As a matter of fact, it must be easy to see that jatis are not occupation groups either, in the past or in the present. No occupation is a monopoly of one specific caste. It is a sociological impossibility that people belonging to an occupation caste (say chamara and kambara castes of Karnataka) practiced the same occupation across time and space for centuries on end. This fact should be obvious to all those who have lived in India. Nevertheless, the caste-occupation nexus is regularly offered as a description of the reality. When this discrepancy is pointed out, it is usually argued that the caste system was an occupation based distinction in the past and only recently the situation has changed, because of colonialism or (colonial-)modernity. However, until and unless one inconclusively shows that castes were occupation based distinctions at some juncture in the past, this line of argument cannot be taken seriously.
It is strange that this argument is made even today in the 21st century despite the glaring examples that even scavenging and cleaning toilets is not done by people belonging to one caste or even by people belonging only to the so-called lower castes.[1] This shows that the roots of our theories of caste lie not in the empirical description of our world but something else.
[1] See the report in a leading Indian newspaper – Hindustan Times (Accessed 3 February 2011):
http://www.hindustantimes.com/Are-Brahmins-today-s-Dalits-in-India/Article1-2219\26.aspx