How do you explain the different ways that Klingons look in the Star Trek universe?

Klingons in TOS, TNG/DS9 and DIS
Klingons in TOS, TNG/DS9 and DIS

In the several Star Trek series, how Klingons look has changed over time...

Star Trek the original series (TOS) shows Klingons as human-like, with brown skin and slanted eyebrows. The Next Generation (TNG) and Deep Space 9 (DS9) show them completely different, with ridged foreheads and long hair. Discovery (DIS) shows them radically different, with black skin and no hair.

How can this be explained?

In a DS9 episode where the main characters go back in time to deal with Tribbles, the producers used TOS scenes and superimposed DS9 characters. Odo asks Worf why the Klingons look so different, and he humorously says "It's a long story. We don't like to talk about it."

What do you think?

How do you explain the different ways that Klingons look in the Star Trek universe?
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