Does Classification Reflect a Biological Relationship Between Humans ana Apes

The fact that different organisms share many traits does not necessarily indicate that they share biological ancestry. Linnaeus's classification alone does not support any of the four competing hypotheses about the origin of modern organisms. However, the tree of relationships implied by Linnaeus's classification forms a

Humerus

Radius and ulna

Carpals

Metacarpals

Phalanges

Figure 8.8 Homology of mammal forelimbs. The bones in the forelimbs of these mammals are very similar, despite the fact that they are used for very different functions. Equivalent bones in each organism are shaded the same color. The similarity in basic bone structure may be evidence of shared origin.

hypothesis that can be tested. If modern species represent the descendants of ancestors that also gave rise to other species, we should be able to observe other, less obvious similarities between them in anatomy and genetic material. And if Darwin is right, we should be able to identify extinct organisms that show a transition between a common ancestor and its modern descendants.

The Anatomical Evidence Figure 8.8 illustrates the concept of homology, similarity in characteristics resulting from common ancestry. Each of the mammal forelimbs pictured has a very different function—bat wings are used for flight, sea lion flippers for swimming, lion legs for running, and human arms for grasping and throwing. However, each of these limbs shares a common set of bones that are in the same relationship to each other, even if they are quite different in size and extent. The most likely explanation for the similarity in the underlying structure of these limbs is that each species inherited the basic structure from the same common ancestor, and the process of evolution led to their unique modification in each group.

Some critics of the theory of evolution counter that a skilled designer—that is, a supernatural creator—could have simply produced these different forelimbs using the same creative base plan. However, some similarities among species appear to be less logical than one would expect from an intelligent designer. For instance, humans contain a number of vestigial traits, which appear to have been modified from functional traits found in an ancestor. These traits either do not function in humans or have a function that is highly modified from that of other descendants of the same ancestor. In other words, vestigial traits represent a vestige, or remainder, of our biological heritage. Figure 8.9 provides two examples of vestigial traits in humans. Great apes and humans have a tailbone like other primates, yet neither great apes nor humans have a tail. Additionally, all mammals possess tiny muscles called arrector pili at the base of each hair. When the arrector pili contract under conditions of emotional stress or cold temperatures, the hair is elevated. In furry mammals, the arrector pili help to increase the perceived size of the animal, and they increase the insulating value of the hair coat. In humans, the same conditions only produce goosebumps, which provide neither benefit.

Humerus

Radius and ulna

Carpals

Metacarpals

Phalanges

Figure 8.8 Homology of mammal forelimbs. The bones in the forelimbs of these mammals are very similar, despite the fact that they are used for very different functions. Equivalent bones in each organism are shaded the same color. The similarity in basic bone structure may be evidence of shared origin.

Media Activity 8.2 Evolution of Whales

Figure 8.9 Vestigial traits reflect our evolutionary heritage. (a) Humans and other great apes do not have tails, but they do have a vestigial tailbone, which corresponds to the functional tailbone of a monkey. (b) The ability to elevate their fur helps many mammals seem bigger and provides increased insulation in cold conditions. The vestiges of this trait in humans appear as goosebumps, which arise under similar conditions of cold and intense emotion but serve no known function.

Figure 8.9 Vestigial traits reflect our evolutionary heritage. (a) Humans and other great apes do not have tails, but they do have a vestigial tailbone, which corresponds to the functional tailbone of a monkey. (b) The ability to elevate their fur helps many mammals seem bigger and provides increased insulation in cold conditions. The vestiges of this trait in humans appear as goosebumps, which arise under similar conditions of cold and intense emotion but serve no known function.

Darwin maintained that the hypothesis of evolution provided a better explanation for vestigial structures than the hypothesis of special creation. Because Linnaeus's classification system showed that similarities among species occurred in both useful and vestigial traits, Darwin argued that the Linnaean system must be a reflection of shared ancestry. A useless trait such as goose-bumps is better explained as the result of inheritance from our biological ancestors than poor design.

The Biochemical Evidence Since Darwin's time, scientists' understanding of the nature of biological inheritance has expanded immensely. Scientists now understand that differences among individuals arise in large part from differences in their genes. It stands to reason that differences among species must also derive from differences in their genes. If the hypothesis of common descent is correct, species that appear to be closely related must have more similar genes than species that are more distantly related.

The most direct way to measure the overall similarity of two species' genes is to evaluate how similar their DNA is. Recall from Chapter 6 that DNA molecules carry genetic information in the sequence of chemical bases making up linear structures. A single gene on a DNA molecule may be made up of a few hundred

(a) Comparing DNA sequences

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