Let me take a crack at it.
The initial breed of short-faced kangaroos had been bred from the standard-issue marsupials, like wallabies, for example, by a group of hyper-advanced entities in an attempt to purify the kangaroos' genetic base. The Earth marsupial gene-manipulation was a crucial portion of a test to see if the methods used there were viable before tests on sapient beings.
Once the method had been perfected, the kangaroos were left on Earth and the entities launched off a galaxy-wide genetic manipulation program, working outside the lines to allow bloodlines with preferable genes to crossbreed in an attempt to create a group of individuals specially born with enhanced abilities, including high-level psionic powers.
However, they were just too good. Some of the psionic science-children were much more powerful than expected, and many of those had so little control over their abilities that they often ended up warping time itself. A certain unscrupulous member of the hyper-advanced entities began using them as weapons, brainwashing them to serve him and using them to erase his enemies from history. However, due to that warlord's lack of thought to temporal consequences, the past of the massive genetic program that created the children was being disrupted, making the timeline unstable, which led to the warlord being discovered. Other members of the hyper-advanced entities managed to capture one of the warlord's time-warping mutants, which had their abilities enhanced even further by its master's experimentation, and, seeing no better choice, imbued the growing fetus of one of their own race with the psionic child's power. The power could not be implanted into an adult, since the sudden introduction of that level of psychic power could potentially destroy the mind of the recipient, and the entities weren't willing to have an insane time-warping entity running around the place.
The imbued child, however, resented his role as the chosen one, and after one of his mentors was removed from history by one of the warlord's attacks, which caused everyone but the time-separated child to forget about him, the imbued child stole a ship and transported himself to Earth, intending to manipulate the past of the breeding program and stop the time-manipulating abilities from ever appearing.
However, in his unstable emotional state, he botched it, and ended up skewing the past results of the program too far, causing it to fail rather than grow in a different direction. His powers spiraled out of control as causality attempted to catch up with the new history, which intersected with the previous time-instability caused by the warlord to set off a temporal chain reaction that ended up destroying large pieces of the timeline.
At the end, almost the entire program was removed from history, and only the first forms of the short-faced kangaroos, early test subjects created to make sure the genetic isolation equipment was working as intended, remained on the planet. Eventually, since the developments that stabilized their genetic structure were removed from history, the short-faced kangaroos began to fall prey to chromosomal diseases, and eventually began to die off. A small group was able to cling to life through beneficial mutations, but the interference of humans led to the extinction of that group as well later on.
Did you really read this far down, or are you just looking to see how long this gets? Don't worry, I'm done. Here you go, anybody that finished reading all that, have a cookie for putting up with me!