Why predictably? Scientific theories have a special rhythm of their own. As intellectual constructs they are not completely neutral, objective formulations. Theories are as dependent on the intellectual climate that produces them as any type of creative work is. The ideal of scientists gathering all of the facts and developing theories based on what the facts tell them is just that: an ideal. It's not real. This doesn't mean scientific theories are as shallow and prone to silly fads as, say, teenage clothing fashions. Let's say that they are much deeper and prone to trends, not fads. At the very least, the intellectual atmosphere of the day decides what the questions are that scientists are trying to answer when they set out to gather "all of the facts."
For example, the question of what caused the mammoths to go extinct would not have occurred to thinkers at the beginning of the Enlightenment. At that time, no one believed extinction was even possible. It went against all common sense, they thought. Look at the difficulty of the task. How could every single member of a species be killed? Even the most virulent plague leaves a few survivors. The greatest hunter can only kill a limited number of prey each day. As he kills, the other prey are warned and flee before his approach. Besides, the world is an enormous place. Even if we were to kill all of a species over here, there will always be survivors somewhere else. Look at the wolf. Europeans had been trying to kill off the wolf since the beginnings of history, but there always more wolves lurking around the edges of civilization.
No, the wise men of the day said, shaking their wise heads, it’s just not possible. Being wise people, they didn’t depend on mere common sense to back up their position--after all, common sense is so, well, common. They had the authority of ancient thinkers and God himself to back up their position.
For those of a philosophical bent, Aristotle's doctrine of plenitude, as interpreted through the lens of seventeen centuries of Christianity, provided the key argument against extinction. The doctrine of plenitude, or the fullness of the natural world, argued that any life form conceived by God must have come into existence as part of his perfect world. Each part of creation is necessary for creation's divine perfection as a manifestation of God's perfect mind. No life form created by God could ever cease to exist, because that would leave an unbridgeable void in the Great Chain of Being. For those more inclined to eschew philosophy and take comfort in a good Biblical verse, there was Ecclesiastes 3:14 -15: "I know that, whatsoever God doeth, it shall be for ever: nothing can be put to it, nor any thing taken from it.... That which hath been is now; and that which is to be hath already been; and God requireth that which is past." In either case, extinction just couldn't be.
When Enlightenment thinkers accepted fossils as the remains of once living things and not just interesting rocks that happened to look like bones or shells, they first tried to make all fossils the remains of known animals. When some fossils were proven to be unfamiliar species--most importantly, Georges Cuvier proving that mammoths were not the same as modern elephants--they argued that these animals must now live in still unexplored regions of the earth. Catherine the Great and Thomas Jefferson, two of the best-informed leaders of the day, both believed that mammoths and mastodons existed on the distant frontiers of their respective countries. But others found the idea that such large land animals could stay hidden highly unlikely. Through the early years of the nineteenth century, the idea of extinction gradually gained acceptance.
The idea of extinction wasn't pushed through by sheer force of the evidence. Other successful ideas helped make it more acceptable. From the earliest days of the Renaissance, European thinkers had had to deal with the fact that the world was a lot bigger and more complex than they had ever imagined. This raised many difficult questions about the literal truth of the first chapters of Genesis. The questions themselves were only very cautiously voiced. The one implication of this questioning that remained unvoiced the longest was the idea that the earth must be much older than mentioned in Genesis in order to account for the size and complexity of the new universe.
Medieval thinkers had never lost sight of the fact the fact that the ancients had believed the earth to be either eternal or subject to eternal cycles of creation and destruction. For a millennium, it was possible to mock such ideas as a sign of unenlightened ignorance. However, after Magellan and Copernicus expanded the known universe, it made sense that more time went hand in hand with more space and more complexity.
More time meant more room for vast changes in nature to develop, either gradually or in forgotten upheavals. This made extinction easier to accept as a part of the greater whole. Additionally, at the same time extinction was gaining acceptance, the idea of ice ages was suggested by Louis Agassiz and quickly gained acceptance. Two years after Cuvier announced his conclusion that the mammoth was an unknown and extinct type of elephant, the first frozen mammoth, that would be recovered and brought back to Europe, was found in Siberia. This showed that the mysterious elephants of Europe and Asia were great hairy beasts more suited to extreme cold than to temperate climates. Again, each idea helped gain acceptance for the other. The idea that the mammoth was a creature adapted to the frozen wastes, who perished when the earth warmed, formed a perfectly suitable narrative for the emerging ideas of the history of the earth.*
This was the first coherent, scientific theory of extinction for the mammoth.** It was a perfect narrative for the nineteenth century. It emphasized the treacherous and dangerous side of nature. Despite forays into sentimental romanticizing of nature, nineteenth century Western culture basically saw nature as something to be fought and tamed. Mankind's ability to change and adapt was seen as progress, a virtue. Our technology gave us the ability to change nature to fit our needs, rather than changing ourselves as nature demanded. The poor mammoth was at the mercy of nature's capricious changes and died out. This extinction narrative would remain unchallenged for over a century.
By the late 1940s, faith in the positive character of technology, the inevitability of progress, and mankind's ability to survive had all taken a blow. The holocaust and the atomic bomb brought on a crisis of self-doubt. Artists and mainstream intellectuals seriously considered the possibility that mankind's innovation might be the source of our own imminent extinction. In 1961, Robert Ardrey published African Genesis, a new narrative of human evolution that emphasized man the hunter, not in its previous heroic mold, but as a vicious carnivore with an instinct to kill and destroy. The next year Rachel Carson published Silent Spring, a book that documented the unexpected negative consequences of indiscriminate pesticide use and which became one of the seminal documents of the environmental movement. Both books were controversial bestsellers and widely discussed for the rest of the decade. The next scientific theory of mammoth extinction reflected these new, pessimistic assumptions of the post war world.
Paul Martin, in a series of papers published in the mid-sixties, argued that mankind and our technology were the cause of the extinction of the mammoths and other large mammals at the end of the ice age. The theory was not entirely new, Cuvier's contemporary Lamarck had believed it, but, it had never had strong evidence to support it. Though there was evidence that people had lived with and hunted mammoths in the Old World, the first evidence that humans and mammoths had lived together in the New World wasn't known until 1929 and the excavations at Clovis, CO. The first iron-clad proof of a mammoth kill in the New World wasn't discovered until 1953 when a mammoth with spear tips between the bones was found at Naco, AZ.
Martin's 1967 paper pulled together a wide array of radiocarbon dates to show that the mammoths and other large ice age mammals all went extinct within about one thousand years of the earliest evidence of humans in the New World (the Clovis site). To add to his argument that humans caused extinctions, he demonstrated the same pattern of human arrival and large animal extinction in Australia, Madagascar, and the Pacific Islands. Martin has a flair for language; he called his theory the Overkill Hypothesis, referred to a blitzkrieg of hunters advancing through the New World, and said, "large mammals disappeared not because they lost their food supply, but because they became one." His presentation was very appealing in a melodramatic decade and soon had many followers.
For the next thirty years the climate and overkill theories battled it out until a third theory appeared suddenly in 1997. In the age of AIDS, it was perhaps inevitable that the someone would suggest a plague as the killer of the mammoths. Ross MacPhee, of the American Museum of Natural History in New York, suggested the possibility of a plague carried by humans or their dogs into the New World killing most of the big game before they could hunt it and forcing the hunters to settle down and become farmers or to change over to smaller game. MacPhee has yet to find any direct evidence to support his theory but gathers samples from each new frozen mammoth discovered to see if it contains any unknown pathogens.
Now we have comets. Like disease and a suspicion of technology, comets have been in the air lately.*** Comets or meteors were an early suspect for the extinction of the dinosaurs, but not a favored one. The preferred idea was that dinosaurs were done in by a combination of climate change and swift mammals.
The leading interpretation of meteors was that most of the big ones were used up in the early days of the universe and that the odds of a dangerous one hitting the earth were, well, astronomical. The unfortunate affair of Velikovsky's cosmic billiards theory of ancient history had further soured most scientists on crediting meteors or comets with causing anything. This changed in 1980 when Luis Alvarez, his father and two chemists published clear evidence of a large cosmic event of some sort happening right at the K-T boundary, the border between the last rocks containing dinosaur fossils and the first without.
The Alvarez theory was hotly debated all through the eighties. Because of the popularity of dinosaurs with children, the debate was more widely known by the public than most scientific disputes. Many scientists specializing in other periods of mass extinction reviewed their evidence to see if other meteors were possible. Some even thought they saw a periodic pattern of meteors and extinctions every twenty six million years. By 1991, David Raup, possibly the leading expert on all extinctions, could wonder in print, "could all extinctions be caused by meteorite impact?" The Alvarez theory is still disputed, though the number of disputants was reduced dramatically at the end of the eighties when a 300 kilometer wide crater of the right age was revealed to lie beneath the Yucatan peninsula.
Other events in the eighties and nineties kept the possibility of cosmic impact on the public mind. Halley's Comet returned in 1986. The spectacular impact of the fragments of Comet Schumacher-Levy 9 with Jupiter in 1993 was the first time a large planetary impact was directly observed and studied by human scientists. This was followed by a couple of big budget summer movies featuring giant impacts.
That an idea is fashionable or rooted on contemporary social concerns and attitudes does not necessarily mean that it is wrong. Though it does mean that Kennett's theory will almost certainly face extra scrutiny. That's probably a good thing. Fortunately, Kennett has evidence:
Evidence for the impact comes from a thin layer of sediment found throughout North America, said James Kennett, a geologist at the University of California in Santa Barbara.
"There are materials with particular chemistries in that layer that collectively provide very strong evidence that the layer was produced by this extraterrestrial impact," he said in a telephone interview.
Kennett said the layer contains tiny spheres of carbon and metals, bits of diamonds, and extraterrestrial concentrations of helium 3 and the element iridium.
The layer dates to 12,900 years ago, he added.
At about the same time, according to the researchers, Earth's climate cooled; mammals like mammoths, mastodons, and saber-toothed cats went extinct; and one of the first American cultures disappeared.
Kennett has a very specific series of events in mind and this leaves many point that can be tested by experts in a variety of fields.
First, he pictures his impact appearing somewhere near the Great Lakes. Naturally, geoleogists will search for more debris from the impact in that area to see if his distribution is correct. Was there really an event centered in that area? If so is there a crater. There doesn't have to be a crater. It could be that the impactor exploded in the atmosphere or that it hit the glacial ice shield, which at that time was just north of there draining into the lakes.
Kennett says the impact would have released a great amount of cold meltwater into the Atlantic, possibly changing the global climate for decades. This theory is already widely accepted. At the time he mentions, the end ice age warming was suddenly reversed in an event called the Younger Dryas. The cold water flood is the most often named cause for the Younger Dryas. Even if Kennett's comet is proven, it must match the date of the Younger Dryas for his scenario to work.
Third, he has thrown himself right into the middle of the climate versus hunting controversy over mammoth extinction. While the impact itself might have been a cause of regional extinction for the mammoths through forest fires and direct devastation of their habitat, it is abrupt climate change of the Younger Dryas that has to do in mammoths and other large mammals in places like South America (where three genera of mastodon-like gomphotheres went extinct).
The fourth and last element of his theory is the newest and perhaps the most interesting. Kennett and his son Douglas Kennett, an archaeologist at the University of Oregon in Eugene, claim that the comet was a major cause of the end of the Clovis culture. The extinction of main big game prey has been the main theory given for the abrupt end of the Clovis culture. This theory works whether they caused the extinction through over hunting, by bringing a plague, or just had the bad luck to arrive as the climate killed all the big game. Losing their prey would have forced a change of lifestyle on the Clovis.
The Kennetts suggest two related but different causes for the end of Clovis culture in different regions. In the West and in what's now Latin America, the Clovis culture died out after losing their prey. But in the East, especially around the Great Lakes, the Kennetts point to an apparent gap between the end of Clovis culture and the beginning of post Clovis culture. They suggest that the impact really did wipe out most life in the area. In their narrative, forest fires, devastated habitat, and fallout killed or drove away all of the people in the East. It was only after the Western Clovis has diversified into new cultures and the Eastern forests grew back that humans recolonized the region.
I'm eager to see what happens when others go over the Kennetts' evidence. The story of the extinction of the mammoths and other large mammals at the end of the last ice age, the arrival of humans in the New World, and the transformation of the environment of the Americas is one of the great mysteries of the historical and earth sciences. So too is finding the right place for cosmic impacts in the history of the earth and its life. Whether the Kennetts' theory survives or not, the discussion should be fun.
* At the same time a counter idea grew up that the mammoth was a creature of a slightly colder time before the ice age who perished when it got too cold. This idea has never had large scale support but has never quite gone away either.
** The religious ideas that extinct species are the remains of prior creations--God using the earth for other projects before man--or that they perished in the flood have always been problematic because they are not mentioned in the Bible. The previous creations idea depends on the possibility of an unmentioned gap of time between God creating the heavens and the earth and the first day of Genesis. The idea that species were killed in the flood directly contradicts God ordering Noah to gather two of every animal and Noah fulfilling God's will.
*** Sorry, I couldn't resist.