Was the idea of caste system in ancient India similar to the modern definition .

I am a 16 year old science student who is interested in understanding his country's past .

In the Vedas and Bhagavad Geeta , it is written about "varna " which was determined by an individual's profession and qualities . It had no connection with their birth according to our religious scriptures . So when did people start to differentiate people based on their birth rather their qualities.

I have also read that the varnas were differentiated on the basis of Aryan genes . I have not found strong evidence regarding this argument that the brahmins , kshatriyas were Aryan people and the shudras were the ancestral Indian people . How true is this assertion ? Were the people differentiated on the basis of their genetic ancestry or varnas were formed after the assimilation of Aryan tribe with the general population .

We see in the Upanishads , there are many verses where saints talked very highly of shudras . Also Krishna says in the BG that any human being irrespective of their varna can worship him . So, why did people discriminate then ?

Also when did jati became a part of Indian social system . Is the caste system related to the varna system or to a person's jati which is defined by birth ?

Can you provide some evidences of caste system among general public from 1000BCE to 700CE in North India .

One more question - Even if the caste system was based on birth , was it similar to the level of atrocities committed in the late 18th and 19th century ?