Apes like Us? The Genetic Case for Taxonomic Reclassification of Chimpanzees

Humankind has historically stood alone in the taxonomic realm as the last surviving member of the genus Homo, yet the evidence is mounting that this isolation may be unwarranted. The taxonomic placement of the chimpanzee (Pan troglodytes) may be much closer to Homo sapiens than previously believed. The discovery of close genetic similarities between chimps and humans combined with the fact that chimpanzees and humans diverged only four to six million years ago (Groves 2001) have prompted some scientists to argue that chimps and humans should be placed in the same genus, moving either chimps to Homo or humans to Pan.

Traditionally, taxonomy in regards to humans used the anthropocentric concept of grades to subdivide the order Primates into groups forming a series progressing from primitive to advanced as estimated by such human-important features as brain capacity and mental abilities(Wildman, et al.). The grade concept traces back to Aristotle's "Great Chain of Being," in which animals are arranged "in a single graded scala naturae according to their degree of 'perfection.'(Lovejoy)" The key differences most frequently cited when separating humans from other animals has historically revolved around the capabilities of the human brain.

John Locke summarized the difference in cognitive ability between between animals and humans as "Beast abstract not." This long-held belief that humans alone are capable of abstract thought began to be challenged by Alfred Russel Wallace, whose studies of Indonesian orangutans led him to conclude that a baby orangutan behaved "exactly like a human child in similar circumstances." Native humans in the area may not have been surprised to hear this, as "orangutan" is a Malay phrase meaning "man of the woods (Sagan, 100)."

In the Canary Islands at Tenerife, home of Wolfgang Kohler's studies of the chimp Sultan, researchers observed two chimps displaying behaviors sometimes thought to be uniquely human. One chimp would hold out food to a chicken to lure it towards him, where the other chimp would start poking the chicken with a length of wire hidden behind his back. The chicken would run away, and the chimps would repeat the game, thus displaying human-like capacities for cooperation, the planning of a future course of action, deception and cruelty (Sagan, 101).

When considering this discovery, the question of human-to-chimpanzee evolutionary connection must be raised. Charles Darwin wrote: "As we have no record of the lines of descent, the lines can be discovered only by observing the degrees of resemblance between the beings which are to be classed. For this object numerous points of resemblance are of much more importance than the amount of similarity or dissimilarity in a few points." The "numerous points of resemblance" to which Darwin referred have taken the form of genetic-level expressions, and these expressions may effectively contradict any argument against chimpanzee reclassification.

Advances in genetic research technologies during the late 20^th and early 21^st Centuries have provided remarkable evidence supporting the decision of the scientists who first named the chimpanzee in 1775, and placed it next to humans under the genus Homo. According to the 2006 Ciccarelli, et al. study, "the [DNA] sequence divergence between human and chimp is low; they most likely would have been assigned the same genus if they had been prokaryotes" (Toward Automatic Reconstruction of a Highly Resolved Tree of Life).

Genetic tests comparing DNA from humans, chimps, gorillas and orangutans reveal striking similarities in the way chimps and humans evolve that set them apart from the others, adding weight to a controversial proposal to abandon the long-used chimp genus "Pan" and reclassify the animals as members of the human family. One important study at the Georgia Institute of Technology compared 63 million base pairs of DNA from different species, where each base is a letter in the animal's genetic code (Sample). Analysis of the DNA specifically targeted examination of the molecular clock, which is the rate at which an animal's genetic code evolves, to show how the span of a generation has changed over millions of years.

These studies demonstrate that although humans and chimps split from a common ancestor in the vicinity of five to seven million years ago (close to Groves' four to six million years), the evolutionary rate between humans and chimps differs by just 3%, which is a much slower rate than that of gorillas and orangutans. The relatively slow speed of the human and chimpanzee molecular clocks shows that the time-space between generations is long, and in regards to human evolution, this has been a characteristic believed to distinguish humans from other members of the Hominidae family. Of the finding, research team member Navin Elango said: "We found that the chimpanzee's generation time is a lot closer to that of humans than it is to other apes." According to the scientists whose study appeared in the journal Proceedings of the National Academy of Sciences, the finding suggests some human traits only emerged one million years ago, a fleeting moment on evolutionary scales (Sample). The published work states:

When two other hominoid species (gorilla and orangutan) were included in the analysis, we found that both the human and the chimpanzee lineages exhibit slower molecular clocks compared to either gorilla or orangutan. This finding contradicts the view that humans differ greatly from all other hominoids in generation-time related life history traits. Rather, it suggests that life history traits that affect generation time may have evolved more than once during the evolution of hominoids, including the recent evolution of human-specific life history traits (Variable Molecular Clocks in Hominoids).

Biologist Soojin Yi said of the results, "[H]umans and chimpanzees should be in one genus, rather than in two different genera, because we not only share extremely similar genomes, we share similar generation time (Sample)." This belief is also held by Morris Goodman, the lead researcher at Wayne State University in Detroit, who stated in 2003 that chimps and humans should be brought together under the same umbrella genus, Homo, after his team of researchers discovered that human and chimp genes are 99.4% identical.

The key genetic similarities that qualify chimps and/or humans for a genus reclassification hinge upon the evolution of a section common to both chimp and human brains called the anterior cingulate cortex, or ACC. To comprehend the importance of the ACC in its connective role between chimps and humans, it is vital to understand in detail what the AAC is, and what it does. In his paper titled "The Anterior Cingulate Cortex: The Evolution of an Interface between Emotion and Cognition", Caltech's John Allman proposes:

[T]he anterior cingulate cortex is a specialization of neocortex rather than a more primitive stage of cortical evolution. Functions central to intelligent behavior, that is, emotional self-control, focused problem solving, error recognition, and adaptive response to changing conditions, are juxtaposed with the emotions in this structure. Evidence of an important role for the anterior cingulate cortex in these functions has accumulated through single-neuron recording, electrical stimulation, EEG, PET, fMRI, and lesion studies. The anterior cingulate cortex contains a class of spindle-shaped neurons that are found only in humans and the great apes, and thus are a recent evolutionary specialization probably related to these functions. The spindle cells appear to be widely connected with diverse parts of the brain and may have a role in the coordination that would be essential in developing the capacity to focus on difficult problems. Furthermore, they emerge postnatally and their survival may be enhanced or reduced by environmental conditions of enrichment or stress, thus potentially influencing adult competence or dysfunction in emotional self-control and problem-solving capacity.

A focused study by Uddin, et al. on the gene expression profiles from the anterior cingulate cortex(ACC) of humans, chimpanzees, gorillas, and macaques was performed to gain further insight about genetic regulatory changes in human and other catarrhine primate brains. The phylogenetic results obtained from the gene expression profiles were astounding.The prevailing traditional expectation was based on a belief that the non-human African apes (e.g., chimpanzees and gorillas) should be more like each other than either should be like humans. Instead, the study found that

...[T]he chimpanzee ACC profiles are more like the human than like the gorilla; these profiles demonstrate that chimpanzees are the sister group of humans. Moreover, for those unambiguous expression changes mapping to important biological processes and molecular functions that statistically are significantly represented in the data, the chimpanzee clade shows at least as much apparent regulatory evolution as does the human clade (Sister grouping of chimpanzees and humans as revealed by genome-wide phylogenetic analysis of brain gene expression profiles).

With this discovery, the scientific evidence supporting reclassification of chimpanzees appears to be without doubt. However, a large number of moral and ethical issues surrounding the idea remain.

The Animal Kingdom is usually kept at arm's length from humankind, and science has generally maintained a policy of "Humans, then everything else" when classifying the Earth's species. When legendary chimpanzee researcher Jane Goodall first presented her findings to the larger scientific community, she described the chimpanzees' personalities, talked about their ability to reason and described their emotions. All of this was considered anthropomorphic and unacceptable (Chimpanzee Central). In 1999 scientists petitioned the New Zealand government to pass a bill conferring "rights" on chimpanzees and other primates, a move that drew derision. Roger Scruton, a moral philosopher, asked: "Do we really think that the jails of New Zealand should henceforth be filled with malicious chimpanzees? If not, by what right are they to be exempted from punishment (Sample)?" Beliefs of some organized religions also factor into the scientific community's decision to consider humans above the rest of the animal kingdom, particularly the Judeo-Christian belief that considers nature to be under humankind's dominion. Yet, classifying chimps in the same genus as humans is not as large of a stretch as some may imagine.

While it is true that humans are indeed a unique species, the placement of humans into a separate genus from any other living organism increasingly appears to be a decision steeped in fear and/or ignorance. The growing body of scientific evidence increasingly refutes many of the common arguments for doing so. Carl Linnaeus, the founder of taxonomy, had this to say in 1788:

I demand of you, and of the whole world, that you show me a generic character...by which to distinguish between Man and Ape. I myself most assuredly know of none. I wish somebody would indicate one to me. But, if I had called man an ape, or vice versa, I would have fallen under the ban of the ecclesiastics. It may be that as a naturalist I ought to have done so (Qtd by Sagan).

The scientific data to support the reclassification of chimpanzees is only beginning to mount, but what was a relative trickle of hard evidence a decade ago is quickly becoming a torrent. Even during the course of this writing, there have been numerous research studies published that add to the growing body of evidence. This subject is literally on the cutting edge of Evolutionary Science, and the future promises answers to long-pondered questions about human origins, prehistory, and ultimately the full understanding of humankind's role as members of the natural world.

Works Cited

Allman, J. M., Hakeem, A., Erwin, J. M., Nimchinsky, E. & Hof, P."The Anterior Cingulate Cortex: The Evolution of an Interface between Emotion and Cognition" Ann. NY Acad. Sci. 2001. 935,107-117.

"Blurring the Line."Chimpanzee Central. The Jane Goodall Institute. .

Ciccarelli, Francesca D., Doerks, Tobias, von Mering, Christian, Creevey, Christopher J., Snel, Berend, Bork, Peer. "Toward Automatic Reconstruction of a Highly Resolved Tree of Life." Science 2006 311: 1283-1287

Darwin, Charles. The Descent of Man, and Selection in Relation to Sex . London: Murray, 1871.

Elango, Navin, Soojin Yi, and James W. Thomas. "Variable Molecular Clocks in Hominoids." Proceedings of the National Academy of Science 103 (2006): 1370-1375. 29 Apr. 2006 .

Groves C. Primate taxonomy. Washington DC: Smithsonian Inst Pr., 2001.

Lovejoy, A. The Great Chain of Being: A Study of the History of an Idea Harvard Univ. Press, Cambridge, MA.1936.

Sagan, Carl. The Dragons of Eden. Tess P, 2004.

Sample, Ian. "Closer to Man Than Ape." Guardian Unlimited 24 Jan. 2006. 3 Mar. 2006 .

Uddin, Monica, Wildman, Derek E., Liu, Guozhen, Xu, Wenbo, Johnson, Robert M., Hof, Patrick R., Kapatos, Gregory, Grossman, Lawrence I., Goodman, Morris. "Sister grouping of chimpanzees and humans as revealed by genome-wide phylogenetic analysis of brain gene expression profiles". PNAS. 101: 2957-2962. 2004.

Wildman, Derek E. and Uddin,Monica and Liu,Guozhen and Grossman, Lawrence I. and Goodman, Morris. "Implications of natural selection in shaping 99.4% nonsynonymous DNA identity between humans and chimpanzees: Enlarging genus Homo" PNAS, Jun 2003; 100: 7181 - 7188.http://www.pnas.org/