by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at firstname.lastname@example.org.
Science writers frequently report 98 percent similarity between the human and chimpanzee genomes, as if that bald datum alone were enough to sate the scientific wanderlust or to send biblical literalists packing for a one-way trip toward a more stable reality. But if neither God nor demon lie in the finer details, the overwhelming elegance of natural selection certainly does. Can small- and large-scale DNA analyses verify the history of evolution? Do they confirm previous conclusions based on anatomy, physiology, archeology, geology, and, of course, the fossil record? Can we exploit vast and recently collected DNA sequences to reconstruct evolutionary relationships between man and various other species, non-human primates in particular? Absolutely, answers Brigham Young University professor and research geneticist, Daniel J. Fairbanks, as he hurls his lay—but no doubt highly motivated—readers into the wondrous, whirling slopes of the double helix.
In 1950, Barbara McClintock detected small segments of DNA that appeared to “jump,” as it were, from one position in the corn genome to another, resulting in major insertion mutations. Following an extended intellectual uproar, McClintock was finally awarded the 1983 Nobel Prize in Medicine for her then-breakthrough discovery of what geneticists now commonly refer to as transposable elements, approximately three million of which reside somewhere in the human genome. Only about eleven percent of these elements, called transposons, actually sever themselves from one location and move to another. By contrast, more typical retroelements, never really skip from place to place, but rather construct RNA copies of themselves that, in turn, are retro-copied back into DNA prior to insertion elsewhere in the genome. All transposons and nearly all retroelements have become relics—so riddled with mutations, in other words, that they can no longer move or replicate. Alu retroelements are important exceptions, comprising nearly ten percent of the human genome.
But the crucial evolutionary feature of retroelements generally is that they once could or now can insert themselves anywhere into the vast DNA text. Therefore, the likelihood that just one would transpose to the same exact position in two different persons by mere chance alone, less said to all members of two or more different species, is infinitesimally small. Yet, in study after study, transposable elements in the human genome have been found at precisely the same locations in the chimpanzee genome 98 percent of the time, and in the DNA of other apes and monkeys at only slightly decreased rates. There can be only one coherent explanation, Fairbanks concludes. These transposable elements, he writes, “became established in the DNA of a common ancestor of humans and chimpanzees, then they mutated in the separate lineages.”
A group of Russian scientists, for example, isolated the fourteen known HERV-K retroelements, decidedly ancient but still transposing in the human genome. Recognizing that a common ancestor is logically implied when even a single retroelement is found in an identical location among two or more species, the Russians compared our HERV-Ks to those of other apes (chimps, gorillas, orangutans, and gibbons), Old World (African and Asian) monkeys, and New World monkeys. Of the fourteen elements, three existed only in the human genome, indicating relatively recent insertion. The other eleven were found at precisely the same positions in both chimps and gorillas. Orangutans shared nine elements with us, and gibbons seven. Old World and New World monkeys had just four and two HERV-Ks in common with humans, respectively. Such evidence is entirely consistent, the author observes, with the primate family tree previously derived from more traditional data, including the fossil record, revealing the tightest ancestral relationship between humans and African apes, followed by Asian apes, and, finally, to a lesser extent, Old and New World monkeys. Confirming results have emerged from additional studies, including those distinguishing Alu elements contained in DNA segments related to hemoglobin and Charcot-Marie tooth disease.
Equally enlightening are the thousands of mostly useless, dead-end pseudogenes that continue to haunt our chromosomal recesses. Many animals—dogs and cats, for example—possess a functional, unitary GULO gene that allows them to produce vitamin C, an extremely useful trick for creatures that do not regularly consume fruits and vegetables. Most primates, on the other hand, have always maintained a vitamin C-rich diet, which explains why human and chimpanzee genomes contain substantially mutated GULO pseudogenes that are 98 percent identical. Unfortunately for us, however, the need for a functional GULO has returned. Alas, natural selection (or, more accurately, the relaxation thereof) does not possess the predictive insight of the typically enscriptured all-powerful and all-seeing God.
Consider as well glucocerebrosidase, or GBA, sequences in primates. Squirrel monkeys possess only the original gene. Orangutans have two functional copies, the first of which resides in the same position as that of monkeys, and the second of which has been copied to a location in close proximity. Although all gorilla, chimp, and human genomes include both GBA copies, their second pseudogene has suffered a 55 base-pair mutation—again, at precisely identical sites in each species. The best explanation for the GBA scenario is, first, that the functional gene duplicated in the common ancestor of apes and humans following the split between the monkey and ape lineages, and, second, that the younger copy mutated into a pseudogene in the lineage leading to gorillas, chimps, and humans after those species diverged from the lineage leading to orangutans. One should infer as well that gorillas, chimps, and humans are more closely related to each other than any of the three is to orangutans, and that all species of ape are more familiar to one another than any ape is to any monkey. A total of 19,724 human pseudogenes had been identified in 2003, and scientists continue to compare them to those of other species. “[T]he same pattern emerges over and over,” Fairbanks instructs. “Once again, we find evidence of our shared evolutionary ancestry with other primates, and more distant shared ancestry with other mammals.”
But even prior to genome sequencing, microscopic examination had yielded similar results. Human and chimpanzee chromosomes can be perfectly aligned, except at a few notable locations. These variances, however, are denoted as rearrangements only because, as it turns out, the relevant DNA sequences are essentially the same. In nine instances, DNA segments were simply inverted over time. Using both fluorescence in situ and DNA sequence comparison methods, scientists have determined that, in chromosomes 1 and 18, the inversions occurred exclusively in the lineage leading to humans, and that, in chromosomes 4, 5, 9, 12, and 15 through 17, they occurred only in the chimp lineage prior to the chimpanzee-bonobo split. In other instances, small sections of one species’ chromosome have merely been expanded with repeated sequences.
In the past, opponents of science education have attempted to exploit the fact that chimps possess 24 and humans only 23 chromosomes. In 1991, however, Yale University scientists resolved the issue by sequencing DNA near the middle of human chromosome 2, discovering that it matched that of the telomeres (DNA segments at the end of chromosomes) of chimpanzee chromosomes 2A and 2B. In addition, the centromere (a constricted segment of DNA within a chromosome) of human chromosome 2 matches the centromere in chimp chromosome 2A, and the corresponding region of human chromosome 2 matches the DNA contained at the centromere in chimp chromosome 2B. “The evidence that human chromosome 2 arose from a fusion” of the two shorter chimpanzee chromosomes “is solid and unmistakable,” writes Fairbanks. “The only reasonable explanation of this evidence is a chromosome fusion that happened after the lineage leading to humans diverged from the lineage leading to the great apes.”
In recent years, human, chimpanzee, and other genome projects have revealed confirming evidence of human evolution that can be characterized only as utterly overpowering. The sad irony, of course, is that, at the same time, generously funded and proudly uninformed creationists continue to crusade against evolution and, in fact, against all scientific realities that conspicuously threaten their fanatical beliefs. Describing himself as a person of “deep religious convictions,” Fairbanks includes two final Stephen Jay Gould-esque chapters exploring the clash of faith and reason along with a much more appropriate appendix outlining the history of biological science from Darwin to the human genome project. Fortunately, however, polemics and politics take a distant back seat to logic and facts in this truly commanding and no doubt timely illumination of human origins, a subject the significance of which simply cannot be overstated.