Photos: 7 Major “Missing Links” Since Darwin

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Tiktaalik: The “Fishapod”

Illustration by Zina Deretsky, National Science Foundation

For the 200th anniversary of Charles Darwin’s birth (February 12, 2009), National Geographic News asked leading scientists for their picks of the most important fossils that show evolution in action—seven of which are presented here, starting with this “fishapod.”

Discovered in Arctic Canada in 2004, 375 million-year-old Tiktaalik had not only gills and scales but traits of a tetrapod (four-legged land animal), including limblike fins, ribs, a flexible neck, and a croc-shaped head.

Why it matters: Tiktaalik is seen as evidence of the period when our aquatic ancestors began moving ashore—along with other fins-to-limbs fossils, such as Acanthostega (Acanthostega picture), the most primitive known tetrapod.

Early Darwin supporters speculated that such fishes had given rise to amphibians. “Acanthostega and Tiktaalik have taken this to a new level,” said geologist Donald Prothero, of Occidental College in Los Angeles.

The discoveries of these and other “missing link” species have helped dispel what Darwin called perhaps “the most obvious and gravest objection which can be urged against my theory” of evolution–the former lack of transitional fossil species.

Archaeopteryx: The First Bird

Illustration by Roy Andersen

First found in Germany in 1861, the 150- to 145-million-year-old Archaeopteryxfossils bear impressions of flight feathers on their limbs and tails.

Why it matters: This earliest known flying bird (illustrated here between a pigeon and an Ornitholestes dinosaur) was discovered within two years of the publication of On the Origin of Species, fulfilling Darwin’s prediction of fossil creatures that would link major species groups—in this case, dinosaurs and birds.

Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County, says this classic missing link shares features with both meat-eating dinosaurs—in the wrist, snout, tail, and pelvis—and birds, “such as asymmetrical, vaned feathers and very long, feathered wings.”

Amphistium: The Halfway Flatfish

Photograph courtesy University of Chicago Medical Center

Revealed in 2008, this 50-million-year-old fossil flatfish’s eyes are not quite on opposite sides of its body but not quite in their modern asymmetrical arrangement, both on one side of the body (modern-flatfish picture). (See “Odd Fish Find Contradicts Intelligent-Design Argument” [July 9, 2008].)

Why it matters: The lack of transitional flatfish fossils—showing the eyes slowly “migrating” over generations—has been used to refute Darwin’s theory of evolution. But “Amphistium and [fellow fossil flatfish] Heteronectes show that, whatever the selective pressures might have been, the asymmetrical flatfish head did evolve gradually,” said Per Ahlberg, an evolutionary biologist at Uppsala University in Sweden.

Ambulocetus: The Walking Whale

Illustration by Shawn Gould

Discovered in Pakistan in 1992, the fossil skeleton of 50-million-year-oldAmbulocetus (“walking whale”) suggests it was able to walk on four legs—on land and in the water.

Why it matters: Explaining the leap from land mammals to whales was another evolutionary headache for Darwin, who proposed bears as possible whale ancestors. Recently unearthed fossils trace whales to a doglike predecessor of hoofed plant-eaters, and genetic analysis has identified hippos as whales’ closest living relatives.

Fossil expert Donald Prothero of Occidental College says Ambulocetus is the “most complete, best studied, and clearest case of something with a whale’s head, the beginnings of an aquatic lifestyle with webbed hands and feet, but still fully quadrupedal.”

Homo Ergaster: The “Turkana Boy” Species

Photograph by Kike Calvo via AP Images

Homo ergaster (shown in a museum display) was a small-brained but tall human species with body proportions similar to our own. Known largely from a 1.6-million-year-old fossil of a child found in 1984 near Lake Turkana, Kenya, the species is often called simply Turkana Boy.

Why it matters: Darwin hardly dared mention human evolution in On the Origin of Species. But the then shocking implication of his theory was obvious. Since the discovery of Java Man, the original “missing link,” in the 1890s, the human family tree has grown rich with fossil evidence of species linking us to ancient apes.

Fred Spoor of University College London said that, whereas fossils of our earliest human ancestors “can very much be seen as apes who adopted a bipedal gait,” Turkana Boy “is a true intermediate between modern humans and other primates.”

Hyracotherium/Eohippus: The Dawn Horse

Illustration by Joe Tucciarone

Known today as Hyracotherium (“hyrax-like beast”), Eohippus (“dawn horse”) was the original name of the first complete skeleton of this primitive, foxlike horse, discovered in the southern U.S. in 1867.

Why it matters: When Darwin went public with his theory of evolution, there was no hard evidence to show how an existing animal had evolved from prehistoric species—until Hyracotherium, kicked off a series of fossil discoveries depicting the evolution of horses over 55 million years.

Paul Sereno of the University of Chicago says Hyracotherium/Eohippus is up there historically with Archaeopteryx. The paleontologist named Eoraptor—the small transitional dinosaur at the root of the dinosaur family tree—with Eohippusin mind.

Thrinaxodon: The Emerging Mammal

Illustration by Johan Scherft

Identified from fossils in South Africa and Antarctica, this archaic proto-mammal emerged on a reptile-ruled Earth some 245 million years ago.

Why it matters: An almost perfect intermediate between mammals and reptiles,Thrinaxodon has played a key role in unveiling the evolution of mammals. Descended from a reptile group called cynodonts, Thrinaxodon was a cat-size burrower that had scales and laid eggs. But, like mammals, it had whiskers, warm blood, and, scientist suspect, a fur coat.

“Thrinaxodon shows mammal-like features beginning to kick in,” said paleontologist Paul Barrett of the Natural History Museum in London. “The origin of mammals is exceptionally well understood, and there is a whole series of fossils forming a nice transformation series that shows how mammals evolved a bit at a time.”

National Geographic

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