This question kind of contains all meaningful questions concerning morality, ethics, right and wrong and in general all conceivable oughts. Even when we ask if we have the right to decide this for some other species, we're simply asking what we truly want - what kind of rights we wish to grant the other species. It's all on us whether we want it or not.
Of course we also have to ask ourselves more specific questions like what we want as a group or as a species and at the same time what we want as individuals, but these are simply extension of the "other species". These are the only meaningful prescriptive questions. Even if you're a religious person, the situation doesn't change very much. Your deity might act as an additional source of prescription to you and you might think you have access to their prescription as a description, but I posit that you are still faced with the exact same questions as the rest of us who don't believe. You need to ask yourself how you wish to value your supposed deity's prescription and determine what, if any, describes your deity's prescription most accurately. It's all on you. Even the methods are exactly the same, your very own reason and judgement.
What is the origin of their will and motivation ultimately? Why they feel thighs ought to be one way instead of the other? My limited understanding is that our motivation is a result of natural selection, history of our species, our personal histories and partly because of the history of our societies. However, going back even further, it would appear that these are simply arbitrary coincidences, some of which have served our species well in the battle for survival, some less so.
These are arbitrary preferences. Generally people prefer to continue to exist and avoid suffering for example, but what is generally considered moral varies from group to group and the extent that it doesn't vary, appears to be mostly simply due to our biological similarity.
Knowing this does not limit us from acknowledging the consequences, for example, we understand it is likely in our interest to grant certain degree of equality to other people, given we don't know how our own lives are going to evolve. We might be the other people one day. It's like gambling, but for a rational person, it's not really gambling, it's investing. It could fail, but it's still better to play the stock market than it is to play the lottery.
Unavoidable conflicts between dissimilar groups are perfectly expected, but luckily most groups are at least somewhat rational and optimize by compromising. However, it is conceivable that conflicts that cannot be solved by negotiation might emerge. These are normally called wars and the worst ones are about survival and extermination rather than anything that could be reasoned.
Once we've determined what we want as a species, we only need to figure out how to best approach that goal. There basically exists only a single method to answer these question for us - science. Science is nothing more than a name for the rational process of fishing out the best model out of all the possible descriptions out there by any means imaginable. Science gives us a description of the universe including its inhabitants - us, and tells us how to best achieve our goals and what outcomes to expect given specific choices. It doesn't tell us what we ought to do. That task is left for us.
Every decision has consequences, typically both good and bad, often unavoidable. There are many ways to make a decision, none of which are necessarily any better than the other. We can aim for least suffering, most pleasure, least boredom, maximized fairness, longest life, maximum number of lives, maximum knowledge, maximum safety, etc. Many of these are mutually exclusive. Most pleasure could mean most suffering. Longest life could mean most boring life. Least suffering almost certainly means minimum number of lives. We can expect negative consequences due to our decisions as well as positive, there will almost certainly always be some of both. Maximizing the number of people is not going to maximize the quality of life for an individual, it might not even maximize the total sum of happiness. Not to mention it probably won't maximize the long term total sum considering all the generations to come who will have to live a life of scarcity when resources have been depleted by the previous generations. Some alternatives are obviously excluded, like maximum suffering. Although, it's not entirely clear we know how to do this either, because what is bad for the humanity now, might be good for the humanity later if we manage to save the planet and its resources for times when they can be more efficiently used. Economic growth tends towards faster depletion and increased rate of destruction. Diseases limit the number of people and save the environment. None of these alternatives are trivially good or bad. Never the less, to deny the nature of this answer, not to mention the existence of these questions and knowledge concerning them is to deny the truth and to deny who we are. We know that doing one thing now will favor some and hurt some others. Not deciding has its own consequences as well. That is the nature of the game. We shouldn't just ignore it.