A recent scientific study created a stir by asserting that humans and chimpanzees belong to the same genus.1 Morris Goodman, an evolutionary biologist from Wayne State University, and his research team generated this excitement when they compared human and chimpanzee genes.2 Goodman's team examined 97 genes that collectively consisted of 90,000 base pairs (genetic letters)—one of the most extensive human-chimp gene-to-gene comparisons yet made—and discovered a 99.4% sequence identity. This similarity led Goodman to conclude that, genetically speaking, chimpanzees are humans and belong in the genus Homo.
Nonscientific readers might see the 99.4% similarity as convincing, but it's unlikely that the scientific community will readily embrace Goodman's conclusion. Genetic comparison is not the sole criterion for biological classification. Humans and chimpanzees have obvious anatomical, physiological, behavioral, and cultural differences that serve as the basis for their assignments to separate genera.
In addition to these significant differences, Goodman's genetic "comparisons" are questionable. The results he seeks are guaranteed by the method he employs. Goodman finds a high degree of genetic similarity because he compares regions of the human and chimpanzee genome already known to be identical. This technique also focuses on a single type of genetic difference: substitutions. A better tactic would be one that compares the entire genome of humans and chimpanzees and considers all types of genetic differences, not just substitutions.
Though these whole-genome comparisons are not yet possible, scientists are close, and preliminary results indicate that humans and chimpanzees are really not so genetically similar, despite Goodman's numbers. For example, one recent study compared five regions of the chimpanzee genome (encompassing 780,000 base pairs) with the corresponding regions of the human genome and found only a 95% sequence similarity when differences called "indels" (insertions/deletions) were considered in addition to substitutions.3 Another study found only 86.7% genetic similarity when segments of human and chimpanzee DNA (totaling 1,870,955 base pairs) were laid side by side.4 This study also included indels in its analysis of human and chimpanzee DNA.
Scientists still lack a clear understanding of the genetic similarities and differences between humans and chimpanzees. But as the comparisons move from single genes to larger regions of the genome, researchers are exposing substantial distinctions. Humans and chimpanzees just don't prove as genetically similar as some once thought, and Goodman's proposed classification scheme seems ill-conceived.
- For example:
- "DNA Demands Chimps Be Grouped in The Human Genus, Say Wayne State Researchers," Science Daily, accessed May 21, 2002
- Randolph E. Schmid, "Chimps May Have Closer Links to Humans," Associated Press, accessed May 20, 2003
- "Chimps May Belong in the Human Genus," United Press International, accessed May 28, 2003
- John Pickrell, "Chimps Belong on Human Branch of Family Tree, Study Says," National Geographic News, accessed May 28, 2003
- Jennifer Viegas, "Study: Chimps Belong in Human Genus," Discovery Channel, accessed May 28, 2003.
- Derek E. Wildman et al., "Implications of Natural Selection in Shaping 99.4% Nonsynonymous DNA Identity between Humans and Chimpanzees: Enlarging Genus Homo," Proceedings of the National Academy of Sciences, USA 100 (2003): 7181-88.
- Roy J. Brutten, "Divergence between Samples of Chimpanzee and Human DNA Sequences is 5%, Counting Indels," Proceedings of the National Academy of Sciences, USA 99 (2002): 13633-35.
- Tatsuya Anzai et al., "Comparative Sequencing of Human and Chimpanzee MHC Class I Regions Unveils Insertions/Deletions As the Major Path to Genomic Divergence," Proceedings of the National Academy of Sciences, USA 100 (2003): 7708-13.