EPQ

Super interesting topic! This, to me, combines a lot of debates in philosophy, law and jurisprudence.
I'd suggest the first place to start is with a literature review - see what kind of arguments and opinions are out there already.
I'm far from knowledgable on this particular topic, but here's a way I'd approach mapping out the literature:

Firstly, what does it mean for anything to belong to anyone?
It'd be good to operationalise the term 'belongs' in some way.

There'll be legalistic definitions, which might tell you who owns something as seen by the law (e.g. whether I fairly purchased it from someone else).
But I get the sense that's not going to be particularly useful - you don't care who 'owns' the Earth as a matter of the law, but more, who should own the Earth.

There'll be definitions based in theories of rights - about what I can claim, and what duties that confers on other people.
John Locke had a 'labour theory of property' that basically claimed that you gain ownership over something by working on it /improving it (so long as there is "at least enough left in common for others"). This idea may be especially relevant to your question - it's a highly controversial theory that people have argued about for hundreds of years.

Or you might want to start with distributive justice - the question of how we should fairly allocate resources between different people.
Then, belonging becomes a normative question - you're not trying to say who owns what, but rather, who should own what.
You'll get different answers for this depending on your theories of justice and ethics.

Having mapped that out, you can begin to connect it to your actual question; does the Earth belongs more to human than animals?
You might want to go a few different ways here, depending on how you want to structure your argument.
You could maybe:
(1) Compare lots of different people's ideas about belonging/ownership (or come up with your own!), argue for the one you think is most plausible, and then apply that concept to your analysis.
(2) Stay agnostic about which concept is best, and instead, compare how different concepts change the conclusions you reach.

For any specific concept of what it means for X to belong to Y, you could start to investigate how humans and animals fit into the picture.
If you have a particular concept, you could see what the necessary and sufficient conditions of ownership are, and then see if either humans or animals have those qualities, and how they differ.
(Maybe humans have this quality and animals don't - maybe human's and animals both do, but in differing degrees. If so, where do you draw the line? What if a specific animal ends up having this quality more than a specific human?)
This is going to start to involve questions of what makes humans and animals different, which gets into a whole other world of literature, which would be useful to map out in a literature review.

So quickly to summarise, I'd say the best way to get started is to begin a literature review, seeing how different scholarship fits in to your question.
I'd begin with looking at:
- what it means for anyone to own anything
- who should own different things
- the ways in which humans and animals are different (that are relevant for the above two questions).

Again, I want to emphasise that I'm not very knowledgable about this topic. Please don't treat this as a perfect guide, and talk to your supervisor about what steps to take. As a philosophy student, this is how I'd personally approach it.
When I did my EPQ, it was on a very different (much less interesting) topic, which required a different approach.
But hopefully, this can give one perspective of a way to approach the question.