The bull has existed in India since the Indus Valley Civilisation (c. 3000 BC - 1700 BC). It was a means of transport and was used to plough fields. Its hide was used to make weapons, as clothing, and to cover battle drums. Cattle-herding was the chief occupation of that time. Bull-worship developed as a result of this dependency.
Ancient deities, like Indra, Agni, and Rudra, are referred to as bulls. In the ritual of Vrisha-utsarga or "freeing of the bull", which is still in vogue, a bull, branded with an auspicious symbol, is let loose on the Kartik full moon night. It is believed to ensure good breeding and fertility.
Being Shiva`s vehicle, the sacred bull, Nandi, is independently worshipped at all Shiva temples.
By the Vedic period, the bull had taken a subordinate position to the cow. The cow had gained importance because of its new-found usefulness. The five products gained from a cow, Panchagavya, were of great significance.But it was only later, when the Manusmriti was being compiled (c. 1 AD - 200 AD), that cow worship had become a popular practice, in thanksgiving for its many benefits: nourishment from milk and milk products, especially for the mind; the means to plough fields; dung for fuel and manure, and, when mixed with mud, a paste to fortify the walls of huts; even urine, believed to have medicinal properties, especially for skin infections.
It is believed that the cow was created on the same day as the Brahmins and is therefore as sacred. Killing it is equated with killing a Brahmin which is considered one of the five deadly sins, or Panchapataka. The cow is also sacred because it provides milk and milk products, which are required at most ceremonies. Killing it is therefore doubly sinful.
Cow-worship is linked with the worship of Mother Earth. Therefore the cow is commonly called Gomata. (Go means "earth" as well as "cow" and mata is "mother").
According to the Puranas, the cow symbolises fertility and prosperity, and is considered a form of Lakshmi. It is also associated with Krishna, whose paradise is called Goloka, or "the place where cows live".In the Skanda Purana, the cow is said to be holy because every part of its body is believed to house a deity. For instance, Indra resides in the tips of its horn, Brahma on its head, the Ashvins in its nostrils, and Yama behind its tail.