A FIN IS A PAW IS AN ARM IS A WING
HOMOLOGY Anatomical, genetic, or developmental similarity among organisms due to common ancestry.
In The Origin of Species, Darwin asked, “What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?” To Darwin, this uncanny similarity was evidence that all these organisms were related—that they share a common ancestor in the ancient past.
The fact that all tetrapods share the same forelimb bones, arranged in the same order, is an example of homology—a similarity due to common ancestry. Before Darwin, comparative anatomists had identified many such similarities in anatomy; what they lacked was a satisfactory explanation for why such similarity should exist. Darwin provided that explanation: homologous structures are ones that are similar because they are inherited from the same ancestor—in this case, an amphibious creature like Tiktaalik. Why is this significant? Think of it this way: every time you bend your wrist back and forth—to swipe a paint brush or hold a cell phone to your ear, for example—you are using structures that first evolved 375 million years ago in fish. As Shubin points out, “This is not just some archaic, weird branch of evolution; this is our branch of evolution” (INFOGRAPHIC 16.5).
The number, order, and underlying structure of the forelimb bones are similar in all the groups illustrated below. The differences in the relative width, length, and strength of each bone contribute to the specialized function of each forelimb. This anatomical homology is strong evidence that these organisms all have a common ancestor at some time in the distant past. The variations in bone shape and function reflect evolutionary adaptations to different environments.
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If they have the same bones, why then do a human arm and a bird wing look so different? Remember that during the process of inheritance mutations are continually introduced into the DNA of genes. Such mutations can produce subtle changes in the proteins encoded by those genes—proteins involved in constructing the bones that make up an arm or a wing, for example. Changes in bone proteins can result in slightly altered bones, for instance making them longer or thinner. When these modified bones are helpful to an organism’s survival and reproduction, the advantageous traits are passed on to the next generation, and populations emerge that have these adaptations. This “descent with modification” (Darwin’s phrase again) results in diverse organisms sharing common—homologous—structures and putting them to different uses.
We can see homology not only in adult anatomy, but in early development as well. Take a look at early embryos of vertebrate animals as diverse as humans, fish, and chickens and you’ll see that they all look remarkably similar. Why should the embryonic stage of a human resemble the embryonic stage of a fish when the adults of each species look so different? Similar embryological structures are further evidence that all vertebrates have a common ancestor (INFOGRAPHIC 16.6).
We can identify homologous structures by tracing their embryological development. Some of our middle ear bones, for example, are homologous with the jaw bones of reptiles and bones supporting gills in fish. We know this because all of these structures develop from the pharyngeal pouches that appear in all vertebrate embryos early in development. This developmental homology is strong evidence that all vertebrate animals are related by common ancestry. Genetic changes over time have introduced modifications in later stages that give rise to distinct species with vast physical differences.
VESTIGIAL STRUCTURE A structure inherited from an ancestor that no longer serves a clear function in the organism that possesses it.
Development helps us solve other evolutionary conundrums as well, such as why reptiles like snakes don’t have limbs like other tetrapods. In fact, snake embryos do possess the beginnings of limbs, but these limb buds remain rudimentary and do not develop into full-fledged limbs (although you can still see stubby hindlimbs in some species of snake today). Such vestigial structures, which serve no apparent function in an organism, are strong evidence for evolution: these now apparently useless features are inherited from an ancestor in whom they did serve a function.
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Zooming in even further, to the molecular level, we find still more examples of homology—and thus more evidence of common ancestry. Scientists have known since the 1950s that DNA is the molecule of heredity, and that it is shared by all living organisms on Earth. Every molecule of DNA—whether from fish, maple tree, bacterium, or human—is made of the same four nucleotides (A, C, T, and G), and all organisms use the information encoded by those nucleotides to make proteins in the same basic way, using the universal genetic code (see Chapter 8). Why should all living things use the same system of decoding genetic information? The best explanation is that this system was the one used by the ancient ancestor of all living organisms, passed on to all of its descendants, and preserved throughout billions of years of evolution.
In essence, DNA serves as a kind of molecular clock: each additional sequence difference is like a tick of the clock, showing the amount of time that has elapsed since the two species shared a common ancestor.