For how long have we been burying our dead? Do we know the first example of an actual burial or entombment, or is it to difficult to know if something was buried or covered over time?
/r/AskAnthropology might be better prepared to answer your question. They typically respond very well to similar questions about prehistoric origins.
The earliest intentional burials seem to be recently discovered specimens in South Africa that are 1.5 million years old. See this news article for a general introduction, and this news article for a more detailed discussion of how much this show ritual. They are much, much older than the next oldest set of specimens of intentional burial, which are 90k to 300k year old. See this old post for a discussion of those.