There's a possibility that humanity is the only sentient species (sentience in this case being narrowed to mean building cities and creating artificial technology) in the galaxy. There may be no other civilization in the galaxy. For some odd reason, humanity could be all by itself, the lone success amongst billions of failures or non-starters (and it's way too soon to even declare ourselves a "success"). However, even if humanity is the lone civilization in the galaxy, it will not remain so.
We have already begun our first steps toward the Offshooting of humanity, when homo sapiens will branch off into many technologically-augmented species. We have bionic limbs, bionic eyes, exoskeletons that can roll back paralysis, implantable technology such as medical sensors, and glasses and contact lenses that will allow us to go online without a phone or computer. While the success of the current generation of computer glasses is very much in doubt, I'm sure that eventually they will be accepted and widespread. It may not be Google who sees that success, but someone will.
But it will merely be the start. As we make strides towards claytronic atoms, utility fog, and DNA manipulation, the very biology of humanity will be ours for the remaking. The very first step will be treatments that eliminate obesity. Now, I have no opinion about obesity either way; however, I do believe that those who are obese would like to truly have a choice as to whether they are obese or not. The obesity treatments will be followed (or possibly preceded by, if forecasts are accurate) by tooth regeneration which will make everyone's smile beautiful. Imagine a world where no one is overweight or suffers from cosmetic defects (such as missing teeth). Some would say this makes for a boring populace; I think it just levels the playing field.
Humanity will eventually use genetic manipulation and cybernetic upgrades to give themselves increased abilities. These upgrades will create the first real Iron Men, the first Captain Americas. Of course, they will eventually find themselves in the possession of criminals, creating the first Banes and Iron Mongers. And finally regular people will possess the technology, just as regular people now can buy guns. Vast debates will rage about the ethics of proliferation, as gang wars become battles between hundreds of criminals clad in power armor or empowered by cybernetic and genetic enhancements. But eventually, even these abilities will be surpassed when humanity finally uses claytronics to redesign the human body so it can be remade at will. We will become a species of shapeshifters.
And that's what will create the Offshoots.
When people are given the ability to shapeshift, humanity will explode into a million subspecies. Some of these species will be based on mythology. We will see real centaurs, real fauns, nymphs, leprechauns, pixies...the list is endless. Fictional species will break into reality. Romulans, Vulcans, Klingons, Ferengi, Shi'ar, Skrulls, Kree, Orcs, Dwarves, Elves, Mon Calamari, Goblins, Gremlins...on and on. Entirely new species based on nothing but the imagination will spring into being. They won't just look the part; the people who choose to become these new species will do so because they want to be these species. They will adopt the clothes, the food, the culture...everything. It will be insane, and Original Humans will try to stop the Offshoots, to no avail.
Some humans will take things too far. They will become the first Changelings, the first Borg, the first Kzinti, the first Daleks and Cybermen, the first Sith, the first vampires, werewolves, etc. The Offshoot Wars will last hundreds of years, maybe even thousands. Eventually the forces of justice and sanity will triumph over the followers of chaos and carnage, and the defeated enemies will be exiled to the farthest reaches of the galaxy. There they will build empires and prepare for the inevitable rematch.
Those who follow religions such as Christianity, Islam, and Judaism will no longer be content to merely wait for their ascension into a Heaven that may or may not exist. Despite fierce and brutal suppression from the orthodoxy, the religious will remake themselves into the very angels they worship. They will create new empires amongst the stars after the last religious war on Earth, exiled by those who choose to not chain themselves to an ethereal faith. All of this is at least a thousand years in the future, maybe even ten thousand. But it will happen. Because once technology catches up to the human imagination, all bets are off.
But what we do to ourselves pales in comparison to what we will do to animals...