The evolution of the house mouse shows the evolution of humankind's impact on the environment.

A house mouse on a pile of grain. Photo: Shutterstock

It has generally been thought that humans only started to have a real impact on the ecosystems around us when we started to develop agriculture. But that hypothesis is slowly unraveling as scientists and researchers learn ever more about our ancient ancestors. A new study has shown that people were having these impacts well before the advent of agriculture, and that they were staying in one place before then, too.

The Natufian hunter-gatherer site in the Jordan Valley in Israel contains a wealth of information about humans who lived around 15,000 years ago. Researchers have found that, when water and game were abundant, humans stayed in the area for extended periods of time. During those periods, the house mouse outcompeted the short-tailed mouse, but when humans began migrating during arid periods and food shortages, mouse numbers balanced back out.

The researchers made this discovery by examining tiny species-related variation in the shapes of mice teeth dating back as far as 200,000 years. With these teeth, the team built a timeline that showed how the populations of the house mouse and the short-tailed mouse fluctuated at the Natufian site and combined that information with data about human mobility in the region.

This may not sound very important, but it shows an early example of commensalism, the adaptation of animals to live near and alongside humans.

“The findings provide clear evidence that the ways humans have shaped the natural world are tied to varying levels of mobility,” said study co-author Fiona Marshall of Washington University. “They suggest that the roots of animal domestication go back to human sedentism thousands of years prior to what has long been considered the dawn of agriculture.”

Commensalism is generally seen as the first step toward domestication of animals. With short lifespans and rapid reproduction, rodents like the house mouse provide a clear view into how this evolution occurred.

“These findings suggest that the hunter-gatherers of the Natufian culture, rather than later Neolithic farmers, were the first to adopt a sedentary way of life and unintentionally initiated a new type of ecological interaction—close coexistence with commensal species such as the house mouse,” said researcher Lior Weissbrod of the University of Haifa. “The human dynamic of shifts between mobile and sedentary existence was unraveled in unprecedented detail in the record of fluctuations in proportions of the two [mouse] species through time.”

This is yet another piece in the complex puzzle of human interactions with nature. Humans have a definite impact on ecosystems in which they live, and the more sedentary humans become in a region, the greater that impact becomes. Continuing research, supported by ever more detailed analysis and technology, is showing scientists just how long humans have been impacting the environment, intentionally or otherwise. In some cases, as with the house mouse, that impact benefited other species, while in others it benefited both humans and animals, such as the commensalism and later domestication of livestock and dogs.

Not every impact humans have had on the environment is has been negative, but they help to put our more destructive tendencies into context.