There are entire scientific disciplines that aim to study how humans behaved through the marks that they left on the earth; anthropology and archaeology. These marks are often left behind as materials that have survived through the renewal process of layers of soil and other top layers of earth’s crust. Tools and other heavy materials are the items that are maintained. These tools and other materials help investigators learn how humans behaved. In more modern contexts, there are written accounts from an individual's perspective that could record them shortly after an event had occurred. Yet in these more prehistoric times, the inferences have to be made by looking at these past remnants. This is how modern observers know generally when humans moved from nomadic cultures to more agrarian cultures. Not only are there more evidence of farming practices, but the earth itself shows differences as there were more farming products made, leaving lesser or greater deposits of certain compounds in the soil and in the atmosphere. This is similar to how antarctic ice can be used as a time capsule device measuring the amount of dissolved carbon dioxide in the ice through many overlapping ice layers. Human influence has not been seen just on the earth itself, but the plants and animals that exist in modern times are often manipulated for purposeful gain by the humans in that area. Domestication of otherwise wild animals are one example of this.