Why humans acquired speech.

When the jungles in Africa thinned out and more Savannah appeared, our
hominid ancestors had to hunt, and travel in open spaces.  One useful
adaptation was bipedalism---so we could travel long distances and free
our hands for carrying meat back to camp and for throwing rocks at
prey.

But why did we start talking?  Observe how a young child acquires
speech.  And one stage a child will point at everything he sees and
name it.
I believe this replicates a stage in human development when it was
helpful for one member of a band of hunters to point out landmarks and
dangers in order to contributed to group success.  Because the members
of the hunting group were genetically related this benefitted the
hominid who did the pointing out and his (and their) genes were passed
on.

Gradually there were improvements in speech, just as there are with
that of a young child.  First adjectives are added, then verbs.  Much
later a past tense (which the Cro Magnons had and the Neandertals did
not.)