Orangutan Genetic Diversity Sheds Light on Humanity’s Origin

In retrospect, it was probably not one of Clint Eastwood’s finest moments, starring along side an orangutan in 1978’s Every Which Way But Loose.

On the other hand, when they look back on it, the scientists who participated in sequencing the orangutan genome will deem it one of the most important accomplishments of their careers. A large international research team representing over thirty institutions recently published a rough draft sequence for the orangutan genome.¹ These researchers also characterized the genetic diversity of orangutans from the islands of Sumatra and Borneo by sequencing and analyzing a portion of the genomes of five orangutans on each island.

Unlike any Hollywood production featuring a great ape, the orangutan genome sequence will have enduring importance, including value for understanding primate biology and human diseases. It also promises to provide information that can guide conservation efforts. And, of course, it will yield insight into the question of human origins.

I discussed the apologetic significance of the orangutan genome on the January 28, 2011 episode of our podcast Science News Flash. My commentary during the podcast focused on how Christians can make sense of the genetic similarities and differences between the human, chimpanzee, and orangutan genomes. The focus of this article will be on the implications of this recent work (specifically, the genetic diversity data for the orangutans on Sumatra and Borneo) on the question, Did humanity originate from an original pair of humans as the Bible teaches?

Genetic Diversity of Orangutans

The orangutan species Pongo abelii and Pongo pygmaeus are found on Sumatra and Borneo, respectively. Conservationists have counted around 7,000 to 7,500 P. abelii individuals on Sumatra and around 40,000 to 50,000 P. pygmaeus on Borneo. Both species are endangered, P. abelii critically so. Evolutionary biologists believe that these two species diverged from a common ancestor around 1 million years ago.

As part of the orangutan genome project, the researchers spot-sequenced the genomes of ten orangutans, five from each island, and compared them. To their surprise, they discovered that the orangutans on Sumatra displayed a much greater genetic diversity than did the apes of Borneo. This result is counter-intuitive because generally larger populations display greater diversity than smaller ones and there almost seven times fewer individuals on Sumatra than on Borneo.

This unexpected result means that geneticists have a limited ability to relate population size to genetic variation within a population. According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.”²

Orangutan Genetic Diversity and the Origin of Humanity

This result has bearing on conservation efforts. It also has implications for humanity’s origin. As Hugh Ross and I discuss in Who Was Adam? numerous studies indicate that humanity originated: (1) recently (around 100,000 years ago, plus or minus 20,000 years or so); (2) at a single location (East Africa)—close to where some Bible scholars believe the Garden of Eden was located; and (3) from a small population of individuals.

Moreover, analysis of mitochondrial DNA (which provides insight into the origin of the maternal lineage) indicates that humanity traces back to a single ancestral sequence which could be interpreted as a single woman. Likewise, characterization of Y chromosomal DNA (which provides insight into the origin of the paternal lineage) indicates that all men trace their origin back to a single ancestral sequence which could be interpreted as a single man.

Others have challenged this interpretation, however, arguing that the genetic data indicates humanity arose from thousands of individuals, not two.³ The chief basis for this claim comes from estimates of the ancestral population size of humans based on genetic diversity.

If the mutation rate is known, the effective population size of any ancestral group can be estimated from genetic diversity of present-day populations. A number of these types of studies do indeed indicate that humans stem from a small population, on the order of a few hundred to a few thousand. That is these studies seem to indicate humanity arose from the thousands of survivors, not a primeval pair. As I wrote in our e-Zine, New Reasons to Believe, it is important to recognize that the population sizes generated by these methods are merely estimates, not hard and fast values. The reason: the mathematical models are highly idealized, generating differing estimates based on a number of factors.

And as the genetic diversity studies on orangutans demonstrate, these models can be misleading, thus raising questions about the reliability of population estimates of the first humans from genetic diversity data of current populations.

In other words, there is no compelling reason to reject the historicity of Adam and Eve.