Read Online Why Evolution Is True by Jerry A. Coyne - Free Book Online Page B
Authors: Jerry A. Coyne
Ads: Link
fulfilled predictions of evolutionary biology is the discovery, in 2004, of a transitional form between fish and amphibians. This is the fossil species Tiktaalik roseae, which tells us a lot about how vertebrates came to live on the land. Its discovery is a stunning vindication of the theory of evolution.
Until about 390 million years ago, the only vertebrates were fish. But, 30 million years later, we find creatures that are clearly tetrapods: four-footed vertebrates that walked on land. These early tetrapods were like modern amphibians in several ways: they had flat heads and bodies, a distinct neck, and well-developed legs and limb girdles. Yet they also show strong links with earlier fishes, particularly the group known as “lobe-finned fishes,” so called because of their large bony fins that enabled them to prop themselves up on the bottom of shallow lakes or streams. The fishlike structures of early tetrapods include scales, limb bones, and head bones (figure 8).
FIGURE 8 . Invasion of the land. An early lobe-finned fish ( Eusthenopteron foordi ) from about 385 million years ago; a land-dwelling tetrapod ( Acanthostega gunnari ) from Greenland, about 365 million years ago; and the transitional form, Tiktaalik roseae, from Ellesmere Island, about 375 million years ago. The intermediacy of Tiktaalik’s body form is mirrored by the intermediacy of its limbs, which have a bone structure in between that of the sturdy fins of the lobe-finned fish and the even sturdier walking limbs of the tetrapod. Shaded bones are those that evolved into the arm bones of modern mammals: the bone with darkest shading will become our humerus, and the medium- and light-shaded bones will become the radius and ulna, respectively.
How did early fish evolve to survive on land? This was the question that interested—or rather obsessed—my University of Chicago colleague Neil Shubin. Neil had spent years studying the evolution of limbs from fins, and was driven to understand the earliest stages of that evolution.
This is where the prediction comes in. If there were lobe-finned fishes but no terrestrial vertebrates 390 million years ago, and clearly terrestrial vertebrates 360 million years ago, where would you expect to find the transitional forms? Somewhere in between. Following this logic, Shubin predicted that if transitional forms existed, their fossils would be found in strata around 375 million years old. Moreover, the rocks would have to be from freshwater rather than marine sediments, because late lobe-finned fish and early amphibians both lived in fresh water.
Searching his college geology textbook for a map of exposed freshwater sediments of the right age, Shubin and his colleagues zeroed in on a paleontologically unexplored region of the Canadian Arctic: Ellesmere Island, which sits in the Arctic Ocean north of Canada. And after five long years of fruitless and expensive searching, they finally hit pay dirt: a group of fossil skeletons stacked one atop another in sedimentary rock from an ancient stream. When Shubin first saw the fossil face poking out of the rock, he knew that he had at last found his transitional form. In honor of the local Inuit people and the donor who helped fund the expeditions, the fossil was named Tiktaalik roseae (“Tiktaalik” means “large freshwater fish” in Inuit, and “roseae” is a cryptic reference to the anonymous donor).
Tiktaalik has features that make it a direct link between the earlier lobe- finned fish and the later amphibians (figure 8). With gills, scales, and fins, it was clearly a fish that lived its life in water. But it also has amphibianlike features. For one thing, its head is flattened like that of a salamander, with the eyes and nostrils on top rather than on the sides of the skull. This suggests that it lived in shallow water and could peer, and probably breathe, above the surface. The fins had become more robust, allowing the animal to flex itself upward to help
Until about 390 million years ago, the only vertebrates were fish. But, 30 million years later, we find creatures that are clearly tetrapods: four-footed vertebrates that walked on land. These early tetrapods were like modern amphibians in several ways: they had flat heads and bodies, a distinct neck, and well-developed legs and limb girdles. Yet they also show strong links with earlier fishes, particularly the group known as “lobe-finned fishes,” so called because of their large bony fins that enabled them to prop themselves up on the bottom of shallow lakes or streams. The fishlike structures of early tetrapods include scales, limb bones, and head bones (figure 8).
FIGURE 8 . Invasion of the land. An early lobe-finned fish ( Eusthenopteron foordi ) from about 385 million years ago; a land-dwelling tetrapod ( Acanthostega gunnari ) from Greenland, about 365 million years ago; and the transitional form, Tiktaalik roseae, from Ellesmere Island, about 375 million years ago. The intermediacy of Tiktaalik’s body form is mirrored by the intermediacy of its limbs, which have a bone structure in between that of the sturdy fins of the lobe-finned fish and the even sturdier walking limbs of the tetrapod. Shaded bones are those that evolved into the arm bones of modern mammals: the bone with darkest shading will become our humerus, and the medium- and light-shaded bones will become the radius and ulna, respectively.
How did early fish evolve to survive on land? This was the question that interested—or rather obsessed—my University of Chicago colleague Neil Shubin. Neil had spent years studying the evolution of limbs from fins, and was driven to understand the earliest stages of that evolution.
This is where the prediction comes in. If there were lobe-finned fishes but no terrestrial vertebrates 390 million years ago, and clearly terrestrial vertebrates 360 million years ago, where would you expect to find the transitional forms? Somewhere in between. Following this logic, Shubin predicted that if transitional forms existed, their fossils would be found in strata around 375 million years old. Moreover, the rocks would have to be from freshwater rather than marine sediments, because late lobe-finned fish and early amphibians both lived in fresh water.
Searching his college geology textbook for a map of exposed freshwater sediments of the right age, Shubin and his colleagues zeroed in on a paleontologically unexplored region of the Canadian Arctic: Ellesmere Island, which sits in the Arctic Ocean north of Canada. And after five long years of fruitless and expensive searching, they finally hit pay dirt: a group of fossil skeletons stacked one atop another in sedimentary rock from an ancient stream. When Shubin first saw the fossil face poking out of the rock, he knew that he had at last found his transitional form. In honor of the local Inuit people and the donor who helped fund the expeditions, the fossil was named Tiktaalik roseae (“Tiktaalik” means “large freshwater fish” in Inuit, and “roseae” is a cryptic reference to the anonymous donor).
Tiktaalik has features that make it a direct link between the earlier lobe- finned fish and the later amphibians (figure 8). With gills, scales, and fins, it was clearly a fish that lived its life in water. But it also has amphibianlike features. For one thing, its head is flattened like that of a salamander, with the eyes and nostrils on top rather than on the sides of the skull. This suggests that it lived in shallow water and could peer, and probably breathe, above the surface. The fins had become more robust, allowing the animal to flex itself upward to help
Similar Books
