In this universe, the main survival tactic and defining trait of a human is their ability to breed with almost every other animal species. They're kind of like parasites. Their reproductive organs are built to adapt to other animals', which has given humans a cruel reputation, a massive ego and a powerful status. They can create children with the limbs, skin, eyesight, and unique abilities that other creatures have, even if it comes at the expense of their regular human functioning (being bipedal, having superior intelligence, the ability to breathe above water, etc.). Usually, human traits are dominant, and the other animals' are recessive, just add-ons or changes that enable better survival. But not always. For example, a human who breeds with a bluebird may produce an offspring who is born with wings instead of arms which enables them to fly (most likely), or a human-looking child with the mental capacity of a bluebird (about a 30% chance), or they just give birth to a pure bluebird (extremely rare, but has happened).
In the current century, there is a medical/social test taken every five years that determines how "human" you are. The scale is on a line from 0-1. About 45% of the population is now at 1, meaning they are totally, indisputably, wholly human. 35% are in the 0.7-0.9 range, meaning they may have small visible features but overall are functioning and looking like a stable human. 19% are still visibly and mentally 0.3-0.6. The final 1% are those with a score of 0.2 or lower; they are considered "non-human" due to their mental functioning or appearance, and, even if they are welcomed and supported, they just cannot fit into society.
People with a score of 0.6 or lower are required to attend special schools that provide support (some schools are better at this than others) for their varying animalistic qualities/needs/desires. Above 0.6, are the people who can attend regular schools or, if desired, schools that are structured for people who still have that little bit of animal blood but don't require counselling, medication or behavioural therapy regarding the animal parts.
I won't get into the nitty gritty details here (class, schooling, mental health, politics, biology, etc.), because this is all the information necessary to understand Atlas and Lou's places in society, and their feelings towards others.