Like a fly that won't shoo away, the question has bugged believers in evolution since well before Charles Darwin outlined the basic theory in Origin of Species in 1859.
If animals gradually change their shapes through time, perpetually giving birth to new species, then where are the fossil remains of transitional animals -- those unfortunate creatures that had to have existed during the demise of outmoded species and before the establishment of new ones? Where are all the "missing links" that, if the theory holds, should be abundant in the fossil record?
So troublesome was the issue, striking as it did at the heart of his central premise, that Darwin devoted a whole chapter of Origin to addressing it. To his death, he held that the lack of hard evidence for evolutionary development between species was the fault of the "imperfection of the geological record." Earth's complete biological history would never be written, he avowed, because nature and time had erased far too many details.
Today's fossil record is dramatically improved over the one Darwin knew, yet it's clear that the precise pathways of evolutionary change will forever be shrouded in mystery, just as Darwin predicted. Some apparently genuine transitional forms have indeed been found, among the best examples being that of a large set of prehistoric mammal-like reptiles belonging to the Therapsid family. But such finds are exceedingly rare. The fossil record remains a murky book of clues, missing many chapters, on how a stupefyingly complex web of life came to be.
So the question is with us still, to the delight of creationists and other critics of Darwinism who depend on the hazy fossil trail to bolster their arguments. Outside fundamentalist religion, the chief criticism has come (since Darwin's day, in fact) from thoughtful scientists who embrace the basic tenets of evolution, yet have trouble reconciling substantial evidence suggesting that some species apparently sprang into being rather suddenly, without benefit of large chunks of geologic time required for their development.
As a consequence, the entire spectrum of modern evolutionary studies is now cast by a different light. Clever new theories have crept to the forefront of international debate on how evolution works. In fact, belief in Darwin's original concept of gradualism, a system invoking the familiar "tree of life" image of graceful, branching development, is virtually extinct in some scientific circles.
Of late, much ado has been made of the theory of punctuated equilibrium, formulated in the early 1970s by two paleontologists, Niles Eldredge and Stephen Jay Gould. New species may arise fairly quickly (over thousands instead of millions of years) from small animal populations that somehow become isolated, the theory postulates. Intermediate stages are thus too fleeting to become fixed in the fossil record.
The new twist is threatening to unseat gradualism as the ideal model for evolutionary theory. Proponents extol the idea's virtues, mainly its capacity to explain the gaps in the fossil record and to suggest answers to nagging questions about how the curious process of natural selection -- never explained very well by Darwin or anyone else -- actually works. Critics of the theory are finding themselves increasingly wedged into intellectual corners, with little new information upon which to base fresh rebuttals.
But to paraphrase Mark Twain, reports of Darwinism's death are highly exaggerated. The fossil record may be full of holes, but the venerable theory of speciation via slow change over vast stretches of time is backed by formidable evidence derived from both the fossil record and the field of population genetics. And once the findings of two FSU paleontologists become well-known, the whole restive world of evolutionary biology may wake up to find that Darwin indeed had it basically right all along.
Drs. Tony Arnold (Ph.D., Harvard) and Bill Parker (Ph.D., Chicago) are the developers of what reportedly is the largest, most complete set of data ever compiled on the evolutionary history of an organism. The two scientists have painstakingly pieced together a virtually unbroken fossil record that shows in stunning detail how a single-celled marine organism has evolved during the past 66 million years. Apparently, it's the only fossil record known to science that has no obvious gaps -- no "missing links."
"It's all here -- a complete record," says Arnold. "There are other good examples, but this is by far the best. We're seeing the whole picture of how this organism has changed throughout most of its existence on Earth."
Pervading any discussion with Arnold or Parker about their work is an almost visible air of excitement, an inescapable feeling of being there>. The young scientists are obviously excited, but they're also keenly aware of the need for caution. Over the years, the field of evolutionary studies has become sensitized by premature revelations of "ground-breaking" hands that claimed to say definitively how some particular cog clicks in the evolutionary machine.
Arnold and Parker aren't ready to make any hard-and-fast claims, but neither are they content to stay mum. They've tapped into one of the richest sources of insight ever found on how evolution really works, and they aren't hesitant to say what their findings suggest about evolution's true character.
Word already is out in the scientific community that what the FSU paleontologists (micropaleontologists, to be more precise) have stumbled onto is, at the least, "intriguing," Parker says. This summer, growing curiosity led to an invitation for the pair to present a paper at a symposium on microevolution, sponsored by the Geological Society of America this fall.
Criticism, when it inevitably comes, will be directed chiefly at the scientists' methodology and, perhaps to a lesser extent, at their choice of model (the animal under study), Parker predicts. "The methods we use are chiefly of our own design," he said. "They require a certain orientation to appreciate. And that's primarily because of the nature of this particular beast."
The study focuses on the microscopic, fossilized remains of an organism belonging to a huge order of marine protozoans called foraminifera. Often heard shortened to "forams," the name comes from the Latin word foramen, or "opening." The organisms can be likened to amoebas wearing shells, perforated to allow strands of protoplasm to bleed through. The shell shapes range from the plain to the bizarre.
Tropical and sub-tropical seas around the globe abound with forams, which are generally divided into two types: the free-floating, planktonic form, which is uniformly small (usually less than a 50th of an inch long); and the benthic or bottom-dwelling variety, which is typically much larger.
The latter type is perhaps best remembered by Earth science students, or by spelunkers who commonly find their wafer-like fossils imbedded in cave walls. The ancient Egyptians used an especially large, now-extinct species, Nummulites, to "tile" the tops of some Giza pyramids.
But it's the planktonic variety that chiefly interests Parker and Arnold. Unlike their oversized cousins, free-swimming forams are found wherever the oceans have, or had, currents -- in a word, everywhere. For nearly a century, geologists have used the animals' tiny, fossilized shells, found in abundance in marine and some terrestrial deposits, to help establish the age of sediments and to gain insight into prehistoric climates.
0nly since the early 1960s, though, have scientists begun to fully appreciate fossil forams' potential as a learning tool for use in evolutionary studies and a host of Earth sciences as well. Advanced deepsea drilling techniques, combined with computer-assisted analytical tools, have ushered in a whole new vista of foram research. Arnold and Parker are two of the first scientists to harness sophisticated technology to a foram project for the express purpose of studying evolution.
It was Arnold, in fact, who in 1982 became the first researcher to successfully marry computers with optical devices to create an efficient, precise way to analyze foram fossils. Before the technique was developed, the field was represented only by a few, extraordinarily dedicated individuals who spent countless hours over microscopes, sorting and analyzing the sand-grain-sized shells virtually by hand.
The apparatus Arnold and Parker now use combines the latest in video technology with a computer specially programmed by the researchers themselves. Although not fully automated (an operator is required), the system is the fastest, most reliable means of foram identification and classification available. The system soon will become far more powerful if its developers succeed in linking it with a scanning electron microscope, as planned.
"There's a nifty passage in Darwin in which he describes the fossil record as a library. The library has only a few books, and each book has only a few chapters. The chapters have only a few words, and the words are missing letters.
"Well, in this case, we've got a relatively complete library. The 'books' are in excellent shape. You can see every page, every word."
As he spoke, Arnold showed a series of photographs, taken through a microscope, depicting the evolutionary change wrought on a single foraminiferan species.
"This is the same organism, as it existed through 500,000 years," Arnold said. "We've got hundreds of examples like this, complete life and evolutionary histories for dozens of species."
Counting both living and extinct animals, about 330 species of planktonic forams have been classified so far, Arnold said. After thorough examinations of marine sediments collected from around the world, micropaleontologists now suspect these are just about all the free-floating forams that ever existed.
The exhaustive species collection also is exceptionally well-preserved, which accounts largely for the excitement shared by Parker and Arnold. "Most fossils, particularly those of the vertebrates, are fragmented -- just odds and ends," Parker said. "But these fossils are almost perfectly preserved, despite being millions of years old. We have the whole creature, minus the protoplasm."
By being so small, the fossil shells escaped nature's grinding and crushing forces, which over the eons have in fact destroyed most of the evidence of life on Earth. The extraordinary condition of the shells permits Parker and Arnold to study in detail not only how a whole species developed, but how individuals physiologically developed from birth to adulthood.
The resulting data base thus holds unprecedented power for evolutionary studies, Arnold said. Not only can he and Parker use it to describe how evolution has worked in a particular species, but they can use it as a standard by which evolution theories -- of which there's a growing number -- may be tested.
"Biologists are overflowing with ideas on the laws of evolution, or principles of evolutionary change, but most of them are simply untestable because of the poor fossil record," Arnold said. "And unless they can be scientifically tested, theories don't really amount to much.
"So, what we have here is our only chance to test a lot of these ideas quantitatively. We'll be able to say, with some degree of reliability, that yes, this or that happens in the forams or no, it doesn't."
Some biologists have long suspected that the evolutionary process works differently, although within certain principles, among different species. In other words, what may be true for evolution in mammals may not be true in molluscs.
"The forams may not be representative of all organisms, but at least in this group we can actually see evolution happening. We can see transitions from one species to another," Parker said.
"And that's a very rare observation." Had Darwin been able to examine the fossil record of forams, he could have fortified many of his arguments on how new species come into being.
The famous naturalist always held that new plants and animals arise from unstable varieties sprung off from old species. Competition among varieties, pressured by the law of "survival of the fittest," inevitably leads to populations that are so profoundly different that they become sexually incompatible (incapable of producing offspring) with populations other than themselves. And voila, a new species is born.
The pattern is exactly what Arnold and Parker have found in the forams. It is but one of a number of observations that the FSU team has made thus far about what arguably is nature's crowning achievement -- the act of speciation itself.
Frustrated by only rudimentary information dug from typical fossil finds, modern-day biologists intent on finding hard evidence on how speciation works have resorted to designing intricate lab experiments using live organisms. Some lines of research based on fruit flies, for example, are now legendary among developmental biologists.
"You could work on fruit flies for an awfully long time and not come close to seeing what we're seeing in a record stretching back nearly 70 million years," Parker said.
"We've literally seen hundreds of speciation events," Arnold added. "This allows us to check for patterns, to determine what exactly is going on. We can quickly tell whether something is a recurring phenomenon -- a pattern -- or whether it's just an anomaly.
"This way, we can not only look for the same things that have been observed in living organisms, but we can see just how often these things really happen in the environment over an enormous period of time."
Adherents of Darwin's theory of gradualism, in which new species slowly branch off from original stock, should be delighted by what the FSU researchers have found. The foram record clearly reveals a robust, highly branched evolutionary tree, complete with Darwin's predicted "dead ends" -- varieties that lead nowhere -- and a profusion of variability in sizes and body shapes. Moreover, transitional forms between species are readily apparent, making it relatively easy to track ancestor species to their descendants.
In short, the finding upholds Darwin's lifelong conviction that "nature does not proceed in leaps," but rather is a system perpetually growing in extreme slow-motion. This means that, in foram evolution at least, the highly touted Eldredge-Gould theory of punctuated equilibrium apparently doesn't work.
In divulging this revelation, Arnold could be forgiven for taking a modicum of perverse glee, the kind a highschool smart-aleck displays when he catches the teacher in a mistake. Gould, now among the most famous scientists in the world, directed Arnold's Harvard dissertation. But there's no room for that here, he says. Arnold maintains a warm professional relationship with his former mentor, who paid his lab a visit when FSU's Distinguished Lecture Series brought him to campus last year. Gould concedes that the forams don't fit his model of punctuated equilibrium, Arnold said.
"He was characteristically pleased to be contradicted with this information. His immediate response was that the forams are probably a special case."
But this response raises an obvious question, says Parker. How many other animals out there are "special?" Are the forams the only organisms that evolve according to the basic precepts laid down by Darwin?
"Since ours is a much better record than most of what Gould and others are looking at, is what they're seeing truly punctuational?," Parker asks. "They may simply be missing fossil records, which, of course, is exactly what Darwin used to say."
But to give Gould his due, both Arnold and Parker describe him as "not being wed to (the idea of punctuational equilibrium) like an apostle is to some kind of creed."
"Steve (Gould) is not convinced that his theory is the truth, the absolute truth," Arnold said. "He simply holds it out as being a possibility for species change that most scientists had overlooked. His main objective was to persuade the scientific community to consider the idea."
Parker credits Gould's idea with piquing his and Arnold's curiosity about the gaps in the fossil record. "It was his establishment of the punctuational model that got people like Tony (Arnold), myself and others to actually look for transitional forms," he said. "Without that impetus, we may never have stumbled onto this."
And in point of fact, in the hands of less scrupulous observers the foram record may have been construed to support Gould's hypothesis about the suddenness of speciation. Darwin would have been shocked to find out just how fast the great family of forams churns out new species, said Parker.
Through dating analysis, he and his colleague have shown that the forams could produce a whole new species in as little as 200,000 years -- speedy by Darwinian standards. "But as fast as this is, it's still far too slow to be classed as punctuational," Arnold said.
Other curiosities are only now beginning to emerge from the mountain of data he and Parker are amassing from the probe into the forams' past. Some of the findings are still too raw to publish, but headed to the scientific press soon will be other surprises about what the foram record says about some of today's most treasured evolutionary theories.
One of the findings already is being described -- perhaps too hastily -- as disproving Cope's Rule, so named for it's synthesis by the American paleontologist Edward Drinker Cope (1840-97). The time-honored evolutionary principle basically holds that all animal groups tend to start out small and increase in size over time.
"We've found out that apparently, lineages don't exactly work that way," Arnold said. "Many of the forams start out small, and essentially stay that way until extinction. Others do manage to wander into dramatically larger sizes, but they're the rare ones."
But the find doesn't necessarily contradict what Cope said, only what many scientists think he said, says Parker. "Cope's observation was simply that there are a few extremely large examples (of individuals) in any given lineage, and these examples always occur at the later stages of the organism's development. And that's apparently true.
"But our findings show that the vast majority of forams start small and end small, even though the mean size increases somewhat due to a few very large specimens. As you get more and more species evolving, some of them eventually manage to get moderately to very large, but most of them don't increase in size at all."
It may be in what the foram record suggests about how life copes with Mother Earth's periodic bouts with annihilation that eventually draws the most attention to Arnold's and Parker's work. The geologic record has been prominently scarred by a series of global cataclysms of unknown, yet hotly debated, origin. Each event, whether rapid or slow, wreaked wholesale carnage on the planet's ecology, wiping out countless species that had taken nature millions of years to produce. Biologists have always wondered how life bounces back after such sweeping devastation.
One of the last great extinctions occurred roughly 66 million years ago, and according to one popular theory it resulted from Earth's receiving a direct hit from a large asteroid. Whatever the cause, the event proved to be the dinosaurs' coup de grace, and also wiped out a good portion of Earth's marine life -- including almost all species of planktonic forams.
This period of mass death, which ended the Cretaceous Period, ushered in the modern chapter of biological development. Earth entered the new era, the Cenozoic, with a wide range of ecosystems virtually devoid of life, yet quite fertile and primed for repopulation.
Like ecologists who study how wildlife recovers from a forest fire, evolutionists are drawn to such incidences of "biological vacuum" in search of clues as to how the earliest forms of life started evolving, when competition wasn't the controlling factor in the process.
Particularly vexing are questions about what forces drive the process of natural selection -- what factors ultimately control the tempo and fate of biological struggle. But because of the traditional fragmentation of the fossil record, answers have remained elusive.
"In most cases of evolution, we're incapable of collecting the basic facts. But here's one case where we are capable," Parker said.
Since the foram record extends through a major extinction event (some of the samples date back nearly 100 million years), it represents the first, grand template against which a flock of pet theories on the beginnings of evolution may now be effectively measured, he said.
"This is the great naturalist experiment," says Parker. "How often is it that you get to almost wipe your slate clean and then watch an ecosystem start up all over again?"
Some scientists have theorized, but never been able to demonstrate, that in the absence of competition, an explosion of life takes place. The evolution of new species is greatly accelerated, and a profusion of body shapes and sizes bursts across the horizon, filling up vacant spaces like weeds overtaking a pristine lawn. An array of new forms fan out into these limited niches, where crowding soon forces most of the new forms to spin out into oblivion, as sparks from a flame.
Other observers, perhaps following Darwin's lead, have envisioned a much more sedate repopulation sequence, with speciation occurring at an immensely slow rate. None of the species die off until their numbers begin to saturate the environment, exhausting its capacity to sustain such proliferation of life.
As revealed by the ancient record left by the foram family, the story of recovery after extinction is every bit as busy and colorful as some scientists have long suspected.
"What we've found suggests that the rate of speciation increases dramatically in a biological vacuum," Parker said. "After the Cretaceous extinction, the few surviving foram species began rapidly propagating into new species, and for the first time we're able to see just how this happens, and how fast."
As foram survivors rush to occupy their new habitats, they seem to start experimenting will all sorts of body shapes, trying to find something stable, something that will work, Arnold said. Once a population in a given habitat develops a shape or other characteristic that stands up to the environment, suddenly the organisms begin to coalesce around what becomes a standardized form, the signature of a new species.
As the available niches begin to fill up with these new creatures, the speciation rate begins to slow down, and pressure from competition between species appears to bear down in earnest. The extinction rate then rises accordingly.
This scenario, Arnold says, suggests that the speciation process is sensitive to how fully packed the biosphere is with other species, not the number of individuals. Ecologists, in referring to a given environment's ability to sustain life as its carrying capacity, generally mean the natural limit, in sheer numbers, of individual organisms that any environment can support, as opposed to the number of different kinds of organisms.
"This is an intriguing concept -- a species carrying capacity, so to speak," Arnold says. "This implies that the speciation process is sensitive to how many species are already out there."
Parker compared this feat to a stand of pine trees being sensitive to the fact that there are no oaks in its midst.
"This implies that it makes an evolutionary difference to that pine population whether any of its neighboring populations are other pines or oaks. I don't believe that kind of sensitivity has ever been demonstrated with living species."
Perhaps if life were any less strange, its fundamental processes any simpler to fathom, scientists would not be so sensitive about their inability to write the definitive book on evolution. It may well be in the abyssal depths of the mystery itself that scientists find their innate compulsion to explain things magnified.
Exactly what new light the findings of Arnold and Parker shed on the evolutionary riddle as a whole is still unclear. Critics may argue that while the FSU findings are interesting, they apply only to a rather peculiar organism and therefore do little to unmask the grander, biological scheme of things.
The FSU paleontologists concede that evolution may in fact be little more than a collection of developmental options, all tailored along the same lines, presented in haphazard fashion before a sea of struggling life. One option may work for this organism, but fail miserably for that one.
"It's very likely that there are going to be some differences in the way evolution works between species," Arnold said. "There are certain guiding principles, however, that we believe should work for all species."
The testimony of the thick foraminiferan record represents a persuasive argument that the principles it reveals are powerful enough to be considered part of evolution's primary resources, he said.
"If someone were to suggest to me that what we've found in the forams doesn't work in rodents, for example, my response would be: 'OK, show me some (evolutionary) data on rodents. You may be right.' The thing is, that person can't show you any data like that, because it doesn't exist."
Parker summed up by describing the foram research with an analogy.
"You walk out of a restaurant and get halfway to your car, only to discover you've lost your keys. It's night. Where do you go look for them? It makes sense to look under the street light. Not necessarily because you think that's where you lost your keys, but because that's the only place you're going to find them if they're there.
"So, we're looking under street lights right now. If at some future time, there's some way of illuminating the rest of the record, we'll start looking in those places, too.
"But right now the illumination is best right where we're looking."
-- Frank Stephenson
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