FROM THE OCEAN
Oddly enough, Charles Darwin stood the scientific world on end by what he found by going to sea 135 years ago, subsequent tinkerers in evolutionary theory have been largely landlubbers. Virtually ignoring seven-tenths of our planet's surface, such investigators focused on land-locked fossil records, establishing evolution's hallowed grounds in the plains of East Africa and the Middle East.
Surely a lack of sophistication in seafaring technology accounts for much of the land-based research effort, although effective deep-sea trawling and sampling devices were in use by scientists in Darwin's day. But even with the advent of powerful sonar gear and deep-ocean drilling techniques in the 1950's, evolutionists' attention remained largely riveted on dusty fragments gleaned from the world's remote hillsides.
In recent years, however, scientists began revisiting the oceans, curious about how certain sea fossils fit models of evolutionary theory synthesized almost entirely from scattered, often puzzling evidence recovered from dry land. Some intriguing results turned up recently in the laboratories of two Florida State University (FSU) marine paleontologists.
Tony Arnold and Bill Parker compiled what may be the largest, most complete set of data on the evolutionary history of any group of organisms, marine or otherwise. The two scientists amassed something that their land-based colleagues only dreamed about: An intact fossil record with no missing links.
"It's all here--a virtually complete evolutionary record," says Arnold. "There are other good examples, but this is by far the best. We're seeing the whole picture of how this group of organisms has changed throughout most of its existence on Earth."
The organism that Arnold and Parker study is a single-celled, microscopic animal belonging to the Foraminiferida, an order of hard-shelled, planktonic marine protozoans. Often shortened to "forams," the name comes from the Latin word foramen, or "opening." The organisms can be likened to amoebas wearing shells, with perforations through which their protoplasm extends. The foram shell shapes range from plain to bizarre.
Tropical and subtropical seas around the globe abound with forams, which are divided into two general types: The free-floating, planktonic form that is uniformly small (usually less than a 50th of an inch long) and the benthic or bottom-dwelling variety that is typically much larger. The later type is perhaps best remembered by Earth-science students or by spelunkers who commonly find the fossils embedded in cave walls. The ancient Egyptians used limestone blocks containing the large, extinct Nummulites to build 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 almost everywhere in the oceans. Their fossilized skeletons, in fact, were among some of the first biological material recovered from deep ocean bottoms by scientists in the 1850s. For nearly a century, geologists have used the tiny fossils to help establish the age of sediments and to gain insight into prehistoric climates.
Only since the 1960s, though, have scientists begun to fully appreciate fossil forams' potential as a tool for use in evolutionary studies and a host of Earth sciences as well. Advanced deep-sea drilling techniques, combined with computer-assisted analytical tools, have ushered in a whole new vista of foram research. Arnold and Parker are among the first scientists to harness sophisticated technology to a foram project for the express purpose of studying evolution.
In 1980, Arnold successfully married 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 their specially programmed computer. Although it requires an operator, the system is the fastest, most reliable means of foram identification and classification available. It soon will become far more powerful if its developers succeed in linking it with a scanning electron microscope.
"There's a nifty passage in Darwin," says Arnold, "in which he describes the fossil record
|By studying forams, Tony Arnold (front) and Bill Parker assembled many evolutionary sequences with virtually no missing links.
"Well, in this case, we've got a relatively complete library," says Arnold. "The 'books' are in excellent shape. You can see every page, every word."
As he speaks, Arnold shows a series of microphotographs, depicting the evolutionary change wrought on a single foram species. "This is the same organism, as it existed through 500,000 years," he says. "We've got hundreds of examples like this, complete life and evolutionary histories for dozens of species."
About 330 species of living and extinct planktonic forams have been classified so far. 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 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," says Parker. "But these fossils are almost perfectly preserved, despite being millions of years old."
By being so small, the fossil shells escaped nature's grinding and crushing forces, which over the eons have in fact destroyed most evidence of life on Earth. The extraordinary condition of the shells permits the paleontologists to study in detail not only how a whole species develops, but how individual animals develop from birth to adulthood.
The resulting data base thus holds unprecedented power for evolutionary studies, says Arnold. 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 for testing evolutionary theories, which are growing in number.
"Scientists 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," says Arnold. "And unless they can be scientifically tested, theories don't really amount to much. So, what we have here is a wonderful opportunity 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 how evolution happened," says Parker. "We can see transitions from one species to another. 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, and perhaps eased a nagging worry about the terribly incomplete fossil record yielded by terrestrial research.
As for the origin of species, the famous naturalist always held that new plants and animals arise from unstable varieties sprung off from old species. Competition among
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Darwin termed the process gradualism, a theory that invokes the slow accumulation of small evolutionary changes over a large period of time, as a result of the pressures of natural selection. What Arnold and Parker found is almost a textbook example of gradualism at work.
We've literally seen hundreds of speciation events," syas Arnold. "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 cannot 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.
Such a revelation flies in the face of latter-day rethinking of Darwinian evolution, which during the past 20 years has tended to gravitate toward a new theory called punctuated equilibrium. First postulated in the early 1970s by paleontologists Niles Eldredge and Stephen J. Gould, this idea refutes the central premise that great amounts of time are necessary to create new species.
Punctuated equilibrium holds that new species may arise fairly quickly (over thousands instead of millions of years) from small animal populations that somehow become isolated. Intermediate stages are too fleeting to become fixed in the fossil record--thus the conspicuous gaps or so-called missing links. (Darwin blamed the "imperfection in the geological record" for the gaps in the fossil record.)
But in the near-perfect record exhibited by the forams studied at FSU, the highly touted Eldredge-Gould theory of punctuated equilibrium apparently doesn't work. The record 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. 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 prepetually unfolding in extreme slow motion.
In the hands of less scrupulous observers, the foram record may have been construed to support Gould's hypothesis about the suddeness of speciation. Darwin would have been shocked to find out just how fast the great family of forams churns out new species, says Parker. Through dating analysis, he and his colleague showed 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," says Arnold.
Other curiosities are beginning to emerge from the probe into the forams' past. One finding is being described--perhaps too hastily--as disproving Cope's Rule, named for its synthesis by American paleontologist Edward Drinker Cope. His time-honored evolutionary principle holds that, within a group, animals teend to start out small and increase in size over time.
"We've found out that apparently, lineages don't exactly work that way," says Arnold. "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."
This 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 mass annihilation that eventually draws the most attention to the FSU paleontologists' 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 Earth's ecology, wiping out countless species that had taken 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 so wiped out a good portion of the marine life--including almost all species of planktonic forams.
This period of massive 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 (and thus competition between species), yet quite fertile and primed for repopulation.
Some observers, perhaps following Darwin's lead, have envisioned a 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.
Other 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 greatly accelerates, 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 fans out into these limited niches, where crowding soon forces most of the new forms to spin out into oblivion similar to sparks from a bonfire.
The ancient record of foram evolution reveals that the story of recovery after extinction is indeed busy and colorful. "What we've found suggests that the rate of speciation increases dramatically in a biological vacuum," says Parker. "After the Cretaceous extinction, the few surviving foram species rapidly evolved into new species, and for the first time we're able to see just how this happens, and how fast."
As the available niches fill up with these new creatures, the speciation rates slow down, and the pressure from competition between species appears to bear down in earnest. The extinction rate then rises accordingly. This scenario, says Arnold, 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 shear numbers, of individual organisms that any environment can support, as opposed to the number of different kinds of organisms or species. "This is an intriguing concept--a species carrying capacity, so to speak," says Arnold. "This implies that the speciation process is sensitive to how many spesies are already out there."
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. Punctuated equilibrium can be a real process impinging on the evolution of other groups of organisms. 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.
Arnold and Parker 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 splendidly for this organism, and fail miserably for that one.
"It's very likely that there are going to be some differences between species in the way evolution works," says Arnold. "There are particular guiding principles, however, that we believe should work for all spesies."
No doubt the relentless search for a better understanding of how life came to be will lead scientists into many ecosystems not yet known or fully explored. Scientists might do well to follow the fossil trail, no matter how hard to track, to its timeless beginnings in the sea.