Conclusion

Wrangham (2023) pointed out that the speciation of bonobos and chimpanzees occurred at a faster rate than those of other great apes. Takemoto et al. (2015) proposed that speciation began with a small population of the common ancestor of chimpanzees and bonobos crossing the Congo River and spreading to its south side around 1.8 or 1.0 million years ago. Genetic studies support the idea that bonobos may have had a single sub-population ancestor (Fischer et al. 2011). If bonobos indeed descended from a small population, their evolution may have started with a small number of genetic mutations that caused a change in physiological or behavioral traits, which then led to the evolution of various other characteristics, including high social status and high level of gregariousness in females and reduced aggression in males. In a small population, even a non-advantageous genetic mutation can spread by genetic drift, and in the case that the mutation is even slightly advantageous, it may be spread and fixed rapidly when the founder population is still small. The idea that the evolution of bonobos was triggered by the development of a single socio-behavioral trait leading to the development of other socio-behavioral characteristics seems more plausible than the coevolution of multiple bonobo traits. In order for the coevolution of multiple traits to occur, many mutations must be selected by long-lasting environmental pressures significantly different from those found in the habitat of chimpanzees on the other side of the Congo River.

One of the candidate traits that could have evolved in the initial stages of bonobo evolution and triggered the development of other traits is a fast recovery of sexual receptivity during the postpartum infertility period. Female bonobos show sexual swellings and copulate as early as eight months after giving birth when E1C levels are increasing but still low (Hashimoto et al. 2022). For example, the evolution of such a trait could arise from mutations that cause an increase in the sensitivity of sexual skin to lower levels of E1C. There is of course a possibility that one of the other traits, such as high social status or high aggregation level in females or reduced aggression in males, was a trigger trait; however, the evolution of these traits may require more complex genetic mutations.

The hypothesis presented in this chapter is still new and needs to be tested. It requires the identification of the related genes by examining the RNA expressed in the sexual skin or ovaries, which is likely impossible due to ethical restrictions. However, a technological advancement that could enable such analyses without an invasive biopsy may reveal the genetic mutations that led to the evolution of the traits unique to bonobos. In addition, if the genes involved in sexual swelling and receptivity are identified, an estimate of the evolutionary age at which those genes appeared may reveal whether females of the last common ancestor of the Pan species and humans also exhibited sexual swellings. An answer to this question is necessary to better understand human evolution but is not feasible based on fossil evidence alone. I hope our understanding of these issues will deepen more quickly without the need to wait for another 50 years of bonobo studies.