Elders, also known across the galaxy as Martians, were an ancient alien species whose society spanned the Milky Way galaxy. They were the native species of the planet known as Harlan's World. When their race was nearly destroyed, several humans known as the Founders came to the planet and carried out a genocide against the Elders.
At one point these aliens discovered the secrets of interstellar travel and managed to establish a massive and prosperous colonial empire. They once dominated currently human-inhabited space for millennia before the dissolution of their highly advanced civilization.
The improper nickname, Martian, was the result of mankind encountering signs of their presence for the first time on Mars. Aside from Sol, there were many worlds and systems which were in their sphere of governance and in which they left multiple artifacts. The discovery of their astrogation charts of habitable worlds in the galaxy as well as new metals and technologies led to the expansion of the human race across multiple worlds in the galaxy, and the creation of cortical stacks. It is currently unknown what led to the dissolution of this advanced civilization, or what ultimately became of them.
Konrad Harlan and several other Meths discovered the habitable alien world now known as Harlan's World. However, they were unable to colonize there unless it was completely devoid of intelligent alien life. At first, it seemed that the Elders were a long-extinct alien race, but the Founders discovered remnants of life forms existing in tandem with the songspire tree. Impatient to colonize the world, Konrad Harlan decided to destroy the remaining Elders. However, one survived by clinging to the songspire tree's roots.
Centuries later, Quellcrist Falconer was placed in a glass coffin next to the songspire tree roots. Over centuries, the Elder who clung to the tree leeched into Quell's psyche, driving her to take revenge on those who had wronged the Elders; the Founders.
The Elder legacy is one of colonization and technological advancement of humanity - as humans used the remnants of Elder technology to advance through the galaxy.
The remnants of the metals and astrogation charts left by this advanced alien civilization launched the expansion of the human race throughout the galaxy through Needlecasting and helped evolve the human species through the development of the Cortical Stack.
Based on the few skeletal remains seen on screen, as well as the Elder encountered in virtual reality, Elders were an extremely tall race with at least four limbs, two wings sprouting from their shoulders, several eyes, and dark skin. Based on comments made by the Elder sharing Quell's stack, they possessed physical strength much greater than that of humans. They were a very militaristic race.