The Kannada language is very old and it is of Dravidian origin. It is
believed that there originally existed a Tamil-Kannada proto-language,
which split into Tamil and Kannada between 8 to 6 BC.
The photo below is an example of the Kannada Script, from a 6th Century cave-rock carving at Badami, 500 kms northwest of Bangalore.
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Badami Rock Carving, Photo Credit : Wikipedia |
From what I have read, there seem to be several eras/types in Kannada language: the proto-Kannada tongue which I listed above, then after that ancient Kannada (Halekannada), middle (Nadukannada) and modern Kannada.
Different historians attribute different years to these terms but broadly they all agree that ancient or Halekannada absorbed various vernaculars (various types of Prakrits) as and when Kannada-speaking people came into contact with them. HaleKannada also absorbed Sanskrit influences, including Panini (4 BC) and other non-Panini versions of Sanskrit.
The Kannada language has been constantly evolving due to political changes, as each new dynasty which ruled the area spoke different languages. The Karnataka region as we know it today was under Nandas (5th & 4th century BC, capital in Bihar), then Ashoka (Maurya empire, also from Bihar); then the Satavahanas (capital in Paithan Maharashtra), then briefly the Pallavas of Kanchipuram (Tamil Nadu), then the Western Ganga dyasty (capital in Talakad near Mysore), then Kadambas (North Canara), Rashtrakutas (Gulbarga) and Chalukyas of Badami (6th to 12th century AD).
With all these shifting centres of power (and language) the hale-Kannada language had ample opportunity to absorb many vernacular Prakrits. With the advent of Muslim rule in the Indian continent, and later the formation of the Deccan Sultanates, Arabic words became part of Kannada. With British rule, English words also crept into the language.
As a result of all these changes, modern Kannada is very different from the original proto-Kannada-Tamil.
As for written Kannada, well the script is descended from the Brahmi script (the earliest known record of Brahmi is Ashoka's edicts, 3rd century BC). The Kadambas and Chalukyas used a form of Brahmi. During the hale-Kannada phase, the script underwent many changes especially because of the need to be able to write new loan words from Sanskrit and other Prakrits. By the 13th century, this script had split into two similar but different calligraphic styles, Kannada and Telugu. In the 1800s, the Kannada script that we see today was recorded by British missionaries.