TETRAPOD LIMBS
The limbs of tetrapods all have the same pattern of bones. Darwin was one of the first to comment that it seems unlikely that this single skeletal structure could be the best one possible for each of the activities it is required to perform in different animals. The explanation for this common structure lies in a common heritage -- a pattern of development laid down in the ancestor of all modern tetrapods and adapted over time, by different environmental pressures, to perform different functions.
If you want to see concrete evidence of evolution, look no further than your hand or your foot. Five fingers, five toes. There's nothing magical about the number, yet five digits at the end of their limbs is a motif that runs through all the animals with four limbs, called tetrapods. Even when there are fewer than five digits in the adult animal -- as in horses' hooves and the wings of bats and birds -- it turns out that they develop from an embryonic five-digit stage. There is nothing inherently advantageous about five digits. Nor is there any environmental pressure that favors five digits on the operating end of four-legged animals' limbs.
Pentadactyly (having five digits) is, in fact, an accident of evolutionary history. All tetrapods descended from a common ancestor that just happened to have limbs with five digits. And over the eons of evolution following that, natural selection worked with variations on pentadactyly rather than starting over again to produce tetrapods with another number of digits, be it two, seven, or 17.
The pentadactyl limbs that tetrapods far and wide all have are examples of homologous structures. The term refers to similarities among species that are inherited from common ancestors. Such similarities are not necessarily functional -- that is, there's no physical reason why the body parts are similar based on the tasks they perform. (When body parts resemble each other for functional reasons, they're called analogous structures.)
Critics of evolution argue that species were created separately in their distinctive forms and didn't descend from common ancestors. But the prevalence of the pentadactyl limb argues just the opposite: That for whatever prehistoric reasons, an ancestral tetrapod had five digits per limb, and all of its descendants did as well. The similarity isn't restricted to the ends of the
limbs -- the bones of the arm, forearm, and hand of different vertebrates form a recognizable pattern, even though they have been adapted to different functions. And aspects of the nerves, blood vessels, and other tissues in the limb reveal other homologous structures.
Homologies are also seen in other structures, and can even be found biochemically, in the very genetic code that stores information for reproducing individuals. These molecular homologies provide some of the best evidence of a single common ancestor for all life on Earth.
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POLAR BEARS AND PENGUINS
Polar bears live in the Arctic, but not the Antarctic. For penguins, the picture is reversed. The pattern of organisms around the globe -- the absence of some species from environments that would suit them, and closer relationships between species that are geographically near each other than between species that inhabit similar environments -- is persuasive evidence of the evolutionary origin of biodiversity.
Darwin, Wallace and the other 19th century naturalists who traveled widely were fascinated by the distribution of animals and plants in their habitats around the world. Why do the Galapagos Islands of South America and the Cape Verde Islands off Africa have strikingly different fauna and flora, despite having similar environments? Why does the Arctic have polar bears and Antarctica penguins?
These patterns impressed Darwin deeply. To him, they argued that species arose in single centers by descent with modification from existing species, and that their geographic range was limited by their ability to migrate to other suitable environments.
The distribution of flora and fauna of the oceanic islands provided Darwin with some of his strongest arguments. The islands contain a small number of species because immigration from the mainland was difficult, he said. Some categories of life are absent altogether, such as batrachians -- frogs, toads, and newts -- even though they would seem to be adapted for such habitats. The reason? They are killed by saltwater, so could not reach the islands by migration. Terrestrial mammals aren't found on oceanic islands more than 300 miles from the mainland. But bats, with their long-distance flying ability, are plentiful.
Another point: Most of the species on islands, while distinct from other species, are most closely related to species on the nearest mainland. Therefore, Darwin said, the island inhabitants must have migrated from the original, mainland area where the species originated. That explains why the species on the Galapagos Islands most closely resemble those on the nearby South American mainland, and those in the Cape Verdes resemble those of west Africa.
Aside from the islands, Darwin was intrigued by unusual distributions of animals and plants across the continents. He concluded that changes in locations of climatic zones over time -- the advance and retreat of glaciers, for example -- could explain some of the patterns in animals' habitats.
Just as intriguing to Darwin, and even more apparent now, is the fact that fossils of possible ancestors of living species are often found in the same parts of the globe where their descendants live today. Darwin observed this in the South American fossils he collected, relatives of today's capybaras and armadillos. Apes today live only in Africa and Asia, and that is where the fossils most resembling modern apes are also found. There are no apes, fossil or living, known from anywhere in the Americas.
These same patterns are just as impressive today. And since Darwin's day, advances in scientific understanding have shown how accurate his conclusions were. For example, plate tectonics, undreamed of when Darwin was forming his ideas, fits elegantly into Darwin's theory as another major influence on dispersal, helping to produce the patterns in the distribution of both fossils and living organisms seen around the world in modern times.
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EVOLUTION OF THE EYE
When evolution skeptics want to attack Darwin's theory, they often point to the human eye. How could something so complex, they argue, have developed through random mutations and natural selection, even over millions of years?
If evolution occurs through gradations, the critics say, how could it have created the separate parts of the eye -- the lens, the retina, the pupil, and so forth -- since none of these structures by themselves would make vision possible? In other words, what good is five percent of an eye?
Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain. Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye, with its many parts and astounding abilities.
Through natural selection, different types of eyes have emerged in evolutionary history -- and the human eye isn't even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an "intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design.
Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through.
Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.
Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.
In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch. For more information see CATCHING THE LIGHT.
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