Most normal creatures don't fight to the death as a matter of course, except in D&D.
Most creatures will, when confronted with a threat (note, not food nor a rock1), run away, as this is the cheapest way of preserving life. Only if they can't run away will they fight. If they fight, they'll fight until they can... run away (see a pattern here?) For creatures in a (nominal) social group, trying to establish social hierarchies, they will also not fight to the death, rather they'll pull their blows and demonstrate capability instead such that one creature can surrender to the other.
In abstract editionless D&D terms, there will be squabbling amongst these tabula rasa as fighting is the only way to establish a dominance hierarchy in the absence of, well, language or other social cues2. The trick is that these fights will start with intimidation and shows of strength. If one side is obviously stronger than the other, the weaker side will demonstrate meekness (see a dog's surrender body language). And place themselves into a vulnerable, surrendering, position, such that they may be spared.
However, this isn't interesting in a system that is primarily about murdering things without remorse and taking their stuff. Instead, narrate (with input from the player) vignettes about these first few days: "show an example of how you make camp, show an example of how you find food..." and neatly ask the player "so how did these savages befriend the player?" Functionally, you'll be iterating through Mazlow's hierarchy of needs until you get to the bit about "self-actualisation" (in D&D terms, murdering people with your friends for fun and profit) that D&D works with, and constraining player choice so that the "you failed to achieve shelter. You die." bits don't occur. Death isn't actually all that fun on a desert island. Alternatively, having the character starve to (almost) death and be rescued by the scared tabula rasa is another way of establishing camaraderie.
By begging the question, and allowing the player to express agency through narrating how he/she dealt with the social dominance situation, you can move onto the bits of the adventure that D&D doesn't absolutely suck at.
Terry Pratchett, Equal Rites:
For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks.
2See, amongst other things, Veblen's conspicuous consumption.