We just met Ethan Peck, the third actor to play Spock on 'Star Trek': How do they stack up?

Spoiler alert: This story contains details regarding Spock’s debut Thursday on “Star Trek: Discovery.”

When “Star Trek: Discovery” tantalizingly described its central character, Commander Michael Burnham, as the adoptive sister of the iconic Spock, it seemed supremely logical that the esteemed Vulcan would eventually appear. He did, on Thursday's episode of the CBS All Access series.

Whether Ethan Peck’s portrayal of Spock is fascinating remains to be seen, although his inaugural episode, "Light and Shadows," closes with an intriguing connection to the beginnings of Spock and "Trek" that suggests promise.

Peck has big shoes – but more significantly, a big mind – to fill in taking on the role originated by Leonard Nimoy in the 1960s series and fine-tuned for today’s generation by Zachary Quinto in the J.J. Abrams-produced films. He may get some time to adjust: the streaming service just renewed "Discovery" for a third season.

More: Star Trek's new Spock: Ethan Peck on what to expect with his 'Discovery' role

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Here’s a look at the evolution of the character, and the three Spock actors:

Classic Spock (Leonard Nimoy)

"Star Trek," 1966-68; "Star Trek: The Animated Series," 1973-74; two episodes of "Star Trek: The Next Generation," 1991; eight "Star Trek movies, from 1979 to 2013

Nimoy is nonpareil. He crafted one of TV and film’s most enduring and indelible characters, becoming the true “Star Trek” breakout and a surprise sex symbol.

His Spock was a double outsider: an alien on a U.S.S. Enterprise populated by humans, but only half Vulcan, as the son of a human mother, setting him apart on his home planet.

Nimoy captured that aloneness, while also embodying Spock’s devastating intellect, scientific rigor, embrace of logic and disdain for emotion, a source of “Trek” comedy when he arched an eyebrow at the florid displays of Captain Kirk and Dr. McCoy.

More: Remembering 'Star Trek's legendary Mr. Spock, Leonard Nimoy

Nimoy contributed to the evolution of Spock, who always had the distinctive pointy ears but sported bushier eyebrows and even a broad smile in the pilot episode, two looks quickly replaced by a more severe physical and emotional appearance.

“It took some time to find the shape of the ears, the color of the skin, the haircut, the eyebrows,” Nimoy told USA TODAY before his appearance in Abrams’ relaunch of “Trek.”

For all of Spock’s Vulcan stoicism, his more emotional human side occasionally broke through, as when he learned that he hadn’t really killed his best friend, Kirk, in the seminal “Amok Time,” which took Spock back to his home planet for an ancient mating ritual that offered a peek at the civilized culture’s savage past.

The first Spock originated the character’s classic and highly meme-able Vulcan traits: the split-finger salute; the telepathic mind-meld; and the nerve pinch, an immobilizing pressure-point maneuver.

Alternate-Timeline Spock (Zachary Quinto)

"Star Trek," 2009; "Star Trek Into Darkness," 2013; "Star Trek Beyond," 2016; upcoming sequel film

Quinto, cast in the role with Nimoy’s blessing, can't improve upon the original, but he's been a worthy successor.

Quinto had the opportunity to learn directly from the master, as his Spock and Nimoy’s much older version (now known as Spock Prime) met in 2009’s “Star Trek,” which justified the encounter with an alternate timeline.

Quinto, who consulted with Nimoy, captured Spock’s physical appearance, demeanor and versatility in dismissing human emotion, ranging from wry to withering. Depictions of Spock's mistreatment by Vulcan children because of his human heritage, and his rivalry with Kirk at Starfleet Academy, further fills in the character's past.

Alternate-Timeline Spock veers off in a significant way, too. Classic Spock had a smattering of intergalactic romantic liaisons, but Quinto’s character has an ongoing relationship with crewmate and fellow Enterprise icon, Lt. Uhura (Zoe Saldana). As that develops, Alternate-Timeline Spock shows a new level of emotion and vulnerability.

Younger Spock (Ethan Peck)

"Star Trek: Discovery, 2019

Because "Discovery” takes place a decade earlier than the original series, Peck’s Vulcan is less evolved than Nimoy's. His introduction Thursday suggests real differences between Younger Spock and the classic.

Although Nimoy, who died in 2015, wasn't involved in casting a role to which he felt “a sense of ownership (and) a blood connection,” there’s a shared pedigree: Executive producer Alex Kurtzman was one of the “Star Trek” film writers who impressed Nimoy with his understanding of Spock.

Most obviously, Younger Spock sports a wild, untamed beard that's seemingly at odds with Spockian precision. The lack of grooming may reflect the character’s internal disarray, as we meet this Spock in hiding – as a fugitive from Federation murder charges – twitching nervously and mumbling repetitively when adoptive sister Michael (Sonequa Martin-Green) finds him in a Vulcan cave.

The contrasting sensibilities of Spock’s mother, Amanda, and father, Sarek, are familiar, but Michael, adopted after her parents were killed, was never mentioned before the new series.

Viewers learn that being half-human was only one of Spock’s early problems. Amanda reveals he had a learning disability, which set him even further apart from other Vulcan youths.

Absent the beard, Peck– the grandson of actor Gregory Peck – fits the Spock physical model, but behaviorally he is the opposite. His evolution promises to make a good ongoing story. Most immediately, Michael must free him from a Starfleet intelligence unit, breaking rules (and maybe a few bones) in a display that would have made Captain Kirk proud.

The piece de la resistance comes at the episode's conclusion, when Michael decodes Spock’s numerical mumblings, which are space coordinates for Talos IV, the forbidden planet at the center of the first “Trek” pilot.

Nimoy's Spock was the only character who moved on to a second pilot episode and the series from that rejected pilot, which was later re-edited into an acclaimed Season 1 two-parter.

“Discovery” can now track where Spock has gone before. How he becomes the character beloved by millions will provide the adventure.

This article originally appeared on USA TODAY: We just met Ethan Peck, the third actor to play Spock on 'Star Trek': How do they stack up?