A tool is an entity used to facilitate more effective action. The most basic tools are simple machines. For example, a crowbar simply functions as a lever. The further out from the pivot point, the more force is transmitted along the lever. A hammer typically interfaces between the operator's hand and the nail the operator wishes to strike.
Tools are the most important items that the ancient humans used to climb to the top of the food chain; by inventing tools, they were able to accomplish tasks that their bodies alone could not, such as using a spear or bow and arrow to kill prey, since their teeth were not sharp enough to pierce many animals' skins.
A telephone is a communication tool that interfaces between two people engaged in conversation at one level. And between each user and the communication network at another. It is in the domain of media and communications technology that a counterintuitive aspect of our relationships with our tools first began to gain popular recognition. Marshall McLuhan famously said "We shape our tools. And then our tools shape us." McLuhan was referring to the fact that our social practices co-evolve with our use of new tools and the refinements we make to existing tools.
Tools that have evolved for use in particular domains can be given different assignations. For example, tools designed for domestic use are often called utensils.
Observation has confirmed that that multiple species can use tools, including monkeys, apes, several birds, sea otters, and others. Philosophers originally thought that only humans had the ability to make tools, until zoologists observed birds and monkeys making tools. Now humans' unique relationship to tools is considered to be that we are the only species that uses tools to make other tools.
Most anthropologists believe that the use of tools was an important step in the evolution of mankind. Humans evolved an opposable thumb — useful in holding tools — and increased dramatically in intelligence, which aided in the use of tools.
Because tools are used extensively by both humans and animals, it is widely assumed that the first routine use of tools took place prior to the divergence between the two species. These early tools, however, were likely made of perishable materials such as sticks, or consisted of unmodified stones that cannot be distinguished from other stones as tools. The beginning of the Stone Age marks the era when hominins first began manufacturing stone tools, and evidence of these tools dates back at least 2.6 million years in Ethiopia. The transition from stone to metal tools roughly coincided with the development of agriculture around the 4th millennium BC.
Mechanical devices experienced a major expansion in their use in the Middle Ages with the systematic employment of new energy sources: water (waterwheels) and wind (windmills).
Machine tools occasioned a surge in producing new tools in the industrial revolution. Advocates of nanotechnology expect a similar surge as tools become microscopic in size.
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