Some of Charles Darwin`s pet ideas were pronounced dead as dinosaurs at the recent North American Paleontological Convention hosted by the Field Museum of Natural History.
Notwithstanding Darwin`s enormous contribution to biology, the 19th Century British scientist inadvertently imposed blinders on paleontologists, restricting the way they look at the fossils they analyze, noted Stephen J. Gould, a Harvard professor and keynote speaker at the convention.
Still, adherents of creationism should sample the 329 papers presented at the meeting before signing up with the professional association of fossil hunters and dinosaur detectives. Virtually without exception, paleontologists have no quarrel with the basic concept of evolution.
Many became scientists because they had pressed their noses up against museum displays as children, enraptured by the thought that a fossil millions of years old could be their distant ancestor.
”As a kid in New York, I felt a sense of wonder staring at the dinosaurs in the Natural History Museum,” said Gould, who, with best sellers like
”Wonderful Life,” has found a popular audience for paleontology, which studies the deeper recesses of the Earth`s history.
When Gould was a graduate student, a century after Darwin published his theories, modern science accepted Darwin`s version of evolution as gospel.
”In the 1960s,” Gould said, ”young paleontologists were taught that our job, whenever new fossils might be uncovered, would be to demonstrate how they provided additional proof Darwin was correct.”
When Gould and a group of like-minded associates became professors, they started taking a fresh look at fossils long stored in university laboratories and museums.
Many simply could not be explained according to Darwin`s assumptions, a realization that forced Gould`s scientific generation to radically revise its account of how life on Earth evolved.
This new version of the story introduces a much stronger element of randomness into the evolutionary process than Darwin allowed for.
It also hypothesizes that life on Earth was shaped not just by natural processes native to this planet, but also by visitors from outer space:
comets that chanced to pass close to the Earth`s orbit and profoundly affected its history.
An intellectual stretch
This upheaval in paleontology is only the latest in a series that stretches back to the Renaissance, when Swiss naturalist Conrad Gesner published ”On Fossil Objects,” the first book with illustrations of curiously shaped rocks and the plants and animals they seemed to resemble.
We now recognize that fossils are the fingerprints, as it were, of ancient organisms whose decaying bodies were replaced by mineral deposits that thus provide a calcified record of life on Earth eons ago.
It wasn`t easy, though, for Renaissance scholars to come to that realization, noted Susan Kidwell, a University of Chicago paleontologist.
When Gesner wrote, naturalists still were struggling to separate their discipline from the tales of mythological beasts and wondrous creatures that had passed for science during the Middle Ages. To suggest that rocks had once been living beings seemed like a regression to superstition and magical thinking.
To see modern life as descending from earlier forms was equally difficult for scientists to grasp.
Even now, the fossil record is punctuated with exasperating gaps that often make it impossible to trace direct connections between present-day species and their predecessors. In previous centuries, the data were slimmer still.
”Paleontological research is the ultimate Sherlock Holmes brand of science,” Kidwell said.
”We try to find the cause of death for `victims` who died long, long ago. Rather than having the whole body, we are lucky if we find a few bones. Usually we don`t have direct evidence of what happened. So we have to construct a circumstantial case by fitting together a piece here, a bit there. It`s no wonder paleontologists are often crossword puzzle addicts too.”
Moreover, many fossils are so radically different from modern life forms that it would seem to require an enormous span of time for those ancient plants and animals to have changed into contemporary species.
But the brain, Kidwell noted, has a built-in resistance that makes it difficult for human beings to conceive of millions and millions of years. A 17th Century Anglican bishop, James Ussher, added up the Old Testament`s genealogy and calculated that the Earth must have been created in 4004 B.C. His conclusion continued to seem reasonable to scientists up to the 19th Century.
Indeed, one of the strongest objections to Darwin`s theory was that it posited an impossibly long history for the Earth. Darwin figured that for evolution to have done its work, the Earth must be about 100 million years old.
We now know his estimate was much too short; modern scientists assume the Earth is about 4.5 billion years old. But even Darwin`s smaller number was beyond the imagination of many contemporaries. The leading physicist of the day, Scotland`s Lord Kelvin, refused to accept the theory of evolution, saying the Earth could be no more than 10 million years old.
Even present-day paleontologists admit their minds boggle at the numbers their science now deals in, Kidwell said. Contemporary paleontology holds that life first appeared on Earth about 3.5 billion years ago.
But for the next 2.4 billion years, or about two-thirds of the time that life has existed, the Earth was inhabited solely by single-cell organisms of the simplest possible design. More complicated creatures appeared only about 570 million years ago, which makes mankind`s whole career on Earth (the first humanoids dating to about 6 million to 10 million years ago) a mere tick or two of the geological clock.
”We paleontologists each have our pet metaphor for dealing with the vast distances on that historical scale,” Kidwell said. ”The truth is, numbers like that are simply beyond human comprehension.”
At the end of the 18th Century, though, a few scholars started confronting the possibility that the Earth might be much older than previously imagined. A French scientist, Georges Cuvier, demonstrated that a fossil elephant, or mammoth, then recently uncovered in Siberia, was anatomically different from living elephants and thus represented an earlier species. The beast`s long-dead fossilized bones, Cuvier argued, ”prove the existence of a world anterior to ours, destroyed by some kind of catastrophe.”
Darwin finds the key
In the first decades of the 19th Century, similar fossil discoveries provided further proof that life on Earth had been evolving over immense periods of time. Indeed, scientists began to see a progressive direction to those changes, the fossil record suggesting that simpler life forms evolve into more complex ones. Finally, Darwin supplied the intellectual missing link, as it were, that seemed to explain why that process took place.
Darwin observed that within each species of plant and animal life, individual variations on a theme are always to be found. The offspring of a single set of parents are not all alike: Some might be taller or shorter, heavier or lighter, than their siblings. Some of those variations, Darwin argued, give their bearers a leg up in life`s struggles. So those variants have a better chance to survive and produce offspring like themselves. Over time, this process of natural selection, as Darwin called it, causes a species to evolve into a successor form.
Of course, Darwin`s theory clashed with the biblical account of God`s having created the world for man`s pleasure. But as Darwinism became accepted, the version of evolution incorporated into schoolbooks still accorded mankind a special place in nature, Gould noted.
As presented to his generation of graduate students, evolution was conceived of as a smoothly running process that connects the single-cell creatures of the far distant past with more and more complex life forms of more recent times. At the head of that unbroken chain of being stood modern man, as if he were the target toward which nature had been aiming through all evolutionary episodes.
That flattering portrait of mankind, though, hasn`t stood up to the test of recent discoveries, noted a number of participants at the paleontological convention. Evolution, they argue, follows a much more complicated formula than Darwin`s.
Gould observed that contemporary research shows that in the Cambrian period, when multicelled organisms first appeared, the Earth was populated by a dazzling array of life forms. Many of them-like Opabinia, a wondrous five-eyed creature-were much more complex than the animals of more recent times. So classical Darwinism, Gould argued, is flawed by its onward-and-upward assumption that life moves inexorably from the simple to the complex.
A roll of the dice
The fossil record also reveals that at critical junctures in the evolutionary process, large numbers of species died virtually simultaneously. All the dinosaurs suddenly vanished from the Earth about 65 million years ago, and such mass extinctions cannot be explained on Darwinian principles, noted David Raup, a University of Chicago paleontologist.
”Natural selection operates slowly and on a totally different scale,”
Raup said. ”Such disasters most likely had extraterrestrial sources.”
Raup argues that such extinctions resulted when comets passed close to the Earth, massively disturbing the planet`s ecology so quickly that older species had no chance to protect themselves by evolving defenses. Other scholars argue that the orderly development of the species was interrupted by the phenomenon of continental drift, as the Earth`s land masses have periodically shifted positions over the ages.
Either alternative means random chance played a large role in shaping the world we now inhabit, Gould argues. Accordingly, we can no longer look upon mankind as the logical outcome of the Earth`s history. Nor should we conceive of evolution as an orderly phenomenon.
Had nature`s dice fallen even slightly differently, life would have taken a different direction, a conclusion that led Gould to chose ”Wonderful Life” for the title of his version of the evolutionary story.
Gould recalled that in the movie ”It`s a Wonderful Life,” Jimmy Stewart is shown how different the lives of his friends and family would have been if he had never existed. The evolutionary process is like that, Gould said, given the role of chance and contingency.
”If we could wind back the tape of history and (do) it over all again, the story of life on Earth would be quite a different one,” he said. ”Humans might have only a walk-on role. Or, maybe, we wouldn`t even make an appearance.”
