|
Pangaea revisited, the Neolithic
reconsidered From Alfred Crosby,
Ecological Imperialism: The Biological Expansion of Europe,
900-1900.
Edited for classroom use by Jeff Littlejohn
In “Pangaea
Revisited,” Alfred Crosby discusses the creation of two worlds -- one in
Africa and Eurasia and the other in the Americas. Although he begins with a short
discussion of Pangaea, Crosby's focus is on the breakup of the
super-continent, the resulting de-centralization of evolution, and the
development, migration, and diversification of human culture.
Crosby 's
article will set the stage for our discussion of Contact.
I.
IT IS
NECESSARY TO BEGIN at the beginning in considering the [history of the
Americas], and that means not in 1492 or 1788 but about 200 million
years ago, when a series of geological events began that brought these
lands to their present locations. Two hundred million years
ago, when dinosaurs were still lolling about, all the
continents were jammed together in one great supercontinent that the
geologists call Pangaea. It stretched over scores of
degrees of latitude, and so we can assume that it had some variations in
climate; but with only one land mass, there would not have been much
variety among its life forms. One continent meant one arena for
competition, and so only one set of winners in the Darwinian struggle
for survival and reproduction. Reptiles, including all the dinosaurs,
were the dominant kinds of land animals in Pangaea -- and, therefore,
the world -- for three times as long as mammals have held that position
since, and yet reptiles diversified into only two-thirds as many orders.
About 180 million years ago Pangaea began to break up
like some immense tabular iceberg rotting in the heat of the
Gulf Stream. First it split into two supercontinents, and then into
smaller units that became, in time, the continents we know. The process
was more complicated than we can describe here (indeed, more complicated
than geologists completely understand as yet), but, in broad terms,
Pangaea broke up along lines of intense seismic activity that later
became undersea ridges. The most thoroughly examined of these is the
Mid-Atlantic Ridge that boils and bubbles from the Greenland Sea to
Spiess Seamount, twenty degrees of latitude and twenty of longitude
southwest of Cape Town, South Africa. From this and other ancient
drowned cordillera, lava poured (and in many cases still pours),
generating new ocean floor and carrying continents on either side of a
given ridge farther and farther from each other. Where these floors,
moving away from the ridges that spawned them, back into each other,
they curl downward into the earth's mantle, grinding and grating,
sometimes scuffing continental mountain ranges up to the skies,
sometimes creating underwater trenches, the deepest features on the
surface of the planet. Geologists, who sometimes have a stony
insensitivity to nuances, call this awesomely vast and eon-consuming
activity "continental drift."
When mammals succeeded dinosaurs
as the globe's dominant land animals and began to diversify into their
myriad orders over the past few score million years, the separations of
the continents seem to have been at their extremes, certainly more so
than today, and there were large inland seas partitioning South America
and Eurasia into two subcontinents each. On these fragments of
Pangaea, life forms developed independently, and in many cases
uniquely. This helps to account for the remarkable degrees to
which mammals diversified and the speed at which they did
so.
Continental drift largely accounts for the differences, often
extreme, between the flora and fauna of Europe and those of the
[Americas and the other Neo-Europes, which have climates similar to
Europe]. A European traveler sailing to [the Americas or] any of the
Neo-Europes must cross one or more of these undersea ridges and
trenches. Europe and the Neo-Europes have not been part of the same
continental mass for many millions of years (except for ephemeral Arctic
connections between North America and Eurasia), years during which the
ancestors of American buffalos, Eurasian cattle, and Australian
kangaroos shambled and hopped down diverging paths of evolution. To
cross these undersea seams is to step from one of those paths to
another, almost to step into another world. (There are seams that are
not under water and do not separate continents, but let us ignore them
for brevity's sake.)
When Pangaea first split into northern and
southern supercontinents, only North America of all the Neo-Europes was
in the same supercontinent with Europe, and so the two have shared the
same latitude and have had anciently similar histories. The floral and
faunal differences between Europe and North America are less striking
than the differences between either of them and the other Neo-Europes.
Even so, their differences were enough to take the breath away from the
Finnish naturalist, Peter Kalm, in Philadelphia and fresh off the boat
from Europe in 1748:
I found that I was now come into a new
world. Whenever I looked to the ground I found everywhere such plants as
I had never seen before. When I saw a tree, I was forced to stop and ask
those who accompanied me, how it was called ... I was seized with terror
at the thought of ranging to many new and unknown parts of natural
history.
Biogeographers have properly designated North America
and Eurasia, including Europe, as different biological provinces or
subregions. After all, Nero threw Christians to the lions, not to the
cougars. As for the other Neo-Europes, there is no doubt about their
deserving biogeographical categories separate from that of Europe. All
three, for instance, have large -- some of them man-sized -- flightless
birds.
The breakup of Pangaea and the decentralization of
the processes of evolution began 180 or 200 million years ago.
For almost all the time since, except for a few instances counter to the
dominant trend (e.g., the periodic rejoining of North America and
Eurasia by reappearances of the Bering land connection, and consequent
intermixing of biota), centrifugal forces have prevailed in the
evolution of life forms. This trend, prevalent since some of our distant
mammalian ancestors made a living by stealing dinosaur eggs, halted
about half a millennium ago (a tiny fraction of a single tick on the
geological clock), and centripetal forces have dominated since. The
breakup of Pangaea was a matter of geology and the stately tempo of
continental drift. Our current reconstitution of Pangaea by means of
ships and aircraft is a matter of human culture and the careening,
accelerating, breakneck beat of technology. To tell that tale, we have
to go back not 200 million years, fortunately, by only a million or
three.
II.
The most adaptable and therefore most
widely distributed of today's large land animals are human beings, and
this has been true of the members of the species Homo sapiens and
their hominid predecessors for roughly six million years (video on human evolution | interactive timeline).
While other creatures had to wait for specific genetic changes to
enable them to migrate into areas radically different from those of
their ancestors -- had to wait for incisors to lengthen into daggers
before they could compete successfully with hyenas on the veldt, or had
to wait for hair to thicken into fur before they could live in the north
-- humans and their hominid predecessors did not. They made not a specific but rather a
generalized genetic change: They developed bigger and better brains
wired for the use of language and for manipulation of tools.
That growth of nerve tissue crammed into the treasure box of the
skull began several million years ago, and as it did, the hominid became
increasingly capable of "culture." Culture is a system of storing and altering patterns of behavior not in the molecules of the
genetic code but in the cells of the brain. That change made the members
of the genus Homo nature's foremost specialists in adaptability.
It was as if the fisherman in the fairy tale to whom the princely
flounder granted three wishes had asked first of all for all the wishes
he would ever want.
These plump-brained apes made use of their
new skills of adaptability to migrate out of their ancestral home
(probably Africa) and across dry Pangaean seams into Eurasia. Ever
since, hominids and humans have migrated; they seemingly have tried to
occupy every crack, cranny, and niche above the low-tide line. Our
ancestors (Homo erectus), with brains, on average, hundreds of
cubic centimeters smaller than ours, increased in numbers, migrated
through out the Old World tropics, and, by 750,000 years ago, moved into
the northern temperate zone, taking up residence in Europe and China.
When modern humans (Homo sapiens) first appeared in Africa around 200,000 years ago, our species quickly migrated around the Old World from Europe and
Siberia to the southern tip of Africa and the islands of the East
Indies. As we went, other species in the Homo genus (like
Homo neanderthalensis) became extinct, leaving Homo sapiens the only
the only living species in the Homo genus. Yet there were whole continents and myriads of islands we had
not explored or settled. We had not yet passed over one of the
expanding, deep-water seams of Pangaea.
III.
These
early humans were about to do something of the same magnitude as moving
from earth to another planet. They were about to leave a world -- the
fractured core of Pangaea, Eurasia plus Africa -- of life forms with which
their ancestors had lived for millions of years and go to worlds where
neither humans nor hominids nor apes of any kind had ever existed,
worlds dominated by plants, animals, and microlife whose forms had often
diverged sharply from the patterns of life in the Old World.
The
new worlds were North and South America and Australia. (To get to New
Zealand, a land mammal had to be a bat or an excellent sailor, and Homo
sapiens arrived there tardily.) Members of the genus Homo had been in
the East Indies for most of the time that the genus had existed; the
waters between those islands were warm and the straits narrow, and the
shallow strait between New Guinea and Australia becomes dry land during
an ice age. Members of our species turned south and walked into
Australia some 40,000 years ago, giving that continent its first large
placental mammal. The second, the dog called the dingo, arrived some
8,000 years ago or even more recently. (These dates and others cited in
this chapter are matters of controversy in which we need not involve
ourselves. We are interested in sequences, not absolute
dates.)
There is evidence that a number of species and even
genera of Australian marsupials and reptiles, creatures considerably
larger than those of historical times, disappeared at roughly the same
time that humans spread through that continent. The temptation is to
promote chronological coincidence to the level of proof and to blame
those extinctions on the invaders, though it strains credulity to claim
that Stone Age human hunters alone killed off Australia's giants. They
may have had assistance from diseases that came south with them from the
East Indies. They did have fire, which Aborigines in historical times
have used to burn over vast areas of the continent annually, and in
ancient times this practice conceivably could have altered the habitats
of the giants sufficiently to make life and reproduction impossible.
Getting from the East Indies to Australia was a matter of
crossing a few warm, narrow straits; getting to the Americas was
something more difficult. The problem was not the cold, foggy, dangerous
waters of the Bering Strait; indeed, that strait has been a broad
highway of tundra for much of the time since the arrival of members of
the genus Homo in Siberia. The problem was the hostility of the climate
in the high latitudes. There were few human beings in Siberia to follow
the herds of caribou and such across Beringia to Alaska, and once in
Alaska the early migrant human fetched up against a continental ice cap
that occupied much of America north of Mexico. There were warm periods
that opened a corridor south from Alaska to Alberta and beyond, but all
in all the pedestrian's passage from Asia to the lush grasslands and
forests of North America was a miserably difficult one.
Humans
probably did not arrive south of the North American ice cap until long
after they set foot in Australia, but in the New World, as in Australia,
there seems to have been a coincidence between the arrival of human
big-game hunters and the extinction of many species of large mammals:
mammoths, mastodons, giant ground sloths, giant buffalos, and horses,
for example. Some individuals of these giant species were incontestably
killed by humans -- we have found stone spear points between the ribs of
fossil mammoths -- but most experts are reluctant to attribute the
extinction of whole species to these human hunters. Again, humans may
have been only part of a wave of invading species, including parasites
and pathogens, that attacked the native fauna. But why would the latter
concentrate on the larger mammals? Why and how would anything but humans
kill mostly those animals representing the largest meals? Homo sapiens
found a hunter's paradise in Australia and the Americas. All three
continents were chock-full of toothsome herbivores utterly inexperienced
in defending themselves against human aggressors, providing the
newcomers with seemingly inexhaustible quantities of protein, fat, hide,
and bone. Expansion of Homo sapiens into Australia and the Americas must
have led to a very large increase in the total number of humans on the
earth. The Americas and Australia were Edens to which God added Adam and
Eve very tardily. "There can be no repetition of this," wrote Francois
Bordes in The Old Stone Age, "until man lands on a hospitable
planet belonging to another star."
IV.
Some 10,000 years ago all the larger ice caps melted,
excepting those in Antarctica and Greenland, and the oceans rose to
approximately their present levels, inundating the plains that had
connected Australia with New Guinea, and Alaska with Siberia, and isolating the avant-garde of humanity in their new
homelands. From that time until the Europeans made a common practice of
sailing across the seams of Pangaea, these peoples lived and developed
in complete or nearly complete isolation. One of the momentary respites
from divergent evolution since the breakup of Pangaea had come to an
end, and for the next few millennia genetic drift and, for the first
time, cultural drift were in perfect consonance with continental drift.
V.
Then humanity made its next
giant lunge, not a matter of geographical migration but of cultural
mutation: the Neolithic Revolution or, more accurately, Revolutions.
According to classic definition, the Neolithic
Revolution began when humans started to grind and polish rather
than chip their stone tools into final form, and it ended as they
learned to smelt metal in quantity and work it into tools that stayed
sharp longer and were more durable than their stone equivalents. In
between, the story goes, humans invented agriculture, domesticated all
the animals of our barnyard and meadow, learned to write, built cities,
and created civilization. The complete story would be a great deal more
complicated, but this definition will serve for our purposes.
The technological
avant-garde of humanity, the peoples of the crossroads of the Old World,
the Middle East, moved down the road toward becoming what we are today
more rapidly than any others. The geographical avant-garde of humanity,
the pioneers isolated in Australia and the Americas, had different
histories. The Australian Aborigines kept to their Paleolithic ways;
they did not smelt metals or build cities. When Captain Cook and the
Australians of Botany Bay looked at each other in the eighteenth
century, they did so from opposite sides of the Neolithic
Revolution.
The peoples of the New
World had their own Neolithic Revolution or Revolutions, most
spectacularly in MesoAmerica and Andean America, but theirs, relative to
that in the Old World, began slowly, accelerated tardily, and spread as
though the Western Hemisphere were somehow less hospitable to the
techniques and arts of civilization than the Eastern. When the conquistadors arrived with iron and steel, the peoples of the
high Amerindian cultures were still in the early stages of metallurgy.
They used metals for ornaments and idols, not for tools.
|