Caste ('jati' or 'varna') in the beginning

As indicated in the Bhagavad Gita (Ch. 4 – V. 13), the caste system in the beginning comprised four general categories of vocations (varna or jati: brahmin, kshatriya, vaishya and shudra) which used to be based on ‘guna’ (qualification / ability) and ‘karma’ (the task or job to be performed), very similar to the jobs currently according to employees' qualifications (‘guna’) matching with their assignments (‘karma’).

The above initial vocational categorization, flexible and logical as based on the qualification ('guna') and assignment ('karma'), however got distorted later becoming rigid on the basis of birth / heredity when children in the newer generations stuck to their parents’ occupations (jobs etc.) to earn money (make a living) quickly and easily, without going through the hassles and spending time to learn and pursue other (unfamiliar and outside one's own family) trades / vocations, which unfortunately also led later (during the post-Vedic era) to the stratification of society on the basis of caste / varna.

Note, the ancient society (tribes or ‘visha’) had viewed varna (occupation categories) initially in terms of their significance functionally only. Vaishya (meaning, belonging to visha or tribe) thus comprised everyone in the tribe (farmers, woodworkers, leather-workers, etc.). In addition, brahmin, as also a part of visha, would speak for the tribe, learn and transmit the Veda (the compiled knowledge) and act as the priest. Similarly, kshatriya, having the ability to lead and defend visha, would be responsible for the security and administration in visha (comprising 'kshatar' or the tribal dominion / area).

In addition to tribe’s three primary constituents (vaishya, brahmin and kshatriya) listed above, sometimes there would be new-comers or migrants (called shudra) to the tribe, who, most of the time, arrived or were brought from outside on foot. The sudra, like any new migrants these days, would initially earn their living by helping and providing services to the original residents of visha or tribe, but over time they (shudra or new-comers) would assimilate and integrate fully in the tribe (visha) to assume the roles as brahmin, kshatrya and vaishya on their own.

The Rig Veda (Book 10: hymn 90.12) also uses anthropomorphic analogy to describe visha (ancient society) functionally: brahmin (with the capability to speak for visha and act as its priest after
learning Veda) being the spokesman (mouth) of visha, kshatriya as the strength (arms) of visha, vaishya as the foundation (thighs) of visha, and shudra (the new-comer) arriving on foot to become a part of visha.

It is obvious from the above that people in the beginning had flexibility in pursuing various vocations and lifestyles, on the basis of 'guna' (qualification / ability) and 'karma' (task to be performed), instead of heredity and birth. A few examples below about people in ancient times opting for very different vocations and lifestyles than their backgrounds originally: Satyakama (in the Chandogya Upanishad) and Valmiki (the Ramayana's author) belonged to shudra category initially, but they learnt Sanskrit and the Veda to become brahmin and eventually acquired the titles as great Sages.

Similarly, Satyavati, a shudra woman in the Mahabharata, became a queen after she married a kshatriya (Shantanu, the king of Hastinapur). Moreover, Satyavati, as well as her daughters-in-law, did not commit 'sati' (suicide by self-immolation) later when they became widows following the deaths of their husbands (king Shantanu and his sons with Satyavati).

Ref.: Subhash C. Sharma, "Vedic vocations (Hindu castes) were not related to heredity (birth)," (yr. 2001):  https://www.oocities.org/lamberdar/_caste.html