The answer is that they did.
We have an entire evolutionary chart and everything. [https://en.wikipedia.org/wiki/Human_evolution](https://en.wikipedia.org/wiki/Human_evolution)
If you are asking why there are none left alive today… we and nature at large killed them off.
To us, they were a threat and survival of the fittest ensued. To other threats, their intellect did not yet outmatch the most clever of predators. That is why all that remains are fossils.
We are, for all our intelligence, still animals. Early homo sapiens would have likely isolated from then warred against Neanderthal and our other predecessors if any remained.
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