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 today
By
Brian Switek
Smithsonian.com,
November 09, 2012
Crocodylians are the last living representatives of the crocodylomorpha,
an even bigger group that originated over 205 million years.
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.
1. Crocodylians
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.
4. Horsetails
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.
5. Lice
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.
6. Brachiopods
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.
7. Ginkgo
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.
9. Coelacanth
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.
