How might similarities and differences in genetic codes, or the proteins built as a result of these codes, be used to determine how closely related different species are?
One way that researchers assessed protein similarities was by harnessing the immune system's ability to recognize foreign proteins. For example, the immune system of a rabbit will recognize a human protein as foreign and will mount an attack against it by making antibodies specific to that protein. If those same rabbit antibodies are exposed to a similar protein — from a chimpanzee, perhaps — they will attack it as well. The more similar the proteins from the two species (human and chimpanzee) are, the stronger this second attack will be. Although variations of this technique were being employed as early as 1904, more sensitive protocols were developed in the 1960s. These more sensitive techniques revealed the remarkable similarity between the proteins of humans and those of other great apes. Expanding upon the work of others and making the assumption that fewer protein differences corresponded to shorter times of separation, Vincent Sarich and Allan Wilson estimated that humans, chimpanzees, and gorillas shared a common ancestor only 5 million years ago — a much shorter length of time than was commonly accepted at the time. Scientists studying the chemistry of DNA moved even closer to actual sequences. Charles Sibley and Jon Ahlquist pioneered the use of DNA kinetics to investigate evolutionary relationships using a technique called DNA-DNA hybridization (see figure, right). Each DNA molecule is made of two strands of nucleotides. If the strands are heated, they will separate—and as they cool, the attraction of the nucleotides will make them bond back together again. To compare different species, scientists cut the DNA of the species into small segments, separate the strands, and mix the DNA together. When the two species' DNA bonds together, the match between the two strands will not be perfect since there are genetic differences between the species — and the more imperfect the match, the weaker the bond between the two strands. These weak bonds can be broken with just a little heat, while closer matches require more heat to separate the strands again. DNA hybridization can measure how similar the DNA of different species is — more similar DNA hybrids "melt" at higher temperatures. When this technique was applied to primate relationships, it suggested that humans and chimpanzees carried DNA more similar to one another's than to orangutans' or gorillas’ DNA.
<span style="color: rgb(55, 55, 55); font-family: 'Lucida Grande', 'Lucida Sans Unicode', 'Lucida Sans', 'Helvetica Neue', Helvetica, sans-serif; font-size: 12px; line-height: 18px; white-space: pre-wrap; background-color: rgb(255, 255, 255);">DNA sequences (and the protein amino acid sequences they encode), change by accumulating mutations over many generations. Therefore, the more similar two DNA sequences are, the more recently they shared a common ancestor. Read about molecular clock.</span>