Genetic Similarities: Wilson, Sarich, Sibley, & Ahlquist (1 of 3)

To investigate how birds are related to one another, a biologist of the 1950s would have carefully studied their anatomical similarities and differences. But today, a scientist working on the same problem could also use the very instructions from which that anatomy was built: its genetic code. DNA sequences form the hereditary links between generations, so it is no surprise that scientists investigating evolutionary relationships have sought to get closer and closer to the DNA that underlies those relationships. However, reading the genomes of entire organisms did not fall immediately from the discovery of DNA in the 1950s. In small steps, scientists came closer to their target.

Scientists first began to zoom in on gene sequences by studying the products of DNA: proteins. After all, if two species are closely related, they should have similar gene sequences, which should then make similar proteins. So before the 1970s, proteins were used as stand-ins for genes in studying evolution.

Testing Similarity Using Antibodies
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 (above left) and Allan Wilson (above right) 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.

Times of divergence and phylogeny of hominoids, as estimated from immunological data.