I've heard that Sanaatana Dharma (Hinduism) exists since the formation of the universe. Were the early humans also Sanaatani (Hindu)?
Gaudapada in his Karika on the Mandukya Upanishad writes in verse III.15 (here - https://www.wisdomlib.org/hinduism/book/mandukya-upanishad-karika-bhashya):
(The scriptural statements regarding) creation as illustrated by examples of earth, iron, sparks, etc., or otherwise, (only) serve the purpose of (ultimately) explaining the unity (of Jīva and Brahman). (Really speaking) multiplicity does not exist in any manner.
*Shankara Bhashya (commentary)*
(Objection)—Before creation all this might have been unborn, one and non-dual; but after creation, all this evolved world and the embodied beings (Jīvas) denote multiplicity.
(Reply)—No, it cannot be so. For, the scriptural passages dealing with creation have another meaning. This difficulty raised here has already been solved by the statements that2 the aggregates (entities) of body, etc., like dream-objects, are produced through illusion of the subject (Ātman) and that creation and the differences of the Jīvas are like the creation and the differences of the Ghaṭākāśas, i.e., the bits of Ākāśa enclosed in different jars. The scriptural3 statements dealing with creation and differences (of the created beings), have again been referred to here in order to show that such statements regarding creation have the purpose of determining the unity of Jīva and Brahman. The4 (theory of) creation has been described in the scripture through the illustrations of earth, iron, sparks, etc., or otherwise; but all these modes of creation are meant for enlightening our intellect so that it may comprehend the identity of Jīva and Brahman. It is just like the story5 of the organs of speech (vāk), etc., being smitten with evil by the Asuras (demons) as described in the chapter on Prāṇa (vital breath), where the real purpose of the Śruti is to demonstrate the special importance of Prāṇa.
(Objection)—We6 do not accept this meaning as indicated.
(Reply)—Your contention is not correct. For7 this story about Prāṇa, etc., has been differently narrated in different recensions of the Vedas. If the story of Prāṇa were literally true, there should have been one version only in all recensions. Different versions of contradictory nature would not have been narrated. But we do come across such different versions in the Vedas. Therefore the scriptural passages recording stories of Prāṇa are not meant to serve any purpose of their own, i.e., they should not be taken literally. The scriptural8 statements regarding creation should also be understood in a similar manner.
(Objection)—There have been different creations in different cycles. Therefore, the scriptural statements regarding creations (of the universe) and stories (of Prāṇa) are different as they refer to the Creations in different cycles.
(Reply)—This contention is not valid. For, they (the illustrations of earth, iron, etc., as well as the stories of Prāṇa) serve no other useful purpose than clearing our intellect as stated above. No one can imagine any other utility of the scriptural statements regarding creation and Prāṇa.
(Objection)—We9 contend that these are for the purpose of meditation so that one may ultimately attain to that end.
(Reply)—This is not correct either; for no one desires to attain his identity with the dispute (in the case of the Prāṇa narrative), or with the creation or destruction (in the case of the scriptural statements regarding creation, etc.). Therefore we have reasonably to conclude that the scriptural statements regarding creation, etc., are for the purpose of helping the mind to realise the oneness of Ātman, and for no other purpose whatsoever. Therefore, no multiplicity is brought about by creation, etc. Anandagiri Tika (glossary)
*1 Before, etc.—*There are definite Scriptural statements regarding creation. These statements are literally true. Therefore multiplicity caused by creation is also true.
2 In Kārikās 3 and 10 (Chapter III), it has been established that the perception of ego and non-ego as separate from Brahman is due to ignorance.
3 Scriptural, etc.—It has been explained, in the previous text that the Scriptural statements regarding creation, etc., are for the purpose of explaining the illusory nature of the universe to those who take it as real. But the purpose of this Kārikā is to enable us to understand the identity of Jīva and Brahman.
4 The creation, etc.—The meaning is that we should not take these Scriptural statements in the literal sense but must get at their underlying significance.
5 Story, etc.— The reference is to the second part of the first chapter of the Chāndogya Upaniṣad. This story cannot be accepted in a literal sense as the organs of speech, etc., being themselves unconscious, cannot quarrel with one another. The significance of the story is to demonstrate the superiority of Prāṇa over other Indriyas (organs). The story referred to here is as follows: The Devas and Asuras, both of the race of Prajāpati, fought with one another. The Devas (Gods) and the Asuras (Demons) are explained as good and evil inclinations of man. The Devas took the Udgīta, thinking that they would be able to vanquish the Asuras with it. The Udgīta stands for the sacrificial act to be performed by the Udgātṛ, the Sāmaveda priest, with the Udgīta hymns. They meditated on the Udgīta as the breath in the nostril, but the Asuras smote the breath with evil. Then they meditated on Udgīta as the speech, the eye, the ear, the mind; but ail these sense organs were smitten with evil by the Asuras. Then they meditated on Udgīta as Prāṇa (vital breath) and the Asuras failed to smite it with evil. Therefore Prāṇa is superior to all sense-organs.
6 We, etc.— We do not accept your explanation, for, the organs of speech, etc., have been designated as gods. Therefore they cannot be insentient matter.
7 For, etc.— This story about Prāṇa has been differently stated in different Upaniṣads. This cannot happen if the story is to be accepted as literally true.
8 Scriptural, etc.— The story regarding creation, as in the case of Prāṇa, has been differently stated in different parts of the Upaniṣads. In some places we read that the Ākāśa was first evolved; again we find that the fire was first evolved and still in another place it is mentioned that Prāṇa was first evolved. Therefore, on account of the contradictory natures of these stories they should not be taken as true. They serve some other purpose, vīz., the establishment of the absence of variety, or the oneness of Ātman (Brahman).
9 We contend, etc.— It is said in the Śruti that the worshipper ultimately realises the oneness of Ātman.
Your question is really of a scientific nature. All accounts of a creation, including scientific, are likely stories. Read the history of science, and the story is always changing, or evolving. Read also Patanjali's Yoga Aphorisms where he describes the evolution of species.