Y: The Last Man – what killed all the men? Top theories explained

"It’s why I think it’s better as a TV series than as a movie because a two-hour movie would have to answer that question in a definitive way and that would be the main mystery. For me the series is about ‘What do people think happened and how do they form groups around that belief system?"

In the series, just two mammals with a Y chromosome survive – Ben Schnetzer’s Yorick and his pet monkey Ampersand, with the pair dragged into a cross-country adventure as they try to work out what spared them, and if there’s a way to save humanity after all.

Notably, in the source comic by Brian K Vaughn there’s no definitive answer to what caused the plague, with a few different theories posited over the course of the story. But Clark does suggest we might be getting some answers in the live-action adaptation, even if we have to wait a while - or for another series.

"I also think it’s not fair to a television viewer to bring up mysteries that you won’t give answers to," she said.

"I have an answer in my head but how it will play out in the show remains to be seen because ultimately I am more interested in how people form their belief systems, the same way that the comic books are."

But what are the options, we hear you cry? Never fear – we’ve listed the best theories below (along with the reasons Yorick may have survived) but beware, we could be dealing in some major spoilers for the TV show. And of course, there’s definitely spoilers for the graphic novel.

Starting with...

Attack on China

The first theory is that secretive US government agency the Culper Ring created a chemical agent intended to stop women from conceiving male children, targeting their Y Chromosome. This was to be unleashed on China to cripple their economy – however instead the agent went worldwide, fulfilling its intended purpose and (as a side effect) killing all the other men as well.

It’s not entirely clear how Yorick and Ampersand survived this, though there may be a clue in the next theory…

Clones made men unnecessary

A third surviving male mammal, Dr Matsumori, is revealed later in the comic, and he has his own theory. A genius scientist, Matsumori believes that his successful cloning trials rendered traditional reproduction obsolete, with “Mother Nature” destroying males as a result – a kind of natural correction that had been building for millions of years.

Matsumori also discovered a chemical compound in the course of his research that “had an adverse effect on the genome of cloned mammals” – he injected this into a lab animal (specifically Ampersand) and sent the monkey to his daughter Allison Mann, in the hope of killing the unborn clone fetus inside her (this story is very complicated). Instead, Ampersand was mistakenly delivered to Yorick and it seems the serum had a protective effect. All those who came into contact with it (Matsumori, Ampersand and Yorick, via cleaning up after his pet) were shielded from the plague.

Other theories are a little more mystical…

A cursed amulet

Yes, really. In the opening issues of the comic Agent 355 removes the sacred Amulet of Helene from Jordan, ignoring an ancient story that a great tragedy would fall on humanity, greater than the Trojan war, if it was ever taken.

It’s a little out there, but this theory does come with an interesting reason for Yorick’s survival – the Bedouin/Middle Eastern wedding ring he buys for his girlfriend Beth was said to be a similar relic, with the mythical power to muddle the holder’s gender to combine him and his wife’s beings. With this in mind, the ring (which Yorick keeps hold of for a long time), could have shielded him and Ampersand from the curse.

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Other theories

Of course, the list of proposed reasons for the plague in the series doesn’t stop there. Others suggested in the comic include a government plot, a version of the Rapture punishing women for original sin (leaving them on Earth), the Earth cleansing itself of the Y Chromosome or even divine retribution for women’s exclusion from the arts (following on from the Bubonic plague).

Some are, suffice to say, more likely than others, but in the comic none are truly definitive, and it could be that the real reason for the plague and Yorick’s survival is some mix between multiple theories.

And who knows? Maybe the TV series will go where the comic didn’t to finally solve this enduring mystery. Why the last man, indeed.

Y: The Last Man episodes 1-3 are streaming on Disney Plus now. For more, check out our dedicated Sci-Fi page or our full TV Guide.