the skeleton to
five Indian tribes who jointly claimed the remains for reburial. But
then a judge in 2002 ruled in
favor of a group of scientists who sued to obtain the remains for
scientific study, since there was no proof for a "cultural
relationship" between the skeleton and present-day American Indians.
Some evidence suggests human occupation of both North and South
Americaearlier
than the Clovis culture, leading to several fascinating alternative
possibilities. Several sites in the southeastern part of the U.S. have been
dated much earlier than Clovis,
but the evidence from most of these sites has been seriously questioned for
technical reasons. However, one remarkably informative early settlement in
Monte Verde, Chile,
makes it difficult to completely accept the traditional
"Clovis-first" view. The site includes remains of dwellings and stone
tools (not Clovis points) as well as medicinal plants, and dates from 12,500
years ago. This early date has led to suggestions that early migrants may have
reached and explored North and South America
by boat, rather than on foot. This would have allowed them to avoid the glacial
barrier, it would have been faster, and it might have been easier, since food
would have been plentiful. Archeological evidence shows that Pacific people
used boats as early as 25,000 or even 40,000 years ago, so the technology may
have been available. There were many unglaciated
pieces of coastline that would have allowed breaks in the sea journey, and
there are several sites along the coast of Canada, California, Peru, Ecuador, and Chile that show
evidence for human occupation between 10,000 and 12,000 years ago.
Unfortunately the sea level is much higher now than then, so much of the
archeological evidence for seafaring at that time may now be deeply submerged
and difficult to find.
In late Pleistocene, during the last 50,000
years, there were mass extinction events in many different parts of the world,
involving at least 200 genera (plural of genus = a group of related species).
But this was different from previous episodes of mass extinction:
1. It was much more
selective, involving mainly the megafauna: the large herbivores (mammoths,
mastodons, huge ground sloths, cave bears, woolly
rhinoceros, other rhinoceroses, etc.) and the carnivores that fed on them, the
dire wolves and saber-tooth cats. There was no accelerated extinction of
smaller terrestrial species, plants, or marine organisms.
The following
disappeared from America,
Europe and Australia:
All herbivores
> 1000 kg
75% of herbivores
100-1000 kg
41% of herbivores
5-100 kg
< 2% of herbivores
< 5kg
2. It occurred at
different times on different land masses:
Time
of start of
major extinction episodes
(years before present)
Africa and S.E. Asia
50,000
Australia
50,000
North
Eurasia
13,000
North
America
11,000
South
America
10,000
West
Indies
4,000
New Zealand
900
Madagascar
800
This excludes any
global catastrophe or climatic change as an explanation.
In all of these
cases except Africa, the extinctions occurred
shortly after the first arrival of prehistoric humans. The first humans
were faced with animals that had evolved in the absence of human predators, and
the animals were probably easily overcome. Therefore, the most plausible
explanation is that these extinctions were caused by overexploitation by human
hunters.
In Africa,
massive extinction does not coincide with the arrival of humans. Humans had
been evolving there for millions of years without causing mass extinctions (they
may not have been as carnivorous as their descendants in other parts of the
world) but it does coincide with the maximum development of advanced early
Stone Age hunting cultures.
Many authors have
remarked that to see what the Pleistocene was like, you should go to Africa. Africa still has
more large herbivores (including elephants, hippos, rhinos, etc.) than any
other place on earth. But, even in Africa, the
big game we see today is only about 70% of the genera that were present in
mid-Pleistocene. About 50 genera disappeared about 40,000 years ago.
It is paradoxical
that the region where humans have existed the longest
(Africa) retained a wide variety of big game
whereas the areas where humans arrived more recently have suffered a more
complete loss. Perhaps the African big game had time to evolve defensive
behavior, whereas species elsewhere were caught defenseless and naive by a
newly arrived advanced hunting culture.
Australia once boasted a
spectacular megafauna including giant wombats as big as grizzly bears
and giant kangaroos. But the continent was colonized by humans (already
Homo sapiens) around 55,000 years ago and subsequently lost all of its large
and medium-sized mammals; in fact all except some
kangaroos. All 19 species exceeding 100 kg and 22 of
38 species 10-100 kg disappeared, along with three large reptiles and
the 450lb flightless bird Genyornis. Miller
et al. used eggshell dating to show that Genyornis disappeared suddenly around 50,000 years ago,
very shortly after the first arrival of humans. This does not necessarily
mean that the animals were simply hunted to extinction. The humans
brought to the continent the use of fire as a hunting tool, and this may have
destroyed so much vegetation that many herbivores were deprived of their food
and could not survive. Although some authors have claimed that the
Australian megafauna was wiped out quickly after the arrival of humans, careful
analysis of the ages of various remains suggests
that man may have coexisted with the Australian megafauna for over 10,000
years. More discussion.
North America. 12,000 years ago, North America had an
amazing Megafauna including condors with a sixteen-foot wingspan, ground sloths as big as hippos, three kinds of elephants, three
kinds of cheetah and five other kinds of big cat, several kinds of pronghorn
antelopes, long-legged, antelope-like pigs, an assortment of camel, llama,
deer, horse, and bison species, giant wolves, giant bears and giant
armadillos.North
America has been called a "super-Serengeti" with more
big animal species than you would find in Africa.
But 11,000 years ago, nearly all of these big animals - 70 species or 95% of
the megafauna - disappeared completely. This is exactly the time when humans (Paleo-Indians) colonized North America,
and their arrival and skill as hunters at that time is documented by the
appearance of artifacts. The disappearing mammals in North
America included all of the following:
*Mammoths
*Mastodons
*Horses
*Tapirs
*Camels
*Four-horned
antelopes
Ground sloths
Peccaries
Giant beaver
Dire wolves
Giant jaguar
Saber-tooth cat
*Some of these
fossils are directly associated with human artifacts in archaeological sites.
The carnivores on the
list were probably not hunted directly, but were dependent on the large
herbivores for food, so soon followed them to extinction. In some cases
accurate dating methods have shown that certain species became extinct at
exactly the times that humans arrived. Giant ground sloths
and mountain goats in the Grand Canyon both went extinct 11,100 years ago,
which is the time that the human hunters arrived (within the accuracy of dating
methods, which is +200 years). There is also direct evidence for killing by
humans. The human archeological sites from 11,000 years ago have stone
projectile points, which were presumably used in hunting the large mammals. One
mammoth skeleton has eight stone spear points among its ribs. Some of the large
mammals were trapped in pits, and some were cornered using fire. La Brea tar pits and the PageMuseum
is an excellent place to see the fossils and reconstructions from this period. Mammoth Trumpet (a newsletter about the first Americans).
Detailed study of
late Pleistocene extinctions in North America
(Martin, 1986) suggests that they happened over just a few hundred years. This
explains why there is so little archaeological evidence for hunting of mammoths
in the New World. The total number of mammoths
from archeological sites in North America is 38; in Asia, where mammoths were
hunted for many thousands of years, there are many more mammoth remains -e.g.
remains of 1000 mammoths at just one site in Czechoslovakia and of 100,000
horses at another site. Paul Martin has suggested that the human population
quickly expanded south from the Bering land bridge, and exterminated the big
game as they went ("Blitzkrieg" model). Martin, P. S.1986. Refuting late Pleistocene extinction models. In Elliot, D.K.
(ed) Dynamic extinction. Wiley and
Sons, NY.1073-130.
Other authors have
disputed the idea that human hunting finished off the Pleistocene megafauna of North America. For example, Donald Grayson, an
archaeologist at the University
of Washington, suggests
that climate shifts and associated vegetation changes could have been
responsible. Grayson disputes two aspects of the overkill hypothesis:
1. Out of the 35
genera that became extinct around this time, only 15 have been shown to have
survived beyond 12,000 years ago. So 20 genera may have disappeared before
human arrival.
2. There is good
evidence for mammoth kills by the Clovis
people, but no evidence that they hunted any other large mammals (he does not
mention the evidence that they hunted two kinds of buffalo).
Paul Martin responds
that the Pleistocene megafauna had survived several climatic changes during the
previous million years, some more severe than the one that occurred at the end
of the Pleistocene. Yet these changes did not cause multiple extinctions.
South
America was also
colonized by humans about eleven thousand years ago, and since that time it has
lost 80% of its genera of large mammals, including ground sloths,
horses, and mastodons.
SURVIVORS FROM THE
PLEISTOCENE
In North America, the
only surviving herbivores of the megafauna are bears, elk, moose, buffalo and
mountain lion. The horse also survived, but only through its domestication and
preservation overseas. The moose was hunted to near extinction but has
recovered to a population of about 1 million. Yellowstone
moose decline due to hunger, not predators, ENN Daily News -- 10-6-1999