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The Unity of Life
As we have seen, Charles Darwin deserves credit for two great ideas. We’ve looked closely at his idea of natural selection, the process that changes the traits of a popula-tion over time and may eventually create entirely new species. His other idea concerns the starting point for this process—namely, that all modern organisms descended, with modifications over time, from a common ancestor. Darwin was correct about this point as well—and as we’ll see, this idea has important implications for psychology.
Evidence in support of this shared ancestry comes from the many (and sometimes surprising) commonalities among species. For example, close examination of the genomes of birds and crocodiles tells us that they have an enormous amount in common—in fact, they’re genetically more similar to each other than either of them is to any other animal group (Janke, Erpenbeck, Nilsson, & Arnason, 2001). Birds and crocodiles, therefore, are each other’s closest living relative; and this helps us under-stand the various traits they share, such as building nests, laying hard-shelled eggs, and caring for their young. Similar commonalities confirm that the closest living relatives of the whales are the even-toed, hoofed mammals, including the hippos (Geisler & Theodor, 2009; Thewissen, Cooper, Clements, Bajpai, & Tiwari, 2009). Our own closest living relatives—a fact evident in shared anatomy, shared behaviors, and very similar genomes—are the common chimpanzee and the bonobo.
In fact, this sharing of attributes extends quite a long way—because ultimately, all of Earth’s living things are derived from a common lineage. This extraordinary unity of life is evident in some remarkable similarities among our planet’s creatures. Here’s a demonstration: Crystal jellyfish have a gene enabling them to make a substance
called green fluorescent protein, and this protein makes the jellyfish glow green in the blue light of the ocean. Genetic engineers recently took a copy of the jellyfish’s gene and inserted it into the chromo-somes of rhesus monkey egg cells (Yang et al., 2008). The scientists fertilized the egg cells in a dish and then implanted them in a sur-rogate mother monkey. The mother gave birth to baby monkeys that make green fluorescent protein—and so, under blue light, the mon-keys emit a bright green glow (Figure 2.14).
Genes transferred between species can not only grant novel functions, they can also restore functions lost as a result of genetic defects. Fruit flies have a gene called apterous that plays a variety of crucial roles in the development of the adult fly. Flies that lack func-tional copies of this gene have, among other defects, no wings. However, when given the human version of the gene (called hLhx2), the flies develop normally (Rincón-Limas, Lu, Canal, Calleja, Rodríguez-Esteban, et al., 1999).
These results provide powerful evidence that diverse species are indeed descended from common ancestors. After all, the fact that a jellyfish gene works just fine inside the cells of a monkey indicates that monkeys and jellyfish are related. Likewise, the fact that a human gene can substitute for one in the fruit fly shows that humans and flies are kin.
The unity of life is essential to psychology. Because of our shared ancestors, we have much in common with other species; and this is why we can learn a great deal about us by studying them. Thus, we can gain an understanding of human memory by studying learning in sea snails; we can learn about the genetic roots of ADHD through studies that involve genetic engineering in mice; and we can develop new medications for mental illness by studying the effect of these pills on laboratory rats. All of these advances rest on the strong biological resemblance between humans and other animals, which is a result of our common ancestry.
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