Creatures That Time Forgot
From a handful of bugs and birds to mountain beavers, species that have changed little after tens to hundreds of millions of years both fascinate and puzzle biologists who study "living fossils"
Lisa W. Drew
A MOUNTAIN BEAVER'S reaction to overheating is hardly what you'd expect from a species that ranks among the animal kingdom's great success stories. Unable to sweat or pant, this muskrat-sized resident of the Pacific Northwest spends most of its time in a cool, moist burrow system. When the creature ventures out to forage, it can get too hot--and at that point all it can do is sprawl, utterly vulnerable to predators, until it cools off.
Yet fossil records indicate that at 40 million years old, the erroneously named species--which is not a beaver at all--is a survivor against all odds. Scientists generally agree that 99.9 percent of all species that have inhabited the planet are now extinct. Most species survive only between 1 and 10 million years. (Homo sapiens has existed for about 200,000 years.) Every single life-form, the statistics say, is eventually doomed.
The mountain beaver, however, has persisted longer than all existing squirrels, rats, chipmunks, mice, true beavers and other living rodents. And although such longevity is rare, it is not unique. Consider, for example, what fossils say about the ages of the sandhill crane (10 million years old), the Gila monster (30 million years old) or the tailed frog (150 million years old). Now consider that these examples are all land animals, relative newcomers compared to aquatic creatures or plants. The oldest living organism on the planet is a 3.5-billion-year-old bacterium that forms mounds called stromatolites in otherwise lifeless briny pools, most notably in Australia.
No wonder Charles Darwin digressed in his conclusion of The Origin of Species in 1859 to point out that some ancient life-forms "may fancifully be called living fossils." The term has stuck. "The living fossils are exactly what the name presupposes," says Cornell University ecologist Thomas Eisner. He has long studied millipedes--the first class of animals to dwell successfully on land 420 million years ago--some of which appear to be identical to their most ancient fossils. "They are fossils in some ways, but of course not entirely," he adds.
In other words, living fossils may look just like their ancient remains, but scientists cannot say how the animals functioned a million years ago. "I'm not sure I believe in the living fossil idea," says ecologist Steven Austad of the University of Idaho, who specializes in research on aging and has studied wild opossums. "You might have something like the millipede that has the same fundamental body form as the fossil but might not in detail be anything like that." Agrees evolutionary biologist David Wake of the University of California at Berkeley, "There is no reason to think that any species is particularly old; there is simply no way to know."
Still, living fossils do boast many features that are clearly primitive, and there is enough similarity between them and their ancient remains to fascinate scientists. For one thing, living fossils usually are at the ends of very long branches in the tree of life, with no close relatives. "What is significant is that the lineage to which they belong is isolated phylogenetically," says Wake. "This means they have no close or very close relatives." That fact could offer guidance to conservationists, he suggests. "In the long-branched taxa, we are preserving something unique."
One example Wake cites is the tailed frog, which is at the end of a branch that split about 150 million years ago from the limb that later sprouted all 4,000 other living frog species on Earth. The tailed frog inhabits banks of clear, cold streams in Pacific Northwest forests. One of its obvious differences from other frogs is a leap-and-gulp hunting technique, which takes much more energy than flicking out a tongue to catch prey. Then there's its tail, actually the male's sex organ, used to internally fertilize females. All but a few of the planet's other frogs fertilize eggs after females disgorge them from their bodies.
Somewhere in those details of the tailed frog's physiology may be clues to why it has survived so long. Or maybe not.
"Sheer luck is a major consideration," says Austad, who points out that even the aging of an individual involves chance. "Some oxygen radical may hit my DNA and cause cancer, and it may hit yours in a different spot and have no impact at all. In the environment there's a lot of random stuff too." For example, whatever killed off the dinosaurs 65 million years ago (climate-changing meteor hits are the leading candidate) spared tailed frogs. "If you were in the wrong place at the wrong time," says Austad, "it wouldn't matter what defenses you had; you still would be blown to cinders."
Mountain beavers also may be lucky. "They don't seem to do anything very well, and yet they have existed for so long," says biologist Dale Steele, who wrote a 1998 recovery plan for an endangered mountain beaver subspecies in Point Arena, California. Although the animals are apparently thriving in pockets of heavily vegetated habitat in many areas of the Pacific Northwest beyond California, they do not seem adept at recolonizing or moving into new territories. Mountain beavers also must have access to large amounts of water because of their primitive kidneys. They even have a low reproductive rate compared to most rodents. Females have litters once a year of just one to three offspring.
"Yet mountain beavers hang in there in some areas," adds Steele, now with the California Department of Fish and Game. "But it doesn't seem as if they have a robust strategy." Largely nocturnal animals that spend 75 percent of their time in their nest chambers under heavily vegetated landscapes, mountain beavers are rarely seen by humans. "Quite often, people in the Pacific Northwest have them in their backyards without even knowing the animals exist," says Steele.
Photo © KEVIN SCHAFER UNLIKELY SURVIVOR: The Hawaiian monk seal has inexplicably beaten the odds; at 15 million years, it is the world's oldest existing pinneped. Most species survive no more than 1 to 10 million years.
Another living fossil that seems to lack a great survival strategy is the Hawaiian monk seal, the oldest living pinneped--the family of fin-footed mammals that includes seals, walruses and sea lions. The monk seal branched off from its ancestors about 15 million years ago. Its physical differences from other seals include a more primitive skull and ears. Behaviorally, monk seals--especially females with young--also seem more disturbed than other seals by human interference. Although the Hawaiian monk seal has many of the same adaptive features as other pinnepeds--such as the abilities to dive deeply for prolonged periods and to exist both on land and in the sea--it isn't clear why this species in particular has survived so long. It seems simply to have benefited from having no significant predators.
But modern monk seals have lost that advantage. When the continents shifted into their present configuration, this species settled at the northwest end of the Hawaiian island chain. There it thrived until humans arrived. During World War II, the U.S. military established facilities at Midway Island and elsewhere along the chain, and for the first time in their history, the seals shared some traditional breeding beaches with people. They also found themselves contending with commercial fishing nets and hooks, which have been known to strangle and pierce the creatures. The new pressures on monk seals, particularly on pups, have caused steep declines in their populations. The species has been federally listed as endangered since 1976.
Aside from luck, another characteristic may explain many living fossils' longevity. "My argument would be that more often than not, the answer is a defensive strategy," says Eisner. For millipedes, defense takes many forms. With exoskeletons, they basically wear their bones on the outside, using the hard material as armor. When threatened, they coil up their segmented bodies to repel attacks. A couple of years ago, Eisner and his colleagues were amazed to discover that one apparently ancient millipede species--"very possibly as old as the oldest"--uses barbed grappling hooks to fend off hungry ants. The detachable bristles ensnare hairlike seta on the ants, often inescapably.
LONG-LIVED:Well-armored armadillos, such as the nine-banded armadillo (above), have survived roughly 50 million years.
For about 50 million years, armor also has protected armadillos. The nine-banded armadillo, which lives in the southern United States and parts of Latin America, bears shields decorated with small bones that protect its back and sides. Not as tough as they may appear, the shields may mostly help protect the creature as it scurries through thick underbrush.
Some animals defend themselves with chemicals, and millipedes, once again, offer classic examples--as do many other arthropods and insects. "Anyone who has collected millipedes in the field is familiar with the odorous and often noxiously irritating fluids that so many of them emit when handled," notes Eisner.
The Southwest's Gila monster, at least 30 million years old, is also aided by chemical defense--as Cecil Schwalbe, a biologist at the U.S. Geological Survey's Sonoran Desert Field Station, can attest. He took weeks to recover from a 1989 bite from one of these reptiles, which deliver a neurotoxin he describes as feeling "like a wave of fire." In addition, the animal has scales on its back decorated with bits of protective bone that are "like a woman's beaded purse," says Schwalbe.
Such defenses only tell part of the story, however. Even the most well-protected living fossils have other adaptations that, along with luck, have contributed to their longevity. A female armadillo, for example, can delay the development and birth of her embryo for as long as 20 months after being impregnated if the offspring's survival is threatened by drought or other problems. Gila monsters can live for as long as three years without food. And their coloring, which varies from yellow and black to red and black, may serve two purposes. "It's like warning coloration out in the open," says Schwalbe. "A potential predator could be warned. But when a Gila monster is in dappled shade under a palo verde tree, that same coloration can be concealing."
Some living fossils that lack obvious strong defenses apparently compensate with other adaptations. One of the mountain beaver's strengths is its ability to stomach almost any vegetation. "Basically," says Steele, "it will eat a number of plants other animals won't mess with because of toxic chemicals." The sandhill crane--which at 10 million years is among the oldest living bird species--also has a broad diet. It eats prey from both salt and fresh water as well as a huge variety of vegetation, including crops and waste grain. And sandhills do well in a range of wet habitats, from tundra to farmland.
Then there is the opossum, which first appeared 70 million years ago. North America's only marsupial, the Virginia opossum probably evolved from its common opossum ancestors about 50,000 years ago. Not only does this animal have some amazing features--such as an immunity to rattlesnake venom that presumably allows it to dine on the vipers--it also can thrive under all sorts of circumstances. "It can live in a lot of different places, eat a lot of different foods and can take a lot of punishment," says Austad. "The fact is it can do well just about any place but deserts."
But those qualities can't completely explain the opossum's survival. A long-lived species might be adaptable, covered with armor, able to squirt poison at enemies and still die off from a simple infection. "These things that seem to have lasted so long must have been protected against disease," says Austad. "They must have good cellular level defenses."
Cornell's Eisner has been pondering that very notion. "My argument is that if you're going to look for medicinals from nature, pick living fossils," he says. "To take some randomness out of the search, start by asking yourself who has been around for millions of years."
Scientists generally agree that 99.9 percent of all species that have inhabited the planet are now extinct.
Field Editor Lisa W. Drew lives in Fairbanks, Alaska.
Related Feature: "Leader of the Flock," National Wildlife, Aug/Sep 1998