Star Trek: 10 CORE Spock Moments

Yesteryear, also written by D.C. Fontana, featured the return of Mark Lenard as Sarek (one of two guest stars to reprise their roles on The Animated Series, with the other being Roger C. Carmel as Harry Mudd) and, via the Guardian of Forever, Spock's travel to the past to meet himself as a child.

Spock's childhood is filled with moments to shape the man that he would become, but it is the interaction with himself from the future, his journey through the Vulcan's Forge, and the loss of his beloved pet Sehlat I-Chaya that truly served as a critical time in his life. 

Cousin Selek, otherwise known as Spock from the future, helped to guide his younger self as the boy embarked on his kahs-wan, despite not being prepared. As they trekked through the desert, they were accompanied by their pet, which in turn led to Spock's life being saved, at the cost of I-Chaya's.

This was Spock's first true encounter with death, as well as the feeling of responsibility for that death. Had he remained at home, I-Chaya would never have faced the Le-Matya that killed him. However, as Spock's older self reminded the boy:

Loss of life is to be mourned but only if that life was wasted. I-Chaya's was not.

Spock ultimately chooses to euthanise his pet, sparing him from any pain. In this moment, the boy begins his journey to becoming the man who travels back in time, prepared to risk everything to save the future. It would not be the last time that Spock time-travelled, protecting a future that needed a logical hero.