The Borg has always referred to other species by numbers (i.e. 8472, 3259 for Vulcans, 5618 for Humans ..etc ). It is assumed according to this that the numbers were incremental based on first encounter with few exceptions. My question is: has there been any reference in Star Trek (canon or non-canon) to the identity of species 1, or who are the first species to be assimilated by the Borg?
While not made explicit, if we can assume that the numbers are sequential, then the first species would be either the Caeliar, Humans, or Kindir, according to the (non-canon) novel trilogy Star Trek: Destiny, Book 3 Lost Souls, where one Caeliar merged with two Humans to form the first Borg, and the Kindir were the first species assimilated after that:
(4527 BCE) After eight days of hiking, the remaining humans returned to Junk Mountain, where they hoped to find the Caeliar alive and still willing to merge with them. However, once there, they found Mantilis apparently empty, although the Caeliar had simply lost the ability to maintain physical cohesion. Sedín - now reduced to a state of pure hunger - possessed all three of them, with Graylock's final thought that “I won't be...won't become...a...cy-“ -Borg. Once Sedín had control over the humans, she realized the female was most precious, and only required one male, so she had Thayer and Graylock kill and eat Pembleton for sustenance. She then instructed them to sleep. Some time later, a mining crew of Kindir native to the planet Arehaz landed on the island; some searching for scientific discovery, some for glory, and some for money. They located the crashed city of Manilis, and were the first to be assimilated by the newly formed Borg.
Discrepancies with the humans being Species 5618 could be explained away by the loss of knowledge of these first few species by whatever also caused them to forget the Vaadwaur (from VOY 6x07, Dragon's Teeth).