The phrase "extinct relatives" refers to the species that were once related to a particular organism but are no longer alive today. These organisms share a common ancestor but have died out over time. Full definition
Late last month, he revealed research results that may provide another stunner about the origins of humans: a previously unknown and now extinct relative of modern man. (science.howstuffworks.com)
(Up to 4 % of human DNA today came from extinct relatives.) (sciencemag.org)
Researchers had previously taken bipedalism as distinctive of humans and their closest extinct relatives, together called hominins, speculating that early hominins adapted to two legs on African savannas some five million years ago, after descending from the trees and outgrowing a phase of knuckle walking similar to that of modern chimps and gorillas. (scientificamerican.com)