The Black Marker sent to earth happened around 65mil BC, while the Tau Volantis aliens lived "more than two years ago", implying that Tau Volantis was not the origin point of the Black Markers but a target of a brethren moon for further assimilation.

The true origin of the first Black Marker can be anything, ranging from an ancient race creating it, leading to their own destruction by convergence, to Necromorphs being an ancient race themselves born to destroy life.

What i personally think is that the first Black Marker spawned naturally due to the first species being evil, bundling up all bad things into a life absorbing organism, that would rage on for millenia to come. 

Here is the most complete theory.                                                                                                                                                                                                                There is an ultra advanced species. Who created black markers. They sent them to planets to create life which was hinted by danik. So the life on earth and tau volantis had been created by black markers. They have commenced the evolution process. Why did they do it in the first place ? They want to harvest it when the pre-created life has blossomed and fed enough to feed ultra advanced species. Moons are nothing more than a harvester machine. When the time comes the black marker hidden in the planets is triggering a necro epidemic to exterminate life on decided planets and lets the convergence (harvesting) begin. As you can see in ds3 moons are manufacturing themselves with the organic material which they pull by reverse gravitation or some kind of force. They prepare themselves as meatballs to serve masters. Tau volantis species are nothing more than a victim like us. This ultra advanced species are seeding life on planets, and when the time comes they harvest it. 

Maybe,it was created as a tool of survival of a dying alien race in hopes of survival,but it gone totally wrong it brought destruction.Or maybe markers were created for specific causes,such as trigerring a beautiful fuana across a planet,but it maybe exposed to a strange material or modified by an intelligent enemy.

My most possible theory is that those MARKERS" were created from special materials that has the power to assimilate life,but maybe they were some type of "robotics" designed for special purposes, but maybe one of them got AI or rebelled,and reprogrammed other markers under their commmand,just like in transfm 4 which galvatron infects other prototypes whith evil chromosomes.

Just as humans can be considered tools for DNA replication, rather than DNA tools for human replication, I think it's interesting to consider the Marker the instigator rather than a tool in the process.  In this view, the Marker would *be* the alien species itself, brought about by a fluke of nature and subsequently evolved on a grand time scale just as our own DNA must have been.

One problem with this idea is the energy it receives beamed to it from outer space.  Perhaps this energy is beamed to it from other Markers far away... or maybe it is a kind of radiation emitted "naturally" from things unrelated to the Markers and their alien kind.

This is how I'd write the markers: as the agents rather than the instruments.

My theroy original black marker is very  giganatic one dead space 2 hints to original black marker .size of brethren moon 3498km and original marker size unknown.

Wow, it's as though this hasn't been done before...oh wait: https://halo.wikia.com/wiki/Precursor

The Black Markers Birth is 150 million years ago,brethren moons insidely Black markers are created by another big technologiest aliens !

Necromoprhs are came from another dimension, very similar like in Stephen Kings novels "Dark Tower". Actualy by my oppinion i think they came from Fallen Angel.