The common ancestor to mice and humans was an inconspicuous rodent-like mammal that scurried along the surface of the earth some 65 million years (myr) before present (BP). It had to be inconspicuous because the earth was ruled by enormous dinosaurs, many of whom would have eaten any small mammal that could be caught. The glorious age of the dinosaurs came to an abrupt end with the collision of one or a few large extraterrestrial objects perhaps asteroids or comets into the earth's surface over a relatively short period of time (Alvarez and Asaro, 1990; Sheehan et al., 1991). Possible sites at which these impacts may have occurred have been identified in the Yucatan peninsula of Mexico and the state of Iowa (Kerr, 1991; Kerr, 1992; Kerr, 1993). It has been hypothesized that the impact resulted in the formation of a thick cloud of dust that dispersed and shrouded the earth for a period of years, leading to a scenario like a nuclear winter with the demise of all green life, and with that, all large animals that depended either directly on plants for survival or indirectly on the animals that ate the plants. At least a small number of our rodent-like ancestors were presumably able to survive this long sunless winter as a consequence of their small size which allowed them to "get by" eating seeds alone. When the sun finally returned, the seeds lying dormant on the ground sprung to life and the world became an extremely fertile place. In the absence of competition from the dinosaurs, mammals were able to become the dominant large animal group, and they radiated out into numerous species that could take advantage of all the newly unoccupied ecological niches. It was in this context that the demise of the dinosaurs brought forth both humans and mice as well as most other mammalian species on earth today.
The Muridae family of rodents, which includes both "true" mice and rats, originated in the area across present-day India and Southeast Asia. Phylogenetic and palaeontological data suggest that mice and rats diverged apart from a common ancestor 10-15 myr BP (Jaeger et al., 1986), and by 6 myr BP, the genus Mus was established. The Mus genus has since diverged into a variety of species (listed in Figure 2.2) across the Indian subcontinent and neighboring lands.
At the beginning of the Neolithic transition some 10,000 years ago, the progenitors to the house mouse (collectively known as Mus musculus, as discussed later in this chapter) had already undergone divergence into four separate populations that must have occupied non-overlapping ranges in and around the Indian subcontinent. Present speculation is that the domesticus group was focused along the steppes of present-day Pakistan to the west of India (Auffray et al., 1990); the musculus group may have been in Northern India (Horiuchi et al., 1992; Boursot et al., 1993); the castaneus group was in the area of Bangladesh, and the founder population bactrianus remained in India proper.
The house mouse could only begin its commensal association with humans after agricultural communities had formed. Once this leap in civilization had occurred, mice from the domesticus group in Pakistan spread into the villages and farms of the fertile crescent as illustrated in Figure 1.2 (Auffray et al., 1990); mice from the musculus group may have spread to a second center of civilization in China (Horiuchi et al., 1992); and finally, bactrianus and castaneus animals went from the fields to nearby communities established in India and Southeast Asia respectively.
Much later (~4000 yrs BP), the domesticus and musculus forms of the house mouse made their way to Europe. The domesticus animals moved with migrating agriculturalists from the Middle East across Southwestern Europe (Sokal et al., 1991) and the development of sea transport hastened the sweep of both mice and people through the Mediterranean basin and North Africa. Invasion of Europe by musculus animals occurred by a separate route from the East. Chinese voyagers brought these mice along in their carts and wagons, and they migrated along with their hosts across Russia and further west to present-day Germany where their spread was stopped by the boundary of the domesticus range (Figure 2.3). Finally, it is only within the last millennium that mice have spread to all inhabited parts of the world including sub-Saharan Africa, the Americas, Australia, and the many islands in-between.
One interesting sidelight of the stowaway tendency of mice is that it is sometimes possible to observe the origin of human populations within the context of the mice that have come along with them. A clear example of this concordance is seen in the domesticus mice that have colonized all of North America, South America, Australia, and sub-Saharan Africa in conjunction with their Western European human partners (Figure 2.3). A more complex example is observed in the Japanese islands where the native mice were long thought to be a separate subspecies or species group referred to in the literature as Mus molossinus. In fact, molecular phylogenetic studies have demonstrated that Japanese mice do not represent a distinct evolutionary line at all.
Instead, they appear to have been derived by hybridization of two other house mouse groups on the mainland nearby musculus in China and castaneus in Southeast Asia (Yonekawa et al., 1988, Figure 2.3). The hybrid character of the mice parallels the hybrid origin of the Japanese people themselves.
Finally, there is the interesting observation of a pocket of mice from the castaneus group that has recently been uncovered in Southern California (Gardner et al., 1991). This is the only documented example of an established natural house mouse population in the Americas that is not derived from the Western European domesticus group. This finding is a testament to the strong wave of 20th century Asian migration to the West Coast of the United States.