Have you ever wondered why the Klingons seen on Star Trek: The Original Series lack the trademark, ridged Klingon forehead made famous in the later series and films?

Let’s teleport to the fictional Star Trek universe — lest mundane thoughts of budget constraints and production design creep into our minds — and focus on a Star Trek–centric answer for this drastic transformation.

According to the collaborative, user-generated Star Trek encyclopedia Memory Alpha, the incongruity is the effect of the Klingon augment virus, a mid-22nd-century biological disaster that threatened to wipe out the Klingon Empire.

Klingons, in their natural state, are the long-haired, ridged-forehead warriors first seen in Star Trek: The Motion Picture. So why exactly did they appear more human-like in The Original Series?

To answer that, we must first look back to Earth’s Eugenics Wars, conflicts that affected the planet from 1992 to 1996, in which selective breeding and genetic engineering led to devastating conflicts that almost plunged the planet into a new Dark Ages. Of course, this is where Khan and his cadre of super beings came from, frozen for centuries until freed by Kirk.

In 2154, Klingon scientists got their hands on several augmented embryos left over from Earth’s Eugenics Wars. They hoped to employ the technology to develop a stronger, faster, smarter Klingon.

The augmentation process did enhance strength and intelligence, while also giving the Klingon test subjects human personality traits and—surprise, surprise—a ridgeless forehead. Ultimately, however, the non-compatibility of Klingon and human DNA meant that test subjects endured an agonizing death soon after augmentation.

While toiling away to stabilize the augmentation process, Klingon scientists enhanced a test subject who, unbeknownst to them, was infected with the Levodian flu. Augment DNA and the virus fused, creating a quickly spreading super virus that infected millions within a month.

With countless masses suffering and dying and the Klingon High Council ready to annihilate the infected planets, a method was developed to stabilize the augmentation virus in its early stages, after the cranial ridges had dissolved and minor neural re-ordering had occurred.

Though in the 2150s the fate of Klingons affected by the augment virus was uncertain, by the 2260s, the time in which The Original Series occurs, the descendants of those survivors, who also bore the augmented traits, had been accepted into Klingon culture to some extent. Augmented Klingons served, for instance, in the Klingon Defense Force, acting as both captains and crew.

By the 24th century, the time in which The Next Generation and Deep Space Nine take place, knowledge of the Klingon augment virus had faded among Federation planets. In Deep Space Nine episode “Trials and Tribble-ations,” for example, when crew members of the space station find themselves 100 years in the past and aboard the Enterprise, Worf is asked why the Klingons they encounter differ so much from those which they are accustomed. Worf replies, “We do not discuss it with outsiders.”