Normative ethics must be informed by nature.
There is no meaning to existence, but there is a purpose to life.
The purpose of life is to pass on one's genes to their children, and then ensure that your children pass on those genes to their children, then you can die.
Since you are human, you can only reproduce with other humans, therefore ethics should at least apply primarily, if not exclusively, to humanity.
It should be applied to all humans for three reasons, first because you, and your offspring, need genetic diversity in the long run, second because as a human, you share genetics with every other member of your species, and third because humans are a species that has it's central advantage in it's ability to co-operate in large numbers, our development of philosophy and science is a testament to this.
Now that we can establish who ethics applies to, and the ultimate end goal ethics seeks to achieve, we can also look to nature to find out what our normative ethical code should be.
You could say that, because we desire pleasure and reject pain, desire happiness and reject sadness, desire knowledge and reject ignorance, and because we desire life and reject death, that we should settle for a utilitarian equation that maximizes pleasure, happiness, knowledge, and life, while minimizing pain, sadness, ignorance, and death.
But this is not always sufficient, as seen when people bring up runaway trolleys and unwilling transplants, so I take one thing out of Ayn Rand, the rule that any action that directly affects the body of another human being, or their property, must be done with that human being's consent.
And so I add this restriction to the utilitarian equation listed above, to place limits upon what actions are justified under it.