Demonic Males - Part 2

Where we last left off, we began to realize that human violence may have some evolutionary or genetic origin, rather than simply being a product of culture or intelligence. Before we begin drawing too many comparisons between humans and chimpanzees, we need to understand the connection there.

Darwin's treatise on human evolution was published in 1871; however, even prior to this book comparisons were being made between humans and primates. As early as the 4th century B.C., Aristotle noted the similarity in anatomy between humans and Barbary macaques (a type of monkey). Much later, in the mid- to late 1800s after gorillas had been discovered, a gentleman named Thomas Huxley demonstrated that humans and gorillas shared more similarities in their anatomy than either did to any species of monkey.

Side note: If you're anything like me, you may not be entirely clear on the intricacies of the various species of monkeys and apes, so a quick explanation is in order. Monkeys and apes are both "simian primates". Monkeys encompass a large number of species, including such well-known examples as the Rhesus macaque, African green monkey, marmoset, and spider monkey. Apes on the other hand, are a much smaller category composed of the lesser apes and great apes (no, not Grape Ape!). The great apes are the category we're interested in because they include chimpanzees, gorillas, orangutans, bonobos... and humans. (well, sorta) And incidentally, monkeys and apes really aren't that closely related.

Although anatomical comparisons were able to show a strong relation between humans and apes, a different type of test was necessary to rule out the possibility of evolutionary convergence (i.e., the similarity in anatomical features as a result of common environmental pressures). At the start of the 20th century, George Nuttall provided that test. Through a clever series of experiments involving producing antibodies to the blood of a particular species and then observing its reaction with the blood of another species, he was able to determine the closeness of their relationship. The more closely related a species the more coagulation would be observed because the antibodies recognized proteins and cells in the relative's blood. His experiments clearly showed a strong connection between humans and apes at the biochemical level, which ruled out the possibility of convergence.

It wasn't until later in the 20th century that sufficiently sensitive experiments were developed to begin probing the relationships between the different species of apes. In the 1960s, electrophoresis results showed that the proteins of the three African apes (gorillas, chimpanzees, and bonobos) and humans were more similar to one another than any were to the proteins of the orangutans. Then in 1984 Charles Sibley and Jon Ahlquist measured the melting temperature of DNA duplexes composed of strands from two different species. The more closely related the species, the more similar the DNA, which means a stronger duplex and a higher melting temperature. Two results came from these experiments. First, humans were most closely related to chimpanzees. Second, and a bit more surprising, chimpanzees are more closely related to humans than they are to gorillas...

Subsequent DNA-related experiments confirmed these findings and also helped establish the evolutionary timeline. Based on the DNA benchmark, our common ancestor with the chimpanzee lived only about 5 million years ago. (Compare that to the divergence between apes and monkeys about 25 million years ago.) The next chapter of the book looks back at these ancestors and tries to understand why we made the transition from a rainforest ape to a woodlands ape.

We've now seen a variety of data suggesting a strong evolutionary link between humans and the great apes, in particular with chimpanzees. Later on this will provide the foundation for a comparison of behaviors in the various species.