What could be the worst impact of cellphone usage to humanity?

That we lose the ability to interact with people face to face.

Other than everybody using one while driving?

The deterioration of our attention span.

All of the above.

Evolving weird shaped thumbs for scrolling.

Total loss of privacy and more invasive surveillance than Big Brother ever dreamed possible.
With the addition of coupled fitness trackers Big Brother will be able to measure some vital signs, heart rate and activity simultaneously and predict when you are under stress or merely thinking of something exciting.

Brain tumors.

That is a debunked myth @Inspired_2write .

People will forget how to do things, like read a map.

I think the total reliance on them is making people feel more important than they actually are.

Like social media posts and even here, our opinions actually matter very little, if at all, to anyone that matters (meaning politicians/ policymakers), yet online keyboard warriors are getting more and more intense.

Not many of the new generations ever mention writing/calling their congressman or anything productive, which I find sad.

We forget how to use our imaginations.