The Top 10 Greatest Survivors of Evolution
Travel back millions of years in your time machine and you’d find some of these species thriving and looking much as they do todayBy Brian Switek
Smithsonian.com, November 09, 2012
When we think about the history of life on earth and the vast changes that have transpired over millions and millions of years—as single-celled organisms evolved into species as disparate as redwood trees, dragonflies and humans—are wonderfully apparent. But, among all that evolutionary change, some organisms have little modified from their distant ancestors. Creatures such as sharks and crocodiles are often viewed as evolutionary sluggards or “living fossils.” While the rest of nature was caught up in life’s race, the coelacanth and duck-billed platypus sat things out.
This perception isn’t quite right. Many species of these living fossils differ significantly from their prehistoric counterparts, and often the apparently archaic creatures are the remaining representatives of lineages that were once more varied and diverse. Still, many of these organisms look as if they belong to another era. Charles Darwin explained why in his famous book On the Origin of Species: Natural selection may have vastly modified other branches in the tree of life over time, but, among organisms like the lungfish, the quirks and contingencies of their habitats and lifestyles remained so stable that there was little evolutionary pressure to change. By chance, these lineages occupied an evolutionary sweet spot. The great Victorian naturalist Thomas Henry Huxley called these creatures “persistent types,” but there is an even simpler name for them—survivors.
Watch any documentary about crocodiles and you’re almost certain to hear the line “They have gone unchanged since the time of the dinosaurs.” That isn’t exactly true. While crocodylians as we know them today—the alligators, gharials and crocodiles that live at the water’s edge—have been around for about 85 million years, they belong to a much more diverse and disparate group of creatures that goes back to the Triassic.
Crocodylians are the last living representatives of the crocodylomorpha, an even bigger group that originated over 205 million years ago. They shared the world with the dinosaurs and came in a startling array of forms. Some—like the 112-million-year-old, approximately 40-foot-long giant Sarcosuchus—looked quite similar to their modern cousins, but there were also formidable ocean-going predators such as Dakosaurus; small forms with mammal-like teeth such as Pakasuchus; crocs with tusks and extra armor such as Armadillosuchus; and lithe, land-dwelling carnivores such as Sebecus. Modern crocs do look ancient, but they are just the remainders of an even older and stranger lineage.
2. Velvet worm
“Velvet worm” is something of a misnomer. Stretching a quarter of an inch to eight inches long, and flanked by rows of stubby legs along their smooth bodies, these invertebrates aren’t worms at all. They belong to their own group, which is more closely related to arthropods, and these inhabitants of the forest undergrowth are part of a much, much older lineage that goes back to one of the greatest evolutionary explosions of all time.
In 1909, secretary of the Smithsonian Institution Charles Doolittle Walcott discovered the fauna of the Burgess Shale—exquisitely preserved creatures from a 505-million-year-old sea. Many of these animals were unlike anything seen before, and the true affinities of many of the weird creatures from these deposits are still being debated. Even so, at least one creature looked familiar. Aysheaia, an invertebrate named by Walcott in 1911, closely resembles velvet worms and may be close to the group’s ancestry. Even though this form lacks some of the specialties seen in modern velvet worms, such as a unique nozzle system that squirts an instant web over prey, the Cambrian creature shared the segmented, stubby-legged body plans with living forms. Frustratingly, the soft bodies of velvet worms don’t fossilize very well so no one is entirely sure when they emerged onto land for the first time. But, if you know what to look for, you can still find them crawling through the leaf litter of tropical forests from Australia to South America.
3. Cow sharks
Most living sharks, from nurse sharks to great whites, have five gill slits on a side. But there are four species of cow sharks that have six or seven gills, a feature thought to be retained for millions of years from some of the earliest sharks. These deepwater, six- and seven-gill sharks are considered some of the most archaic of all shark species.
The evolutionary story of sharks is primarily one of teeth. With the exception of rare fossils that preserve remnants of soft parts, teeth are usually all that is preserved from cartilaginous shark bodies. An articulated specimen of the early shark Doliodus problematicus pushes the shark’s existence back to at least 409 million years ago, and they are probably even older than that. The lineage to which today’s six- and seven-gill sharks belong, however, is more recent. Based upon isolated, saw-blade fossil teeth, paleontologists think cow sharks have existed for at least 175 million years. These deepwater sharks are opportunistic feeders—taking whatever they can—and may have had a stable role as a deep-sea cleanup crew, scavenging on the bodies of marine reptiles during the Mesozoic and shifting to marine mammals after the time of the dinosaurs. We know very little about the appearances of these ancient sharks, but their roughly bladed teeth hint that they have been consummate deep-sea carrion feeders for millions of years.
Long-lived lineages of animals often get most of the attention, but there are some survivors among the plants, too. Horsetails must be some of the greatest. These archaic plants are often found growing in patches along streamsides and other wet habitats. Place a dinosaur toy among them, and the prehistoric model will look quite at home.
The reason why horsetails are considered so ancient comes from two lines of evidence. Living horsetails are unique among plants in that they reproduce via spores rather than seeds. Other plants likely gave up this method of reproducing millions and millions of years ago, but, old though it may be, the spore technique makes horsetails resilient and very difficult to remove from places where they are considered weeds. Horsetails also have a very deep fossil record. Though they make up small parts of forests now, enormous horsetails once made up entire forests in the days before modern trees evolved. In fact, much of the world’s coal, which originates from 360- to 300-million-year-old Carboniferous deposits, are the remnants of horsetails such as Calamites that could have grown to be over 100 feet tall.
Not all the great survivors are charismatic. Some of evolution’s greatest success stories are parasites, but few have stuck in there longer than lice.
Although louse fossils are rare, in 2004 paleontologists announced that they had found a 44-million-year-old feather louse that was strikingly similar to lice that live on the plumage of waterbirds today. The record of lice probably goes back even further. Last year, researchers used the few known louse fossils along with genetic comparisons between living lice to determine when major lice lineages evolved. Feather lice, in particular, seem to have split from their hitchhiking relatives sometime between 115 and 130 million years ago—right when little mammals were scurrying through the Cretaceous undergrowth and feathered dinosaurs were flocking around on land. Since feather lice evolved to feed on early birds and feather-covered, non-avian dinosaurs, they have had to change little to keep up with their hosts.
Pick up a brachiopod and you might think you’re looking at an ordinary clam. A shell split into two halves, called valves, protects the invertebrate, but in the case of the brachiopod, these two halves are unequal in size. That’s how they got their common name—the unequal proportions of the shells make some of the creatures look like old oil lanterns, hence the name “lamp shells.”
Whether found in gravel, attached to kelp or clinging to the rock of a continental shelf, brachiopods are relatively rare today. There may be around 100 different genera now living, but over 5,000 are known from a fossil record spanning 530 million years. By about 488 million years ago, brachiopods had become the dominant shelled animals in the seas—they were so thick in some places that their shells compose most of the sediment other fossils are found in—but that all changed with the worst mass extinction of all time. This was the Permian mass extinction, which some paleontologists rightly call the “Great Dying” for its catastrophic effect on the planet’s fauna. Though the exact triggers are still debated, about 251 million years ago a huge amount of greenhouse gases were dumped into the atmosphere, and the oceans became highly acidic. Brachiopods suffered, giving a foothold to the mollusk ancestors and cousins of modern clams and cockles. Brachiopods have hung on in whatever crevices they could attach to but never managed to regain their dominance.
Ginkgo trees aren’t quite as archaic as horsetails, but a record of over 175 million years is nothing to sneeze at. Today these trees are represented only by one species, Ginkgo biloba, but this tree with fan-shaped leaves had its heyday when ferns, cycads and Jurassic dinosaurs dominated the landscape.
Modern Ginkgo trees are not very different from those that herbivorous dinosaurs may have fed on. A recent Paleobiology study by Wesleyan University paleobotanist Dana Royer and colleagues found that Ginkgo trees seem to do best in disturbed habitats alongside streams and levees, a habitat preference that may have been their downfall. Scientists know from living Ginkgo trees that they grow slowly, start reproducing late and are generally reproductive slowpokes when compared to more recently evolved lineages of plants that live in the same places. Ginkgo trees may have simply been out-bred by other plants when suitable habitats opened up, but this makes it all the more remarkable that one species managed to survive to the present day.
8. Duck-billed platypus
The duck-billed platypus truly looks as if it belongs to another era, if not another planet. In fact, when 19th-century European naturalists first saw stuffed specimens sent from Australia, some scholars thought the animals must be a joke. But evolution wasn’t kidding—here was a mammal with a duck-like snout and a tail like a beaver and that laid eggs.
Monotremes, like the platypus, are strange mammals. These archaic, egg-laying forms last shared a common ancestor with marsupial and placental mammals over 175 million years ago, and rare fossils from Australia indicate that there have been platypus-like forms since 110 million years ago. Though often reconstructed with a narrower-snout, the Late Cretaceous Steropodon was a close cousin of early platypuses. A much closer relative to the modern platypus, known as Obdurodon, has been found in more recent rocks spanning about 25 to 5 million years ago. This animal is different from its living relative in retaining adult teeth and some particular skull characteristics, but the skull shape is strikingly similar. Rather than being a new kind of creature that evolved after the dinosaurs, the duck-billed platypus is truly a more archaic kind of mammal with roots that go far deeper than most other mammals on the planet.
Coelacanths were supposed to be dead. As far as early 20th-century paleontologists knew, these distant fishy cousins of ours—categorized as “lobe-finned” fish because of their fat fins supported by a series of bones similar to those in our own limbs—had gone extinct by the end of the Cretaceous, about 66 million years ago, along with the mosasaurs, pterosaurs, ammonites and non-avian dinosaurs. But it in 1938 Marjorie Courtenay-Latimer, a curator at South Africa’s East London Museum, recognized a very strange fish lying on a dock after getting a tip about something strange from the deep. As it would turn out, the fish was a living coelacanth—she might as well have found a living Tyrannosaurus.
Paleontologists have discovered fossil coelacanths younger than 65 million years old since 1938, but, since these were unknown when the fish was re-discovered off South Africa, the discovery of a living member of the group immediately catapulted the fish to fame. Two species have since been recognized, and they are different than their prehistoric relatives—enough to belong to a different genus, Latimeria—but they are still quite similar to their prehistoric cousins. Creatures recognizable as coelacanths go back to about 400 million years ago, and these fleshy-finned fish were the evolutionary cousins of lungfish and our own archaic forerunners—the very first vertebrates to walk on land were specialized lobe-finned fish related to the recently discovered Tiktaalik. Like many other organisms on this list, though, living coelacanths are the last of a once more widespread and varied lineage.
10. Horseshoe crab
There is probably no animal that epitomizes the title of “survivor” than the horseshoe crab. With their shield-like carapaces and long, spined tails, these arthropods look prehistoric. When masses of one species, Limulus polyphemus, congregate on Mid-Atlantic beaches in the warmth of early summer, it is difficult not to imagine the scene as something from the deep past.
Exactly when, where and how horseshoe crabs evolved remains a matter of ongoing investigation, but the group of arthropods they belong to is thought to have diverged from their arachnid cousins around 480 million years ago. The basic horseshoe crab body plan has been around since then, although not exactly in the form we now know. The newly named, 425-million-year-old Dibasterium durgae looked roughly like a horseshoe crab from the top, though if you were to turn the arthropod over, you would have been greeted by a nest of double-branched legs used for both breathing and locomotion.
Over time, other horseshoe crab species developed other odd adaptations. Creatures like the boomerang-shaped Austrolimilus and the double-button horseshoe crab Liomesaspis represent the extremes in the group’s variation, but it is true that horseshoe crabs as we know them today have been around for a very long time—the 150 million year old Mesolimulus looks like it would fit right in on a Delaware beach. Horseshoe crabs have continued to change since then, of course. The modern Atlantic horseshoe crab is not found in the fossil record, and the specific group of horseshoe crabs to which it belongs only has a record of about 20 million years. Still, the changes within the group have been astonishingly slight when viewed against the big picture of evolution. Since the time of the horseshoe crab’s origin, the world has seen several mass extinctions, the rise and fall of the non-avian dinosaurs and shiftings of continents and climates so drastic that the world truly is a wildly different place. All the while the horseshoe crabs have been there, crawling along the seafloor. May they will continue to do so for millions of years to come.