Sibley and Ahlquist Discover Human-Chimpanzee Genetic Relationship

Date 1984

Charles Gald Sibley and Jon E. Ahlquist showed that the DNA of humans is 99 percent identical to that of chimpanzees, making chimpanzees the closest living relative to humans, with a common ancestor dating back only five million years.

Locale New Haven, Connecticut

Key Figures

• Charles Gald Sibley (1917-1998), professor of biology, ornithologist, and taxonomist
• Jon E. Ahlquist (fl. late twentieth century), professor of biology
• Vincent M. Sarich (b. 1934), professor of anthropology
• Allan C. Wilson (1934-1991), professor of biochemistry

Summary of Event

The consensus among anthropologists before 1980 was that humans evolved from a common ancestor with apes approximately 15 to 20 million years ago, and the earliest humans were represented by Ramapithecine fossils dating back to that period. Ramapithecus was considered to be an early human because its jawbone fragments were more rounded like the human jaw and less like the rectangular ape jawbone.

New evidence emerged during the 1980’s, however, that indicated a much more recent human origin from a common ancestor with apes. In 1984, Charles Gald Sibley and Jon E. Ahlquist reported their finding that the deoxyribonucleic acid (DNA) of humans is 99 percent identical to the DNA of chimpanzees a similarity typical of species that are much more closely related than humans and chimpanzees had been thought to be. Sibley and Ahlquist suggested also that chimpanzees are more closely related to humans than they are to gorillas, which was surprising because it had been assumed, based on appearance, that the opposite was the case. Regardless of whether chimpanzees are closer to humans or gorillas, Sibley and Ahlquist’s results showed that all three species separated from one another relatively recently, possibly as recently as 5 million years ago.

Sibley and Ahlquist based their conclusions on comparisons between human DNA and DNA from chimpanzees and gorillas. DNA, the chemical form of genes, has a “double-helix” molecular structure, similar in shape to a ladder twisted into a spiral. The two sides of the ladder correspond to two strands of DNA wound around each other, and the rungs correspond to chemical bonds holding the two strands together. The strands of the double helix consist of chemical subunits called nucleotides, and all genes are constructed from only four different kinds of nucleotides. The chemical names of the nucleotides are adenine, guanine, cytosine, and thymine, abbreviated A, G, C, and T, respectively. An average gene consists of a continuous linear strand of hundreds of nucleotides, and it is the exact number and order of nucleotides that makes each gene and each species’ genetic code unique. The entire set of human genes contains approximately three billion nucleotides. Sibley and Ahlquist discovered that approximately ninety-nine out of every one hundred nucleotides are exactly the same in the genetic codes of humans and chimpanzees.

A universal feature of DNA is that the bonds holding the two strands of nucleotides together can connect only a G on one strand with a C on the other, and vice versa, or an A with a T; no other combinations are possible. The attractions between G and C and between A and T are so specific that these nucleotides attach to each other spontaneously to hold together the two strands of the double helix. Sibley and Ahlquist used this specific binding of DNA strands to compare the DNA of different species. They started by heating the DNA until the two strands of the double helix separated. Normally, as heated DNA cools, the two strands bind back together as every G matches up with its original C partner on the opposite strand and every A matches with its T. Sibley and Ahlquist compared DNA between two species by melting apart the DNA of both species, mixing them together, and then measuring how well they bonded to each other as the mixed pool of DNA strands cooled. If two species were to have identical DNA, the nucleotides of the two strands would match perfectly and bind strongly. If one species has DNA that is 1 percent different from the DNA of the other species, then ninety-nine out of one hundred nucleotides match, and the mixed DNA reassociates with 99 percent of the strength of identical DNA.

Scientists measure the strength of binding between two strands of DNA by determining the temperature required to melt them apart. If more nucleotides match, the binding is tighter, and it takes a higher temperature to separate them. If fewer matches are made, there is weaker binding across the two strands. Mismatched DNA has weaker binding and melts apart more easily and thus separates at a lower temperature. Overall, each one-degree drop in melting temperature when the DNA of two species is mixed corresponds to approximately 1 percent mismatching between the two species’ genetic codes. Given that when the DNA of humans is mixed with chimpanzee DNA the strands separate at about one degree below the temperature at which human DNA separates from itself, there is approximately a 1 percent difference between the human and chimpanzee genetic codes. Sibley and Ahlquist estimated also that the genetic difference between humans and gorillas is approximately one-third greater than the difference between humans and chimpanzees.

Sibley and Ahlquist’s discovery of the small difference between human DNA and the DNA of chimpanzees reveals more than a close genetic relationship between the two species. It also provides an estimate, independent of the fossil record, of the time that has elapsed since the two species diverged from a common ancestor.

Estimates of the ages of fossils are based on radiometric dating. In this method, the amount of radioactive decay in a fossil since it was formed is measured and the rates of decay, which are constant for specific elements, are used to calculate the fossil’s age. Because many radioactive elements decay at very slow rates over millions or billions of years, it is possible to date fossils that are millions or billions of years old.

Fossil records are the only remains of extinct species, but species that are alive now can be thought of as “living fossils,” because all organisms carry DNA in their cells that is inherited from distant ancestors. Humans and chimpanzees, for example, have been evolving as separate species only long enough for 1 percent of human DNA to change. If humans had been separated earlier from chimpanzees, the DNA would differ by a greater percentage, as it would have had more time to change. The DNA of living species, in short, represents a “molecular clock” that carries a record of the divergence of all related species from their common ancestors.

The exact time that two species have been evolving apart can be calculated if the rate at which changes in DNA evolve can be determined. Sibley and Ahlquist estimated that, overall, the total DNA of a species changes at the rate of approximately 1 percent every 5 million years. The rate of 5 million years of evolutionary divergence per 1 percent difference in DNA comes from two sources: One is direct measurement of the DNA mutation rate, and the other is calibration of DNA comparisons against the fossil record in species for which good fossil records of ancestral origins exist and living descendants have also survived whose DNA can be analyzed. For example, the fossil record of two related flightless birds, ostriches in Africa and South American rheas, indicates that these species separated in their evolution approximately 80 million years ago. Their DNA is approximately 16 percent different, which corresponds to about 5 million years of independent evolution per 1 percent change in DNA. Given that human DNA is approximately 1 percent different from chimpanzee DNA, it follows that this difference represents about 5 million years of divergent evolution from a common ancestor.

Significance

Anthropologists were reluctant to accept Sibley and Ahlquist’s conclusions. Molecular taxonomists argued for a recent divergence of humans from apes, but most anthropologists believed in a very ancient human origin. The two schools of thought might still be divided except for the 1979 discovery of new fossils showing Ramapithecines to be ancestors of orangutans, not humans. The original Ramapithecine fossils were only jawbone fragments, but the more recently discovered and more complete specimens were nearly identical to modern orangutans and showed that orangutans were well on the way toward their present form 15 million years ago. This revision left the fossil record without any ancient human ancestors until the relatively recent “Lucy” (Australopithecus afarensis), who dates back to 3.8 million years ago. With evolutionary reconstructions based on fossils and DNA comparisons suddenly in agreement, the consensus among anthropologists shifted toward accepting a common human and chimpanzee ancestor approximately 5 million years old.

Sibley and Ahlquist’s demonstration of the close genetic relationship between humans and chimpanzees was anticipated by earlier evidence from investigators such as Vincent M. Sarich and Allan C. Wilson, who demonstrated extremely close molecular resemblances among humans, chimpanzees, and gorillas as well as significant differences between these three species and other apes. Previous studies, however, were based on only one or a few specific proteins or genes, or on comparisons of total DNA that were not exact enough to date the origins of different primate species. Sibley and Ahlquist, in contrast, had spent years carrying out approximately twenty thousand comparisons of DNA among bird species, and they used the expertise they gained in their earlier research to conduct the most definitive study of primate DNA comparisons up to that time.

The 1980’s were an exciting period for the study of human evolution, as the picture of how humans evolved became more complete and more secure because it was based on two consistent but independent reconstructions from fossils and DNA comparisons. Although 5 million years of evolutionary divergence between humans and chimpanzees is consistent with the fossil record, debate has continued among molecular taxonomists about the exact calibration of DNA differences to evolutionary time. Some have argued that 1 percent DNA difference corresponds to as little as 2 to 3 million years of divergence, whereas others estimate 6 to 8 million years. Further DNA comparisons and refinement of techniques can provide increasingly precise estimates, and additional fossil discoveries can confirm or challenge the predictions of molecular taxonomists.

The primary and most profound change that occurred in the view of human evolution during the 1980’s was the recognition of a much closer evolutionary relationship between humans and great apes than had been previously recognized. Ten years of progress in the study of human evolution reduced the evolutionary distance between humans and apes by more than 10 million years, to a mere 5 million years. Given that 5 million years is a short time in the history of life on Earth and that chimpanzees and humans remain approximately 99 percent genetically identical, Sibley and Ahlquist’s research showed that, biologically, chimpanzees are nearly human and humans are barely different from chimpanzees.

Bibliography

Campbell, Bernard. Human Evolution: An Introduction to Man’s Adaptations. 4th ed. New York: Aldine, 1998. Introductory college-level textbook by an anthropologist presents an objective, comprehensive, and detailed overview of human evolution. Includes glossary, extensive bibliography, appendix listing human fossil discoveries, and index.

Gribbin, John, and Jeremy Cherfas. The Monkey Puzzle: Reshaping the Evolutionary Tree. London: Bodley Head, 1982. Well-written account of human and primate evolution as it was revised to reflect the evidence from molecular taxonomy. Reviews the molecular evidence extensively, from the very beginnings of such work up to, but not including, the work of Sibley and Ahlquist. Includes bibliographic references and index.

Lewin, Roger. “DNA Reveals Surprises in Human Family Tree.” Science 226 (December, 1984): 1179-1182. Research news article in the leading American scientific journal describes Sibley and Ahlquist’s results and techniques in detail and in relatively nontechnical language. Written in response to Sibley and Ahlquist’s original paper and includes a sense of the initial reactions of the scientific community.

‗‗‗‗‗‗‗. Human Evolution: An Illustrated Introduction. 5th ed. New York: Blackwell, 2004. Brief, general introduction to the topic, written in nontechnical style, includes a survey of fossil and molecular evidence, including Sibley and Ahlquist’s work, and discussion of cultural evolution. Includes glossary and index.

‗‗‗‗‗‗‗. In the Age of Mankind. Washington, D.C.: Smithsonian Books, 1988. Beautifully illustrated book provides a comprehensive summary of human evolution. Includes excellent photographs and index.

‗‗‗‗‗‗‗. “My Close Cousin the Chimpanzee.” Science 238 (October, 1987): 273-275. Research news article reflects the status of Sibley and Ahlquist’s discovery three years after it was first reported. Describes the general consensus in support of Sibley and Ahlquist’s main conclusions and additional evidence supporting their views. Focuses in large part on the question of whether chimpanzees are more closely related to humans or to gorillas.

Pilbeam, David. “The Descent of Hominoids and Hominids.” Scientific American (March, 1984): 84-96. Well-written account of the changing view of human evolution as a result of both the molecular evidence and the reassignment of Ramapithecus from the human to orangutan lineage, with an emphasis on the fossil evidence. Pilbeam was originally one of the strongest supporters of the view that humans diverged from apes 20 or more million years ago, but he shifts his opinion here to support the more recent divergence point of 5 million years.