Missing links" refers to fossil evidence of evolutionary transitions that has not been found. The term makes little sense today, as practically all "missing links" have been found. This is particularly amazing when one considers the low probability that any individual, or even any species, might be preserved. Fossils have been found that confirm most of the transitional states between major (and minor) groups of organisms.

For example:

- The transition of reptiles to birds required the evolution of feathers, the loss of tail vertebrae, the loss of teeth, and the development of the furcula or breastbone, among other things. A missing link might have feathers but have reptilian skeletal features such as tail vertebrae and teeth. Numerous fossils of feathered dinosaurs have been found, as well as primitive birds that had teeth. The most famous of these fossils is archaeopteryx.

- The transition of reptiles to mammals required numerous changes, some of which can be preserved in fossils. One of these is the transition of some reptilian jawbones into mammalian ear bones, and the relocation of the jaw joint. A missing link might have intermediate jawbones and two jaw joints, one reptilian and one mammalian. A series of such fossils, the therapsids or mammal-like reptiles, has been found that displays these features, including some with two jaw joints.

- The transition of terrestrial mammals to whales required, among other things, the loss of hind legs, the transition of front limbs from legs into paddles, and the migration of the nostrils to the top of the head. A missing link might have small limbs and a skull intermediate between that of terrestrial mammals and modern whales. A series of such fossils has been found.

It is not always certain that the transitional fossil or fossils are actually the direct ancestors of the modern forms. Archaeopteryx is a good example. There were other birds that had more modern characteristics and that lived soon after Archaeopteryx. Therefore Archaeopteryx itself may not have been an ancestor of modern birds. It was probably a cousin, not an ancestor. Archaeopteryx and the true ancestor of modern birds descended from a common ancestor, and Archaeopteryx had evolved less than the ancestor of modern birds at the time that Archaeopteryx lived. Archaeopteryx is very clear confirmation of a transitional state between Mesozoic dinosaurs and modern birds. Feathered dinosaurs lived at the very time that one would expect them to have lived, if birds evolved from dinosaurs, but scientists are not sure which feathered dinosaur might have been the true ancestor.

In some cases, the evolution of a complex adaptation might appear improbable or even unbelievable. This is one of the major claims of intelligent design. In such cases, the discovery of a fossil "missing link," or the living descendant of a "missing link," can demonstrate that the complex adaptation did, in fact, evolve, even if an observer could not have imagined how. One example of many is the complex set of adaptations displayed by desert cacti such as the saguaro of the American southwest. Such cacti can survive and flourish in desert areas because they carry out photosynthesis and store water in thick green stems rather than leaves. In order to do this, cacti underwent several evolutionary modifications. Their leaves were reduced to small and temporary structures that did not carry out photosynthesis; stomata, the pores normally found on leaves, developed on stems instead; bark development was suppressed on the stems, allowing them to be soft and green rather than hard and brown; instead of many small branches, they developed large, thick branches that stored water; and they developed a modified form of photosynthesis known as CAM . . .

Free term papers are not written to satisfy your specific instructions. You can use our professional writing services to buy a custom written research paper, term paper, or essay on Science at affordable price. CustomTermPapers is the best solution for those who seek help in writing term papers, essays, and research papers related to Science and other relevant topics.