Since humans existed we started seeking truth and self-realization but what about other beings or humans of the early stone age?

Self-Realization can only be achieved in a human form. This is because only in human form, one can charge (bind new) as well as discharge (leave old) karmas, whereas in all other life forms, one sensed living beings onwards, they only discharge karmas. The ability to charge karmas benefits us humans as we can accumulate merit karmas. Through the merit karma, human beings can rise spirituality higher and attai
Self-Realization.

The path to the truth and Self-realisation has existed since time immemorial. The science of Self-realisation and the ultimate truth has been around since the existence of mankind from the very beginning.

For some who were extremely fortunate during the early period, they had attained the knowledge of the Self, the ultimate truth and gained the final liberation from an Arihant. But for many during that period, they had chosen to enjoy the worldly life, as a result, decided to forego the gift of Self-realisation, thus extending their cycle of birth and death.

Who are Arihants and why can only an Arihant grant the final liberation?
The title of Arihant is given to:
the One, who exists in a living form;
them, who have annihilated all their inner enemies of anger, pride, deceit, greed and sexuality, the key adversaries of the Soul;
those living fully enlightened beings, whose presence turns every place they dwell into, a place of pilgrimage, and whose devotional viewing grants final liberation to the one who has attained Self-realisation.
Only Arihant, the most elevated beings, can grant the final liberation!

Final liberation means freedom from the perpetual cycle of birth and rebirth; complete freedom from karmas.
The twenty-four Tirthankaras on Earth were indeed Arihants when they were living during the earlier period; but they have now attained the ultimate liberation of an enlightened being. They have achieved the final liberation and have now become Siddhas- formless and totally free from karmas, and they reside in the realm of liberated Souls.

Before His departure, Lord Mahavir, the last of the twenty-four Tirthankaras, had said that the cycle of twenty-four Tirthankaras on Earth was ending and therefore there would be no more Tirthankaras on this planet for this time cycle. Lord Mahavir asked people to worship the living Tirthankaras in Mahavideh Kshetra, because there are alive there!
Is there a present Arihant in this current period?
There is, but not on our Earth. Only those who are currently present and living can be called Arihants.
At present, there are twenty Arihant Tirthankaras in Mahavideh Kshetra (another world within our extended universe). Due to an everlasting spiritually elevated environment in Mahavideh Kshetra, Tirthankars are able to incarnate there. However, they are unable to do so on Earth because of the current declining time cycle of our world.

Of the twenty Arihant Tirthankaras, Lord Simandhar Swami resides in the closest proximity to Earth and has a unique karmic obligation to facilitate the liberation of the beings on our Earth. So for us, Simandhar Swami is the Arihant of the current times!
Param Pujya Dadashri, the founder of the movement of Akram Vignan - the spiritual science that has opened up the path to step-less freedom, has explained that Lord Simandhar Swami’s idol must not be considered a mere idol, it is a living representation of His actual presence. Dadashri described that the benefit of Lord Simandhar Swami’s devotional viewing is that we bind further merit karmas, which facilitates one’s ultimate liberation!

However, prerequisite to attaining the final liberation from an Arihant, one must first acquire the knowledge of the Self (Self-Realization) from a living Gnani (enlightened being), a vital link to reaching Simandhar Swami, thus initiating first step towards final liberation.

Today, Akram Vignan is the best chance for us mankind to attai
Self-Realization in a matter of just two hours time through the direct grace of the Living Gnani. So let’s not miss out on such a unique opportunity of attaining the knowledge of the Self and thus procuring the ultimate truth of the pure Soul.

To know more: https://www.dadabhagwan.org/path-to-happiness/spiritual-science/who-am-i-realize-your-true-self/what-is-self-realization/