How have organisms including humans evolved? This has been a question tackled by a number of scientists over many decades and is being investigated by using diverse approaches and targets ranging from fossils to genomes. I am using body morphology, that is “shape”, as a marker for evolution, and focusing on macaques, which also belong to the order Primates like humans. Primates have evolved over a long period of time, but there should have been triggers that led to the diversity we see today. By using the genomes, where traces of evolution are hidden, the relationship between the genomes and body shape as axes, and an approach of “crossing between species”, I will find a way toward elucidation of this difficult question.
Primates including humans branched from a common ancestor and have evolved into human (Homo sapiens), chimpanzee, gorilla, orangutan, Japanese macaque, etc. A phylogenetic tree shows such and other branching of evolution, and the distance between two branches denotes the similarity of the two species (Fig. 1). In a phylogenetic tree, species split from others just as many branches grow from a trunk. This is a result of divergence, which is a process of new species formation caused by one or more changes in the genome that result in physical and/or physiological changes that inhibit the organism from mating with those of the original form. On the other hand, diverse phenotypes may be formed when two different species, which had separated at a certain time point in the past, cross with each other resulting in a new combination(s) of genes. For example, in the genus Homo, there have been species other than humans or H. sapiens, such as H. erectus (Peking man) and H. neanderthalensis (Neanderthals), but they are all extinct today and only H. sapiens remains. Homo spp. were believed to have evolved independently from each other, but ancient genome analysis suggests that there were genetic exchanges between Neanderthals and groups of H. sapiens in areas outside Africa. In other words, present humans may possess some characteristics of other extinct species.
A characteristic that appears due to a change in a gene is likely to remain if the characteristic allows the species to adapt to the environment and disappears if it does not. On the other hand, whether a characteristic remains in later generations or not is also affected by more than random chance. So far, I have investigated relationships between body shape/ divergence, and environment/region in over 20 macaque species including the Japanese macaque. The species of the genus Macaca are widespread, ranging from the tropics, to the subtropics and to the temperate zones, suggesting that a strong relationship is likely to exist between shape and environmental adaptation. In particular, the face, where sensory organs for sensing changes in the environment such as eyes, nose and mouth concentrate, is estimated to have undergone big changes in order to adapt to each of these diverse environments. However, the relationship between the shape of the face and the environment is difficult to see because changes in the face shape are connected to the size of the body. I analyzed the relationship by correcting for the effects of body size and investigated the relationship between face shape and environment. The results showed a trend; species that have a long face live in the tropics, and species that have a short face live in the subtropics and temperate zones. A comparison of the relationship with divergence showed that changes of face length had occurred several times. Therefore, such regional mutations are not a coincidence but are likely to be a result of adaption to the environment (Fig. 2). Such evolution, which involves evolution of similar traits in different lineages, is called parallel evolution. However, we do not know how such differences in face shape could have occurred. Elucidating the genetic mechanisms of parallel evolution is an important and future topic.
Evolution does not only involve divergence but also crossing. In Primates including humans, much genetic evidence of crossing has been discovered, but little is understood on how crossing occurred and what effects it had on evolution. When we try to study human evolution, we face a restriction, i.e. very few clues are available for investigating the effects of crossing because Homo species other than H. sapiens are extinct. I thus focused again on the Macaca species which are relatively close to the genus Homo and can be used to investigate interspecies crossing. In cross-hybrids of the Macaca species, I investigated the relationship between the shape of the skull and the degree of hybridization and found that the skull of a hybrid shows an exterior shape that is intermediate of its parents and an interior shape (shape of a cavity structure called the maxillary sinus) strongly reflects the shape of only one parent. In other words, the exterior and interior are governed by different genetic mechanisms, suggesting that a new type of macaque could emerge by crossing which will result in a new combination of genes. To investigate the relationship further in detail, I am currently measuring the three-dimensional morphology of the skull based on CT image data and analyzing the relationship with genome data (Fig. 3).
Natural hybridization (interspecies crossing in natural environments) has been observed in Macaca species in southeast Asia, such as between M. mulatta and M. fascicularis. Recent technological development has enabled us to analyze the genome without capturing the animal but from its excrement. I aim to approach the mystery of evolution by conducting field studies in south and southeast Asia where many macaques live, carefully investigating mutations in the genome and skull shape, and uncovering how divergence, crossing, and resultant morphological changes have taken place.