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Book 4

"Brother, Can You Spare a Banana?" 
James G. Petropoulos, Science Writer

One may well ask that question of the next chimpanzee one meets. Recent research shows that humans (Homo sapiens) and chimpanzees (Pan troglodytes) are 99.4% genetically compatible, although (based on fossil and genetic evidence) the two species diverged five to seven million years ago. So close are the similarities that it has been suggested that chimpanzees be reclassified as genus Homo. Yet it is clear that chimpanzees and humans are physically and mentally quite different. Of the great apes, the chimpanzee is by far our closest relative. The gorilla is less closely related, and the orangutan (despite it's almost human face) even less so. Genetic research on all four species is beginning to yield information on what exactly makes humans different, and in time, perhaps will shed light on what makes us human.

A look at the physical differences between chimps and humans helps illustrate these minor genetic differences. Chimpanzees are arboreal; though omnivorous, they live on a diet consisting chiefly of fruit; they are four to seven times as strong as humans; they are more agile but less dexterous than humans. Humans, of course, are bipedal, very much omnivorous, predatory by instinct, and have superior intellect and communication skills. It has been proposed that the genetic differences between chimpanzees and humans are largely due to the two species' differing lifestyles, which can perhaps explain why, after five to seven million years, chimpanzees still live in trees and humans do not.

The most easily recognizable genetic difference between humans and chimpanzees is the number of chromosome pairs. Humans have twenty-three pairs, chimpanzees have twenty-four. However, this difference is deceptive. Findings suggest that somewhere along the course of human evolution, two pairs of "chimp" chromosomes fused and rearranged themselves into our familiar twenty-three. The genetic information contained in those "fused" chromosomes has both human and ape counterparts.

The genetic differences scientists are concentrating on may surprise many readers.

Andrew G. Clark of Cornell University recently completed the most comprehensive comparison study to date of the genetic differences between humans and chimpanzees. Using a supercomputer, a partial chimpanzee DNA map of eighteen million sequences was lined up with the genomes of a human and a mouse, to determine which human genes were evolving most quickly. It was hypothesized that if natural selection favored certain genes, perhaps these genes were part of what made us "human." Starting with 23,000 genes, the final field was whittled down to 7,645 human genes that most differed from chimpanzees and mice. Clark and his team isolated genes that determine sense of smell, digestion of protein, development of long bones, hairiness, and hearing. Clark's conclusions were that at some point, human olfactory senses and amino-acid metabolism genetically diverged from those of the chimpanzee, presumably enabling early humans to better smell the types of food they sought, and to better process the proteins found therein. These findings coincide with archaeological evidence that humans began eating meat about two million years ago. These genetic mutations may have been brought about by a new ecological niche created by climate changes.

The genes that determine amino-acid metabolism explain why we are able to digest more dietary proteins, and also lend a clue as to what may have resulted when this newfound digestive ability triggered changes in other proteins. Findings published by the RIKEN Genomic Sciences Centre in Japan support the hypothesis that certain proteins (for example, those affecting brain tissue) may have been genetically altered over time by our change to a more carnivorous diet. As man began chemically altering his food by cooking it and adding other proteins to his diet, such as dairy and legumes, further changes in human proteins may have been triggered, resulting in modern humans.

Another discovery was made by the Howard Hughes Medical Institute. A gene known as ASPM was isolated. Mutations in ASPM affect the size of the cerebral cortex, the part of the brain most closely associated with our "humanity." This protein is much more complex in human form than in apes, and might be a key factor in the evolution of the large human brain.

Clark speculated that genes connected with the sense of hearing might also have contributed to the divergence between humans and chimpanzees, and that these genes may be at the root of human language and communicative ability. He concluded, "Perhaps some of the genes that enable humans to understand speech [involve] not only the brain, but also hearing."

One such gene, alpha-tectorin, determines the makeup of the tectorial membrane of the inner ear. It is known that mutations in this gene cause congenital deafness in humans; perhaps the fine-tuning of alpha-tectorin millions of years ago enabled early humans to understand more complex speech. The difficulty in training chimpanzees to understand human language suggests that perhaps their hearing isn't quite as acute as our own. Our advanced ability to communicate-very much a part of our humanity-may all be due to a gene acting upon an obscure ear protein!

Clark warns, however, that the biological differences between humans and chimpanzees are not necessarily the result of one or even many particular genes, but the hypotheses raised by his experiment certainly merit further study.

Seemingly minor differences in our genetic makeup have resulted in two very different critters, and science is only now beginning to sort it all out. There are yet other differences. Chimpanzees are more genetically diverse than humans; among humans, those living in Africa are more genetically diverse than non-Africans. One theory is that all humans living today sprang from a single female ancestor living in Africa some two hundred thousand years ago. This theory is based not on nuclear DNA, but on mitochondrial DNA. This is another story for another day, but we can be sure that the 1 percent difference between humans and chimpanzees is more astonishing than the 99 percent similarity.

So go ahead and ask a brother chimp if he can spare a banana...chances are he'll have no idea what you're talking about!


Cornell News, December 18, 2003

University of Chicago Hospitals article, "Human Brain Still Evolving," September 8, 2005, article on "Mitochondrial Eve"

Wildman et al. "Genomics in Humans and Chimpanzees"

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